28 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

[4] Watts, C. B., Jr., and Setzer, L. E., CAA Type I Course Line [9] McKnight, F. S., 4 Preliminary Study of Operational AdvantagesComputer. Civil Aeronautical Administration Technical De- of Pictorial Navigation Displays. Civil Aeronautical Administra-velopment Report No. 152, January, 1952. tion Technical Development Report No. 241, June, 1954.

[5] Setzer, L. E., The CAA Type III Portable Pictorial Computer, [10] Blount, E. M., Dowling, C. E., Kav, H., McCormick, R. E.,Part I-Development and Initial Tests. Civil Aeronautical Ad- and Sellers, E. R., Technical and Operational Evaluation of theministration Technical Development Report No. 172, October, Type IV Pictorial Display Equipment. Civil Aeronautical Ad-1952. ministration Technical Development Report No. 242, June,

[6] Setzer, L. E., The Type IV Rotatable Panel Pictorial Computer, 1954.Part I, Development and Initial Tests. Civil Aeronautical Ad- [11] McCormick, R. E., and McKnight, F. S., The Type V Pictorialministration Technical Development Report No. 195, May, Computer with Automatic Chart Selection, Part If-Technical and1954. Operational Evaluation. Civil Aeronautical Administration Tech-[7] Setzer, L. E., and Leake, P. H., The Type V Pictorial Computer . Twith Automatic Chart Selection, Part I-Development and Initial mcalDever Lopm nteprt N. 24, Juner,19 .ETests. Civil Aeronautical Administration Technical Develop- [121 Seibert, V.SLi, Sauner ., and Setzer, L. E., A Dual-ment Report No. 199, June, 1954. control Corse Line Com.puter, CAA Type IA. Civil Aeronautical

[81 Blount, E. M., Kay, H. A., McCormick, R. E., McKnight, F. S. Administration Technical Development Report No. 244, July,and Yost, M. H., The Type III Portable Pictorial Computer- 1954.Part II-Technical and Operational Evaluation. Civil Aeronauti- [13] RTCA SC-54-Operational Analysis of the Need for and Use ofcal Administration Technical Development Report No. 209, Course Line Computers in Air Traffic Control, preliminary re-June, 1953. ports.

An Evaluation of High Frequency Antennasfor a Large Jet Airplane*

0. C. BOILEAU, JR.t

Summary-The ability of aircraft cap-type high frequency anten- ILIAISON SYSTEMnas to act as efficient radiators is a subject of much current interest.In most instances the fixed-wire type hf antenna is incorrectly as- The hf commun1icatio system is primarily intendedsumed to be superior to flush cap-type antennas. In this paper a per- for lonig range communication. The particular systemformance comparison of five different hf communication antenna evaluated operates in the frequency range of 2 to 24 mcconfigurations for a large high speed aircraft is presented. These and consists of the following components as shown inantennas include two fixed-wire types, a conventional tail-cap typer . .>,,5,. , e ' ~~~~~Fig. 1; the anteniia aiid itS lightniing arrestor-feed svs-and two "L-gap" tail-cap configurations. The impedance vs fre- - squency characteristic is explored over the 2-24 mc range for each an-tenna. The antenna system efficiency of each antenna is computedusing the antenna impedance characteristic and considering thetransmission line, coupling network, antenna feed and lightning pro- ANTtNNAtection system, and the antenna radiation pattern. This antenna sys- /tem efficiency is then used as a basis for comparison of the five an- POWPQ TPANWIN tf{ICItNCY DW XTtennas. POwte OUT

ANT[NNA 5Y$TEN tfffC1tNCY=PTtG X P.-Pt.

W1 tITH THE ADVENT of jet aircraft flying atspeeds in excess of 500 mph and at altitudes AQNT fftD

above 40,000 feet, the selection of the proper UNMATtlNGIThigh frequency (liaison) antenna configuration for useon a particular aircraft is, in a large part, based on the J G-8/U COAXIAL CABLt

T2ANSM1551ON LINtaircraft structural and operational requirements as well KA AN/.2C-21as on the antenna electrical characteristics. The struc-tural considerations of the antenna; e.g., corona and Fig. 1-Liaison system.voltage breakdown protection and feed point location,s , . , . s ~~~~~~~~~~~tem, an anteiina impedaiice nmatching unit located atalso play an important part in the proper choice. On all teantenna, a receiver-transmitter unit located remoteof Boin Aipln Copn' hig speai*rpae from the antenna, and the nlecessary interconnecting rfflushf anenshv.ente hieo h eo and control cables. M4aximum rf power output of trans-

d ynamcistadthe lectrial engneer.mitter iS 150 watts, amplitude modulated 100 per cent.

* Manuscript received by the PGANE, July 5, 1955; revised man- To achieve the long range communication, sky-wTaveuscript received Novrember 15, 1955. transmission is utilized. Operationally this results in an

This paper was presented to the National Conference on Aero- infint nube of cobnain of trnmsso anautical Electronics at Dayton, Ohio, on May 9, 1955. l lenme fcmlaln ftas1so a

t Boeing Airplane Co., Seattle, Wash. rameters involving the ionospheric condition and the

1956 Boileau: High Frequency Antennas for a Large Jet Airplane 29

location and position of the aircraft and the ground ANTENNA SELECTIONstation at anly of the frequencies in the 2 to 24 mc range. Considering the present state of the high frequencyAll of the peculiarities of the hf system have been antenna art and the exacting structural, operational, andanalyzed and grouped together inl the Air Force liaison system design requirements, the series-fed flush cap-antenna specification (MIL-A-9080) prepared, in con- type antenna, utilizing the vertical fin, is deemed to bejunction with the Air Force, by Stanford Research the most feasible completelv flush type hf antenna con-Institute.' The specification places emphasis on different f f pparts of the hf svstem in the two frequency ranges 2 to t e eoration of flushwingctap an n

I tors eliminated consideration of flush wing-cap antenna6 mc and 6 to 24 mc. In the 2 to 6 mc range, because of configurations although it may have been possible tothe similarity of the antenna radiation patterns of all o r

I obtain comparable electrical performance using apossible antenna configurations for airframes of this size smaller isolated section on the wing tip than that re-and because this is the range in which the antenna im- quired on the vertical fin. Shunt fed antennas werepedance characteristic makes it difficult to transfer eliminated both by the structural and system require-efficiently the rf energy from the transmitter to free ments. It should be noted that certain semiflush faired-space, the specification is based on power transfer and in configurations are currently being considered atmatching circuit efficiencies. In the 6 to 24 mc range the Boeing for large jet aircraft hf antenna applications, butantenna impedance may be matched with reasonable these configurations are beyond the scope of this paper.efficiency. The antenna radiation pattern, however, doesvary throughout this frequency range. Therefore, theadditional factor, radiation pattern efficiency, is con-sidered in the 6 to 24 mc range. Hence two efficiencies STA8,are defined:Two to 6 inc: power transfer efficiency (P.T.E.): The

ratio of the power radiated into space to the power input TTAZ MASat the transmission line. In this evaluation the losses of

* * * ~~~~~~~~~~~CENTEQ-FEDZIG-ZAG L-GAPD !1,fmthe transmission line and the antenna structure have /TAr I ZA Lbeen neglected. Therefore, power tranisfer efficiency isthe matching unit efficiency when the unit loads into theantenna impedance uncorrected for antenina structure TOP-FED ZIG-ZAG L- Plosses.

FULL TAIL CAP

Fig. 3-Proposed liaison anitennas.

Since the airplane dimensions and the wavelength ofoperation are of comparable magnitude, the currentsexcited on the airframe by an hf antenna contributesignificantly to the radiation. To provide the necessary

___ X__ ___ - __ - __ - - coupling between the isolated section of the airframe(the antenna) and the remaining aircraft skin and toprovide an antenna impedance that may be successfullymatched, a considerable portion of the airframe must beisolated. To establish the optimum over-all flush hf an-tenna configuration considering weight, structural feasi-bility, and electrical performance for integration intothe vertical fin of a large jet aircraft, three different cap

Fig. 2-Radiation pattern efficiency sector of effective propagation. antennas were evaluated electrically. These cap an-tennas include a conventional full tail cap antenna and

Six to 24 mc: antenna system efficiency (A.S.E.): The two L-Gap antennas. See Fig. 3. The L-Gap antennasproduct of power transfer efficiency and radiation pat- were studied because of the obvious structural ad-tern efficiency (R.P.E.) where radiation pattern effi- vantage provided by this configuration; the isolatingciency is defined as the ratio of the power radiated into gap does not cut through the rudder and its balancethe solid angle useful to communication, to the total mechanism, and it does not cut the fin main spar thusradiated power. This useful solid angle is ± 300 about eliminating the heavy structural splicing of metal-tothe horizontal plane of the aircraft. See Fig. 2 above, dielectric material-to metal. Electrical advantages in-

clude the ease of mounting fin-top located antennas and1 E. J. Moore, "Factor of merit for aircraft antenna systems in running-lights without the use of filters across the hf

the frequency range 3-30 mc," TRA~NS. IRE, PGANE-3, pp. 67-73;A\ugust, 1952 antenna isolating gap. The L-Gap antenna iS unique in

30 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS March

that the effective electrical length canl be varied, anid g __° _ _ _ __ __ __ __ _hence, the reactance characteristic, by movement of the =to_olllfeed-point around the periphery of the isolated section. |

10Three different L-Gap feed point locations were con-sidered: bottom, center, and top. Only two of theseL-Gap antennas were evaluated, the Center-Fed and -o --,0the Top-Fed. The Bottom-Fed antenna was discarded F Oearly in the investigation because the reactance charac-teristic was similar to the Top-Fed antenna reactance ' -4 l' 0 Z4

characteristic while the resistance characteristic was t - T Z - G (ucI-much poorer.

In addition two fixed wire anitennas were evaluated g j °| i1| -.for comparison purposes. One, the "reference antenna," E M0-owas specified in the military specification used as an__ __ _______evaluating means. The other fixed wire antenna, the f MtN T| |liaison fixed wire, is mechaniically equivalent to the % -100-present B-47 fixed wire hf antenina which is considered 2 _ - - - -to be an aerodynamically feasible antenna. These an- -z 4 c 8 zo 1I820 VQC(?C)_tennas are pictured in Fig. 4. ToP-+eD ZIG-ZAG L- GAP

LAXQEF0RENCE _ _ _

~~~~~~~~~ANTENNA '.->/8,W 100__

LIAISON -10 -X

Fig. 4-Fixed wire antcnnas.+2 4 6 16 I 1 X14 6 18 D0 22 24

EVALUATION FULL TAIL CAP

The cap antennia impedance characteristics are pre- Fig. 5-Impedance characteristics of proposed liaison antennas.sented in FPg. 5. A comparison of these three character-istics indicates the relative performance of the hf system Fig. 6 presents ani interesting comparisonl, that of thein the 2 to 6 mc range. In this frequency range the reac- isolated area and isolating gap length for the Full Tailtive component is approximately equal for the three Cap and the L-Gap antennas. Since the radiation re-antennas; however, the resistive component does vary sistance is directly proportional to the isolated area anidand is a good indication of the power transfer efficiency inversely proportional to the isolating gap length whenthat can he expected. Since the power transfer efficiency the antenna is short with respect to a quarter wave-is directly proportional to the magnitude of the resistive length, the reasons for the observations made abovecomponent, the Full Tail Cap antenna will be the best regarding the 2 to 6 mc range are apparent.choice electrically in this portion of the liaison band. The feed point impedance of the Liaison Fixed WireAbove 6 mc the reactive components do vary. The antenna is shown in Fig. 7. The longer length of this

Center-Fed L-Gap and the Full Tail Cap antennas have type antenna results in the many resonant conditionassimilar reactance characteristics. The reactance starts throughout the frequency range. Again, in. order to keepat 2 mc at - 500 ohms and becomes series resonant at the reactance negative, the antenna far-enld is alter-abo!t 23 mc going positive above this frequency. The nately open and short-circuited so that the particularTop-Fed L-Gap antenna starts at the same point at 2 impedance matching unit will be adequate.mc but becomes series resonant at 16 mc. Since this is an In order to establish the power transfer efficiency ofL-Gap antenna it is possible to short-circuit the far end each antenna the antenna impedance, as giveln in Figs.of the antenna which causes the parallel resonant con- 5 and 7, is transformed to the characteristic impedancedition to occur at this frequency. Therefore, the reactive of the rf cable (50 ohms) by the network shown in Fig.component will be positive above 16 mc with the open 8. In the calculation, the total network losses are as-Top-Fed L-Gap antenna and negative above 16 mc using sumed to be in the inductance, L, of the matching unit.the short-circuited Top-Fed L-Gap. The particular Capacitance C2 and switch S (Fig. 8) allow the matchingmatching unit considered for this liaison system requires network to transform the values of antenlna impedancea negative reactance component for proper operation, that have a small reactive component, less than -60hence, the placing of an electrical short-circuit at the ohms. This limitation of the L-type impedanlce matchingbottom end of the Top-Fed L-Gap antenna is necessary network, inductance L and capacitance Cl, is deter-for impedance matching purposes. However, one un- mined by the range of these variable elements.expected advantage provided by the short circuit at the The limiting reactive vJalue onl the antenlna im-far end is a higher radiation pattern efficiency. pedance, -60 ohms, is a rough approximation since this

1956 Boileau: High Frequency Antennas for a Large Jet Airplane 31

the "minimum" curve. In these calculations the "refer-ence antenna" is assumed to have a constant P.T.E. of90 per cent, an optimistic value.

Figs. 10 and 12 have been drawn as continuous curves

when in actuality the discontinuous nature of the an-

tenna impedance characteristic would require the powertransfer efficiency characteristic to be discontinuousalso. The curves do give a useful average value at those

/,'a, | frequencies where the antenna far end condition changesfrom open to short-circuited. In addition the insertionof capacitance C2 in the circuit of the antenna im-

/ X z' | pedance matching network at a particular frequency asrequired by the antenna impedance characteristiccauses a noticeable discontinuity in the P.T.E.

AQEA GAP LtNGTtlfULL TAIL GAP 51.7 Q. f. 10.4 PT. 5XCtNTtQE-ftD ZIG-ZAG L-GAP 24.43Q1FT. IaOlfT. L

Fig. 6-Comparison of antenna isolated area and isolating gap length. C C2 OI2&; tANTENNA

L CC3 - C

value varies with frequency and the antenna impedanceresistive component. Considering the Full Tail Capimpedance characteristic, Fig. 5, capaci-tance C2 will bein the circuit from 17 mc to 24 mc. The value of the l------------------------

capacitance C2 is chosen to change the antenna reactive tUA MATCGtING AIPTON LIGHTNING Ulfcomponent to -60 ohms at 24 mc. Hence, at 17 mc the UNIT AIMtJTOQ ffED FILTEQreactance of capacitanice C2 is greater than required Fig. 8-Cap type antenna feed system and matching uniit schematic.

causing the reactive component of the impedance char-acteristic, presented to the basic L-type transforming The radiation pattern efficiency of the five hf an-network, to be discontinuous at this frequency. The tennas is shown in Table 1. An item of particular interestshunt capacitance of the uhf filter, which allows the use in these data is that the fin-cap antennas, in general, haveof a fin top uhf antenna and is necessary only for the a higher radiation pattern efficiency than the wireFull Tail Cap antenna, is considered a part of the net- antennas.work for this antenna only. It should be noted thatno dielectric losses of the cap antenna structures and no TABLE Iseries losses of the fixed-wire antennas were considered. ANTENNA RADIATION PATTERN EFFICIENCY (PER CENT)

-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Full Top Fed Liaison Fixed Refer-

ti. Scale Center L-Gap Full Wire ence_ _ _ Fre- Fed Tail Fixed

quency L-Gap CpWire§ZZ5 X _ _ (Mc) Open Shorted Cap Open Shorted Open

10~ ~ ~ - 6 56 58 66 53 39M 10 > > <l'--te 'CONDITOM8 53 54 63 3129

1 1--L 1 =o lo10 52 57 60 60 44__ __ ____ __ __ __ ~S~MTt 12 57 41 61 50 48

/ ~~~ ~~~ ~~~~145454 57 37 46$00 16 48 51 55 57 43 43

//- L Ar18 49 52 58 48 57 4720 52 55 56 57 41 51

2 4 68 1012 14 1618I 0 22 24 22 53 54 58 56 45 48fRtQU5NCY (Mc) 24 54 53 59 57 62 52

LIAISON fIXED WI_-Fig. 7-I mpedance characteristic of liaison fixed wire antenna.

Another consideration in the design of these antennasThe antenna system efficiency for each antennla, and is the peak rf voltages developed during transmission.

the specification values for A.S.E. are shown in Figs. 9 The presence of this voltage, when the aircraft is at thethrough 12, on the following page. The antenna system high altitudes involved, can lead to serious corona andefficiency for each antenna was computed at every even voltage breakdown problems.mc from 6 to 24 mc. The average is the arithmetic aver-age of the value at these frequencies. The fixed wire "ref- CONCLUSIONerence antenna" establishes the comparison value of the Fig. 13 presents the results of this evaluation andaverage and minimum A.S.E. These values are shown on points out the relative merits of these antennas. Con-}Figs. 9 to 12 and are the 40 per cent "average" line and sidering the 2 to 6 mc range, although no antenna meets

32 IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATlONAL ELECTRONICS March

660L t4- t ! 1 L ' _ _ . _; [ _

ci I ,I

i_|_ e_ C y,___,_-,, _ 4__.F

O !4 6 8 h2N000UNCY (MC)-2 4 6 e

F88QLIOCY (MC)- 2 2

Fig. 9-Antenna system efficiency Center-Fed Zig-Zag L-Gap. Fig. 10-Antenna sy,stem efificiency Top-Fed Zig-Zag L-Gap.

420 04 u

;0! 4 60610 IZ 14 l 1 DB 2 2 24. 4 0 03 D 14 16 i 0 22 24e0§QU5NCY (Mc) - VR8eU18NCY (MC)-

Fig. 11-Antenna system efficiency Full Tail Cap. Fig. 12-Antenna system effciency Liason Fixed Wire.

POWtQ ANItNNA ANTENNA the specification average (60 per cent), the Full Tail CapTP?ANWEQl JYJTtM ItK antenna most nearly approaches the specification re-tftICItNCY tffIlCItNCY VOLTAGE quirement, as would be expected. In the 6 to 24 mc

2-6 MC 6-24 MC AT 20MC range, the greater portion of the frequency range, theAvUPAGt(.)lMINIMUM(.)[ AvCOAGt(%) (voLTJ) F'ull Tail Cap and the T'op Fed L-Gap antennas meet

rPECF1CA10N 60 25 40 he specification. The Center Fed L-Gap and the LiaisonfDECIFICATION ~~~~~~~Fixed Wire antennas do not meet any of the specifica-fULL TAIL CAP 53 32 45 4780 tion criteria. This is indicative of the exacting require-

Top-ffD m|nents of thle specification which, in our interpretation,ZIG-ZAG L-GAP 371 17 42 5840 mleans that it is intended as a design goal. It is con-

sidered that, with the system components available atCf:NTtD-ffD 39 19 | 35 |5790 this time, the present state of the high frequency an-

tenna art and the design limits of today's aircraft, theLIAIJON fIXfD WIPE 57 24 37 4600 performance which can be expected from a completely

flush hf antenna system for large jet aircraft is indicatedFig. 13-Merits of the antennas. by the data presented here.

C7(~K~D