all about antennas

8/18/2019 All About Antennas

1/13from the archives of Bob Grove MONITORING TIMES 1

No subject is more widely discussedin the radio field as antennas, andwith good reason; after you select

your radio equipment, no accessory is moreimportant. There are many myths surroundingantennas, and we’re going to put them to restin this series.

Radio Waves:

Some Basics When we connect a wire between the twoterminals of a battery, electric current flows.This current generates a combined electric andmagnetic energy “field,” a zone which extendsat the speed of light into space. When we breakthe circuit, the energy field collapses backonto the wire. If we reverse the connections

back and forth rapidly, each successive pulse’selectrical (positive and negative) charges andmagnetic (north and south) poles reverse aswell. This simulates a basic radio wave whichconsists of a magnetic and electric field vibrat-ing simultaneously, or in phase. The electric field (“E” for electro-motiveforce, measured in volts) is parallel to theaxis of the wire, while the magnetic field(“H” named after researcher Joseph Henry) is

perpendicular to it. This field is described aselectromagnetic. Familiar illustrations depict-ing radio waves as wavy lines or crosshatchedarrows are graphic representations only. Thereare no “lines of force” as implied when ironfilings line up during magnet demonstrations;those filings line up because they all becomelittle magnets, attracting and repelling oneanother. Radio waves are only a continuousfield of energy which, like a beam of light,is strongest at its source, weakening withdistance as it spreads its energy over an ever-widening area. In fact, radio waves and light waves differonly in frequency over a continuous electro-magnetic spectrum, with higher-frequencylight having greater energy and the ability to beseen by some living organisms. Scientists evenrefer to an antenna as being illuminated byradio energy. Radio waves can be reflected by

buildings, trees, vehicles, moisture, metal sur-faces and wires, and the electrically-chargedionosphere. They can be refracted (bent) by

boundaries between air masses, and they can be diffracted (scattered) by a ground clutter ofreflective surfaces. Radio and light waves travel through thevacuum of space approximately 186,000 miles(300 million meters) per second, but when they

pass through a dense medium, they slow down;this velocity factor, is given as a specifica-tion for transmission lines. When we specifyantenna and transmission line lengths, theseare electrical wavelengths which are shorterthan free-space wavelengths because of thisreduction in speed.

Propagation We refer to the behavior of radio wavesas they travel over distance as propagation.Ground waves stay close to the earth’s surface,never leaving the lower atmosphere. They areseverely attenuated (reduced), rarely reachingmore than a few hundred miles even under

ideal conditions. Surface waves, thelowest ground waves, often reach-ing their destination by followingthe curvature of the earth. Spacewaves are the line-of-sight groundwaves which travel directly fromantenna to antenna. Space waves at VHF and UHF,when encountering abrupt weather

boundary changes, experiencetemperature inversions and ductingas well as other influences that canfunnel signals into significantlyextended ground wave coverage.At the upper reaches of our atmo-

sphere, ultraviolet rays (UV) fromthe sun ionize (electrically charge)the air atoms, lending the nameionosphere to this highest zoneof the earth’s atmosphere. Radiowaves which reach these ionizedlayers, averaging 25-200 mileshigh, are called sky waves The lowest regions of theionosphere, the D and E layers,are influenced directly by sunlight;their effects begin at sunrise, peakat noon, and disappear after sunset.They absorb radio signals. In otherwords, the longer the wavelength

(that is, the lower the frequency), the more theabsorption. This explains why daytime recep-tion below roughly 10 megahertz (MHz) is so

poor. But, the E layer also reflects shorter-wavelength (higher frequency) signals back

to Earth; the higher the frequency, the morethe reflection. This is what provides distance(DX) on the higher shortwave frequencies.Most DX, however, is produced by the nextregion up, the F layer, which retains its elec-trical charge well into the night, reflectingsignals back to the earth over great distances.All of these solar influences increase duringthe maximum sunspot cycle every 11 years,then gradually diminish again. The earth itself can reflect radio waves,allowing a phenomenon called multihop;combinations of earth reflections and iono-spheric refractions producing as many as fiveskips! More skips than that would be attenu-ated by ionospheric absorption and terrestrial

ALL ABOUT ANTENNAS

Part 1 of a Series

By Bob Grove W8JHD, Publisher, Monitoring Times

Signal propagation is a combination of ground waves and sky waves.


2/132 from the archives of Bob Grove

scattering, rendering the signal unreceivable.Internet sites like www.hfradio.org/propa-gation.html publish continuously-updatedradio propagation forecasts, and a variety of

prediction computer programs are availableelsewhere, allowing the user to plan ahead forthe most productive use of the spectrum. Tropospheric scattering in the E and F2layers is fairly common in the 30-50 MHzspectrum, especially during the daytime andduring sunspot peaks. It favors the east in

the morning and the west in the afternoon.At VHF and UHF, ionospheric propagationis rare. Some sporadic E skip, lasting from afew minutes to an hour or more, may occur inthe 50-200 MHz range. It is caused by erraticclouds of ionization at an altitude of 75-100miles.

A similar phenomenon is produced whenmeteors enter the E layer. At such high speeds,the meteor vaporizes, producing an ionizedtrail which is capable of reflecting VHF signals

back to earth 1000 more miles away, mostdramatically in the 50-80 MHz spectrum. It is estimated that some 200 tons of me-teor material, from visible to dust size, strikes

the earth every day; much more vaporizes inthe upper atmosphere. Because of this constant

bombardment, there are completely automatedsystems relying on this technique for long-distance data transfer.

Patterns The shape of the field of energy emit-ted by a transmitting antenna, as well as thegeometric response by a receiving antenna, isknown as its pattern. It may be a simple donutshape surrounding the axis of the wire as in a

half-wave, or smaller, dipole, called a doublet,or it may be multi-lobed, as in a multiple-wavelength antenna, called a longwire. The elevation pattern, variously calledradiation angle, takeoff angle, maximum am-

plitude elevation, and launch angle, is affected by height above ground, length of the antennaelement(s), and the presence of nearby metal,including other antenna elements. It is an in-tegral part of an antenna’s gain characteristicswhich we will discuss in a later issue.

The higher the frequency, the shorter the

wavelength and the easier it is for a signal to get through an opening in an absorptive or reflective enclosure.

Nearby trees, buildings and hills take theirtoll, too. Locating an antenna inside a large

building with steel frame and metal reinforce-ments may attenuate signals up to 25 dB atVHF and UHF, according to one study. Brickwalls, slate or tile roofs can account for 6 dB,even more when wet. Shorter wavelengths(900 MHz) get through small windows inshielded walls where longer wavelengths (150MHz) do not.

Location, Location,

Location…

The Radio Horizon Radio waves, like light waves, follow theline of sight. Because of the curvature of theearth, higher antennas “see” a farther horizon.Assuming a flat, unobstructed terrain, the visualhorizon is about 8 miles for a 30-foot-elevatedantenna, increasing to only 16 miles at 120 feet!

Notice the square law effect: it requires roughlyfour times the height to get twice the distance.Once an antenna is high enough to “see” pastnearby obstructions, it takes at least double that

height to notice any improvement. The lower the frequency, the more radiowaves are capable of following the curvature ofthe earth beyond the visual horizon. Typical base-to-mobile communications ranges are about 50miles in the 30-50 MHz band, 30 miles at 150-174MHz, 25 miles at 450-512 MHz, and 20 miles at806-960 MHz. Obviously, these distances willvary depending upon radiated power, receiversensitivity, antenna gain, elevation and location. Although the higher the antenna the better,coax cable losses may compromise any signalimprovement; the higher the frequency, the worsethose losses. For example, at 450 MHz, extend-ing a 30-foot antenna to 60 feet could increase

signal strengths by 5 dB, but if you are usingcommon RG-58/U coax, signal strengths may

be attenuated by the same amount, resulting inno improvement at all!

At 800 MHz, using this small diameter, lossyRG-58U, signals would get worse with height!Worst of all is thin RG-174U which has all the badcharacteristics; in long lengths at UHF, you mightas well short-circuit your antenna connector! Always use low-loss cable such as thefollowing, listed in increasing performance:RG-8/X, RG-8/U, Belden 9913, or 1/2” foam(Andrews), Heliax (all 50 ohm cables); or RG-59/U, RG-6/U or RG-11/U (72 ohm cables).

Terrain, trees, wiring, metal siding, nearby buildings and other reflective surfaces all affect antenna performance. The lower the antenna, the more obstructed it is likely to be. A basementwould be a very poor antenna location. Signals are unpredictably reflected by metal and wiringin and on the walls and ceiling; nearby electric and electronic appliances invite interference to reception; soil absorbs transmitted energy and also reflects signals upward; and signals come mostly from overhead (there aren’t many there) rather than from the horizon.

Antennas are designed to favor certain directions, both for transmitting and receiving.The lower the frequency, the more the signalis capable of following the contour of the terrain, and the less likely it is to be absorbed by trees and foliage. One study showed that with dense trees and vertical polarization, attenuation at 30 MHz is about 3 dB, increasing to10 dB at 100 MHz.



Last month we examined some of the char-acteristics (peculiarities?) of radio wavesand the importance of proper placement of

an antenna. This month we’ll take a close look atthe antenna itself.

What is a Ground? The earth plays an important role in radiosignal propagation, but grounding your radioequipment is not one of them. While attachingthe chassis of your radio to a buried conductor inmoist soil may protect you from electrical shock;drain off static-charge buildup; help dissipatenearby lightning-induced spikes, and even reduceelectrical noise pickup, it will not make receivedor transmitted signals stronger. Radio waves travel through space, notthrough the ground except at very close ranges orat extremely low frequencies. They are intercepted by the antenna’s metal element(s), not by the soil beneath it which absorbs and dissipates the signalas heat.

A good electrical ground consists minimallyof two eight-foot metal rods, at least ten feet apart,connected to the radio equipment by a short lengthof heavy braid. Moist, mineralized soil is best; dry,sandy soil is worst. A radio-frequency (RF) ground, on the otherhand, is more extensive. A vertical antenna may be thought of as a center-fed dipole turned on itsend, and the lower half removed so that we canmount the remaining element on the ground where

the coax will be attached. But we must somehowsupply that missing half of the antenna. If we simply bury the needed wire in theground, the energy that would radiate from thatelement is absorbed by the mineralized soil,simply heating it. Such an antenna is sometimesreferred to as a “worm warmer!” Instead, we construct a counterpoise on orabove the soil, a metallic surface emulating a“perfect” (reflective) earth, composed of radialwires connected to, and extending outward fromthe coax shield at the base of the antennas. How many spokes of wire, and how long?AM broadcast stations use at least 120 radials for

transmitting purposes; you should use at least 161/8-wavelength wires to avoid power losses fromsoil absorption. Because current is at its maximum at thefeed point, density of metal around the base of theantenna is more important than the length of theradials. If you have 100 feet of wire, ten 10-footlengths are better than two 50-foot lengths. Thisis not so critical on receive-only antennas. Even a single quarter-wavelength wire pro-vides counterpoise effect; it may be run randomly

or even coiled loosely in some cases. Such a wireis often connected to the chassis of the transmitterif it is hot during transmitting as evidenced by painful RF burns when touching the equipment,especially your lip to the mike! The inverted V antenna is a good example ofhow to keep the high-current feed point away fromabsorptive and reflective earth by elevating it tothe apex of the antenna. The ends of the droopingelements (high-voltage points) come to withina few feet of the ground where their capacitiveinteraction with the soil may cause some lengthdetuning of the antenna, but little signal loss. Don’t confuse a ground-mounted, counter- poised vertical with an elevated ground-plane

antenna. On the ground we are trying to preventradiation from being absorbed by the soil; an

elevated ground-plane antenna, however, behavesmore like a dipole in free space, with the radialssupplying half of the antenna and forming the pattern.

Construction and Size Two neighboring shortwave listeners decideto erect antennas to monitor 41-meter (7.1-7.3MHz) international broadcasting. One neighbor,using rocks as counterweights, throws about 50feet of small-gauge hookup wire over a coupleof tree limbs; it sags in a number of places, hasno insulators other than its plastic covering, andaverages some 30 feet in the air. At the center cutof the wire he has soldered a 50-foot length of TVcoax which he runs down to his receiver. His neighbor, a purist, erects two 30 foottelephone poles 60 feet apart, stretching 66 feetof heavy gauge, silver plated, uninsulated wire between porcelain insulators. The antenna is inan open yard with no trees. At the center he care-fully attaches a commercial coax connector, fromwhich he runs a 50-foot length of large-diameter,low-loss, RG-8/U coax. Does the purist hear signals any better? Nope. Assuming identical environment and an-tenna orientation, reception will be virtually thesame. The difference in signal strengths between50 and 66 feet is imperceptible. The plastic-coatedwire insulates it from the moist tree limbs, buteven if it touched, the resistance of the treeswould not contribute significant signal loss. Signalabsorption by foliage at 7 MHz is minimal; theresistance of the thinner wire is less than one ohm;and the difference between 50 feet of RG-58/Uand RG-8/U at 7 MHz is a mere fraction of a dB. For receiving purposes, an antenna may bethick or thin; its texture may be solid, stranded ortubular; its composition may be any metal (gold,

steel, copper, lead or aluminum); it may be cov-ered with insulation or left bare. All signals willsound virtually the same. Even if signal strengths were reducedconsiderably, they would still be just as audible, because at shortwave frequencies, once there isenough signal to be heard above the atmosphericnoise (static), a larger antenna will only capturemore signal and noise. The S-meter may readhigher, but you would hear the same signal abovethe noise audio with the “deficient” antenna bysimply turning up the volume control. So why bother with good construction prac-tices? Heavy gauge, stranded wire will withstand

All About Antennas

Part 2By Bob Grove W8JHD

A good ground system utilizes short, large- gauge wire to connect radio equipment com-monly to at least one deep ground rod.

A good radial counterpoise (A) is always pref-erable to using lossy Earth (B) in a verticalantenna system.

The inverted V is a popular dipole configura-tion.

The radial counterpoise on a ground mountedvertical (A) prevents soil absorption of the radiowaves; the radials of an elevated vertical (B)are part of the antenna itself and help shapethe pattern.



ice, wind loading, and flexing better than thin solidwire, and it will radiate transmitted power moreefficiently. Commercially made center insulatorswith built-in connectors are more rigid and waterresistant than soldered connections and they can be easily disconnected for servicing or inspection.Sturdy, insulated suspension is more durable overtime, and keeping antennas away from tree foliagemay avoid some signal loss at higher frequencies.

Skin Effect A thin, hollow, metal tube is just as efficientin conducting and radiating radio-frequencyenergy as a solid wire of the same diameter andmaterial. This is because RF energy barely dips below the surface of the conductor, and the higherthe frequency, the shallower the depth. The largerthe surface, the less resistance, which would waste power as heat. Skin depth varies inversely withthe square root of the conductivity and the per-meability (magnetic attraction) of the metal; the better the conductor, the deeper the skin effect.At microwave frequencies (10 GHz), the skindepth of silver, an excellent conductor, is 0.64micrometers (µm), while that of aluminum, a poorer conductor, is 0.80 µm.

Iron is a very poor conductor and has high permeability; its skin depth is only 1/7 that ofcopper, making it a poor choice as a conductor atradio frequencies.

Antenna Size The energy-intercepting area of an antennais called its aperture (another similarity to light asin the aperture of a camera lens) or capture area;the larger its aperture, the more signal it captures.Curiously, a large antenna is not necessarily bet-ter at transmitting (or receiving) than a smallerantenna. If a small element can be designed to be just as efficient as a large antenna, and radiates the

same pattern, there is no benefit in using a largerantenna unless it can be configured to offer gain,which comes from shaping the directionality ofthe antenna. Similarly, all antennas of the samesize (wire dipoles, folded dipoles, fans, trap anten-nas, cages, or any other) radiate the same amountof power. Their relative advantages come from pattern directivity. The U.S. Coast Guard found several de-cades ago that a five-foot antenna was adequatefor HF reception 100% of the time. Remember,the purpose of an antenna isto detect enough signal toovercome the receiver’s owninternally-generated noise;

once that is accomplished,more signal only means moreatmospheric noise with itsattendant interference fromstrong-signal overload. Below approximately50 MHz, atmospheric noise(static) becomes increas-ingly worse the lower wetune. This background hissis a composite of thousandsof lightning strikes occur-ring simultaneously, aroundthe world. Once we detectenough signal to overcome

the receiver’s own self-generated circuit noise, alarger aperture will only increase the atmospheric

noise right along with the signal. If the noise islocally generated (power lines or an electrically-noisy neighbor, for example) a beam or loopantenna can be rotated away from the source ofthe noise to null the interference, hopefully towardthe direction of the signal as well. As we tune upwards from 50 MHz, atmo-spheric noise diminishes; therefore, larger and better-matched antenna sys tems do improvereception because they help overcome receivernoise, which can be higher than atmospheric noiseat VHF and UHF frequencies. Ultimately, oncethe aperture is great enough to overcome receivernoise at these higher frequencies, larger aperturewill only pick up more noise (just as at the lower

frequencies) so directivity should be the goal for better reception.

Antenna Gain Signal improvement may come from a largeraperture, or from intentionally distorting (shaping)the field to produce a narrower pattern. Whilelarger aperture increases background noise as wellas signal strengths, directivity favors one or moredirections at the expense of others. This reducesoverall pickup (better signal-to-noise-ratio), con-centrating on a target direction for receiving and/or transmitting, and reducing reception interfer-ence from the sides and back. Such pattern re-direction often refers to front-to-back rat io and side-lobe rejection ,describing how improvement in one directionis accompanied by the desirable loss in otherdirections. The pattern can be shaped by adding parasitic elements, which are unconnected butsecured to the boom, called reflectors and direc-tors (see Yagi below). Feed point mismatch doesnot affect an antenna’s gain or pattern. Adding a second identical antenna separated by ½ wavelength and connected in phase, known

as stacking will increasetransmitted and received sig-nal strengths by 3 dB, regard-less of the original gain. Thus,two 1-dB-gain antennas will

provide 4 dB total gain, andtwo 20-dB-gain intercon-nected antennas will provide23 dB total gain. Antenna performance isusually compared to a half-wave dipole reference. Somemanufacturers compare thegain of their antennas to an“isotropic” radiator which is atheoretical (and nonexistent)antenna that has a spheri-cal radiation pattern. Thisgives manufacturers a 2.1 dB

higher gain claim than if they compared it to a realantenna: a half-wave dipole. Unless the claimedgain figure is followed by dBd or dBi, referencinga dipole or isotropic radiator in free space, it ismeaningless and suspect. Assuming we run the transmission line awayat right angles from the antenna for at least aquarter wavelength, the location of the feed pointcauses very little distortion of the pattern, but theimpedance selection varies dramatically. Is a good transmitting antenna always a good

receiving antenna? Yes, if its aperture is largeenough to capture enough signal to overcomereceiver noise. The law of reciprocity states thatif an antenna system efficiently radiates a signalinto space, it will just as efficiently deliver anintercepted signal to a receiver. Is a good receiving antenna a good trans-mitting antenna? Not necessarily. If randomlyerected, it may be susceptible to power loss dueto impedance mismatch. Its pattern will be unpre-dictable and reactance may shut down a transmit-ter with built-in protection against mismatches.

Arrays Depending upon its thickness, taper andlength, a mass of metal, brought within one-quarter-wavelength of a radiator (the drivenelement, connected to the feed line), will interactwith the field, focusing (there’s that light analogyagain!) the energy to produce directivity or gain. Probably the best known of these combina-tions is the Yagi-Uda array, named for the twoJapanese scientists who developed the antennain 1928. While Uda actually did all the develop-mental work, Yagi published the results, so theantenna, as fate would have it, usually bears hisname alone.

Curiously, the Japanese did not use the Yagiin World War II. The modern Yagi consists of a half-wave-length driven element, a single rear reflector about5% longer, and one or more forward directors

about 5% shorter. The elements are usually spaced0.15-0.2 wavelengths apart. Depending upon the number of directors, aYagi may have six to twenty decibels (6 - 20 dBd)gain over a half-wave dipole in free space.

There are many computer programs availablein handbooks and on the Web for designing Yagias well as other effective antennas.

Next Month :What do we mean by “matching” an an-

tenna? What is “impedance?” Is it possible to re-motely “tune” an antenna for best performance?Stay tuned for the next thrilling installment!

Atmospheric noisedecreasing with frequency.

Stacking any two identical antennas,regardless of their individual gain,will increase the total gain by 3dB.

The Yagi is a popular beam antenna with forward gain.



All About Antennas

Part 3By Bob Grove W8JHD

Last month we discussed the many physi-cal aspects of antenna design, not onlyin construction but in location. Now

let’s take a close look at some of the electricalconsiderations.

Matching the System The term “impedance matching” alwayscomes up when referring to an antenna andtransmission line. To impede means to oppose,so what is being opposed in an antenna system? When a battery is connected to a light bulb,the resistance of the filament is the impedance,dissipating the opposed energy as heat and light.Ohm’s law reveals that there is a simple relation-

ship between resistance, voltage and current.When a transmitter is connected to an antennain free space, RF energy is radiated into space;the voltage and current are controlled both by theantenna’s radiation resistance and any capacitiveor inductive reactance which may be present. Why does an open circuit like a dipoleaccept and radiate power? An antenna is a spe-cialized form of transmission line; it is coupledto space, which has an impedance of 377 ohms.The center feed point impedance of a half-wavedipole, however, is much lower than that.

Resonance The impedance of an antenna is a combina-tion of radiation resistance, conductor resistance,and reactance. Radiation resistance is desirable;it’s what accepts power and radiates it into space.Conductor resistance, however, wastes power asheat. Reactance opposes incoming energy; it iscaused when an antenna is too long or too shortat a particular frequency, so that when the wave(signal voltage) traveling along the antenna isreflected from the ends, it returns to the feed point “out of phase” with the incoming wave. A half-wave antenna is naturally “reso-nant”; an arriving signal travels that half-wavelength in half its cycle, then reflects back inthe other direction, finishing that cycle when itreturns to its starting point, the electromagneticequivalent of a vibrating guitar string. Measurements will reveal maximum cur-rent (and minimum voltage) at the center, andmaximum voltage (minimum current) at the endsof the wire. A multiple-half-wave (full-wave,wavelength-and-a-half, etc.) antenna will have astanding wave on every half-wavelength section.

Radiation Resistance An infinitely-thin, half-wave dipole in freespace (at least several wavelengths away fromother objects) would have a center feed pointimpedance (radiation resistance) of 73 ohms.Constructed of normal wire the impedance is

closer to 65 ohms; if thicker tubing, 55-60 ohms.This impedance rises as we move the feed pointoff center. If we use a folded dipole (See figure.)the feed point impedance rises to about 300ohms. Proximity to the earth’s surface also alters

the feed point resistance of a horizontal dipole,typically dropping from 100 to nearly 0 ohms asthe antenna is lowered from 0.33 wavelengthsto the earth’s surface, and fluctuating between60 and 100 ohms at heights between 0.33 and 1wavelength. Vertical dipoles fare better, since their pat-

terns do not radiate directly downward wherethey would interact with the earth. Once elevatedat least 0.25 wavelength, their impedance re-mains a relatively constant 70 ohms.

A vertical antenna with drooping radialshas lower impedance, nominally 50 ohms; ifthose radials were horizontal (at right angles tothe vertical element), the feed-point impedance

would be about 35 ohms. If 50-ohm coax is attached to an antenna’s50-ohm feed point, we have a perfect (1:1 ratio)impedance match, but if that 50 ohm coax is at-tached either to a 25 or 100 (50/25 or 100/50),is that bad? No. Is 3:1? No.

The simple fact is that if there is no resis-tive loss in the feed line or antenna (of coursethere always is), 100% of the generated powerwill be radiated by the antenna regardless of themismatch. What really happens with an impedancemismatch? Some of the signal voltage reflects back from the antenna junction through the coaxto the transmitter where it is re-reflected and

eventually radiated into space. The higher thevoltage (that is, the worse the mismatch), themore power is absorbed by the resistive insula-tion, heating it. That’s where low-loss coax isimportant. An impedance mismatch does not produce radiation from the feed line. When receiving, all signal voltage gath-ered by a perfectly matched antenna is fed tothe receiver, but with a mismatch, the reflectedsignal is radiated back into space. In practice,this is usually of minor consequence, especially

at HF and below, where atmospheric noise is adominant influence on signal interference.

The Transmission Line In the early days of radio when open-wiretransmission lines were common, the voltagefields produced by standing waves would lightup bulbs and deflect meters brought near thelines; nowadays, with the near-universal use ofcoaxial cable which encloses the electrostaticfields, such measurements are not as easy. Connecting an unbalanced line (coax) to a balanced antenna can cause RF currents to flowon the outside of the line, but these are not stand-ing waves. So what gives a transmission line itscharacteristic impedance (surge impedance)? Afeed line can be considered as a radio-frequency,low-pass filter consisting of an infinite numberof series inductances shunted by an infinitenumber of parallel capacitances. The impedance of this distributed network

is theoretical, based upon the dielectric constantof the insulation, the spacing of the conductors,no losses, and infinite length. While the most common feed line imped-ances are 50, 75 and 300 ohms (TV twin lead),there are more than two dozen commercially-available impedances from 32 to 600 ohms.

So why 50 or 75 ohms? Why have we chosen impedance standardslike 50 and 75 ohms for coax? For transmitters,the best power-handling capability is at 77 ohms,while the best voltage tolerance occurs below30 ohms. 50 ohms is a good compromise and it

matches several standard antenna designs. For receiving purposes, 75 ohms is op-timum for low coax losses, so it was adopted by the cable TV industry. Conveniently, it alsomatches several antenna designs. The impedance a transmitter or receiver“sees” when it is mismatched to a length oftransmission line connected to an antenna is acomposite of the length of the line along withits losses, the SWR (see “Traveling Waves” below), and the load (feed point impedance ofthe antenna to which it is connected). If they areall properly matched, however, the impedanceis determined only by the characteristic of theline.



Magical line lengths Trick No.1: The impedance measured atthe bottom of an electrical-half-wavelengthtransmission line (or any whole-number mul-tiple of a half-wavelength), regardless of thecharacteristic impedance of the feed line, is thefeed point impedance of the antenna.

For example, if, at some frequency, anantenna has a feed point impedance of, say,143 ohms, then we will read 143 ohms at the bottom of a 50-, 72- or 300-ohm, electrical half-wavelength line connected to it. Keep in mind that this is an electrical half-wavelength; we must multiply the free-spacehalf-wavelength by the velocity factor of thecoax. For example, a half-wavelength at 14 MHzis 33 feet; using coax with a velocity factor of66% would mean that you would actually cutthe line to a length of 22 feet.

Trick No.2: We can use a quarter-wave-

length piece of transmission line as an imped-ance-matching transformer using the formula:

For example, by substituting actual valuesin the solution below, if we wish to attach a 100-ohm antenna to a length of 50 ohm cable, we caninsert a quarter-wavelength matching stub of 70(often marked 72 or 75) ohm cable.

= = 70.7 ohms

Don’t forget to multiply the free-spacequarter-wavelength by the velocity factor and

shorten the length of the cable accordingly. Forexample, a quarter-wavelength at 14 MHz is234/14, or 16.7 feet; using coax with a velocityfactor of 66%, the actual physical length would be cut to 11 feet. If the line needs to be physically longer, useodd multiples of the quarter-wavelength and thetransformation will remain the same.

But remember, most antennas exhibit avery narrow frequency bandwidth for a givenimpedance, so all this magic occurs only aroundone frequency; on single-element antennas likedipoles and verticals, it also works on odd-har-monic multiples, although the match degradesas we increase the number of multiples. Remember as well, to take into accountthe velocity factory of the coax. For example,

if a half-wavelength at 7 MHz is 66 feet in freespace, then the electrical half-wavelength ofcoax with a velocity factor of 67% would be 44feet.

Standing Waves or

Traveling Waves? When the system is non-resonant, thewaves reflect back from any point where theimpedance changes, passing across each other

in phase. Typically, these changes occur wherethe transmission line attaches to the antenna. Early instrumentation could not detectwhich waves, forward or reflected, were beingmeasured; their composite voltage was shownon a voltmeter, periodically distributed alongthe transmission line. They were assumed to bestanding waves and the name has stuck. The comparison of those summed voltage peaks to the minimum voltages interspersed between them is called “voltage standing waveratio” or “VSWR.” Engineers prefer to measurethe “voltage reflection coefficient,” the comparison of the reflected voltage to the incidentvoltage at any one point on the line.

Since power (measured in watts) is a product of voltage times current, as the currentrises, the voltage falls (and vice versa); thus, thecurrent peaks are half way between the voltage peaks. The ratio of the current peak to minimumis the same as that of the voltage, so “VSWR”is usually shortened to “SWR” to accommodate both units. For example, if a 200-ohm resistive antennafeed point is attached to a 50-ohm line, we wouldhave a 4:1 SWR. The presence of inductive orcapacitive reactance adds further to an antenna’simpedance.

When transmitting , the high voltages produced by high SWR may arc across the feedline insulation or tuning components, and highcurrent may waste energy by heating the feedline conductors. Since these are stationary pointson the line for any particular frequency, thetransmitter (or matching device) may experienceeither high voltage or high current, dependingupon the length of the line.

In a receiving system, antenna-to-transmis-sion-line mismatch will also produce losses inthe transmission line; additionally, any imped-ance mismatch between the receiver and the

antenna system will reflect power back to theantenna where it will be re-radiated back intospace.

Feedline loss Single-wire feed, popular in the early 1900s but now virtually abandoned, matched best athigh-impedance feed points (hundreds or eventhousands of ohms); it was commonly used tooff-center-feed antennas in the early days ofradio, often with an SWR exceeding 10:1, butthey were efficient radiators. The lowest-loss transmission line commer-cially available is open-wire, parallel feed lineknown as “ladder line.” It accommodates high power and high SWR with virtually no loss.

Disadvantages of open-wire feeders in-clude:(1) A separation requirement between it and any

nearby moisture or metal by two to four timesthe separation of its two wires to avoid someSWR increase resulting from interaction withits unenclosed field;

(2) Unbalancing the line by allowing one wire tocome closer than the other to nearby metaor moisture;

(3) Inability to bend at sharp angles without ad-ditional reflective losses;

(4) Impedance mismatch when attaching tostandard low impedance antennas andtransmitters (except when used in multiplesof a half-electrical-wavelength long at specificfrequencies);

(5) Balanced matching requirements when usedwith unbalanced equipment (like every trans-mitter made!);

(6) Vulnerability to electrical noise pickup ifslightly unbalanced;

(7) Changes in characterist ic impedance fromrain, ice and snow;

(8) and rarity of parallel-line connectors on radioequipment.

Solid-dielectric, parallel feed line like TVtwin lead may also be used for receiving andlow-power transmitting provided all the caveatsregarding open-wire feeders are observed.

Because its closer conductor spacing con-fines its field more, it may be brought within twoor three inches of nearby metal or moisture. Butthe plastic insulation on inexpensive TV twin-lead disintegrates with time, collecting moistureand residue in its cracks, making it lossy. Coaxial cable, on the other hand, mayapproach the efficiency of open wire, may be run underground or through metal pipe, iselectrical-noise resistant, and mates easily withconventional connectors.

Here are the reasons that most coax islossier than open-wire feed line:(1) Its conductors are smaller, offering more

resistance to waste the current as heat.(2) The dielectric (insulation) surrounding the

conductor dissipates some power; the higherthe frequency, the higher the dissipation.

These two factors explain why large diam-eter, foam dielectric, short length, coax cablesare preferred, especially for transmitting. Thereis also a safety reason: coax doesn’t radiate itsenergy.

Of course, mammoth coax is wasted ifsmaller will do; after all, in house wiring, we



don’t use enormous #4 bus wire when #12 safely passes all the current that is required. So what is the best coax? Generallyspeaking, the bigger the better, with aluminum-sheathed hard line taking the prize. But willyou know the difference between that and, say,Belden 9913, foam dielectric RG-8/U, RG-213/U or RG-214/U? Not unless you are runningat least 100 feet at 1000 MHz or higher, or aretransmitting more than 1000 watts. For receiving purposes, or for transmitting

up to 200 watts, it’s even easier. Since we aren’tdeveloping high voltages, we can use smaller-diameter cable, just so long as it’s not lossy.

Generally speaking, coax with a high veloc-ity factor rating suffers the least loss. Below 30MHz use RG-58/U, RG-59/U, RG-6/U, or RG-8/X for runs of up to 100 feet. For VHF/UHF to1000 MHz, use any of these but the RG-58/U. Don’t let 70 ohm (instead of 50 ohm)impedance throw you; you won’t hear the differ-ence for receiving, and the impedance mismatchfor a 50 ohm transmitter is only 1.4:1 which isinconsequential, resulting in a loss of less than0.2 dB, which is imperceptible. Generally speaking, the thinner the coax,

the poorer the cable. Skinny RG-174/U should be used only for the shortest runs (a few feet). Never use shielded audio cable in place of

coax for radio frequency work; it is very lossy,has dreadful shielding, inviting interferenceduring reception, and radiation during transmis-sion. Its reputation for causing radio-frequencyinterference (RFI) when used to interconnectdigital accessories is notorious! But even good coax deteriorates withtime; foam-dielectric coax, initially superior in performance, loses grace first, falling victim tomoisture intrusion. Many experts (especiallycable vendors!) recommend replacing coaxevery five years.

So, how can we tell if the coax is still good?One way is to short-circuit the far end of thecable and attach the near end to an SWR meterwhich, in turn, is connected to a low-powertransmitter. The short will reflect 100% of the power reaching it, sending it back to be regis-tered as reflected power. The higher the SWR,the better, because it means that energy is not being absorbed along the way. Replace coax thatshows a short-circuit SWR lower than 3:1. An easier test is to attach an ohmmeter onits high resistance scale across the shield andcenter conductor of the coax, leaving the farend open. There should be no reading on themeter (several megohms resistance). If there is

a reading, the lower the resistance, the worsethe coax. It may be showing the consequencesof water intrusion or corrosion.

A high SWR between the feed line andantenna may appear as a low SWR at the trans-mitter. Corroded or loose connectors, lossy cableand other resistive agents can all contribute to adeceptively low SWR reading. Since no cable is 100% efficient, the SWRmeasured at the transmitter will always be lowerthan the actual mismatch at the antenna; the poorer (lossier) the cable, the lower (and moremisleading) the reading. Only by connecting an SWR meter directlyto the antenna feed point can we get a true SWR

reading. Use good cable and that SWR differ-ence is but a few percent. So how does transmission line loss indecibels translate to percentage of power loss? Ifsystem impedances are matched properly, a 1 dBloss uses up 20% of the power; 3 dB represents50%; and 6 dB attenuation means that 75% ofthe power is being used to heat the coax, whethertransmitting or receiving.

In an unmatched system, line losses areeven worse. High-SWR voltages dissipate more power in the transmission line’s dielectric (insu-lation breakdown), current peaks dissipate more power in the conductor (resistive losses) and both effects are aggravated by rising frequency.The result is that power is being wasted as heat. For example, a 6:1 SWR in 100 feet of RG-8/U at 14 MHz produces only a 1 dB loss, but at450 MHz it becomes 6 dB, and at 900 MHz, 8dB. With poorer-quality cable, losses are much

worse. It pays to use good cable! Keep in mind that these are coax losses; ifyou use open-wire feeders, the loss at 10:1 oreven 20:1 SWR is insignificant. Such high SWRwas present on early, micro-power earth satel-lites, but we heard those fine 23,000 miles below,demonstrating once again that SWR alone hasnothing to do with radiation efficiency. Contrary to popular myth, high antennaSWR does not radiate any more harmonics ortelevision interference (TVI) than a 1:1 SWR,assuming that the transmitter is properly tunedon frequency. Keeping SWR to a minimum by propertransmission-line impedance matching is a pre-

ventive against damage, especially to moderntransceivers with marginal power specifications. Automatic power-reduction circuits oftenkick in with an SWR as low as 2:1, makingmatching a requirement to achieve full output power.

Tuning the System Antenna tuners, antenna tuning units(ATUs), transmatches, couplers and match- boxes are different names for the same thing:combinations of adjustable capacitors and coilsto compensate for inductive and capacitive re-actances in the antenna system. Transmatches

(the preferred term) also provide adjustable impedance transformation between the receiver ortransmitter and line, and some provide balanced-to-unbalanced matching as well. Every length of metal has some frequencyor frequencies at which it is naturally resonant;that is, the inductive reactance equals the capaci-tive reactance, thus mutually canceling the reac-tance and leaving only the radiation resistance.If an antenna is too long for it to be naturallyresonant at some desired frequency, we say it is

inductive; a series capacitance can “tune out”that inductive reactance which opposes theincoming RF power. Conversely, an electrically-short (capaci-tive) antenna can be adjusted by a series induc-tance. Contrary to a popular notion, a loadingcoil does not “add the missing length” to ashort antenna; its inductive reactance cancelsthe antenna’s capacitive reactance. We can alsoneutralize these reactances with a transmatchconnected at the transmitter output. Quoting antenna guru Walt Maxwell,W2DU, when the transmatch is properly tuned,“...the entire system is made resonant...all re-actances in the system are cancelled...the net

reactance is ZERO! In addition, by obtaining aconjugate match at the antenna tuner, a conju-gate match is inherently obtained at any other junction in the system where a mismatch existed prior to obtaining the match with the tuner.” A transmatch is adjusted to provide capaci-tive and inductive values of equal magnitude, but of opposite phase, to the returning reflected power, thus re-reflecting it back toward theantenna in phase with the transmitted power. We don’t electrically alter any reactance inthe antenna system, we merely neutralize theireffects, thus matching all impedances in the process. All that is left is the antenna’s radiationresistance, so all power is radiated.

This “tuned feed-line” approach can beused with single wire, open parallel line, twinlead, or coax equally well. Since a transmatch istypically connected between the transmitter (orreceiver) and feed line, it can only impedance-match those two points; it has no affect whatso-ever on matching the feed line to the antenna. Wewould need to connect the transmatch betweenthe feed line and antenna feed point to producea match there.



Just because it’s a transmatch doesn’t meanit’s a good transmatch. Flimsy construction andsmall-gauge wire may mean additional losses,especially at higher power levels. High-powertransmatches are invariably more efficient thanthe low-power variety.

Efficiency Efficiency is a commonly misunderstoodconcept in antenna system design; it is simplythe percentage of transmitter-generated signalwhich is radiated by the antenna, or receivedsignal voltage which is delivered to the receiver.If there were no resistive or insulation losses, anyantenna and feed line would be 100% efficientwhether or not they are properly matched.

Balanced or Unbalanced?Most elevated, horizontal antennas are fed

at or near the center; they are said to be balanced, both from a standpoint of symmetry as well asreference to ground.

Most vertical antennas are unbalanced,often making use of radial systems as an artificialground reference. There is nothing inherentlysuperior about one over the other; it is merelya question of whether they are best fed by twinlead (balanced) or coax (unbalanced). Balun(balanced-to-unbalanced) transformers, whichwe will discuss later, as well as transmatchescan be used to match balanced to unbalanced

circuit elements, and to match impedances. What is the penalty for misbalancing thefeed point? It may cause some RF current toflow on the surface of the feed line, or some strayradiation from the feed point, producing somedistortion in the pattern’s symmetry, affectinggain somewhat.

Next Month The last part of this series. Choosing anantenna to match the task. How about acces-sories? Final take-home points.

The term “polarization” refers to the rela-tive position of the electric component of theradio wave with respect to the earth’s surface.A vertical antenna, often referred to as a “Mar-coni” (or “whip” if short), has its element(s),and therefore its electric field, perpendicularto the earth. A horizontal antenna, variouslycalled a “Hertz”, “flat top” or “Zepp” (afterthe trailing antennas on the Zeppelins), hasits element(s) and electric field parallel to theearth. Neither polarization is inherently superior.The choice is made on a basis of practicalconsiderations such as the likelihood of ahorizontal antenna causing television interfer-ence (TVI); the possibility of interaction withnearby metallic masses in the same plane; adesired pattern; reduction of noise pickup from power lines and accessories; the area available,or ease of mounting. Vertical antennas have only one mounting point and, properly placed, radiate uniformlytoward the horizon in all compass directions;their low angle of radiation favors distant com-munications. Horizontal wire antennas must be elevatedat least a half-wavelength above the earth fora low angle of radiation and reception; theyutilize at least two suspension points, and radi-ate primarily at right angles to the wire axis.It is more practical to make a long horizontalantenna than a tall vertical antenna because ofthe support requirements. At high frequency (shortwave), there islittle difference in performance between prop-erly installed horizontal and vertical antennas.Distant signals arrive with mixed polarizationfrom multiple reflections, and sometimes eventhe compass direction (azimuth or bearing) isunpredictable. For VHF/UHF, vertical polarization is therule since mobile communications dominatethis part of the spectrum, and it is easiest tomount a whip antenna on the vehicle. Except

in the city where buildings reflect signals, shortrange VHF/UHF communications retain theiroriginal polarization.

Dipoles The most common basic antenna is thehalf-wave dipole, a length of wire at low fre-quencies, or tubing at VHF/UHF, which is cutat the center and connected to a transmissionline. Such an antenna matches coax well for

about +/-5% of its design center frequency, but the impedance steadily rises beyond that,requiring an antenna “tuner” (transmatch) fortransmitting. On odd harmonics (3rd, 5th, etc.) of thefundamental design frequency, the impedancelowers again, making the antenna multibandeven without a tuner. A half-wave dipole is a half-wavelengthlong only at one frequency; that same length isa full-wavelength at twice the frequency, and aquarter-wavelength at half the frequency. The theoretical length in feet of a half-wave dipole in free space is found by dividing492 by the frequency in megahertz. But sup-

port insulators and wires at theends (“end effect”) makes theantenna about 5% capacitivelyshorter. Divide, instead, 468 bythe frequency in megahertz;Thus, a 7 MHz, half-wavedipole would be 67 feet long. Since it is more convenientat VHF and UHF to calculate ininches, divide 5616 (468 times12 inches) by the frequency inmegahertz. Thus, a 146 MHzdipole would be 38 inches long. At its design frequency and below, the radiation and receiv-

All about Antennas Part 4:

Choosing Antennas

By Bob Grove W8JHD (all graphics courtesy the author)

The polarization of an antenna is simply its relationship to the surface of the earth.

The gain of an an- te nna ma y co me fr om in cre as in gits aperture (A) or narr owing it s pattern(B).



ing pattern is perpendicular to the element, butas the harmonic multiple increases, the patternchanges, and the lobes now favor the ends ofthe antenna with resultant gain. This should betaken into consideration for widefrequency applications. Such an antenna can be erected to favorground-wave communications at the lowerfrequencies, and sky-wave DX at higher fre-quencies. While it may be tempting to erect thelongest dipole we can, consistent with availablereal estate, a quarter-wave dipole captures only3 dB (half and S-unit) less than a half-wavedipole. We won’t hear much difference. For

transmitting, if properly matched, the radiated power is virtually identical.

The “Longwire” Many shortwave enthusiasts mistakenlyrefer to a random wire antenna as a “longwire,” but it doesn’t qualify unless it is at least one fullwavelength long at its operational frequency.Thus, a 150 foot antenna is just a half-wavedipole at 3 MHz, but it is a longwire above 6MHz.

So why doesn’t a long, horizontal short-wave antenna make a great scanner antenna?For one thing, as you use a given length antennaat higher and higher frequencies, it has large

A 67-foot dipole is half-wavelength at 7 MHz, but 1-1/2 wavelengths at 21 MHz. As the fre-quency increases, wavelength decreases.

A halfwave dipole (A) as seen from above has a classical figure-eight pattern at right angles to the curve. The same antenna at twice the frequency, now a full wavelength, has a clo-verleaf pattern.

numbers of lobes and nulls, making it verydirectional. Not only that, but most VHF/UHFsignals are vertically polarized. Even if it were suspended vertically, thelongwire antenna pattern would favor VHF/UHF signals off its ends—above and below,not off its sides. And finally, impedance mis-match would cause considerable signal loss inthe transmission line at VHF and even more atUHF.

Ground planes The ground plane antenna may be thoughtof as a vertical dipole in which the bottom ele-ment is replaced by an array of horizontal (ornearly so) elements, or even a sheet of metal,simulating a perfectly conductive earth. Like the dipole, a ground plane designedfor a specific frequency will experience anincrease in feed-point impedance and radiationangle toward its ends as the applied frequencyincreases, reducing effectiveness on harmonicoperation, except for working or monitoringoverhead aircraft and orbiting satellites!

Mobile Antennas In an automotive environment, the vehicle body is the ground plane; its contours and an-tenna placement influence the radiation (andreception) pattern of the signal. Directivitygenerally favors the mass of metal—a roof-mounted whip has a basically omnidirectional pattern, while a rear-bumper mount favors theforward direction of the vehicle. At frequencies above 10 MHz or so, thesurface area of the vehicle and the length re-quirements for a vertical antenna are practicalfor efficient operation; a quarter-wave whip isresonant, exhibiting a feed point impedance ofabout 36 ohms, a good match for conventional

50-ohm cable. But as frequencies lower, the electrically-short antenna possesses less and less radiationresistance (less than one ohm at 2 MHz) andmore and more capacitive reactance whichmust be cancelled by a series inductance (load-ing coil). Base-loading the whip requires less induc-tance than center- or top-loading because theupper whip section’s capacitance with the car body reduces its own capacitive reactance; butthe radiation resistance remains low (in somecases a few ohms), so that coil and transmissionline resistances contribute a proportionately-higher loss.

The angle of the radialelements affect the feed point impedance of a ground plane vertical.

Raising the position of the loading coilchanges the current distribution along the an-tenna, increasing the radiation resistance, butthe longer loading coil requirement introducesmore resistive loss. A position approximately2/3 the way up the whip may be an optimumcompromise. But as the frequency of operation lowers,choosing a high-power-rated coil with lowresistive loss becomes increasingly important,even for receiving and low-power transmitting.

When all resistive losses are kept low and

the reactances are tuned out, the typical inputimpedance of an HF mobile antenna remains inthe 10-20 ohm range. A broadband impedance-matching transformer is recommended fortransmitting. In a poorly-designed mobile antenna in-stallation, resistive losses may be high enoughto match the impedance requirements of theradio and coax, negating the need of a matchingnetwork—but the performance is awful!

Grounded Antennas It is actually possible to connect one endof an antenna to a solid earth ground with ex-cellent results. This is the principle behind theBeverage antenna. Why doesn’t the ground trickle off all thesignal voltage? Because of standing waves. Weare dealing with high frequency alternatingcurrent, not DC. The element is long enoughto utilize reflected signal voltage to cancel anyshort-circuiting to ground. Just as with a dipole or vertical in whichone element set is connected to the groundedshield of the coax transmission line, this an-tenna detects the voltage difference betweenthe elements, and that is what is sensed by thereceiver as a signal. That is why a car body, in spite of thefact that it is “grounded” to the frame, and

thus to the radio, can be used as an antennaat VHF and UHF frequencies. Arriving radiowaves create standing waves – voltage pointsthat can be tapped for their energy and fed toa receiver, or reciprocally, will accept energyfrom a transmitter and radiate it into space. However, the unpredictable signal patternsas well as interference from ignition noise andautomotive electronics make the car-bodyantenna a poor choice.

Traps For multiband operation, a trap (a coil andcapacitor in parallel) may be placed between

The mobile radio enthusiast has a number of antenna location choices with rooftop the best.Other spots include rear bumper, trunk lid, rear and front cowls, and rear window.



sections of an antenna to provide automaticselection of appropriate lengths for given fre-quencies. The trap is high Q (sharply tuned)to the resonant frequency of the length closestto the feed point, providing several hundredohms impedance isolation from the adjoiningsection(s). At other (non-resonant) frequencies, thecoil simply adds slight electrical length be-tween the adjoining elements, all of which now

add to the total antenna length. In this manner,a combination of elements and traps allowresonant operation on several bands withoutthe need of a transmatch. The sections are arranged infrequency order with the highest frequencyclosest to the feed point.

If a transmatch is available, a trappedantenna is undesirable since it suffers fromthe traps’ resistive losses, gaps in frequencycoverage, more components to fail, and highercost than a simple dipole.

Active or Passive

Antennas? With one singular exception, all antennasare passive; that is, they have no amplifyingelectronic circuitry. They simply reflect, re-fract, radiate or conduct the electromagnetic

energy which reaches them.The exception is the active (voltage probe

or E-field) antenna which consists of a short(a few inches to a few feet) receiving elementcoupled to a wideband, small-signal amplifier.It is not used for transmitting. While active antennas may have smallsize and wide bandwidth, and can deliverlarge signals to the receiver, they have theirdisadvantages. They are expensive, they re-quire power, they may burn out or degrade in

performance from nearby lightning or strongsignals, they generate noise and intermodula-tion interference (“intermod”), and they areusually placed close to interference-generating

The Beverage is an example of a high performance, low frequency, grounded receiving antenna.

Traps are used to isolate antenna elementlengths for multiband operation.

electronic appliances. Don’t use an activeantenna if an adequate passive antenna isavailable.

Invisible Antennas Appearances or deed restrictions some-times require a hidden antenna. Receivingantennas are much less demanding and easierto hide, but even transmitting antennas can

be inconspicuous. Of course, VHF and UHFantennas, because of their compact sizes, areeasier to hide than HF antennas, but even HFantennas can be unimposing. An attic crawl space is the first recom-mendation provided the antenna can be sepa-rated from large metal surfaces and electricalwiring. Always use low-loss, well-shielded coaxtransmission line to prevent appliance noise

pickup during receive, and stray radiationduring transmit. A balun transformer andferrite-bead choke may be useful as well. Wire antennas may be run along base-

boards, ceiling molding, behind curtains,

and even under eaves, rugs or carpeting. Ifoutdoors is accessible, a thin, high wire is vir-tually invisible, especially if it is covered withgrey (neutral color) insulation; run it from theroof to a tree. A ground rod would be virtuallyinvisible by its nature. A wire antenna in a tree is also incon-spicuous. It can be run vertically up the trunk,suspended in the branches, or even constructedas a wire array for gain and directivity. Anantenna element doesn’t have to be perfectlystraight. The coax feed line can be trenched

just beneath the soil. Resourceful hams, SWLs and scanningenthusiasts have often resorted to make-doantennas. Bed springs, filing cabinets, raingutters and downspouts, aluminum windowframes, curtain rods, disconnected telephoneor power lines, metal flagpoles, aluminum lad-ders, fences, wheelbarrows, grocery carts, andeven vehicle-mounted antennas coax-fed intothe radio room have been called into service!

Next Month: Now that we’ve discussed an-tenna systems, what recommended accessoriescan improve both transmission and reception?

And finally, what are the take-home pointsthat mean the most? Don’t miss next month’sconclusion to this MT exclusive series onantennas!



In the four previous issues we’ve covered just about every aspect of antennas andtheir permutations. In this final chapter

we’ll discuss the best ways to connect them toradio equipment, and review the most importantfacts to remember. At audio frequencies, any kind of connec-tor will work as long as it can handle the voltageand current. But at radio frequencies, connectordesign is critical. Audio connectors, such asRCA phono connectors and earphone plugs, become increasingly deficient with frequency

and have no specific impedance characteristics.They are usable up to about 30 MHz for receiv-ing purposes. Motorola connectors, developed for AMcar radios, were grandfathered into VHF usewhen FM coverage was added to those radios.Early VHF converters needed Motorola con-nectors in order to interface with the car radios;then mobile scanners adopted them since thecar antenna could be called into limited servicefor local scanner reception. Marginal in per-formance at VHF/UHF frequencies, Motorola plugs have been abandoned by scanner manu-facturers in favor of infinitely superior BNCconnectors.

The BNC was named for its configura-tion and inventors – it is a (B)ayonet design by (N)eill and (C)oncelman and is excellentfor applications through at least 1,000 MHz(1 GHz). Neill also contributed his initial tothe N connector, developed during World WarII for military UHF communications. It is anexcellent waterproof choice through at least2 GHz, but harder to assemble since it is de-signed for large RG-8/U and R-213/U cables.Concelman’s initial adorns the less popular Cconnector of World War II.

The PL-259 (the so-called “UHF” con-nector), developed during the 1930s, is stilla favorite for HF ham and CB transceivers aswell as shortwave receivers. It works well upto at least 50-100 MHz. Low-cost, TV-type F connectors workextremely well through at least 1 GHz, but theyrequire solid-center-wire cable, and adaptorsare always needed to interconnect communica-tions equipment and accessories. Some imported nickel-plated connectorsare poorly made; their loose fittings and out-of-

tolerance thread pitches can add noise, intermit-tent performance, signal loss, and electrolyticcorrosion to the system. On the other hand, alab test at Grove Enterprises showed only afraction of a dB loss at 1 GHz from five differ-ent, imported, nickel-plated adaptors cascadedin series. But, to be safe, it’s always better to choosea branded, silver (or gold) plated connector oradaptor if available. Better yet, use a cable withthe correct connectors instead of adaptors.

Preamplifiers A preamplifier (“pre-amp” or “signal

booster”) is simply a small-signal amplifier placed between the antenna and receiver. Whenintegrated with a small receiving antenna, thecombination is called an active antenna (previ-ously discussed). A preamplifier connected to a poorly-located antenna will not perform as well asa well-placed, larger “passive” (unamplified)antenna, but it may be the only alternativewhen the better antenna is not practical. If ashortwave receiving antenna is at least 20 feetlong and in the clear, a preamplifier is probably

unnecessary. A preamp must have a lower noise figure(self-generated “hiss”) than the receiver, orthe only thing it accomplishes is increasing both signal and noise, just as if you had merelyturned up the receiver’s volume control.

It must have wide dynamic range – theability to amplify weak and strong signalsequally without becoming overloaded and thusgenerating spurious signal products, known asintermodulation (intermod), which interferewith normal reception. At VHF and especially UHF frequenciesand above, where transmission line losses may become significant, a preamplifier mounted atthe antenna will boost signals above the losscharacteristic of the line. Still, the preamp isvulnerable to all the problems described above. Even when working perfectly, a preampcan cause the receiver to overload and generateintermod of its own, desensitizing it to weak

signals, aggravating images, or even damagingits delicate RF amplifier circuitry. Use it as alast resort.

Splitters and combiners A splitter is essentially a broadband RFtransformer which allows one signal source to be equally divided into two or more paths; thisallows, for example, several receivers to operatefrom one antenna. Since a typical two-way splitter is an RFvoltage divider, each output will be reduced by3 dB, half the original power level. Connected in reverse, a splitter becomes

All about Antennas Part 5:

Accessories, Connectors and AdaptorsBy Bob Grove W8JHD

The most common antenna connectors and their maximum recommended frequencies.

A preamplifier should only be used in excep- tionally weak signal areas.



a combiner, allowing two signal sources toadd commonly. This allows, for example, twoseparate-frequency antennas to be used simul-taneously with one receiver. But, if the two antennas have a similarfrequency response, they can produce destruc-

tive interference (signal canceling) from certaindirections, while providing 3 dB overall gainin other directions. Basically, they comprise adirectional array – a “beam” antenna. TV splitters marked “V/U” or “VHF/UHF” or “54-890 MHz” actually work reason-ably well from the low HF range (typically 3MHz) up through 1 GHz. While there are transmitter split-ters and combiners, those made forreceivers are far more common andless expensive. They will also allowlow power – a few watts – to passwithout much problem, but higher power levels will heat the fine wind-

ing and saturate the small ferrite core,wasting power and even destroyingthe device.

TV Splitters Splitters used to couple two ormore TV sets to a common antennasystem are quite suitable for general- purpose receiving installations. Mostare marked with their recommendedfrequency application. Those intended for poweringsatellite-dish low-noise block down-converters (LNBs), marked 950-1450

MHz, should be avoided for short-wave and scanning, but those marked5-900 MHz work well from shortwaveright up through VHF/UHF scanning. Such splitters are usually DC passive; that is, their windings areelectrically interconnected so thatvoltage may transferred through thesplitter to activate an antenna-mount-ed preamplifier or down-converter. However, if you have voltageon the line but don’t want it to getthrough your DC-passive splitter,and don’t have a DC-blocked splitteravailable, there are accessory F-to-F

fittings that contain a DC-blocking capacitorthat may be attached to the splitter. Some split-ters offer a combination of passive and blocked ports on the same device. Conventional installations which have noDC power requirements on the transmissionline work just fine with either DC passive orDC blocked splitters.

Balun transformers The term “balun” (a contraction for “bal-anced to unbalanced”) is a wideband RF trans-former that allows an unbalanced transmissionline (coax) to be used with a balanced antenna(dipole or beam). Many transmatches include balun circuitry. Balun transformers may incorporate astep-up ratio (typically 4:1) to allow low-impedance coax to correctly match high-impedance antenna feed points, or a simple 1:1ratio to connect coax (unbalanced) to a balancedfeed point. Instead of a 1:1 balun transformer, a ferrite bead RF choke at the antenna-coax feed pointwill reduce RF on the feed line by absorbingunbalanced power. Like splitters, VHF/UHF TV balun trans-formers can be used with VHF/UHF scanners,shortwave receivers down to about 3 MHz, andeven for low-power (a few watts) transmitters. If a balun transformer is connected be-tween the feed line and antenna, a transmatchwill still resonate the entire system – balun,

antenna, feed line,tuner, and interactiveenvironment (tower,nearby wires, trees,rain gutters, etc.). But the tuner doesnot change the anten-na’s natural feed pointimpedance; if therewas a mismatch be-tween it and the balun

or feed line before thetune-up, it will remaineven after the tuner isadjusted to resonanceand shows a 1:1 match. A balun works properly only with resistive(non-reactive) loads; system reactances cancause impedance transformation ratios differentfrom what was intended. Therefore, it is not agood idea to use a balun over a wide frequencyrange on a narrowband antenna. Baluns also addlosses, due to wire resistance and possible coresaturation during transmit.

Attenuators It may seem self-defeating to make re-ceived signals weaker, but under some condi-tions it is advisable. For example, if you livenear several broadcast transmitters, or a high- powered paging transmitter, or if most signalsin your area are quite strong, they may be toohot for your receiver or scanner to handle.

Receivers may “comeapart” under these conditions, gen-erating spurious signals (intermod)or even desensitizing, making weak-signal reception virtually impossible. If your outdoor antennacauses either of these symptoms, anattenuator may be the prescription;some receivers and scanners havethem built in. But if an attenuator islikely to make desirable weak signalsunreadable, try a filter.

Filters A filter is a tuned circuit whichallows only certain frequencies to pass; their generic names imply whatthey do. A low-pass filter passes lowfrequencies and attenuates higherfrequencies, while a high-pass filter passes high frequencies and attenu-

ates lower frequencies. A band-pass filter passes aspecific range of frequencies, whilea band-reject (“suck-out”) filter at-tenuates a swath of spectrum. A trap, or notch filter, attenuatesa very narrow range of frequencies,while a peaking filter passes a verynarrow range of frequencies. Filters can be used with trans-mitters to reduce harmonic and otherspurious signal radiation, or withreceivers and scanners to selectivelyreduce strong, interfering signal

Balun transformers are used for match-ing coax to antennas.

Types of filters.

Splitters are used to feed one antenna to two(or more) receivers; they can also be used in reverse to combine two (or more) antennas to operate one receiver.


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frequencies. No filter is perfect in its characteristics;desired signals near the edges of the designrange (cutoff frequencies) will also be attenu-ated somewhat. Even at the center of its attenu-ation range, some strong signals may still getthrough. A credible manufacturer will publishthe response curves of his filters to reveal theirlimitations.

Antenna switches It is often desirable to select among twoor more antennas for optimum reception ortransmission. For receiving purposes, or evenfor low power (a few watts) transmitting, TVcoax antenna switches work admirably fromDC through 1000 MHz. CB-type antennaswitches work fine up to about 30MHz, for both receiving and transmitting. For higher power, especially at higher fre-quencies, select a commercial coax switch ratedfor the frequency range and power required.

Biohazards It has long been known that electromag-netic radiation can produce effects in biological

organisms; of particular concern to radio hob- byists is the influence of radio waves on thehuman body. If we consider the adult as an antenna,it has a natural resonance between 35 MHz(standing, grounded) and 70 MHz (insulatedfrom ground). Body parts, too, are resonant – the head ataround 400 MHz (700 MHz for infants). Sincethe body is a lossy conductor, it dissipates muchof the induced energy as heat. But in most casesit is not thermal affects that are of the greatestconcern. On-going controversy revolves aroundwhether cancers may be induced by low-level

AC fields from power lines and associatedequipment and appliances, as well as transmit-ted radio fields. Virtually all studies suggest that powerlevels under 100 watts or so into an elevatedoutdoor antenna are safe. Concern mountswith indoor, attic, mobile, and low-elevation,directional, transmitting antennas. Until all the facts are known, it is best tofollow these guidelines:(1) Keep away from antennas and open-wire feed

lines that are transmitting.(2) Elevate well overhead any transmitting an-tennas, especially directional beams whichconcentrate their energy.

(3) Operate nearby transmitting antennas (mo-bile, attic, in-room) at low power (nominallyno more than 25 watts).

(4) Operate RF power amplifiers with their coversin place.

(5) Hold hand-held transceivers away from yourhead by using extension mikes.

(6) Stay at least two feet away from power trans-formers.

Take-Home Points –

Facts, Not Fiction As we close this five-part series on antennasystems, here are some of the most important parts to remember:

1. Except for very thin wires, most antennasare efficient radiators. Virtually all losses inan antenna system occur in the feed line.

2. A high standing wave ratio (SWR of 3:1,6:1, etc.) merely indicates the presence ofpower reflections on the feed line due toimpedance mismatch. If there are no loss-es in the feed line, all reflected transmitterpower will be returned to and radiated bythe antenna.

For receiving systems, all captured signalpower will be returned to the receiver. Ifthere is an impedance mismatch betweenthe receiver and transmission line; how-ever, reflected signal power will return tothe antenna where it will be re-radiatedback into space

3. Reflected power does not flow back intothe transmitter and cause damage oroverheating. If damage occurs, it is dueto mistuning the amplifier.

4. A low SWR reading only means that thetransmitter, feed line and antenna systemare impedance-matched; it does not nec-essarily mean that everything is workingproperly.

Corroded or intermittent connectors,ineffective grounds, lossy cable and other

resistive agents can all give a deceptivelylow SWR. Unless an antenna is broadbandby design, a low impedance maintainedover a wide frequency range without retun-ing is particularly suspect.

5. Neither an antenna nor the feed lineneeds to be self-resonant (no inductive orcapacitive reactances) to perform properly. Virtually any antenna and its feed line, nomatter how reactive, can be brought toresonance by a properly designed trans-match.

6. Using low-loss transmission line, and atfrequencies below 30 MHz or so, signalsexperiencing an SWR of at least 3:1 andperhaps as high as 5:1 will be indistin-

guishable from signals produced by aperfect 1:1 impedance match.

7. Adjusting a transmatch at the radio posi-tion does not alter the reactance or im-pedance of either the antenna or the feedline; it brings the entire mismatched andreactive system into resonance by “conju-gate matching,” introducing reactance-canceling capacitances and inductancesof its own, so that the attached receiver ortransmitter senses only a resistive load.

8. A large antenna does not radiate more

power than a small antenna, nor is morepower radiated from a particular configu-ration (dipole, vertical, beam, quad, cage,bowtie, rhombic, loop, etc.). But a largeantenna does radiate a more concentrat-ed, directional field than a small antenna,and it captures more signal energy duringreception.

9. No transmission line needs to be a specificlength if a transmatch is available. Adjust-ing the length of a feed line does not alterthe SWR, just the impedance measured atthe tuner/feed line connection.

10. High SWR in a coax feed line does notcause RF currents to flow on the outsideof the line, nor will the coax radiate. High

SWR on an open wire feed line will notcause the feed line to radiate as long asthe currents are balanced, wire spacing issmall compared to wavelength, and thereare no sharp bends.

11. Assuming low-loss feed line, an SWR meterwill read the same at the antenna feedpoint, anywhere on the feed line, and atthe transmitter.

12. Raising or lowering an antenna to adjustits feed point impedance has no significanteffect on power radiated, only the shape ofits elevation pattern. However, raising thepattern between 3 and 20 degrees fromthe horizon can improve DX communica-tions.

13. A frequency meter or dip oscillator con-

nected at the bottom of a feed line cannotmeasure the resonant frequency of theantenna; it measures only the combinedresonance of the antenna plus the feedline.

14. A balun transformer on a transmittingantenna will match impedances correctlyonly if it is used within its power limitations;excessive current may saturate its core,wastefully heating the balun while givinga deceptive SWR reading.

15. A loading coil on a short antenna doesn’tadd missing length, it adds inductive re-actance to cancel the capacitive reactanceof the short antenna.

16. A transmatch doesn’t “fool” the transmitter

or receiver into “thinking” it is connectedto the correct impedance any more thanan AC wall adaptor “fools” a radio into“thinking” it is getting 12 volts DC when itis plugged into 120 volts AC. In both casespower and impedance transformationsreally occur.

RECOMMENDED READING

The ARRL Antenna Book , published by the Ameri-can Radio Relay League, 225 Main St.,Newington, CT 06111.

Antennas J.D. Krauss, second edition, 1988;McGraw-Hill Book Co.

Filters are used to remove unwanted signalinterferences.

Antenna switches should be chosen care- fully, both to handle high power when used for transmitting, and for low loss when used at VHF/UHF.

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