microwave source

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Klystron Oscillator

Reflex Klystron

Traveling Wave TubeBiological effect of microwaves


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Microwave Source

High-power microwave sources use specialized vacuumtubes to generate microwaves.

These devices operate on different principles from low-

frequency vacuum tubes, using the ballistic motion of

electrons in a vacuum under the influence of controllingelectric or magnetic fields.

These devices work in the density modulated mode,

rather than the current modulated mode. This means

that they work on the basis of clumps of electrons flyingballistically through them, rather than using a continuous

stream of electrons.

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Microwave Source

High-power microwave sources use specialized vacuumtubes to generate microwaves.

These devices operate on different principles from low-

frequency vacuum tubes, using the ballistic motion of

electrons in a vacuum under the influence of controllingelectric or magnetic fields.

These devices work in the density modulated mode,

rather than the current modulated mode. This means

that they work on the basis of clumps of electrons flyingballistically through them, rather than using a continuous

stream of electrons.

3


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Klystron Oscillator

- A klystron is a vacuum tube that can be usedeither as a generator or as an amplifier ofpower, at microwave frequencies.

-Is preferred tube for high-power, high-stabilityamplification of signals at frequencies fromUHF to about 30 GHz


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Klystron Oscillator

- A klystron is a vacuum tube that can beused either as a generator or as anamplifier of power, at microwavefrequencies.
http://en.wikipedia.org/wiki/Image:Klystron.enp.gif


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Two cavity Klystron Amplifier


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Applications

As power output tubes

1. in UHF TV transmitters

2. in troposphere scatter transmitters3. satellite communication ground station

4. radar transmitters

As power oscillator(5 50 GHz), if usedas a klystron oscillator


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Reflex Klystrons

The reflex klystron has been the mostused source of microwave power in

laboratory applications.


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Construction

A reflex klystron consists of an electron gun, a cavity with apair of grids and a repeller plate as shown in the abovediagram.

In this klystron, a single pair of grids does the functions ofboth the buncher and the catcher grids.

The main difference between two cavity reflex klystronamplifier and reflex klystron is that the output cavity is

omitted in reflex klystron and the repeller or reflectorelectrode, placed a very short distance from the singlecavity, replaces the collector electrode.


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Working

The cathode emits electrons which are acceleratedforward by an accelerating grid with a positive voltage on itand focused into a narrow beam.

The electrons pass through the cavity and undergo

velocity modulation, which produces electron bunchingand the beam is repelled back by a repeller plate kept at anegative potential with respect to the cathode.

On return, the electron beam once again enters the samegrids which act as a buncher, therby the same pair of grids

acts simultaneously as a buncher for the forward movingelectron and as a catcher for the returning beam.


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Reflex Klystron oscillator


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PH001 Unit 2 Lecture 6 12

Working

The feedback necessary for electrical oscillations isdeveloped by reflecting the electron beam, the velocitymodulated electron beam does not actually reach therepeller plate, but is repelled back by the negative voltage.

The point at which the electron beam is turned back canbe varied by adjusting the repeller voltage.

Thus the repeller voltage is so adjusted that completebunching of the electrons takes place at the catcher grids,the distance between the repeller and the cavity is chosen

such that the repeller electron bunches will reach thecavity at proper time to be in synchronization.

Due to this, they deliver energy to the cavity, the result isthe oscillation at the cavity producing RF frequency.


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Performance Characteristics1. Frequency: 4 200 GHz

2. Power: 1 mW 2.5 W

3. Theoretical efficiency : 22.78 %4. Practical efficiency : 10 % - 20 %

5. Tuning range : 5 GHz at 2 W 30 GHz

at 10 mW


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Applications

The reflex klystrons are used in

1. Radar receivers2. Local oscillator in microwave receivers

3. Signal source in microwave generator of

variable frequency

4. Portable microwave links

5. Pump oscillator in parametric amplifier


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Traveling Wave Tube

Traveling Wave Tube (TWT) is the most versatilemicrowave RF power amplifiers.

The main virtue of the TWT is its extremely wide

band width of operation.


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Basic structure of a

Traveling Wave Tube (TWT)


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Basic structure

The basic structure of a TWT consists of a cathode andfilament heater plus an anode that is biased positively toaccelerate the electron beam forward and to focus it into anarrow beam.

The electrons are attracted by a positive plate called thecollector, which has given a high dc voltage.

The length of the tube is usually many wavelengths at theoperating frequency.

Surrounding the tube are either permanent magnets orelectromagnets that keep the electrons tightly focused intoa narrow beam.


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Features The unique feature of the TWT is a helix or coil that

surrounds the length of the tube and the electron beampasses through the centre or axis of the helix.

The microwave signal to be amplified is applied to the

end of the helix near the cathode and the output istaken from the end of the helix near the collector.

The purpose of the helix is to provide path for RFsignal.

The propagation of the RF signal along the helix ismade approximately equal to the velocity of the electronbeam from the cathode to the collector


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Functioning

The passage of the microwave signal down thehelix produces electric and magnetic fields thatwill interact with the electron beam.

The electromagnetic field produced by the helixcauses the electrons to be speeded up andslowed down, this produces velocity modulationof the beam which produces density modulation.

Density modulation causes bunches of electronsto group together one wavelength apart and.these bunch of electrons travel down the lengthof the tube toward the collector.


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Functioning

The electron bunches induce voltages into the

helix which reinforce the voltage already present

there. Due to that the strength of theelectromagnetic field on the helix increases as the

wave travels down the tube towards the collector.

At the end of the helix, the signal is considerably

amplified. Coaxial cable or waveguide structures

are used to extract the energy from the helix.


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Advantages

1. TWT has extremely wide bandwidth. Hence, it

can be made to amplify signals from UHF to

hundreds of gigahertz.

2. Most of the TWTs have a frequency range of

approximately 2:1 in the desired segment of the

microwave region to be amplified.

3. The TWTs can be used in both continuous andpulsed modes of operation with power levels up

to several thousands watts.


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Performance characteristics

1. Frequency of operation : 0.5 GHz 95 GHz

2. Power outputs:

5 mW (10 40 GHz low power TWT)

250 kW (CW) at 3 GHz (high power TWT)10 MW (pulsed) at 3 GHz

3. Efficiency : 5 20 % ( 30 % with depressed

collector)


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Applications of TWT

1. Low noise RF amplifier in broad band microwavereceivers.

2. Repeater amplifier in wide band communication linksand long distance telephony.

3. Due to long tube life (50,000 hours against th forother types), TWT is power output tube incommunication satellite.

4. Continuous wave high power TWTs are used introposcatter links (due to larger power and largerbandwidths).

5. Used in Air borne and ship borne pulsed high powerradars.


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Biological effects of microwaves

Electromagnetic radiation in the 1 mm to 1 m

wavelength range (300 MHz to 300 Ghz) is

referred to as microwave radiation.A part of which is known as radiofrequency (RF)

radiation, which covers 0.5 MHz to 300 GHz

range and is considered in the context of

adverse biological effects.


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Ionizing and non ionizing radiations

of electromagnetic energy


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Ionizing radiation

Ionization is a process by which electrons arestripped from atoms and molecules and this canproduce molecular changes that can lead todamage in biological tissue, including effects onDNA, the genetic material.

This process requires interaction with high levelsof electromagnetic energy to ionize biologicalmaterial, this include X-radiation and gammaradiation.

The energy levels associated with RF andmicrowave radiations are not great enough tocause the ionization of atoms and molecules,therefore, it is a type of non-ionizing radiation.


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Non ionizing radiation

Microwave energy is non-ionizingelectromagnetic radiation.

Ionizing radiation messes up molecules, non-ionizing radiation merely heats them.

In general, it does not have sufficient energy tokick an electron off an atom thus producingcharged particle in a body and cause biologicaldamage.

The only proven harmful effect from exposure to

microwave (or RF) radiation is thermal.RF radiation can enter deep into the body and

heat human organs.


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Effect of microwaves in human body

The blood vessels are dilating and the blood flow

increases substantially as the thermoregulatory

mechanism is activated in order to keep the bodytemperature constant.

With rising body temperature the metabolic rate rises,

which may lead to Stress-Adaptation-FatigueSyndrome.


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Effects produced by the electromagnetic

waves at different frequency levelAbove 10 GHz (3 cm wavelength or less) heating

occurs mainly in the outer skin surface.

From 3 GHz to 10 GHz (10 cm to 3 cm) the

penetration is deeper and heating higher .From 150 MHz to about 1 GHz (200 cm to 25 cm

wavelength), penetration is even deeper andbecause of high absorption, deep body heatingcan occur.

Any part of the body that cannot dissipate heatefficiently or is heat sensitive may be damaged bymicrowave radiation of sufficient power.


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Measurement of Microwave exposure The microwave energy exposure is measured in terms of

SAR (Specific Absorption Rate) or PD (Power Density).

SAR is the energy which is absorbed in a unit of mass or

volume of the body per unit time. The standards that limit microwave exposure were set at

0.4 W/kg SAR for occupational and 0.08W/Kg for publicexposure.

The averaging time for determination of SAR was 6minutes. Power density is the energy absorbed per unitarea in unit time. The high power microwaves definitelycause some adverse effects in the human system


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Effects of Microwave energyPower

level(mW /cm2)

Long-term effecton human body

Remarks

0.01 Nothing

0.1 Nothing

1 Nothing5 Nothing Accepted standard for microwave

oven leakage

10 Nothing Accepted standard for maximum

continuous exposure to radiated

emissions (cell phones, etc.)30 You can feel heat

100 Cataracts can be

produced

Summer sunlight is at this level

1000 Pain is induced


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What do Microwave Ovens, Cell Phones and

Cordless Phones have in common?They all emit... Dangerous MicrowaveRadiation!

The GOOD NEWS is... with Microwave radiation

you can...Boil waterCook meat

Fry eggs

Do you know YOUR Brain can be FRIED???


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The BAD NEWS is...

with Microwave radiation you can...

Fry Your Brain

Your head and brain heat up significantly when you talk

on your cell phone or cordless phone.


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Want proof?

After 15 minutes of using a cell phone, the orange, red and pink showsignif icant, dangerous HEAT. Most heat is generated in your ear canal,

which is directly connected to YOUR BRAIN


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After 15 minutes of using a cell phone WITH the BIOPRO

Harmonization Chip applied to it, the green and blue colorsshow cool tissue.

Your head's temperature remains normal, providing you with theprotection you deserve.


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Proof enough?

Some scientists estimate that you are now exposed daily to

100 million times the electromagnetic frequency (Micro

wave) radiation of your grandparents.So.

AVOID FREQUENT USE OF

CELL PHONES! ! !


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