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TRANSACTIONS ON ANTENNAS A N D PROPAGATION, VOL. AP-34 , NO. 9. SEPTEMBER 1986 1143

Axial Mode Helical Antennas

Abstruct-The radiation characteristics of helical antennas operating inA comparison

and a helicalparasitic helix HAP) is proposed in order to enhan cethet is foun d that the gain of the H A P in which the parasitic

x is wound froma point diametrically oppo site to hat of on eand one-is about 1 dB higher than that of thehelix.The ratio of the frequency band in which the HAP

3 dB s1: l .g .

INTRODUCTIONAXIAL MODE helical antenna has beenused as acircularly polarized radiator with a wide bandwidth. Theinformation for designing this antenna s given by[13, who has derived an approximate expression for the

King andWong [2] have also presented thegainof the axial mode helical antenna with its turns35. Their experimental research revealsasa function of frequency and he peak gain as aof turns, together with adiationLee and Wong evaluated the gain, assum-

the current along the helix to be a single outward travelingof constant amplitude [3]. They discussed the discrep-surements made by King and Wong.It is well known that we can enhance the power gain by

us tothiso explore the possibilities of increasing the powerwith the axial length remaining unchanged.First, attention is paid to comparison between a monofilarantenna [4]-[9] and a bifilar helical antenna [lo]. On

the theoretical current distributions, the radiationr gain of the bifilar helix isof the monofilar helix. However,r helix has the disadvantage of a complicated feedingexcited with antiphase

Because thecurrent distribution of the axial modemonofilar

Manuscript receival Scptember 17, 1985; revised March 29, 1986H. Nakanoand Y . Samada are with the College of Engineering, HoseiJ. Yarnauchiswith Tokyo MetropolitanTechnical College, 1-10-40IEEE Log Number 860913.5.

3-7-2 Kajino-cho, Koganei, Tokyo 1 8 4 , Japan., Shinagawa-ku, Tokyo, 140 Japan.

wave region [111, [12], we propose a helical antenna with aparasitic helix whichis wound only onhe surface wave regionof the driven helix. Detailed studies show how he currentdistribution and the power gain are affected by the location ofthe parasitic helix (HAP). It is demonstrated that the proposedhelix can realize an enhanced gain, while maintaining a simplefeeding system. The ratio of the frequency band in which theproposed helix has an axial ratio of less than 3 dB is found tobe 1:1.8.

MONOFILARN D BIFILARELICALNTENNASA previous study [ l l ] has numerically evealed hat the

current distribution of a monofilar helix operating in the axialmode is composed of two distinct regions. One is a region Cwhere the current decays smoothly from the nput to theminimum. The other is a surface wave region S where thecurient has nearly constant amplitude nd approximatelysatisfies the in-phase condition or the endfire radiation. To bemore exact, the phase velocity in the region S does not exceedthat of the in-phase condition.Fig. l(a j shows the configuration and the coordinate systemof a monofilarhelical antenna to beused as a referenceantenna in the following discussions. The number of helicalturns nd is eight; the pitch angle CY is 12.5 ; he circumferenceof the helical cylinder h s 10 cm, which corresponds to onewavelength 1 X) at a frequency of 3 GHz; the wire radius p is0.5 mm. The helical wire of two turns near the open end istapered to reduce the reflected current with onsequentimprovement in he axial ratio [ 5 ] [131. In he present analysisthe antenna is treated as a helix mounted onan nfinite groundplane allowing the use of the imageheory, i.e., as a balancedtype helix, and is fed by a delta-function generatoi of 1 V .The radiation pattern (electric fields ofEo ndE@ , hich iscalculated from the theoretical current distribution of Fig.l(b), is illustrated in Fig. I(c) together with the experimentaldata. It should be noted thathe theoretical current distributionis obtained using an integral equation [ 5 ] . Since the radiationpatterns at each azimuthre similar to each other, it is shownoronly one principal plane. The average half-power beamwidth(HPBW)of Eo and EG omponents at = 90 plane s about52 , and the maximum sidelobe level is about - dB. Theaxial ratio indicates an excellent value of 0.7 dB (experimentalvaluewas 0.8 dB) on thehelical axis. The power gain iscalculated to be 10.6 dB from

where I 0) s the theoretical current at the antenna input, R t isOO18-926X/86/0900-1143 01 OO 986 IEEE

Authorized licensed use limited to: HOSEI UNIVERSITY KOGANEI LIBRARY. Downloaded on October 1, 2009 at 01:36 from IEEE Xplore. Restrictions apply.

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1144r .

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AF-34, NO. 9 EPTEMBER 1986

xY

a r m l e n g t h 11.5 c mcb

experi ment al E e a *=,-(C)

Fig. 1 (a) Configuration and coordinate system. (b) Current distribution.(c) Radiation pattern of a monofilar helical antenna. The number of helicalturns nd = eight, pitch angle Y = 12.5 , circumferenceof helical cylinderCh = 10 cm, wire radius p = 0.5 mm and frequencyf = 3 GHz.

the resistance of the antenna input impedance (= 0.5 V/I O)A ) , and ro is the distance from the origin to a field point asshown in Fig. l(a).Now consider enhancing the power gain. The most well-known means s to increase the number of helicalurns nd withsubsequent increase in the axial length. The behavior of thetheoretical power gainof the monofilar helix is shown in Fig.2, as a function of the axial length. It is noted that the powergain is close to the experimental result by King and Wong 2],although the present antennabacked by an infinite groundplane reflector has the configuration parameters different fromthose of their antenna backed by a avity. The difference in thegain shown in Fig. 2 is about 0.5 dB. The slightly lower valuein the computed gains partially due to the employment of thetwo-turn end taper. Fig. 2 also shows that the power gain inthe present antenna increases, for instance, from 10.6 dl3 to13.1 dl3 as the nd is increased from eight to 16. This simplemeans of increasing the powergain by the helical turns,however, forces us to unreasonably expand the axial length ofthe antenna from 1.7 X to 3.5 X (from 17.2 cm to 34.9 cm at 3GHz)The increased power gain can also be realized by addinganother helical wire or by a bifilar helical antenna [lo], asshown in Fig. 3(a). The two helical wires with the same pitchangle are wound from two points diametrically opposite to

/ * - -

e 16 t urns

Of m m f l l a r h e l l c o l antenna ucked bye m e r l m t a l e s u l t b y Klng and Wonglz~cavity ; P l t c h angle Q=u . ~t

16 t urnse m e r l m t a l r e s u l t b y Klng and Wonglz~Of mm fl la r he l lco l an tenna ucked bycavity ; P l t c h angle Q=u . ~t

taeeredSecilan ; turns1 , 1 1 1 , 1 , 1 , 1 , 1 , 1 11 . 6 2.0 2.4 2.8 3 . 2 3 . 6 4.0 4.4

O x i d l eng t h 0Fig. 2. The theoretical power gain of a monofilarhelicalantenna as afunction of the axial length. The circumference of helical cylinder Ch s 1 h .

nx ZY

(C)Fig. 3. (a) Configuration ind coordinate system. (b) Current distribution.(c) Radiation pattern of a bifilar helical antenna. The number of h e l i dturn nd = eight, pitch anglea = 12.5 . circumferenceCh= 10cm, wireradius p = 0.5 mm, and frequencyf = 3 GHz.

each other, and the two nputs are excitedwith antiphaserelation.The theoretical current distribution of the bifilar helix isshown in Fig. 3@), where the number of helical turns n,, thepitch angle cy and the wire radius are the same as those used inFig. l(a); nd = eight, CY = 12.5 andp = 0.5 mm. Since thecurrents on the two wires are the same in the amplitude, onlyAuthorized licensed use limited to: HOSEI UNIVERSITY KOGANEI LIBRARY. Downloaded on October 1, 2009 at 01:36 from IEEE Xplore. Restrictions apply.

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et al.: AXIAL MODE HELICAL AhTEhWA

ound to be similar to that observed inmonofilar helix in that the two distinct regions C and SExaminationof he adiation pattern shown in Fig. 3(c)that hemainbeam is narrower than hat of theelixhown in Fig. l(c). The half-power

of the bifilar helix is 44 , while that of52 . Consequently, an increased power12.3 dB is obtained in the bifiiar helix. This value isrom 1) in which he coefficient of 30 in the60 because of the bifilar helix, and1.7 dB higher than the power gain of the monofilar helix. Itincreased power gain is realizedlength remaining unchanged.From the above-mentioned results, itcanbesaid hat aof winding two helicalwiies and exciting the twonputspowerare forced to use a hybrid circuit to excite

HELICALNTENNA WITH A PARASITICELIXIt is interesting to consider the case where one wire of ais not excited, i.e., used as a parasite whose inputthe current must be nduced on thethe driven

bution of Fig. 4(a) in which the one wire of the bifilarin Fig. 3(a) is not excited. The standing wave isble on he driven and parasitic helices. The form of themarkedly different from those of thein Fig. l(b) and the bifilar helix in Fig. 3@).Fig. 4(c) is the radiation pattern evaluated using the currentshown in Fig. 4(b). From the radiation pattern theo be 12.1dB, which is close to that ofbifilar helix hown in Fig. 3(a). It should be noted,, that the input impedancedoes not keep constant ver

as shown in Fig. 4(d). Since theof the input impedance s caused by the standingon of the current, it is necessary to reglize ag wave distribution of the current on both the drivenhelices.

It houldbe ecalled hat he current distribution of aoperating in the axial mode is divided into twoC and S. Because the decaying wave regionC acts asand he surface wave region S as a director, wean idea thata parasitic helix should be wound onlyn thehe driven helix establishes a surface waveWe designate this type of helical antenna as a HAPFig. 5(a)).Fig. 5(b)shows the current distribution on a HAP composed

nd = eight, whiche monofilar helix shown in Fig. l(a), and thehelical turns of n = six. The latter isnd from a point diametrically opposite to that oftwo turnsis found that the form of the decayingregion on the driven helix remains unchanged despite

1145X

Z

YY

(a)

1

d r i v e n h e l i xarm length 7 7 . 5 cm

-3L Jarm length 71.5 cm(b)

experinental

Z i n = R i n + j X i n

U3 -200-d

II -

-400 I I I I2 .0 2 .5 3.0 3 . 5 4 . 0f requency G H z )

(d)Fig. 4. (a)Configuration and coordinate ystem. (b) Current distribution.(c) Radiation pattern at 3 GHz. (d) Frequency characteristic of the nputimpedance of a bifilar helical antenna where the onerm is not excited. Thenumber of helical turns n = eieht, Ditch anele a = 12.5 .circumferencehe parasitic helix. The decaying wave region again h = 10 cm, wire adius p = 0 5 mm.- - .~


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1146 IEEE TRANSACTIONS ON ANTENNAS Ah?) PROPAGATION, VOL. A P - 3 4 NO. 9, S E P T E m E R 1986x d r i v e ne l i x should be notedhat the use of a parasitic helix contributes to, . p a r a s i t i c h e l i x the enhancement of the power gain with an advantage of asingle feed of the antenna.The enhancement of the gain can be explained as follows.The denominator in (1) remains unchanged despite of addingthe parasitic helix, since the current value at the input of theHAP is nearly the same as that of the monofilar helix. This

means that the higher gain is caused by the increase in thenumerator, or the electric field. Since the current inhe& surface wave region approximately satisfies the in-phasea condition for the endfire radiation, the increase in he current

amplitude leads to the increase in the electric .fieldnhe3 ' : direction of the Z-axis. It should be recalled that the sum of the1

x YY

a5 a4 m

current amplitude in the surface wave region on the drivenhelix and that onhe parasitic helix is arger than the amplitudeon the monofilar helix.Consequently, the gain is enhanced byZTTP. length 7 7 . cm the additionf the parasitic helix.

arm l e n g t h 5 7 . 0 cm(b)

I(C)

Fig. 5 . (a) Configuration and coordinate system. (b) Current distribution.(c) Radiation patternof a helical antenna with parasitic helix. The numberof driven h elical turns nd = eight, the number of parasitic helical turns n p= six, pitch angle a = 12.5 . circumference h= 10 cm, wire radius p= 0.5 mm, and frequencyf = 3 GHz.

acts as an exciter for the remaining turns of both he driven andparasitic helices. The remaining turns of each helix have thecurrent of relatively constant amplitude and act as a director.The relative phase velocity s defined as he ratio of hephase velocity of the current on the wire to the light velocity.Both the relative phase velocities of the currents of the drivenand parasitic helices on the surface wave region are almost thesame as a value of 0.82 of the monofilar helix shown in Fig.l(a). In addition, thesumof he current amplitude in thesurface wave egionon the driven helix and thaton heparasitic helix is larger than the amplitude on the monofilarhelix.Big. 5(c) illustrates the radiation pattern. The HPBW iscalculated to be 47 , which s narrower than hat of themonofilar helix shown in Fig. l(c). An increased power gainof 11.4 dB not obtainable in the monofilar helix is realized.Although the power gain of 11.4 dB is somewhat lower than12.3 dB obtained in the bifilar helix shown in Fig. 3(a), it

LOCATIONF A PARASITICELJXIn the previous section, a prototype of a helical antenna witha parasitic helix onan infinite ground plane or a H P has beenmade at a single frequency where the circumference of thehelical cylinder corresponds to one wavelength. In practicalapplication, it is desirable to keep an increased gain over awide range of frequencies.The fundamental concept for designing the HAP is that theparasitic helix should be wound only over the surface waveregion of the driven helix in order to establish a traveling wavedistribution of the current. Hence it is important to choose thelocation of a starting point of the winding of he parasitic helix.In this section two types of AP'S are investigated from thestandpoint of requency characteristics. One is aH P n whicha parasitic helix is wound from a point diametrically oppositeto that of one and one-half turns of the driven helix of n d =eight turns. The other is wound from a point of two turns,which is identical to the HAP shown in Fig. 5(a). The two

HAP's are designated as type (A) and type (B), respectively. Itshould be noted that the type (A) is made on the basis of theshift of the minimum inhe current distribution. The end pointofheegion C or the minimumpointofhe currentdistribution shifts slightlyoward the feed oint as thefrequency s increased. As shown n Fig. 6 , the minimumpoint at 3 .7 GHz is located near a point of one and one-halfturns.Fig. 7 shows the frequency characteristics of the powergains nd the axial .ratios ofwoypes of HAP's. Forcomparison thoseof the monofdar helix shown n Fig. l(a) arealso given by the lines labeled as M . It should be emphasizedthat the gain of type (A) is about 1 dB higher than that of themonofilar helix, and that the bandwidth of type (A) is widerthan that of type (B). The increase in the gain of type (A) isexplained by the fact that it has the desirable currentdistribution which is dominated by a traveling wave ofa nearlyin-phase condition for the endfire radiation over a wide rangeof frequencies, as shown in Fig. 8(a). On the other hand, asindicated in Fig. 8@), type (B) shows standing wave behaviorin the current distribution at higher frequencies, with subse-quent narrowness of the bandwidth.


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el al.: AXIAL MODE HELICAL ANTENN A 1147

n r i v e n h e l i x, p a r a s i t i c h e l i x----_- . 7 GHz

3.0 GHZ/ r L.\

I ; arm length1 . 5 turns 77,s m2 turns

6 . Frequency characteristic of the current distribution of a monofilarhelical antenna. The number of helical turns nd = eight, pitch angle a =12.5 , circumferenceCh = 10m,and wire radius p = 0.5 mm

13

12

l l-Ec 100

g 9a

8

7

2.0.5 3.0 3.5 4.0

7 t 4a2.0 TYPe A){ heoretlcolo oex~erlmntalI, iYPe B) ----theoretlcol 1 -M ---- theoretlcol1

frequency GH z )(b)7. Frequency characteristics. (a) Power gains. (b) Axial ratios of type(A) and type (B) . Pitch anglea = 12.5 , circumferenceC h = 10 cm, andwire radius p = 0.5 rnm.

In summary, it can be said thatype (A) has a good choice ofocation for the starting pointof the winding of thehelix nd realizes a higher power gain over aof 1 1.8 , where the axial ratio remains withindB, having he advantage ofa single feed. For furtherimpedance characteristics of type (A) andlix are shown in Fig. 9. The input impedancetype (A) is on the order of 160 60 ohms in the resistiveand -70 35 ohms in he reactive value over the

arm length 77 .5m

arm length 62.1 cm(a)

n .dr ivenel ix

arm length 7 7 . 5 cn

-4 p a r a s i t i ce l i x .- ._-./. - 2

mrlX-Ill10I:am

Fig. 8. (a) Current distribution of type (A). (b) Current distributionof type(B). Both are at 3.7 GHz.


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a= -10a theoreticalI

0 0 o oexperimental1-,,+ monofilar helix - - - - - theoreticalI ,

I I I2 . 5 3 .0 3 , 5 4.0frequency (GHZ)

Frequency characteristics of the input impedance of type (A) and amonofilar helical antenna.16 I I I I I I I

T n e ( A ) , p = nd - 1 . 5-- - -. - TYPe(B1 , np = nd - 2.0mnofiiar h e l l c a l antenna -L

: n u m e r of drlven hellcal turns

,r

1 110 12 111 16 18 20nd turns

10. Power gains as a function of the number of helical urns. Pitch angle(Y = 12.5 , circumference C h = 10 cm, wire radius p = 0.5 nun, andfrequencyf = 3 GHz.

So far we have referred to the three antennas of type A),type (B), and monofilar helix, fixing the number of the drivenhelix turns nd to be eight. The power gains when he numberofthe driven helix turns is changed are shown in Fig. 10. It isfound that type A) isabout 1 dB higher than hatof hemonofilar helix at any number of the driven helix turns.CONCLUSION

The radiation characteristics of axial mode helical antennashavebeen calculated using the theoretical current distribu-tions. Although a bifilar helix has a higher gain han amonofilar helix, it requires a complicated feeding system witha hybrid circuit. To eliminate the complexity and maintain anincreased power gain, a helical antenna with a parasitic helixhas been proposed and discussed.Since theparasitic helix should be wound n a surface waveregion of the driven helix, specialattention is paid o the

location of a starting point of the winding of he parasitic helix.It is found thathe gain ofaHAP n which the parasitic helix iswound from a point diametrically opposite to that of one andone-half turns of the driven helix is about 1 dJ3 higher than thatof the monofilar helix. The ratio of the frequency band inwhich the H A P radiates a circularly polarized wave within an

1148 IEEE TRANSACTIONSN ANTEhWAS AND PROPAGATION, VOL. AP-34,O. 9. SEPTEMBER 1986

axial ratio of 3 dB is calculated to be 1 :1.8.ACKNOWLEDGMENT

We would like to express our indebtedness to Mr. H.Mimaki for his kind assistance in arranging the data in thisPaper-

REFERENCESJ . D.Kraus, Antennas. New York: McGraw-Hill, 1947.H.E. King and J. L. Wong, Characteristics of 1 to 8 wavelengthuniform helical antennas, ZEEE Trans. Aniennm Propagat. vol.K. F. Lee and P. F. Wong, Directivities of helical antennas radiatingin the axial mode, Proc. Znst. Elec. Eng. pt. H, ol. 131, pp. 121-122, 1984.R. C Johnson and H. asik, Ed., Antenna Engineering Handbook.New York: McGraw-Hill, 1984, ch. 13.H.Nakano and J . Yamauchi, Characteristics of modified spiral andhelical antennas, Proc. Znst. Elec. Eng. pt. H,.ol. 129, pp. 232-237, 1982.H. akano, J. Yamauchi, H. Mimaki, and M. Sugano, The balancedhelical antenna radiating right- andeft-hand circularly-polarized-waves, ZECE Japan vol. 6343, pp. 743-750, 1980.T. ShiokawaandY. Karasawa, Radiation characteristics of axial-mode helical antenna, ZECE Japan vol. 63-B, pp. 143-150, 1980.K. F. Lee, P. F. Wong, and K. F. Jam, Theory of the frequencyresponses of uniform and quasi-taper helical antennas, IEEE Trans.Antennas Propagar. vol. AP-30, pp. 1017-1021, 1982.R. G . Vaughan and J . B. Andersen, Polarization properties of theaxial mode helix antenna, ZEEE Trans. Antennas Propagat. vol.A. G . Holtum, Improving the helical beam antennas, Electron.H. Nakano nd J. Yamauchi, Radiation characteristics of helixantenna with parasitic elements, Electron. Leti. vol. 16, pp. 687-688, 1980.H Nakano, T. Yamane, and J. Yamauchi, Directive properties ofparasitic helixand its application to circularly polarised antenna,Proc. Znst. Elec. Eng. pt. H, ol. 130, pp. 391-396, 1983.D. . Angelakosand D. Kajfez, Modifications of the axial-modehelical antenna, Proc. ZEEE vol. 55,pp. 558-559. 1967.

AP-28, pp. 291-296, 1980.

AP-33,pp.10-20, 1985.VOI. 29, pp. 99-101, 1960.

Hisamatsu Nakano (73, for a photograph and biography please see page840 of the August 1984 issue of this TRANSACTIONS.Yuji Samada was born in Mie, Japan. on Septem-ber 22, 1961. He received the B.E. and M.E.degrees in electrical engineering from Hosei Uni-versity, Japan, in 1984 and 1986, respectively.He joined the Sony Co. Ltd., Tokyo, in 1986.Mr. Samada s a member of the Institute ofElectronics andCommunicationEngineersofJa-pan.

Junji Yamauchi (%I), for a photograph and biography please see page 40of the August 1984 issue of th i s TRANSACTIONS.

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