Jones Antenna Handbook - World Radio History

ANTENNASAntenna Theory An antenna is an electrical conductor,suspended in air and insulated from ground,which either radiates or receives radio-frequency energy. Sometimes the ground isused in conjunction with the antenna conductors, as will be defined later. The effectiveness of an antenna depends uponnumerous electrical and mechanical factorsof design, all of which are discussed underantenna types.An antenna can be compared to any tunedcircuit, except that its capacity and inductance are distributed along the wire insteadof being lumped, as in a coil and variablecondenser. Every antenna system has afundamental wavelength or resonant frequency of its own. It should always be operated at its resonant frequency, becausethe efficiency is many times greater in thiscondition.The physical dimensions of the antennaat resonance bear a certain relation to thewavelength. In order to simplify the explanation of radiation in space, it can becompared with waves in water produced bythrowing a stone into a still body of water :there will be waves which have peaks andtroughs, similar to those produced in spaceby radio waves. The radio waves, in effect,require a physical amount of space betweenthe peaks and troughs (condensations andrarefactions) and therefore the antenna ismade of such a length that it is equal to onepeak or one trough of a wave in physicallength.the space required is very small. For anantenna, the wire may be stretched out intoa straight line so that it is nearly a full halfwavelength long. For an antenna consistingof a straight wire one-half wavelength longelectrically, the physical length will be approximately 5% shorter than the electricallength, because it is impossible to secure awire having zero diameter supported inspace without end insulators.FEl!!DFR/f"CEOERTO TtU,NSUITlEA'ANTENNACOUPl,.UfFIG.«!Single Wire Antenna for Transmitting.Radio waves travel with approximatelythe speed of light, which is 300 millionmeters per second. This provides a convenient method for expressing the resonantfrequency in terms of wavelength, or vicevcrsa.300,000,000F . -----where F is the frequency in cycle per second . is the wavelength in meters.Radiation FieldINSUI.ATOftLINSULATO,t-:;-GROUNDFIG 0 lConventional Antenna for Receiving.A complete wave consists of one peak andone trough, defined as one wavelength. Anyresonant circuit is the electrical equivalentof one-half wavelength; in the case of atuned circuit the constants are lumped and A wire connected to any source of oscillating electrical energy will radiate radiowaves due to the varying intensity of theelectrical field surrounding the wire. Thefield closest to the wire is called the induction field, which oscillates to-and-fro; thatpart of the field which escapes forms theenergy in the radiated field which is urgedoutward and diffused in all directionsthrough space. Any wire supported in spaceand within range of the radiated field willintercept the energy and will have inducedin it a radio-frequency voltage, which isdetectable as an incoming signal by receiving apparatus.Radio waves are transmitted from an antenna through space in two general typesof waves. One is called the ground ·wave,7

Jones Antenna Handbookwhich follows along the surface of theground, and is rapidly attenuated for veryshort waves. The ground wave is useful inlong-wave radio communication, also forvery short distance work on ultra-shortwavelengths. Best broadcast reception isalways had when the receiver picks up onlythe ground wave, which means that normallyit must be within a 100 mile radius of evena high power transmitter. Fading effectstake place at greater distances, due to theinterference between the ground wave andthe sky waves.That portion radiated upward from theantenna is known as the slty wave, since itis reflected back to earth by ionized layers intht! upper atmosphere known as The Kennelly-Heaviside Layers, as shown in Fig. 3.nelly-Heaviside Layers. Obviously the timerequirt!d for a ground wave to reach thereceiver will be less than that of a highangle sky wave, resulting in variations ofsignal strength at the receiver. \Vhen twoor more waves from these different pathsarrive at the same instant (in phase) thesignal strength will be greatest. If the timelag is great enough so that one wave tendsto neutralize another ( out of phase), thesignal intensity will decrease, resulting inthe phenomena known as fading.The rate of fading varies with frequency,and even small changes of frequency sometimes have entirely different rates of fading.A modulated wave from a radiophone station consists of a band of frequencies beingtransmitted, and this variation of fadingwithin this narrow band results in distortion in the received signal. This effect isknown as selective fading, because the sideband frequencies may be stronger at a giveninstant than the carrier ignal at the receiving point, resulting in bad distortion ofaudio quality in the output of the receiver.Electrical PropertiesFig. 3Reflection of Radio Waves from the Heaviside Layer Around the Earth.At very low angles of radiation, tht!waves start out practically tangent to theearth's surfact!, penetrate into the ionized!avers and are bent back to the earth at avery distant point. Higher angles of radiation are bent back to earth at shorter distances until a ct!rtain high angle is reachedfor any particular frequency which will nothe bent back to earth. This angle varieswith the season of the year, frequency andtime of day. At angles slightly less thanthis value at which the layers arc penetrated. the radio waves can be carried aroundone of the upper layers to extremely greatdistances before being bent back to earth,no matter what the angle of propagation.The Kennelly-Heaviside Layer is a strataoi ionized air molecules, of which the ionization is due to the ultra-violet radiationsfrom tht! sun. This stratospheric layer liesabove the earth at rlistam:es of less thanone hun lrerl up to several hundred mileselevation. The relative density of the layersis not comtant, but varies from year to yearand seems to rlepcnd upon sun-spot activity.The time required for the sky waves toreach the receiver varies in accordance withthe number of reflections to and from earthand the changes of ionization in the Ken-8 A wire stretched out into space has .fodttctance of the same type as that producedbv wire wound into a coil. This antenna\;ire also has a distributed capacity to nearby objects, such as the ground. As in anyelectrical circuit, inductive reactance andcapacitive reactance impede the flow of current in either a transn:iitting or receivingantenna. At resonance, the inductive reactance is equal and opposite to the capacitive reactance, with the result that the electrical current is onlv limited bv the resistance. The resistarice consists of severalcomponents, such as wire resistance, dielectric losses from nearby objects, ground resistance, insulator losses and radiation resistance. The latter is a fictitious termwhich is useful in expressing the powerradiated hy the antenna. It is that resistance which would consume the same amountof power that is radiated into space by theantenna; the power lost in other forms ofresistance is wasted. Short-wave antennasgenerally have a very high ratio of radiation resistance to loss resistance and aretherefore very efficient.In a resonant antenna. standhi q wa'/Jcs ofcurrent a11d voltage exist. In a typical halfwave antenna the current is maximum atthe center. and zero at the ends. The radiofrequency voltage is max;mum at the endsand minimum at the center. These standingwaves exist because an impressed radiowave will travel out to the end of the antenna am! be reflected back toward thecenter. since the end is an open circuit corrcspondin;.: to a large mismatch of im-

Antenna Theorypcdancc. The resonant antenna is oi suchlength that the rellected wave will be inphase with each succeeding impressed wave,or oscillation, resulting in a standing wavealong the antenna wire.Standing wavesproduce more actual radiated power intospace from an antenna than when the valuesof voltage and current arc uniform and oflower value all along the antenna wire.Radio-frequency feeders to an antenna aregenerally designed for uniform distributionof current and voltage along their entirelength (no standing wave). In other words,the feeders should not radiate because theantenna proper alone should be the radiatingmedium.c .c-:,;;,VDLTAGIEJ, . , ,).,.-· I -.,/,,,,./."CtNTEA.',,, i'---------.;.,-,,-----:,I- ". . w -'- ' ---.,1ANTllallllHA'\.'",. .,. VOL'TAGESM(lWING HOW 5TANDIN WAVES [lllST ON A AE'.SOtd lll'TANT[N' IACVJIRENT IS M., lllllolUM AT Ct:NTER VOLTAGIE: IS MAXIMUM AT EN09FIG. 4The impedance along a half wave antennavaries from a minimum at the center to amaximum at the ends. The impedance isthat property which determines the antennacurrent at any point along the wire for thevalue of radio-frequency voltage at thatpoint. The main component \If this impedance is the radiation resistance; normallythe latter is referred to the center of thehalf wave antenna where the current is amaximum. The square of the current multiplied by the radiation resistance is equalto the power radiated by the antenna, andfor convenience these values arc usually reierred to the center uf a half wave sectionof antenna.The curve in Fig. 5 indicate, the theoretical center point radiation resistance of ahalf wave horizontal antenna for variousheights above ground. These values arc oisome importance in matching radio frequency feeders to the antenna in order toobtain both a good impedance match and anabsence of standing waves on the feeders.A transmitting antenna usually consists ofa wire of definite length which may begrounded, ungrounded or connected to acounterpoise. A ground made hy either adirect or capacitive connection acts as a reflector to the aerial wire, therefore completing the circuit. \Vith a direct groundconnection, the antenna may b ' either anelectrical quarter wavelength or odd multiples of quarter wavelengths; the ground././,.frLI/ .VIV -! .' -.---HOIII.Z(,)NT'I 1"1',-DOIJ8LE:Tt1 U, . TW \/Cl No:;.Tt 5Fig. 5Radiation Resistance of Half Wave Horizontal Antenna for Various Heights AboveGround.acts as a subterranean reflector, furnishingquarter waves to the antenna to give halfwaves or multiples of half waves for thedesired resonant effect. A very short wirecan be loaded to an electrical quarter waveby means of a loading coil to ground ; awire over a quarter wave long can be reduced to an electrical quarter wave bymeans of a series condenser to ground.l --------------, OV RTERw VECONOSNUAFig. 6A counterpoise which consists of one ormore wires in a network insulated fromground will often reduce loss resistanceswhich might occur when the quarter waveantenna is connected to poorly conductingearth. The counterpoise in the case of anetwork of several wires acts as a condenserplate with high capacity to earth, with theresult of lower loss in t hc antenna system;for this reason the counterpoise should befairlv close to the ground.Fig. 7 shows a vertic al antenna with anelaborate ground wire system buried underthe surface of the earth for the purpose ofobtaining low loss resistance connection toground. This system is more generally usedthan the counterpoise.9

Jones Antenna UART RWAVtABOVEGROUNDFig. 7&ROAD510tDirectional Properties The radiation field of an antenna is moreintense in certain directions, depending uponits height above ground, as well as thelength of the antenna al)d the tilt or angleof the antenna wire with respect to ground.A short antenna (up to a half

Jones Antenna Handbook By Frank C. Jones 1937 EDITION Published by Frank C. Jones and distributed by Pacific Radio Publishing Company, Inc. Post Office Box 3278 .