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Research and Development Technical Report ECOM-3333

MULTI BAND HF ANTENNA

by

,.. .... Whitman

September 1970

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ECOM UNITED STATES ARMY ELECTRONICS COMMAND· FORT MONMOUTH, N.J.

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Reports Control Symbol osn-1366

TECHNICAL REPORT ECOM-3333

MULTIBAND HF ARl'ENNA

By

Robert E. Whitman

Task No. lH664nOD5352133

September 1970

DisrRIBtn'ION STATMENI' (1)

Thie document has been approved for public release and sale; its distribution is unlimited.

US ARMY ELECTRONICS OOJltWID

ABSTRACT

An experl.111mtal analysis in the 2 ltfa to 12 MHz frequency range is

perfol'll9d on a multiband HF antenna which contains a two, three, or

four dipole array. The antenna design equations and bandwidths are

determined. The effects of dipole, feedpoint, and frequency separation

on the design parBJ1Bters a.re noted. Inverting the dipole array from the

coanonly used configuration is shown to significantly ill:prove the

antenna.' s gain.

ii

TABLE OF CONTENTS

I. DTRmUCTI<li

II BJPIIDIDiTAL ANALYSIS

A. Dipole Reeonmce and Bmdwidtb

1. Tbr• Dipole Antenna

2. Two Dipole Antenna

3 POllr Dipole Antenna

B. llaent Separation

C Dipole Frequency Separation

D. Gain

I. Imerted Maltib111d Antenna

IU. COIICLUSICIIS A?I) RICOMMBNDATICIIS

'• Conclusions

B. Beet I endaticna

PIOURBl

TABLB I

nOURB 2

TABLE II

nomm 3

TABLE III

LIST OP Il,LUSTRATiafS

111

PAGE NO. l

l

1

l

4

4

4

4

7

9

9

9

10

2

)

s 6

8

9

I • INTRODU CTI~

An experirlental anaJ7sis in the 2 MHz to 12 MHz frequency range has been aade on a 2, 3, end 4 dipole multiband anumna eimilar to that presented by the Stanford Research Institute in "A Field Ouida of Simple HF Dipoles," March 1967, to detercine i te deeign par&lll8ters and opera­tional liJl:1 tations. Several antennas were tested to determine element :resonance length, dipole bandwidth, lllinimum frequency separation, mini.mull dipole separation, and gain. Figure 1 shows a pictorial view of a 3 dipole IIUltiband antenna. Various canponenta and dimensions are labeled by letters and will be referred to throughout the text.

II. EXP~TAL ANALYSIS

A.. Dipole Resonance and Bandwidth

1. Three-Dipole Antenna

Resonant lengths of elements A, B, and C were determined by me38Urlng the antenna input impedance at the end of a 40 foot, RG-S8c/U transmission line using a 0R-16o6A radio frequency bridge and plotting the results on a Sm1 th Chart nonuali zed to 50 ohms. Elements of random length were constructed into an antenna configuration of Figure 1 such that el9118nt A > B ) C. Element lengths were made reasonably different to prevent serious coupling between dipoles. Dipole separation D was chosen as S feet. P'eedpoint separation E was chosen as zero. (Although these separations were randomly selected, they are considered ideal parameters tor ease in construction of the antenna.) With the antenna elevated to a center height of 2.5 feet, input impedance •asurements vere made for several antennas which had varying dipole lengths. Table I sho-..s a partial list of the d&ta obtained. The dipole length, its resonmt trequeney, its length rel.t1tive to the resonant free space wave length, and bandwidth detenlined by a 118.XiJIUl'll VSWR of 3 a.re presented. An average or resonant lengths was made to determine the design equation for each dipole. The resultant design equations are listed below.

462 Upper Dipole Length • 2A • t(KRz) ft., (lo.rest frequency)

485 Middle Dipole Length • 2B • f'(MHz} ft., (Intermediate

frequency) 493

Lower Dipole Length • 2C • f{KHz) ft., (Highest frequency)

E: 0 inches, D: 5 feet.

These design equations are close to but not equal to those stated by the Stanford Research Ineti tute 'Where a feedpoint separation of 14 cm was used.

;;"

Insulator

\)

len

lement.s

n

FIGURE 1

EE DIPOLE ,TIBAND

------==::::===~ ~ -~----, 'B

-·--

Rope 1:ictension _/

D -1'..

NNA

TABLi I

DIPOLE RF.SCIUNC& AND BANDWIDTH

Element A Element B El.ement C

Length f(ac/s) %(,-o) BW(i) Length f(:mc/s) i(ro) 'B\ii\i) Length t~tt.e/s) 'li\r o) 'r>'fl (,:J>)

185.3 1 2.5 94.4 8 86 11 .5.58 97.5 4.65 10.1 1 7.06 101 2.7

16:, 1 2.78 93.5 7.15 : 108 1 4.45 98 4.6 76.3 1 6.38 99 3.0

159.4 1 2.91 9h.J 7.25 86 1 5.65 99 5.13 67.4' 7.35 100.8 2.72

185.3 1 2.5 94.4 8 9l .4 I 5.25 99.5 6.3 70.1 1 7.06 101 J.12 uJ ii

185.3' 2.5 94.4 8 94' 5.1 97-5 - n.1• 6.9 100 2.61

165.3' 2.11 93 7.8 112.8 4.28 98 4.2 95.3 s.14 100 2.75

98• 4.7 93.7 ll.5 72.8 1 6.75 100 4.2 58.S• 8.51 101.2 3.06

134 1 3.4 93.0 7.5 108 1 4.48 98.2 3.6 76.3 1 6.37.5 99 J.O

AVERAGE 9).8 8.1.5% 98.5 4. 7% 100.2 2.87i

2. Two-Dipole Antenna

The input impedances for a veral two dipole antermas were measured. The resultant design equations and dipole band-ddth.s were shown to be identical to the upper and intermediate dipoles of the three dipole antenna.

3. Four Dipole Antenna

The four-dipole array performed similarly to the three­dipole antenna with the fourth (shortest) dipole operating identically to the shortest dipole of the thre dipole antenna. The bandw.idth of the two lower dipoles averaged between 2 • .5 to J.5 %, depending upon the frequency separation. (In gen ral., the greater the frequency separation between adjacent dipoles, the greater is the bandwidth.) Figure 2 shows a typical impedance plot of a four-dipole antenna. Adjacent dipoles are e parated in frequency by approximately 30%.

B. Element SeJ?aration

Dipole separation dietances D and E were varied independently to dete:nnine what effect they h on antenn operation. With distance D fixed at .5 feet, impedance nts were recorded for feed point separations of 0, 5 and 10 inches. The easu.re ts were repeated for D separations of 2.5 ft. and 10 ft. Th results of these measurements are shown in Table II. It is noted th t the longer dipole (el ent A) reu.ins unaffected in resonance and bandwidth to all the changes in the antenna configuration. Ho ver, each shorter dipole showed increasing dependence to a change in th se antenna parameters. In general, the shorter the separation the ef f ct th parameters have on the lower dipoles resonant length d b dwidth. It 1 noted that a D separation greater than 5 ft. or approximately .o6 of the upper dipole half length has no appreciable ff ct on any of the dipole characteristics. However, a separation of 2 • .5 rt. or .OJA severely reduced the C dipoles bandwidth. An increase in the feed point separation to 5 inches slightly improved the bandwidth of the B d C di.pol s. An increase to 10 inches caused no further improvement. Tho incre e in th feed point separation did affect th shorting f ctor u d to cal cul ete th dipole resonant length ~s shown 1 n Tahl II.

Due to the narrow bandwidth and the variation in the shorting factor of the C di pol which occurs for diff rent di pol acings, it is reco1'11Tlended that the input VSWR of the third and fourth dipoles be verified by measurement for a rrul ti band ant nna which contains three or more dipoles. The design equation for these dipoles hould be u d as a guide only for calculating their r sonant 1 ngth • Nonna.l ari ti.on in ground conduc-tivity and dipole pacing c c 1m wmc variations which can result in dipol resonance outeid the 3 :1 VSWR circl •

C.

The minimum f uency par tion or pr dictable antenna operation

4

FIGURE 2

INPUT IMPiIDANC~ OF A FOUR DIPOL~ MULTIBAND ANTENNA

TABLE II -BANDWIDTH AND SHORTING FACTOR

VA.-.UATION ,-lITH D AND E VARIATION

D E DIPOL& A DIPOLE B DIPOLE C (ft) (in) BW(%) S.F. Bi-1(%) S.F. BW(%) S.F •

2.s 0 7.6 .93 4.2 .985 2.9S • 985

s 0 7.8 .985 4.2 .98 2.75 1.0

10 0 6.8 .935 5.o .97 2.65 .99

2.S 5 7.2 .935 h.5 .91 3.0 • 97

°' I 5 5 1.2 .935 4.h .97 3.16 .98

10 5 7.2 -935 5.2 .96 3.17 .98

2.5 10 7.4 .935 4.35 .95 J.4 .96

5 10 7.4 .935 4.2 .96 J.4 .965

10 10 7.6 .935 5-3 .955 3.6 .965

waS detennined by continuously adjusting dipole lengths to increase tJieir frequency separation until each dipole just operated in accordance 111th the parameters derived from Table I. Input impedances were measured for a two dipole mu.l tib and antenna w1 th A and B element lengths of 50 feet. Klement B was shortened in 2 foot steps until the resulting Smith chart plot indicated proper resonance and b dwidth. The resulting frequency separation which gave proper dipole operation was 25%. A third dipole, element C, identical. in length to the adjusted element B, was at.tached t,o the multiband ant nna. edance measurements were again recorded as element C was reduced in 2 foot steps. The frequency separa­t,ion which resulted in proper operation for dipole C was 28% below the resonant frequency of dipole B. Figure 3 a.bows the resulting ~th Cha.rt plot for the three dipoles separated in frequency by 25% and 28%. It is noted, however, thllt frequency separations less th&n this amomit can be used. The useable band"1dtb w1 thin a VSWR of 3 becomes increasingly narrow for reduced separation.

D. Gain

Vertical incidence gain of the elements in the ltibsnd antenna was measured by- cceparing the recei vod ignal from the ml ti band antenna t,o that signal received by a dipole cut to half wave length resonance. Vertical incidence gain ;r s a.sured in lieu of azimuthal gain since the horizont:u dipole antenna is predonlinantly used to launch and receive vertical incident sky wave signals. The received signal was generated by a horizontal dipole located 37 les from the receiving site. The two receiving antennas were erected parallel to each other and separated by approxim:itely 500 feet. E ch anttl'Ul was fed to a central receiving station through 2.50 feet of RG-58C/U tran mission line. The equi?ll8Ilt used t,o record the received signals consisted of two R--390 receivers for band­width control and two URM-8.5 field strength tars for signal strength measurement. The field strength fluctu tions re recorded and the analysis of these recordings w used to determine the tiband antenna gain compared t.o the reeon t dipole. Tb results of the t st are tabu­la-ted in Table III, Colwnn a. It is noted th t a severe reduction in gain occurred in the int di ate and lo r di.poles. A econd test performed two weeks later confi d th se results. The loss in vertical incident gain is attributed to the longer dipole ele nt acting as a reflect.or t,o the incoming signal. An additional gain test was performed with the nmJ. tibsnd antennii in an inverted configuration to place the smaller dipole, el nt C, s the upper radiator. The results are tabu­ia t,ed in Table III, Column b. T overall antenna gain is :,bown to be 91.gnificantly iJllproved. Due to ite greater length, the lower dipole was not. affected t,o the s extent as the smaller dipole was in the f oraer test.

7

------------Element A Element B Klement C

U 'O

1 0

FIGURE 3

I UT I MINIKlJM FRE

EDANC ENCY S

FOR ARArICN

FREQ. (Mlfa)

2.5

5.25

1.0

TABLE III

GAIN

REF. DIPOLE SIG. STRENGTH (db)

0

0

0

E. Inverted Multi band Antenna

MOLTIBAND ANTENNA SIG. STRENGTH (db) a. b. ( inverted)

0

-4.9

-6.5

-2 (lower dipole)

-2.2

-1.8 (upper dipole)

Due to the advantage in gain achieved by inverting the antenna, additional measurements were made to detennine if any changes had occurred in the antenna parameters when the antenna was configured such that element A < B ( C. It was found that the design parameters did not change nu.meri.cally, but have reversed their order such that each dipole retained its former design length and bandwidth.

Upper Dipole Length • 2A • f{~J ft., (highest frequency)

~-Middle I>.1.pole Length • 2B • f(MHzJ" ftl, (Intermediate frequency)

462 Lower Dipole Length • 2C • f{MHz) ft., (lowest frequency)

III. CONCLUSIONS AND RECCHOOIDATIONS

A. Conclusions

Th& multiband HF antenna has been found to be a practical means of providing efficient HF conmunication on several frequencies without antenna adjustment. Multiband antennas having two, three and four dipolea were teated in the 2 MHz to 12 MHz frequency range. The bandwidtha of the third and fourth dipoles ( highest frequencies) were ebown to be severely reduced and the deaign equation for these dipoles listed in paragraph IIA should be used as a guide only. For predictable operation, adjacent dipoles should have a frequency separation of approx:1.mately 28% or greater and a pcysical separation of five feet or .OJA which ever is larger at the terminal ends of the dipoles.

9

Tbs antenna erected 1n an inverted configuration {shortest dipole on top) exhibits significantly improved gain charaoteristios. Tba COJIIIODly used non-inverted JIUl ti band antenna suf'f ers severe gain 1oaa 1n the lower dipoles when the antenna is receiving vertioall.y­i.Doident aignale.

B. Recomnendatioos

It is reoOJIID9ndad that the inTerted lllltiband antenna be used 1'or applications where several preset channels are required. It has 1-diate a:pplloation for use a.a a base station antenna where a change i.n trequeney tor dq and night time ooJDm1 cation is required. The anterma lendl!II itself to field •~dient fabrication. Three or mre dipole llllltibsnd antennae ahould b& UBed onl;r when such equiJaent as a radio .trequeney bridp or th.ru line watt meter is av&i.lable to assure proper antenn~ input impedance.

10

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~e~ on a li.?llenta1. 'l'he ~tiband Hfal.Ysis in the 2 MHz to 12 MHz frequency range is performed feedJ,01~nna. design sntenna which contains a two, three, or four dipole arr,q • ;he dipo{' &nd fre~quations and bandwidths are determined. The effects of dipole, lltt)ro~e t e array fro ency separation on the design parameters are noted. Inverting

he antenna, m the commonly used configuration is shown to significantly .q gain.

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