what exactly is a tesla coil

8/6/2019 What Exactly is a Tesla Coil

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o what exactly is a Tesla Coil?

So what exactly is a Tesla Coil?

"But Captain! Ye cannae change the laws o' physics!"

Chief Engineer Scott, USS Enterprise

here remains nagging away in the back of the mind the idea that the exact mechanism of operation of asla coil has still not been fully disclosed. This bothered me not at all to begin with, as my interest layth the spark transmitter and the Tesla coil was a mere dummy load and nothing more. Eventually,wever, I had to concede that the blasted thing would not leave me in peace until I had got to thettom of it. I pondered for an excessive amount of time over this, until gradually the concept began to

merge from the murk. All this Tesla coil stuff seems vaguely familiar from somewhere . . .

an remember from 25 years ago, one of my chemistry teachers asking the class "What would happenyou put a bottle of electrons in the middle of the lab?" He then proceeded to give the wrong answer,at everything in the lab would immediately rush towards the bottle because of its net charge. Actually,u and I know the right answer to that question. The electrons have so much less inertia thanerything else in the lab that it is the electrons which do the moving, bursting out of their bottle andshing in all directions towards the induced positive charges on the surface of each earthed object in thecinity. The important thing of course is the charge q, i.e. the number of electrons stuffed into that

ttle. There are 96500 coulombs in one faraday of electricity, and each coulomb is equal to 6,24 x 10ectrons. The more electrons in the bottle, the greater the repulsive force developed between them, theeater the induced charges in the surroundings, and the greater the attractive forces between theectrons and their surroundings.

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he Tesla coil, it turns out, is very closely related to another high voltage machine, the Van de Graaf nerator. Perhaps it would be more appropriate to state that relationship the other way around, since thesla coil predates the Van de Graaf by at least half a century.

xcluding the rotary spark gap, if any, a Tesla coil is a Van de Graaf generator with no moving parts.he rotating insulated belt which carries the unidirectional charging current in the Van de Graaf isplaced, in the case of the Tesla coil, by the rotating vector of the sinusoidal oscillation which carriese bidirectional charging current. The essential physics of the charging of isolated electrodes is theme for both. The dielectric breakdown of air is but little different, a relatively minor modificationing required on going from dc to low frequency ac, to high frequency ac. The Van de Graaf generatorsimply a dc version of the Tesla coil. Whereas it has been found progressively more difficult tocrease the charging current of Van de Graaf machines to much above a few milliamps, the chargingrrents associated with Tesla coils are measured in amps. This is because the charge q on the topectrode makes the journey from earth and back again in microseconds, and current is simply the rate of arging, dq/dt. As the frequency is reduced, the current for a given charge falls, and this helps toplain why Tesla coils tend to get more efficient as the frequency drops, as the resistive in-phase i 2Rsses for the circulating, wattless power in the secondary system (and primary) are generally smaller.nce the current falls with frequency, it also explains why the charging current for a Van de Graaf of e same top electrode capacitance is so much less at dc than for a Tesla coil at ac.

ctually, this is not the whole story and in fact there is something unexpected about the role of sistance during the exchange of energy between the primary and secondary. Whilst resistance certainlyimportant in causing losses in circulating, wattless power in a tuned circuit, in the electromagneticnsfer of that power between circuits, we have a different situation. The losses in connecting a chargedpacitor to an uncharged capacitor via a resistor are independent of the resistor value, and depend onlythe relative capacitor sizes and the magnitude of the charge. The efficiency of the transfer of energy

tween primary and secondary therefore depends very little upon the efficiency Q, provided it isough to ensure magnetic coupling which will be the case if Q L is at least five.

is very unfortunate that the symbol Q can have two meanings, either a measure of the efficiency of apacitor, coil or tuned circuit, or alternatively a quantity of charge in coulombs. In what now follows, Qd q are both amounts of charge. The following derivation is due to the late Professor Cotton of ottingham University.



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C 1 is charged to a voltage V 1 and a charge Q is stored on its plates, and C 2 is uncharged, i.e. V 2 =

ter a time, a charge q will have been transferred to C 2. The potential differences across the capacitors

ll be:

d hence

herefore

tegrating

herefore

ow q=0 when t=0; therefore



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hus

ate of change of energy = i 2R

ence for the total energy lost in the resistance R we have



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om which it can be seen that the amount of energy lost in the transfer is independent of the value of thesistance R, the product i 2R remaining constant as R is varied. It is the relative sizes of the twopacitors and the charge on C 1 (or in other words the voltage to which C 1 is charged) which determine

e efficiency of the transfer. (The late Professor Cotton's contribution ends here and Q reverts to being aeasure of efficiency.)

his obviously has application to the Tesla transformer:

the two circuits are linked by an ideal transformer, with coupling constant k=1, transformation ratio T

d no leakage inductance, nor reactance, nor resistance, the transformer and C 2 can be replaced byother capacitor, C 2'.

he value of C 2' is now given by the equivalent of C 2 as seen in the primary circuit by C 1 and R. Thismply C 2 multiplied by the square of the transformation ratio, T 2. In other words, the amount of energy

st in each cycle of oscillatory power transfer depends only on the magnitude of the charge beingnsferred and the relative sizes of the capacitors. Evidently then, the most efficient transfer overall willcur in one half cycle, i.e. the most efficient transfer of energy corresponds to critical damping. Thisll occur if in one half cycle the top load on the secondary has sufficient charge deposited on it to causeelectric breakdown of the air. Frequently, however, this is not the case, and a number of cycles elapsesth the charge on the topload increasing at each successive (positive or negative) peak of the oscillation



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fore dielectric breakdown can occur; in other words, the secondary system acts as an integrator for ac,ther like the inverse of logarithmic decrement. We might thus call it logarithmic increment.

owever, whilst coil resistance has little if any effect on the efficiency of energy transfer betweenimary and secondary capacitors, it should be clearly remembered that the losses in the coils do mostrtainly effect the amount of energy stored in the tuned circuits as circulating, wattless power. Now thises not matter much with spark transmitter-driven coils because that energy doesn't circulate for more

an say half a dozen cycles prior to spark breakout, but with valve transmitter-driven coils it's afferent story, of which more later. Thanks to the fact that in the spark transmitter-driven coil, theimary capacitor does the job of wattless energy storage when it is charged by the power supply, energycumulation by the Tesla secondary is not required, and the most effective spark-driven coils arenerally those which have a large energy storage in the primary and good quenching at the gap. A largecondary topload is vitally important in these systems, as we shall see.

n de Graaf generators usually don't give the spectacular spark display of a Tesla coil simply becausee charging rate is too low and as the charge on the top electrode raises the potential to breakdown it

mply begins seeping away by corona discharge and an equilibrium is rapidly reached with the chargingte, no spectacular breakouts ever occurring. The charging rate with a Tesla coil - particularly a spark nsmitter-driven one - is so high that no chance is given for equilibrium to be attained prior to spark eakout. On occasion you may see film footage of the insulating pressure vessel being lifted off a smalln de Graaf when it is still running. Under an inert atmosphere, e.g. sulphur hexafluoride, the chargethe top electrode of a Van de Graaf can reach similar proportions to a Tesla coil, and then as the

essure vessel is lifted and the gas drifts away, very briefly, you get a short-lived explosion of sparks,st as with a Tesla coil.

he physical similarities do not end there. Whilst it appears at first sight that the Tesla top load is notsulated from ground, the turns of the secondary coil do in fact perform the same function as the stack equipotential rings often used in Van de Graaf generators, i.e. they provide a graded potential

fference along the length of the support. In fact, as the number of turns increases, so each turnproximates more nearly to an equipotential ring. If we consider the top electrode to be instantaneouslyarged, then it can be seen that the self-inductance of the coil will act as a perfect insulator on anstantaneous timescale because it resists changes in current flow, and the turns are indeed equipotentialngs under these conditions.

A Wee Diversion

has been noticed that whilst sinusoidally-driven Tesla coils (e.g. powered by valve power oscillators)ow as expected no signs of net polarisation, spark-driven coils do on occasion show a net dc offsethich registers on an electrostatic voltmeter placed in the neighbourhood of the working coil. Whilst itpossible that there is some polarisation effect at the sparking top load this is unlikely, due to thetreme voltages and if indeed this were the case it might be expected not only to occur with more coils,



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cluding sinusoidally-driven ones, but to be of the same polarity each time, neither of which is true.

far more likely explanation lies in the nature of the ground contact to which the base of the secondaryconnected.

q The Frenchman, Professor Edouard Branly invented his 'coherer' over a century ago, and itdepends for its operation on as-yet-unexplained conductivity mechanisms between conductingparticles under damped wave excitation. This effect is entirely absent with sinusoidal stimulation.

q It is well known that if it is desired to measure the resistance of a ground connection, that ac mustbe used and not dc, else electrolytic polarisation takes place and the ground resistancemeasurements are totally false. Moreover, they vary with polarity.

q It is also known, thanks to the work of Fessenden, that a thin contact point of wollaston wirebarely dipping into an electrolyte (nitric acid) solution has the ability to rectify radio frequencyoscillations.

seems likely that a combination of these effects is responsible for partial rectification of the excitationplied to spark discharge driven Tesla coils, and that the difference in ground connection from one coilanother is responsible for the variable reports of net dc polarisation of Tesla coils. The exact nature of e ground connection, where the metal meets the soil, will vary tremendously with geology andoisture content and will be very different from one location to another. If this is the true explanation,en it would be expected that the coils showing this effect would also show a significant secondrmonic component, since this is to be predicted by Fourier transform theory if rectification iscurring. Interestingly, whilst these polarisation effects have been reported with "quarter wave" coils,ey are apparently absent from "half wave" coils.

hether or not this be the case, it may also prove possible to introduce and control a deliberate net dcfset by installing e.g. a thyratron in the earth lead, by means of which proportional control of thesulting unidirectional pulsed charging current could be effected by the usual means applied to dimmer

witches and the like.

return to the charging of isolated electrodes. Classical physics tells us that there is a voltage gradientoduced at the surface of a charged conductor, and that ionisation of the air will occur when thisadient has a value of between 2 and 3 megavolts per metre (20-30kV/cm). The variables are pressure,mperature and particularly humidity, and frequency of the voltage if it is alternating. The gradient ate surface may also be influenced by the total charge, fairly obviously, and also by the geometry of therface, a sharp radius enabling ionisation at a much lower potential than a gently rounded surface. For, there is a simple relationship between the breakdown potential and the radius of the conductor.

we assume that we need three megavolts per metre to bring about ionisation, then we can calculate:



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ote - the capacitances of the spheres are shown for convenience and of course do not depend on theeakdown voltage)

Breakdown Voltage, kV Sphere Diameter, cm Capacitance, pF

50 3,3 1,8

150 10,0 5,7

200 13,3 7,4

250 16,7 9,3

300 20,0 11,1

350 23,0 12,8

400 26,7 14,8

450 30,0 16,7

500 33,3 18,5

600 40,0 22,2

750 50,0 27,8

1000 66,7 37,0

hilst the capacitances of toroids will vary considerably according to major and minor diameters, theeakdown voltage of a toroid of a particular minor diameter will be very similar to a sphere of equalameter, the curvature of the sphere or toroid determining the surface potential gradient. The aboveble may thus be used to estimate the breakdown voltage (noting that it will vary according tomperature, pressure, humidity, frequency) of a given toroid. Example: a toroid has a minor diameter of x inches. This is about 15cm. From the table, a sphere of 13,3cm has a breakdown voltage of 200kVd a sphere of 16,7cm a breakdown voltage of 250kV. The toroid thus will have a breakdown voltagethe order of 225kV dc.

e now need to ask how much energy is needed to produce this voltage, and if we are dealing with aark transmitter-driven coil then according to the accepted formula it will be CV 2 referred to thecondary capacitor (losses are responsible for making this approximate when referred to the primarypacitor). However, we may wish to ask a slightly more intelligent question: how many electrons do weve in the bottle, and is there a better way of getting them there?



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om this it can be seen that the charge q increases with capacitance. If C increases (from the middleuality) then q 2 must increase for the same amount of energy. The potential required to stick theectrons in the bottle will decrease, i.e. the work required to put the electrons in the bottle will be lessigger bottle = more room for the electrons = less mutual repulsion). For spontaneous breakout of aark, the top load must then be given a smaller minor radius if it is a toroid, or a sharp spike or elevated

mall sphere should be attached.

ith a large secondary capacitor, there will be more electrons in the bottle for a given energy.

his is the best reason for large top capacitances on Tesla coils. It has little to do with coupling oratching, and nothing whatever to do with the theory propagated for years (around eighty years) that theark length produced by the secondary has something to do with the Q of the secondary. That theory is

ongly applied to spark transmitter-driven Tesla coils, it applies to double tuned intermediateequency transformers (IFTs) which, though related closely to Tesla coils, are in subtlety very different,gely because of the fact that IFTs are fed by continuous waves, and it predicts the largest sparks for

e smallest secondary capacitances. A couple of IFTs are shown below. The coils and capacitors can beearly seen.

he theory given above is in agreement with experiment, that it is the total charge on the top electrodehich determines the spark length and this increases with secondary capacitance for spark transmitter-iven coils. The only useful function performed by Q in spark transmitter-driven Tesla coils is toovide for an efficient transfer of energy from primary to secondary under conditions where an ironre cannot be used for obvious reasons. Here it is sufficient for the unloaded Q of the secondary to be



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y at least ten, since all it needs to do is couple the wattless power accumulated by the primarypacitor. Hence my old 500VA coil secondary, battle scarred by direct strikes and with a measuredloaded Q of just 35 gave just as good results as when it was new.

owever, valve-driven coils need a high secondary Q. This is because the secondary system of a valve-iven coil has to perform the wattless power accumulation which in a spark transmitter-driven coil isrformed by the energy storage on the primary capacitor prior to spark discharge across the primary

p. In the case of valve-driven coils, a small top load will prove beneficial because the Q - important invalve-driven circuit - decreases with increasing capacitance, just as with an IFT. Here, spherical topads are likely to be better than toroids, since a high breakdown voltage will allow a large charge priordielectric breakdown of the air, and the small capacitance will keep the Q high. Evidently there is ampromise to be made between the maximum charge for a given energy and the large Q which iseded to allow the necessary charging current to be built up over a large number of cycles.

his is one of the reasons why the theory developed for IFTs cannot be applied in the same way to spark nsmitter-driven Tesla coils - the IFT is a "continuous wave" system whilst the spark transmitter-

iven Tesla coil is in essence a "one-shot" system. The other reason why the theory which wasveloped for intermediate frequency transformers cannot be applied to Tesla coils generally, in exactlye same way as it is to IFTs, is simply due to the difference in the form of the secondary capacitors. Ine IFT the secondary capacitor is of the conventional multi-plate type, and any net charge on the "live"ates induces a corresponding opposite charge on the "earthed" plates, which are physically very closethem. The electrostatic fields from these charges are equal and opposite throughout all space, exceptthe very close vicinity of the plates, and it is only between the plates and in the dielectric where a

gnificant field exists. (This is analogous to the cancellation of fields found in parallel conductornsmission line systems.) Even this is minimised by the presence of the dielectric, since a dielectric

orks by reducing electric fields by polarisation. There is thus no net field external to the capacitor of y consequence, and the only voltage which is measured is the potential difference between the plates,hich increases as the capacitance is reduced.

the Tesla coil, the secondary capacitor is an isolated electrode, effectively in "free space". Theectrostatic field developed by any charge on this electrode does indeed induce opposing charges onrrounding objects in the vicinity, but these are removed by some considerable distance and the localld is not counteracted by them.

he local field generated by the charge q is given by:

here q is the charge in coulombs, r is the radius in metres, 4 0 = 1,11265 x 10- 10 and E is the

ectrostatic field in volts per metre. Thus we have the apparent paradox that the biggest sparks are



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oduced in the spark transmitter-driven Tesla coil when the potential across the Tesla secondary is lowd the top capacitance large. It is the charge q in the bottle which "does the damage", exactly as for then de Graaf generator. The spark length is dependent on the charge. If we have two Tesla coils, one of

hich has double the charge on its topload than the other, the greater charge will produce a spark 1,4mes (square root of the charge ratio) as long, to a good approximation.

he potential in volts at any distance from this charge is:

m still puzzled over the exact mechanism which determines spark length for a given q, undoubtedly itvolves around an equilibrium of forces, but am reasonably assured that this will follow given thatfter eighty years of misunderstanding) the mechanism of operation of the Tesla coil is now plain. I amormously relieved that it can be explained in terms of known theory.

"Great are the deeds of the Lord!They are studied by all who delight in them."

Psalm 111 v 2

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Maximising Power 1

The Spark Transmitter. 2. Maximising Power, part 1.

"I think a famous French mathematician and physicist was guilty of only slightexaggeration when he said that no discovery was really important or properly

understood by its author unless and until he could explain it to the first man hemet on the street."

Sir J.J. Thomson

y means of a suitable condenser, inductance, spark gap and high voltage source it is possible tonerate power at r.f. (radio frequency). Having generated power at r.f. it is necessary to ensure that it is

nsferred efficiently to the load, be that an aerial or Tesla coil, where it can do something useful. Theestion of "how?" is now one to be considered in greater detail.

aximising Output.

onsider a generator connected to two resistors in series. Call the resistors R 1 and R 2. If the generator

oduces an emf E volts, then the voltage across R 2, call it E 2, is equal to:

d the power P 2 dissipated in R 2 is equal to:

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Maximising Power 1

his value is a maximum when R 2 = R 1 and the power dissipated in R 2 then becomes:

ow imagine that R 1 is inside the generator itself. The above expression for the power P 2 in R 2 is the

aximum which can be obtained by connecting a generator having internal resistance R 1 to the

sistance load R 2. This is a very important result and is generally called the maximum power theorem

demonstrates that the maximum power transfer can only occur when the two resistances are equal, ande generator and load are then said to be matched.

esistances, Reactances, Impedances and Resonance.

hat has been stated above is not only true for circuits containing pure resistance. In a circuit containingcoil and/or a capacitor, there will not only be the resistance of the coil to consider, but the reactancescoil and capacitor too. The reactance of a coil or capacitor can be thought of as the resistance of thatmponent to alternating current (ac) and because the frequency of ac can obviously vary, the reactancea coil or capacitor is frequency-dependent, or in other words, the resistance of a coil or capacitor to

e passage of ac depends on the frequency.

he reactance is given the symbol X and is measured in ohms as is a resistance. The formulae forpacitive and inductive reactance are:

his begs the question, what is the real difference between a resistance and a reactance if both areeasured in ohms? It is necessary in ac circuits to take account of the relative phases of voltage and

rrent, and the difference between a pure resistance and capacitive and inductive reactances is one of ase. In a purely inductive circuit, the voltage leads the current (this is called a positive phasefference and is shown by +j) and in a capacitive circuit, the current leads the voltage (a negative phasefference shown by -j, j being the square root of -1). In a purely resistive circuit, there is no phasefference. In other words, the difference between resistive and reactive ohms is not real but imaginaryorry about that.) Only resistances consume power, because for power to be consumed, the current andltage within the component must be in phase. Reactances do not consume power (aside from anysistance they may have) but take it from the supply over part of a cycle and give it back over the rest.e might then amend our formulae to take account of this, and, whilst doing that, we can rename the



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Maximising Power 1

f bit, which appears with monotonous regularity, as the Greek letter .

evitably in a circuit containing reactance there will always be resistance too. The combination of actance and resistance is called "impedance". This too is measured in ohms and is given the symbol Z.the coil or capacitor is efficient, the resistance can often be neglected (this is particularly true of pacitors) and the impedance is then numerically equal to the reactance. In other cases we mustlculate the effect of the resistance, and in a circuit where there is a combination of significantsistance, capacitive reactance and inductive reactance, we must calculate the net result thus:

there is more capacitive reactance than inductive reactance, the value of Z will be somethingultiplied by negative j; if there is more inductive reactance, Z will be something multiplied by positiveWhenever the squares of the inductive and capacitive reactances do not come to zero under that squareot sign, there is net reactance (shown as +j or -j) present in the impedance Z, and Z is said to be aactive load.

ow, as we said above, only if the generator is suitably matched to the load will the maximum power bensferred. In the case of reactive loads, it is not enough for the generator to have the same impedance -

e impedance Z of the generator must be the "conjugate" of that of the load - conjugate effectivelyeans "equal and opposite" - and if the impedance of the load contains positive (inductive) reactanceen the impedance of the generator must contain negative (capacitive) reactance of equal magnitude.sssst - this of course means that the load and generator together are resonant!]

appily, this complexity is greatly reduced in the case of resonant tuned circuits as loads, because theactances at resonance of capacitor and coil have equal and opposite phase differences and the loadpears to the generator to be a simple resistance - the impedance Z of the tuned circuit as a whole is are resistance because the squares of the capacitive and inductive reactances under that square root signm to zero.

have to confess a marked objection to the various statements sometimes seen that the reactancesancel out". The reactances are not sentient beings! Coils and capacitors do not sit and conspiregether to reduce their reactances to zero as the resonant frequency is approached! The reactancesthin each of the components are still very much present at resonance as is the relative phase differencee to them (if you were to take the components out of the circuit and measure their individual



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Maximising Power 1

actances at that same frequency you would see it) but the net phase difference across the inputminals becomes zero because phase differences, unlike reactances, do cancel and all the generatores is a pure resistance. Hence the conjugate impedance required of the generator to get the most powerto the load is also a pure resistance. As we saw in the paragraph above, a reactive load must be drivena reactive generator having a conjugate impedance, and a vital consequence of the maximum power

eorem is therefore demonstrated by tuned circuits themselves.

he Maximum Power Theorem inside Tuned Circuits.

ny tuned circuit at resonance obeys the requirements of the maximum power theorem. This is because,resonance:

he impedances are "conjugate"; in terms of the reactances we know that X L = -X C. This equationrresponds to two physical realities:

The capacitor is fully charged and there is no current passing through the coil. This situation isstable and the capacitor spontaneously discharges via the coil, creating a current in it which is linkedth a magnetic field. The energy stored in the capacitor (potential energy) is exchanged for the energysociated with the magnetic field and flowing current (kinetic energy) in the coil. In other words,

he capacitor is the generator, and the coil is the load.

The current flowing in the coil is at a maximum and so is the associated magnetic field. The capacitorfully discharged. Unfortunately, because of this, at this very moment the power supply fails. Theuation is unstable. The current flow ceases momentarily, the magnetic field gradually collapses, and arrent is induced in the coil, passes out of the coil and charges up the capacitor. Kinetic energy ischanged for potential energy, the above formula is effectively written the other way around, the coils become the generator and the capacitor has become the load.



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Maximising Power 1

nce the impedances of coil and capacitor are conjugate, the maximum power is transferred fromansferred from, not consumed by!) one to the other at the condition of resonance. If there was nosistance in the circuit, this exchange or oscillation would continue indefinitely because the current andltage are out of phase in each branch of the circuit and no power is consumed; the damping andcrement would be zero and the Q infinite. Moreover, if the exchange of energy is attempted at anyher frequency than the resonant frequency, the impedances are not conjugate and less than theaximum power will be transferred from one to the other.

wer is not consumed by the tuned circuit, except in any resistance which may be present, but isnsferred from one component to the other and back again. This enables the tuned circuit to act as art of "accumulator" for radio frequency power and in a transmitter which is sending say 500W to therial and taking say 750W from the power supply, there may be over 7kW circulating in the tank cuit. The phase difference between voltage and current at resonance is equal and opposite in the coild the capacitor and so at the terminals of the circuit there appears to be no phase difference at all. It'sll there for each component - you just can't see it - the reactances don't "cancel", but the phasefferences do. The tuned circuit at resonance appears to be a pure resistance because of the cancellation

the phase differences at the terminals, but it obviously is not exactly the same thing as a puresistance because a resistive load by definition consumes power and converts it to heat, light, radioaves, mechanical energy etc. This is particularly evident in a parallel tuned circuit as shown, since atsonance the apparent resistance is very high (theoretically infinite).

his in fact tells us a lot about loads which are not at resonance but contain net reactance - a reactivead receives power from a generator for part of the alternating cycle, and gives it back over anotherrt. Since the voltage and current are not in phase, power is not actually consumed except by thesistive component of the load. Such unconsumed, circulating power is called "wattless power" because

doesn't do any work. Electricity meters still measure it however, and you still pay the bill for it, so it isexcellent idea to minimise the wattless power taken by any electrical appliance, which of course we

n do by cancelling the phase difference present at its input terminals. This is called "power factorrrection", is generally achieved by connecting appropriately rated capacitors across the mains input toe appliance, and a purely resistive load on the mains supply has a power factor of one, meaning thereno wattless power for which you are being charged. Not only is this advantageous to the consumer,t it would be a disaster for the power company if the distribution system was feeding a huge reactivead with massive out of phase components (remember the maximum power theorem!) and powermpanies require that all large loads are power factor corrected. All appliances which you buy and

hich have a significant reactive component to their load characteristics are power factor corrected bye manufacturer, so please don't try "tweaking" them!

he next installment will examine the meaning of Q and how to achieve an impedance match byductive coupling.

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Maximising Power 1

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Maximising Power 2

The Spark Transmitter. 3. Maximising Power, part 2.

"What may seem to the uninitiated a simple explanation, may appear to theexpert as a most round-a-bout way of stating the ideas; the technical words are

the short cuts."

C.R. Gibson, F.R.S.E.

nloaded Q and Loaded Q.

he parameter Q has been mentioned several times and it is now appropriate to consider what is meant by

The definition of this quantity is simply the ratio between reactance and resistance, or in other words,attless power divided by in-phase losses. It can also be expressed as the amount of power circulating in asonant circuit divided by the amount of power which must be added per cycle to keep the oscillations atnstant amplitude. There is a variety of definitions therefore, but all with the same underlying meaning;is a measure of efficiency and is frequency-dependent.

nloaded Q refers to the Q of the coil, capacitor or tuned circuit itself with no other factors considered.his might reasonably be the Q value measured on a Q meter, or the Q of a Tesla secondary operatedthout ionisation of the air or spark breakout.

aded Q might refer to the same component or circuit, but now under operating conditions such that it ispected to be coupled in some way to a resistive load in which power is being dissipated. Thus it couldthe Q value of a valve transmitter output tank circuit when that transmitter has been properly adjustedfeed an aerial, or, what is effectively the same thing, the Q of a Tesla coil primary circuit when the coilin operation and sparks are being drawn from the secondary.

he difference comes about very simply because coupling a tuned circuit to a load reflects the resistancethat load into the tuned circuit (how much depends on the coupling). If Q is the reactance divided by the

sistance, then if the resistance goes up, Q comes down. In fact, operating a spark transmitter or Teslail in the unloaded condition is asking for serious trouble: if the primary circuit has nowhere to dump itswer which might come about through serious mistuning, all of that "output" will remain in the primarycuit, causing severe overheating at the gap with damage to the sparking surfaces and probableeakdown of the capacitor insulation. The same is true of valve transmitters, where the circulating outputwer results in a badly overheated anode, and in modern semiconductor circuits where it results in deadnsistors if the protection circuitry isn't quick enough. The efficiency of an output tank circuit is simply:



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Maximising Power 2

d the relationship between the voltage, power and Q L of a tank circuit under pure sinusoidal conditions

given by:

here P is the power, is 2 times the frequency, L the tank circuit inductance and Q L the loaded Q.

hus high unloaded Q, Q U , is necessary to ensure low losses in the components themselves, and low

aded Q, Q L , to ensure that there is not a build-up of power circulating in the tuned circuit, but is

nsferred to the load and thereby to ensure reasonable overall efficiency. Simply adjusting the couplingtween the primary and secondary will vary Q L within reasonable limits. (More about coupling and the

upling constant k later.) Tight coupling gives low Q L, loose coupling gives high Q L. In a transmitter,

s value of Q L matters for another reason, namely that it gives added harmonic suppression and

nsmitters generally compromise with a Q L value of between say 5 and 15. Lower than 5 and the

rmonic suppression suffers. Higher than 15, and the components in the tank circuit begin to get hot.

he reason for this is not hard to see. If Q is defined as wattless power divided by losses, then the lossese not restricted to heating losses - a tank circuit feeding an aerial has a power loss to the aerial, and theattless power is that being transferred between the coil and capacitor (remember the maximum powereorem inside tuned circuits). If Q L is 15, what this means is that there is fifteen times as much

culating, wattless power as there is being delivered to the aerial. There will, of course, be fifteen timesmuch heating loss as for operation with Q = 1 because whatever the circulating current, it will also pass

rough whatever resistance is present in the tuned circuit and being in-phase in the resistance, willoduce i 2R losses. A 500W transmitter at full output may take 750W from the supply if it is efficient; if e loaded Q of the tank circuit is 15, then there will be 15 x 500 = 7,5kW circulating as wattless power ine tank circuit. Likewise, in a Tesla secondary there may be a very large wattless power circulating, builtover a number of cycles, which is periodically dumped into a large spark as in-phase power as soon as

e wattless power accumulated is sufficient to bring about ionisation of the air.

a spark transmitter or Tesla coil, increasing the loaded Q up to this point restricts the power transfer to arrower range of frequencies and if the balance is right, the maximum current in the aerial (or Teslacondary) will increase, yielding better results, always assuming the primary voltage developed is within



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Maximising Power 2

e primary capacitor rating! But it isn't quite as simple as that, because the value of Q L also sets the lev

circulating current in the coil and hence the magnetic field and hence the efficiency of power transfer;L values less than about 4 are quite difficult to extract sufficient power from without an excessively high

upling constant and close proximity of the coils. The really big (read megawatt) transmitters obviouslynnot afford to use a high loaded Q in the tank circuit and operate with Q L of 3 or 4 at most; in such

ses, special techniques are employed to obtain efficient coupling and good harmonic suppression. Let's

ok at an example of valve technology (modern transistor technology tends to do things differently for ariety of reasons) in which a valve output circuit is delivering power to a transmission line of 50 ohmspedance. Let us assume it transmits on 200kc/s and that we want to design for a loaded Q of eight.

rstly we decide on the reactance of the tuned circuit components. This is simply Q L times the

nsmission line or load impedance, so it's 8 x 50 = 400 ohms. Now using the standard formulaeckwards we calculate the values of L and C at 200kc/s which will give 400 ohms reactance:

hese values of L and C will not only resonate at 200kc/s, but when coupled to a load of fifty ohms via ak coil will allow a loaded Q of eight to be readily achieved without the output link being so close that

ere is a danger of high tension flashover. This evidently has serious application to Tesla magnifiers, ase shall see later.

easuring Q.

e a Q meter! These occasionally turn up at ham radio rallies. If you have an oscilloscope and a signalnerator (couple both very loosely to the tuned circuit) you can find the resonant frequency and the twoquencies, one either side of resonance, which give 0.707 times the voltage maximum at resonance, call

em f 0 for resonance, f 1 and f 2 for the lower and the higher frequency respectively (the other coil -

imary or secondary - must not be in the vicinity when this measurement is made, i.e. the measurement isbe made on the uncoupled system). The unloaded Q is then equal to the resonant frequency divided bye bandwidth:



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Maximising Power 2

ccounting for resistance.

the real world, we must modify the equation we had for a tuned circuit which described the energyored in the capacitor as being equal to that in the coil, so as to include the losses due to resistance. Wew have:

rearranging:

is now possible to see the importance of high unloaded Q in the primary inductance of the spark nsmitter. (This of course applies also to the secondary.) The potential energy stored on the capacitor is

vided between the kinetic energy of current and magnetic field in and around the coil on the one handattless power) and the heat energy dissipated in the resistance (in-phase power) on the other. But there's

ore to it than that. Remembering that the resistance is proportional to the number of turns on the coilumber of turns multiplied by by diameter equals length of conductor) and the inductance isoportional to the square of the number of turns, as we add turns to the coil, the inductance increasesore rapidly than the resistance. In other words, coils of high inductance tend to be more efficient thanils of low inductance because the ratio of inductance to resistance is higher (at a given frequency thiseans that Q U is higher) and hence the division of energy between inductance and resistance favours the

ductive kinetic energy rather than resistive heating losses. This inductance helps overall efficiency whenu consider that there is also the resistive loss associated with the primary spark gap to be taken intocount.

was often noticed by radio amateurs after around 1912 (when, following the loss of the "Titanic" theyere banished to wavelengths shorter than 200 metres / frequencies higher than 1.5Mc/s) that at thesegher frequencies it was difficult to get a spark transmitter to operate efficiently because of the need toe small coils, and lead lengths had to be kept very short to ensure that most of the inductance was in theil (which being coupled to the aerial, would do some good) and not in the interconnections where itould be wasted. Spark transmitters and Tesla coils alike benefit from plenty of inductance in the primarycuit to help offset the resistive losses in the spark gap. However, there is indeed a limit to the usefulnessmore turns. As the inductance goes up, the current goes down, and the resistance of the gap is inverselyoportional to the current.

a coil acquires more turns, so the radio frequency resistance of the coil is increased over and above thate to the additional length of wire by the "proximity effect" between all those turns, which tends toduce the cross-sectional area of the wire over which the current can flow. Then there is the "skin effect"



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Maximising Power 2

hich constrains high frequency currents to flow on the outside of the conductor. This is more noticeableth thick wires than thin wires, and the ratio between the rf resistance of a thick wire and its dc resistancelikely to be large, resulting in the curious fact that there is an optimum wire size for a given size of coild a given resonant frequency. So in practice there is always an upper limit to which inductance of a coiln be increased, for a given frequency of operation, size of wire, spacing, and overall length andameter, before the efficiency starts to fall off. Happily, in the case of primary windings, this limit isldom approached. With Tesla secondaries, 600 - 2000 turns may prove useful before anything untoward

noticed, and this will vary widely according to the variables stated above.

he photo above shows a low loss transmitting inductance at the Rugby station in the early 1920s. Theale of things can be gauged by the two workmen.

he proximity effect is very hard to predict, but the skin effect is much simpler. In copper wire at 20C,e skin depth in centimetres is given by:



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Maximising Power 2

you imagine a circular wire, the skin depth is represented by a thin ring around the circumference andthe current may be thought of as flowing in it. This is not the physical reality; the current actually

creases exponentially with depth and there are major phase changes too, but the skin depth is theckness of a ring in which all the current could flow and give the same observed resistance, assuming nooximity effect (which sadly is only true if there are about ten wire diameters between turns!)

he skin depth is proportional to the square root of the resistivity of the wire and resistance wirecordingly suffers less from the skin effect than copper, since a resistance wire must be much thickeran a copper wire at a given frequency before the effect becomes noticeable. Accordingly, where wirese thick enough to exhibit skin effect, there is less difference between the rf resistance of copper andsistance wire than the dc resistance difference between the two might lead you to expect, since the lowerckness of the copper skin in which the current is flowing tends to be compensated for by the greaterckness in which the current flows in the resistance wire.

he resistance of copper wire at 20C of diameter d centimetres in ohms per centimetre at radio frequency

he solutions to the problems of proximity and skin effect were worked out by a number of early wirelesschnology researchers, and originally it was German wireless engineers who, around the time of the Firstorld War, came up with the solution called "litzendraht". Litzendraht, or litz, consists of a numbernything from three wires to several thousand) of individually insulated wires braided together, thedividual wire diameter chosen such that the skin effect is negligible in each wire, and braided together inch a way that each wire has a reasonably equal share of the inside and outside of the composite wire sormed. For the sort of frequencies commonly encountered in Tesla coils, 0.08mm (44swg) is a fairmpromise and coils having an extremely high Q can be wound with litz made up from 44swg wire. It istonishingly expensive to buy (currently around 12 for one ounce) and for anything except the small

ils used in wireless work, it is best to "roll your own" even if that means that the optimum braiding islikely to be achieved. Whilst it is now extremely expensive, using litz has become very much easiernce wires ceased to be enamelled with real enamel. In my Father's day, litz of 50swg was in common user frequencies around 1-2Mc/s, and the enamel had to be carefully scraped with a sharp razor blade fromch 0,025mm strand before it could be soldered, resulting in a certain amount of "litz rage" when largembers of wires had to be treated - and broke. Small diameter litz of more than 27 strand wascordingly none too popular. These days, with plastic insulation, you just solder it using a hot iron whichrns clean through the covering.



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Maximising Power 2

owever, Q in the secondary can be made sufficiently high for practical purposes without the complexitylitz simply by using wire of the optimum thickness; though a coil can always be made more efficient atquencies under 1Mc/s by using litz, it can become critical on the efficiency of the braiding of theands and the insulation which spaces each strand. Calculations of optimum thickness of a single strand

ere yet again done around the time of the First World War and books on wireless technology of pre-W2 vintage often give ideas and references should you be keen to try. A particularly valuable paper ons whole topic was written by Professor C.L. Fortescue and published in the Journal of the Institute of

ectrical Engineers, pp 933-943, 1923. This, and the references in it, and more particularly the criticismsthe discussion at the end of the paper itself, are a good guide to the design of coils - and also a guide toe pitfalls.

hilst the skin effect is essentially conquered by these techniques, proximity effect can only be overcometrial and error spacing of the turns. It often transpires that the losses associated with the proximity

fect are more acceptable than the extra length of wire and extra resistance if the turns are to be spaced,d this is particularly the case with large diameter single wires. With wires of optimum diameter or litz,acing is usually effective, but litz is in a sense automatically spaced because of the insulation around

ch strand and the overall covering, usually silk (real or artificial).

nother invention from this same era was the ferrite core. Ferrite cores concentrate the magnetic fieldoduced by a coil into a small volume, and a greatly increased inductance can be obtained from a smallil by using a ferrite core. Since the inductance has been increased without significantly increasing thesistance (you must be careful to provide a small amount of clearance between the ferrite and the coil ore ferrite will increase resistive losses) the Q of the coil improves dramatically, values in the highndreds being common. Unfortunately, ferrite cores in Tesla coils are generally not a good idea, and thisr several reasons:

1. The coil response now extends down to dc and is no longer confined to the resonant frequencyalone! Basically, the ferrite core reduces leakage inductance and increases the coupling constant k to something a lot closer to one. This makes for an exceptionally vicious pulse transformer whichresponds to all oscillating magnetic fields regardless of frequency, rather like an iron cored audiotransformer and with similar characteristics of broad frequency response with a high frequencyhump due to resonance at the top end. Not only do you get the benefits of the decaying oscillatorysinewave rf discharge, but you get the massive wallop from the initial dc pulse out of the capacitor.Whereas with a small air cored Tesla coil of say ten watts it is perfectly safe to take the spark discharge into a piece of bare metal held in the hand, if it is ferrite cored the result will be extremepain if not risk to life. I found this out the hard way, fortunately through the handle of a well-insulated screwdriver. Though separated from the metalwork by one inch of clear plastic, I couldfeel every single spark like a stinging slap on the palm of my hand. Mercifully, I was expectingtrouble and took the precaution of using the screwdriver; I dread to think of how it would have feltif I had been using a bare piece of metal instead. The conventional, air cored coil I usually usedwith the same power source gave a spark which I could take to a key or metal rod with nothing felt.

2. The ferrite core has a saturation limit, or in other words, for a particular frequency there is a limit



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Maximising Power 2

to how much power you can couple through each cubic inch of ferrite. The more powerful the coil,the more ferrite is needed and this, like litz, is not cheap.

3. Ferrite is not a perfect insulator. Some grades are actually of quite low resistance, only a fewhundred ohm-centimetres. In my experiments I simply potted the ferrite (which was a highresistivity grade, being one inch diameter interference suppression beads intended for slipping overhalf inch diameter cable) in beeswax and wound the coil on a thin former over that, and there was

no trouble with insulation breakdown. That's all well and good for a low-powered baby coil of tenwatts input, but I wouldn't expect it to hold for a kilowatt!

oils using ferrite cores are not a good idea for these reasons. If you simply must try it, make a baby coilth not over ten watts input, and under no circumstances try to take the discharge into a bare piece of etal in your hand, or I promise you will regret it. This caution is of less importance if the source is anusoidal generator rather than a spark transmitter.

he next section looks at the vexed question of the ideal conductor for radio frequencies.

ack

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nductive Coupling of Tuned Circuits

The Spark Transmitter. 4. Inductive Coupling of Tuned Circuits.

"All is riddle, and the key to a riddle is another riddle."

R.W. Emerson

ill we ever get to the load? Well, it's getting closer . . . trust me, I'm a doctor.

ductive Coupling.

aving generated power at rf, it is necessary to transfer it from the primary circuit where it originated to

e secondary circuit where it will do something useful. Remembering the requirements of the maximumwer theorem, it is clear that the conditions required are those of an impedance match.

In the diagram, the primary and secondary coils are coupledinductively by means of a mutual inductance M. This has a reactanceat the operating frequency X m. For perfect matching, the resistance

coupled into the primary circuit must be equal to the resistive loadwhich the generator expects to see. If the load is itself reactive, thento get a perfect match two out of the three reactances X

p, X

s, and X

must be variable. Fortunately, if primary and secondary are tunedcircuits at resonance, things become greatly simplified and we havethe possibility of obtaining a perfect match purely by varying themutual inductance coupling. This we do according to the equation:

here R p and R s are the resistances associated with both circuits. (The value of the mutual inductance Mn then be calculated if we know the frequency of operation.) Well, what exactly are these resistances?r the primary circuit, R p is the resistance associated with the generator (remember the maximum

wer theorem!)

ut what is R s? That is a harder question to answer. It also seems likely, for a Tesla coil secondary, that

will vary according to whether a spark is being produced from the top electrode or not. At least withaerial it will be constant. With a resonant aerial in fact it's fairly easy, being equal to the sum of the

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sistive losses in the aerial, the ground and the radiation resistance. The radiation resistance is atitious resistance, which if it was included in the aerial would cause as much power loss throughating as the loss of power due to radiation from the aerial. So in the case of a Tesla secondary, whate need here is a "spark resistance" which would cause the same amount of power loss as is caused byark discharge from the top electrode. It is by no means easy to see what value this should be. We can,wever, state with certainty that it is unlikely to be very useful trying to reduce the rf resistance of thecondary to a value much below that of the ground connection, as the two act in series.

rtunately, we can be ignorant of the exact requirements here and adjust the mutual inductanceupling by the physical separation of the primary and secondary coils and, according to time honouredreless practice, "tune for maximum smoke"! [You can usually tell when you've damaged an electronicmponent because the smoke they put in it at the factory leaks out. I don't know who it was whovented smoke as a means for indicating faulty components but all I can say is, it's a jolly clever idead I wish I had the patents on it.]

ne of the reasons (by no means the only one, nor, as it happens, anything like the best one) why Tesla

condaries give bigger sparks with larger top capacitances is because there is often insufficient mutualductance coupling for perfect impedance matching (this may be because decreasing the separationtween the coils causes sparking into the primary) and when the top capacitance of the secondarycreases, the resonant frequency drops, more turns are needed on the primary (which may be too smallrelation to the primary capacitance) and the effective size of the primary is increased, therebycreasing the proportion of input power converted to rf, and increasing the mutual inductance andproving the match. It may also happen that reducing the secondary reactance (corresponding to theop in frequency) improves the impedance match and we get a better power transfer from that causeo. (We'll come to the best reason for big secondary capacitances later.) But there's more.

he mutual inductance coupling between primary and secondary can be related to their self-inductancemeans of the coupling constant k:

otice that since k is defining the relationship between magnetic flux linkages in the circuit, it can never

greater than 1. A value of 1 means that all the flux produced by the primary is linked with thecondary and vice versa. A value of k greater than 1 would mean that more than all of the fluxoduced by the primary is linked with the secondary and thus values of k greater than 1 (and I haveen people claim it!) means you have a problem! In fact, k = 1 is never achievable! The closest you are

kely to get is in the output transformers of high quality valve amplifiers where primary and secondarye split into interleaved windings, and in specialist types of instrument transformers where thenstruction is similar. Power transformers used for supply distribution are also quite good, fortunatelyr the supply companies and the end user. Neon sign transformers and welding transformers areamples of designs where the value of k varies considerably with the load and is sometimes a lot less



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an 1.

ansformers of this latter type are called "magnetic leakage transformers" because the design is suchat a large proportion of the flux generated by each coil can escape from the magnetic circuit associatedth the other coil. Under load, the proportion which "leaks" increases. This gives intentionally poorwer regulation and ensures that when a short circuit is placed across the secondary (the striking of the

elder's arc, the conductive breakdown of the neon gas, or the flashing over of the primary spark gap)

e output voltage is suddenly reduced until the "fault condition" is removed. Mr. Melville Clapp-stham in the USA can be credited with the introduction of this type of transformer for spark nsmitters, and his Model E transformer has a prominent place in wireless history. Similar results canobtained from a power transformer of the closely coupled type if there is an external inductance

hoke) in series with the primary and this external choke provides the necessary "leakage inductance".

he coupling constant is independent of the number of turns in a coil. The number of turns in a coiltermines the magnetic field which will be produced for a given current. The coupling constant isncerned with how the lines of magnetic force produced by one coil interact with another coil, and

nce the coupling constant between two air spaced coils depends only on their physical size andsposition in space. Hence to obtain the best coupling between primary and secondary in an air-corednsformer we can only change the size and spatial relationships of the coils. With a tapped coil it maynoticed that changing the tap position changes not only the self-inductance but also the couplingnstant. This is of course because when the tap is moved to a different position, the effective size andatial relationship of that coil are changed as well as its self-inductance.

hen we have the critical coupling, which exists when the voltage output is optimised, then we have anditional relationship between k c the critical coupling constant and the Q values of primary and

condary:

he value of coupling constant is important in a spark transmitter because the tightness of couplingtermines the rate at which the primary loses power to the secondary, and hence determines decrement,mping, sharpness of tuning (loaded Q) and intensity of current at resonance (and hence secondaryltage in a Tesla coil.) Remember those nice graphs showing the logarithmic decrement and loaded Q?

he graph of decrement = 0.09 and loaded Q = 34.6 corresponds to the critical coupling constantving a value of k c = 0.17, which, from the records left by the old-time spark wireless operators, is

ound the maximum which can be used with a quenched spark gap of multi-plate construction. Hencer a critical coupling constant of k c = 0.17, the product Q pQs must be 34.6. We can of course split that

oduct between a wide range of possible Q p and Q s values! If both are equal to Q = 5.88 (the square

ot of 34.6) the decrement of each circuit individually is given by the graph of = 0.53.



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he diagram shows the effect of varying the coupling on the frequency distribution (read 'logarithmiccrement') of a spark transmitter. As the coupling is increased much beyond 20%, k = 0.2, the

equency spread increases dramatically, indicating that the logarithmic decrement has increased andat loaded Q has decreased. The square of the current, plotted on the y axis, also plummets drastically.

he next diagram shows that, in order to get the highest possible secondary current, the primary andcondary circuits have to be slightly detuned. In each case the primary circuit remains tuned to aavelength of 650m, whilst the aerial circuit (secondary) is varied from 500 - 650m. The best result isr 585m. This was obtained in an experimental test circuit chosen to demonstrate the effect clearly, ande best detuning is here about 11%. For the average aerial and coupling k=0.17, the detuning wasrmally about 3%.



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urves like these are recorded by coupling an rf ammeter into the circuit. I have seen circuit diagrams inhich the meter is placed directly in the primary circuit, but evidently these were low voltage circuitsossible with the quenched gap which will operate on just a few hundred volts) and it is more usual touple the meter to the aerial circuit and then indirectly by means of a coupling loop. A thermocouple

mmeter would be the instrument of choice, but these curves were recorded most likely with a hot wireeter, whose deflection is proportional to the square of the current - hence the plot of I 2 on the y axis.

he mutual inductance coupling ensures that everything critical to the operation of the spark transmitterr Tesla coil) is dependent on just about everything else, and that is why trying to find the globaltimisation for a Tesla coil to give the biggest spark for a given input power is so very difficult. It isn'tat we have such an enormous number of variables - it's the interdependence of all of themmultaneously on each other!

l of which goes to show how very complicated the inductively coupled spark transmitter (or Teslail) really is. It's a nice demonstration of the fact that there is not necessarily a direct correlationtween the number of components in a circuit and its complexity of operation. The spark transmittercuit is one of the simplest - just seven components (power transformer, primary capacitor, primary

ductance, primary spark gap, secondary inductance, secondary capacitance and mutual inductance)d yet a detailed description of its operation would require a lot more space than this and cartloads of gher mathematics. Any electrical circuit can be broken down into just four fundamental 'units' -ductance, capacitance, resistance and mutual inductance - and with just seven components, this circuits the lot.

s a mere radio ham tinkering outside my sphere of professional competence I can only scratch therface. I am left gasping with admiration at the achievements of the old timers who built and operatedark transmitters and Tesla coils often without a clue as to the frequency of operation or technicalowledge much above Ohm's law. They did it, of course, by a combination of knowing inside out whatere was to know, by meticulous method and by sheer patience and dogged determination.

h, by the way. We have now arrived at the load.



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e're not through yet though. The next section looks at transmission lines and magnifier circuits.

ack

omepage



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he Induction Coil

The Induction Coil

"Now does my project gather to a headMy charms crack not, my spirits obey, and time

Goes upright with his carriage. How's the day?"Prospero, "The Tempest"

t last! It lives! My novel sidetone generator is complete! (Cue thunder & lightning,aniacal laughter, clanking chains, baying hounds etc.)

e wanted one of these things since I was a lad, it has only taken thirty years but I've finally gone

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he Induction Coil

d done it. The details of what to do and how to do it were obtained mainly out of reprints of evant constructional articles from Lindsay Books ( http://www.lindsaybks.com ) One or twoeaks I had to discover for myself.

he induction coil primary was wound with a couple of layers of 1,7mm wire. This can be seenotruding from the centre of the coil to the left in the photo below. Around 200 turns were used

total. The coil covering is heavy photo card from a stationers, later on I sprayed it with aerosolax polish and it attained a deep shine, just like the ebonite sheet which was once used for this job.he primary core is packed with about 11 ounces of 1mm iron fence wire, which was annealed in arbecue charcoal fire.

he primary former is a length of cardboard tube, and both end cheeks on the primary formere also short lengths of cardboard tube which were a good slip fit, stuck on with PVA wood glue.his slides inside a length of Tufnol tube, 1,5 inches outside diameter and approximately an eighthan inch wall thickness obtained from RS Components ( http://rswww.com ) which also provided

fnol sheet for the base and main coil end cheeks, and more half inch Tufnol tube for the EHTandoffs also seen in this picture. The phosphor bronze balls used as discharge electrodes are half ch diameter, sadly no longer available, but brass will do as well and small doorknobs (reallyended for chests of drawers) can be obtained from several chains of DIY warehouses, in fact Ied a one inch doorknob as the discharge ball on the Tesla coil. All other metal supplies andany hand tools (e.g. taps and dies) used for projects on these pages, I obtained from Macc Modelngineers, 45a Saville Street, Macclesfield, Cheshire, SK11 7LF, tel. 01625 433938 - their service isst rate, and they are now on the internet!

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he Induction Coil

ou can find MACC here.

he silver contacts I used for the interruptor, around a quarter of an inch diameter, were from aece of silver I've hoarded for years. This I melted, cast into an ingot, er - lump, and thenachined to size. Silver has the curious property that, when molten it absorbs oxygen from the, which is expelled again when the metal solidifies. The silver castings are riddled with minute

ow-holes from this cause, irritating but harmless.

he picture above shows the ever-versatile Unimat 3 being used to wind the "pies" of thecondary, and a variac was used to regulate the speed. The winding gear is a simple thing madeom a couple of thin Tufnol sheets with a brass spacer, and it runs on a rod which passes throughe Unimat headstock as the pies are too big to swing over the lathe bed. The wire was guided bynd, moistening my fingers with linseed oil. The secondary is constructed from around 2lbs of swg enamelled and double silk covered wire. I was lucky to find this as a production over-run ate Scientific Wire Co. ( http://www.wires.co.uk ) otherwise it would either have been unobtainablehave cost a fortune. It's worth making a telephone or email enquiry as not all they have is on

e website. The coil gives (unloaded) a spark of an inch and a bit between half inch phosphoronze balls. It is very hot and will ignite paper and card rapidly. The completed pies are shownlow. A ruler gives an idea of scale.

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he Induction Coil

wo of these were lost in the initial test when the insulation (here paper disks) blew through. Thesulation was replaced with three layers of card soaked in melted beeswax per pie, and molten

ax mixture was painted on around the seal between the Tufnol tube and the card washers. Thees were insulated with a molten mixture of beeswax and rosin (from a musical instrument shop)st as the originals. This was brushed on with a fine paintbrush whilst winding the coils; theotor was slowed to a crawl whilst doing this. The unevenly-insulated pies were then cookedntly in an oven at around 50C just to soften the wax mixture until the insulation becamemogenous. The pies, part-assembled on the Tufnol tube (with the initial paper insulation) areown below.

mount the coil I chose a piece of West African mahogany from South London Hardwoodsttp://www.slhardwoods.co.uk ) and I also bought a couple of sticks of ebony. The latter were usedmake handles for the coil discharge electrodes, for the core of the Ruhmkorff commutator

witch, and for the pillar on which the Tesla secondary stands. Hard rubber, called ebonite, wased for this, but dry wood seems OK at these voltage and power levels. To my knowledge, there is

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he Induction Coil

st one compounder of ebonite left in the UK, in Manchester, and it's fine if you want a few tons!

he design of the radio frequency part of this project called for months of experimentation withe part-made induction coil, at that time screwed to a piece of very tatty plywood whilst manysign changes were made. It was decided to use a primary of two layers of eighth inch brassiler band from Macc, supported on wooden dowels which were soaked in melted beeswax. It is

sential that the holes for these are drilled exactly vertically in the baseboard, otherwise theimary will look very uneven and will not contact all of the octagonally-spaced supports. Heree Unimat is being used to sink these holes.

ore woodwork was needed when it came to hiding the Fizeau condenser across the contacteaker points. The condenser used was an old WW2 vintage Dubilier nitrogol 2uF 400V unit. Itd already been established that 1uF was not enough, and the voltage rating is essential to copeth the back emf. Here's the inletting nearing completion. I placed a ruler across the bottom of s shot to give an idea of scale, but the divisions aren't very visible, unfortunately.



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he Induction Coil

he wood looks quite pale prior to French polishing with Liberon button lac dissolved in

ethylated spirits. It was rubbed down with sandpaper between coats until all the wood poresere filled, and thereafter rubbing down with Liberon 0000 steel wool and linseed oil. It's the firstme I've ever tried doing it and it's neither easy nor perfect to look at - but not a bad job for avice. The near-finished baseboard, together with a few components, is shown below. Thetagonally-spaced dowels for the boiler band primary can clearly be seen, and the condenser isell fitted in its hole. Aerosol wax polish should not be used on shellac finishes, as the solvents in itll dissolve the French polish which took you so long (and so many attempts!) to get looking good.

se the solid stuff in tins instead.

ottom left in the above photo is part of the Ruhmkorff commutator switch. This was a mostfuriating thing to make, requiring some of the most dubious soldering I have ever done (with a-year old 230W thermostatic iron!) but we got there eventually . . . final polishing was with some

mazing Japanese Tamiya abrasive paper, down to 2000 grit, which I got from Brouckx model

op in the Koning Albertstraat, Hasselt, Belgium. The components of the switch appear here.



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he Induction Coil

he Tesla secondary was around 700-800 turns of spare wire left over from the induction coil,ound an inch and a quarter diameter and seven and three quarters long. It's very unevenlyound, the turns even cross over at a few places! The coupling coil is an unknown number of rns (try until it works!) of 36swg tinned double silk covered copper, also from Scientific Wire,ound on a cardboard pot which once held Fair Trade cocoa powder. The condenser is a cheat. I

as going to use an ex-Admiralty WW2 vintage 28kV 0,0011uF mica condenser, but its lossesmpered spark production (the input to the induction coil is only 30-70 watts or so - my QRPdio friends will cringe - but that's not much to make sparks with, especially given theefficiency of induction coils) so I used ten 0,01uF 1kV polypropylene units from RS Componentsseries, hidden inside the wooden box. The odd value of the ex-Admiralty mica is due to its beingactly one standard Admiralty "jar" of capacitance in old units! The output from the Tesla coil isown below.



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he Induction Coil

he sparks are maybe 1,5-2 inches long. I have changed the colour balance of the photo, as theiginal is too blue. This is still not right, but it's closer to the purple which your eye actually sees.here's an evident difference in the spectral sensitivity between eye and film, especially at lowht levels. In operation, the most important thing is to ensure that both the interruptor contactsd the sparking balls are kept clean. Every ten minutes or so of operation necessitates a break for

eaning with abrasive paper. The setting of the spark gap is also very critical to success, and thearks from the Tesla coil make a strange squeaking noise when all is tuned correctly. A goodound connection is important, and a large metal object (I have used a big metal case maybe fiveet by two by two) is better than the ground rod I use for my amateur radio transmitter.

he only problem with this thing is the copious quantities of ozone emitted (honestly, you can'tve it switched on for more than a minute without needing to leave the room, it's that bad!) Withe fundamental around 2Mc/s, plus the emissions of the interruptor, it does cover all amateur

nds though . . . simultaneously!

ack

omepage



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he Quenched Spark Gap

The Quenched Spark Gap

"...and here's the practice o' it."

Unknown

he quenched gap, as popularised by Telefunken, was a great success owing to the very high rate of witching which lent increased efficiency to their transmitters. It was said that a 500 watt Telefunken set

th its quenched gap could outperform a Marconi set of 2.5kW with its rotary gap, and this was mostlywn to the efficiency of the Telefunken gap, an early and poor quality illustration of which wascluded in "Alternator, Arc and Spark."

decided at the outset of my attempt to replicate a spark transmitter that I wanted to use a multi-plateenched gap and hence had to set about making one. At the time, I could not obtain copper sheet in the

zes I wanted, nor copper rod at all, so the whole thing was made from brass. The electrical and thermalnductivities of a metal are related through the Wiedemann-Franz law which states that the ratio of ese conductivities is a constant, independent of the metal and varying only with temperature.egarding electrical conductivity, the difference is not enough to cause concern as the skin effect givesme degree of compensation (as will be shown later) and the periphery of each gap is large, butgarding the thermal conductivity of brass I was a little concerned that the inferior thermal properties

ght be the ruin of my plans. I need not have feared. For the relatively short time that I fire the thing, the gap never gets more than tepid at the most; however, had I the choice and was starting over again

would use copper, although it is much less pleasant to machine. Aluminium would also be a goodoice for the cooling flanges, though probably not for the gaps.

ttp://home.freeuk.net/dunckx/wireless/quenched/quenched_gap.html (1 of 7)6/20/2011 2:48:08 PM


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he cooling flanges are about 15cm/6 inches in diameter and are made from 1,6mm thick brass sheet.he eight flanges had to be cut by hand using a 52 tooth-per-inch piercing saw with occasionalbrication with cutting fluid. This took a long time as may be imagined, each flange having acumference of 47cm/18 inches, all eight amounting to sawing a line 3 metres/12ft 6 inches long,d consumed several weeks' worth of evenings and weekends, with considerable wear-and-tear onbow and wrist joints and many tired fingers! The blades are fortunately not expensive, which is just asell as each blade lasted little over one flange.

he next stage was to mount all eight flanges together, drill a 1/8 hole through the centre of all eight onriend's drill press (thanks Dennis, G7OGN) pin them to prevent their moving relative to one anotherd attack the edges with a file until they were all about even; an electric drill with carbide roughingsk speeded this process considerably, but flying burrs necessitated good eye protection withlycarbonate safety goggles. Following the smoothing of the edges and removal of many sharp burrs

th suitable care to avoid impaled fingers, not always successfully, the eight flanges were mountedgether on the trusty Unimat 3 and three holes drilled at 120 , each hole starting out one eighth inameter, and by progressive stages being enlarged to half an inch, pinning these holes in turn to preventtation of the flanges between or during drilling operations. Drilling these three holes necessitatedeping one hand on the Unimat motor (for those who are not aware, the Unimat 3 motor is notntinuously rated) and switching off when it became too hot to hold, which was often. This was a long,

ow, laborious business, even with plenty of cutting fluid. That brass sheet is tough old stuff, and there'sx 1,6mm = 12,8mm (a bit over half an inch) of it in total to be got through.



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he next job was by way of light relief. The brass cooling flanges were now supported by nailing themry loosely to a piece of scrap wood through their centre holes and having masked their centres (wheree electrical contact with the gaps would be made) spray painting them with black matt barbecue paint.here are chemical methods for blacking brass, which I would use if I had access to the chemicals (myD is in chemistry!) but sadly I didn't at this time so had to make do with paint. If the authenticity bug

tes very deeply, I may just strip all the paint off and do the job properly, but I doubt even I could telluch difference.

her tasks performed, but not photographed, were:

q the cutting and drilling of end plates from Tufnol paper/phenolic composite (takes the edge off aplane in minutes, giving the blade the appearance of having been attacked by a file) thiscomposite being chosen as it looks very like Bakelite;

q turning, cutting with a die, hardening and tempering the pressure screws and their mounting nuts,which are used to compress the gaps tightly together;



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q turning thick fancy brass washers to place under the nuts of the tension studding;

q cutting and trimming Tufnol tubing to act as insulators over the studding, the studding andinsulator passing through the three half inch holes in each cooling flange;

q

q sawing with an eighty-teeth-per-inch piercing saw blade mica washers out of two inch mica disks1/16 thick. These had to be smoothed down to a flat, parallel profile and this was very tediousindeed, being accomplished by a combination of rubbing the washer against a piece of sandpaperstapled to a board (and losing half the fingerprints from the tips of my fingers in the process -

ouch! - and the blood on the mica doesn't help its insulating properties either) and holding thewasher in the three jaw chuck of the lathe, which was free to rotate, and bringing up upon it asmall grindstone spinning at 4000rpm in the drill chuck (shown above) - unfortunately the softmica rapidly plugs the grain structure of the grindstone. I started with forty disks and ended upwith about fifteen washers between 1-1,1mm thick from which to select the best ones for the gaps- obtaining complete uniformity was impossible;

q



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q turning twenty brass disks, one and a half inches diameter and around a quarter of an inch thick to act as the electrodes for the ten gaps. Ten gaps was the number selected on the basis of 1kVper gap. This operation was photographed but unhappily the flash unit mistimed and the fault wasnot noticed until the job had been done and the film then developed. A sketch of an electrode disk is appended. The recessed groove was cut later as a result of experience and it very effectivelyprevents the spark from "walking" its way through the insulating washers. The height of theprotruding sparking surface was matched to the washer giving an overall gap length of around8mm for 10kV, or 0,8mm/ 1/32 inch per gap. The rim was progressively reduced in thickness inorder to increase the pressure applied to the washers (pressure = force / area) and improve thesealing, but obviously the strength of the brass limits the extent of this thinning process. Thesegap electrodes were silver plated by a local firm on the basis that a chapter on spark transmittersin an old book I have says the electrodes were silvered - the silvering lasted mere minutes undernormal cond

what exactly is a tesla coil

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