PROCEEDINGS OF THE I.R.E.

Microwave Omnidirectional Antennas*HENRY J. RIBLETt, ASSOCIATE, I.R.E.

Summary-This paper describes briefly a number of radiatingelements which have proved useful in the design of high- and low-gain omnidirectional antennas at wavelengths of 3 and 10 centime-ters.

INTRODUCTIONr HE SUCCESS of microwave radars for militaryT purposes has induced considerable interest in the

development of omnidirectional microwave an-tennas. During the war these antennas, in conjunctionwith suitable transponders, were used for homing andidentification purposes, and now they are seeing servicewith communication systems.

Active work on microwave omnidirectional, antennasbegan at the Radiation Laboratory in 1942. A largenumber of individuals have contributed to the develop-ment of these antennas. This paper summarizes thework done at the Radiation Laboratory and is, there-fore, a report on the activities of a group of men in theLaboratory rather than an account of the efforts of theauthor alone.The use of centimeter wavelengths places a different

emphasis on design than is encountered in low-frequencyapplications, so that, in deciding the place of these an-tennas in the general art, the following cQnsiderationsshould be kept in mind:

(a) High frequencies involve close tolerances whenmeasured in inches, so that designs which are inherentlyreproducible are required.

(b) High-gain antennas can be made without exces-sive size, and this fact, coupled with relatively narrowradar bandwidths, shifts the design emphasis in manycases from bandwidth to pattern quality.

(c) Antennas with gains, relative to an isotropic an-tenna, ranging from 3/2 to 20 are of importance, so thata premium is placed on radiating elements or bays witheasily variable impedances.

(d) Feed lines are large compared with radiating ele-ments, so that older antenna forms usually have to bematerially altered in order to be usable.

(e) Pressurizing and weatherizing means are electri-cally large, and hence become an integral part of thedesign.The first efforts toward high-gain microwave omnidi-

rectional antennas were in experiments with a biconicalhorn antenna.' However, it was soon realized that, for

* Decimal classification: R326.8. Original manuscript received bythe Institute, April 5, 1946; revised manuscript received, September4, 1946.

t Formerly, Radiation Laboratory, Massachusetts Institute ofTechnology, Cambridge, Massachusetts; now, Submarine SignalCompany, Boston, Massachusetts.

This paper is based on work done for the Office of Scientific Re-search and Development under Contract No. OEMsr-262.

1 W. L. Barrow, L. J. Chu, and J. J. Jansen, "Biconical electro-magnetic horns," PROC. I.R.E., pp. 769-779; December, 1939.

the narrow-band problems encountered in radar prac-tice, an array arrangement was much more economicalof space and was more easily weatherized.The considerations listed above have placed an em-

phasis on the development of a radiating element which,for a given polarization and frequency, has the followingproperties: (a) gives by itself a uniform azimuth pat-tern; (b) is small, mechanically reliable, and susceptibleof quantity production; (c) when coupled to a line, hasan impedance, either pure series or shunt, which is suffi-ciently flexible so that it can be used in a single-elementantenna or in a high-gain antenna with many bays; and(d) has by itself an elevation pattern which does notdiffer too greatly from that of a dipole.When an element is found which satisfies these condi-

tions, for a given application, the design of arrays witha multiplicity of elements is straightforward. In all theapplications which have been encountered, the peak ofthe radiation pattern should be on or near the horizontalplane. This condition is insured if pure series or shuntelements are spaced Xg/2 apart on the feed line. In thiscase the pattern can easily be calculated and the im-pedance behavior is simple, since, if the elements are inshunt or series on the line and are spaced Xg/2 apart,it will be found that they are essentially in parallel orseries with each other and with the shorting stub.

GROUND AND SHIP ANTENNASFig. 1 shows a die-cast element which has been very

useful for horizontal polarization in the 10-centimeter

Fig. 1-Tridipole element.

band. It illustrates well the properties which a micro-wave omnidirectional element should possess. Its azi-muthal pattern has a uniformity which is better than2 to 1 in power2 from 9 to 11 centimeters. It has beenpossible to eliminate the need for insulators or dielectricsupports by the use of three-wire feeds. The flexibilityof impedance is made possible by the fact that the ex-citing pins may be fastened directly to the inner conduc-

2 All pattern data will be given in terms of power.

474 May

Riblet: Microwave Omnidirectional Antennas

tor or capacitively coupled thereto. Fig. 2 shows a ments, capacitive coupling has been convenient. In thecompleted antenna using these elements. It has separate latter case, over a 2 per cent band the standing-wave

Fig. 4-Omnidirectional antenna using X/2 cylinders.Fig. 2-Tridipole omnidirectional antenna.

Fig. 3-Cylindrical element.

transmitting and receiving sections. Experience indi-cates that one, two, or three elements may be directlycoupled to the line with reasonable bandwidths, i.e.,standing-wave ratio < 1.4 over a 10 per cent frequencyband.3 For an antenna with as many as fourteen ele-

3 All standing-wave ratios are measured in terms of voltage.

1947 475

Fig. S-Omnidirectional antenna using X/4 cylinders.

PROCEEDINGS OF THE I.R.E.

ratio < 1.4, while its elevation pattern, which has a halfwidth of 8 degrees and side lobes less than 5 per cent, isunchanged. These elements have been die-cast in threesizes for the 10-centimeter band and in two sizes for1000 and 700 megacycles. The latter have been providedwith capacative stubs at both the input terminals andends of the dipoles, making possible a reduction in therelative diameter of the elements and greater patternuniformity. I

For vertical polarization in the 10-centimeter band,the basic idea shown in Fig. 3 has proved to be satisfac-tory. This employs three-wire lines exciting radiatingcylinders. These cylinders may be either a half-wave-length long and fed from both ends, as shown in Fig. 4, orless than a quarter-wavelength and fed centrally inpairs, as shown in Fig. 5. It has been found that each ofthese schemes has its own advantages. For a six-elementantenna having solid cylinders the standing-wave ratio

1947 Riblet: Microwave On

Omnidirectional antennas at 3 centimeters have beenhorizontally polarized almost exclusively. At this fre-quency, tolerances are so close and feeding lines of prac-tical size are relatively so large that attempts to buildthis type of antenna with dipoles have on several occa-sions in the past led to considerable difficulty.

Fig. 8 shows a completed 3-centimeter-band omnidi-rectional antenna. The upper of the two antennas acts as

Ptnidirectional Antennas

gives the elevation patterns of a 12-bay antenna overthe operating frequency band. Fig. 11 shows the stand-ing-wave ratio as a function of frequency for both a 12-and 3-bay antenna. To avoid the need for an externalfeeding line, a series of antennas using a double coaxialfeeding line have been designed. These use either five orsix slots on a 1-inch coaxial line with alternate elementsstaggered.

Fig. 9-Slotted Eo wave guide.

a receiving antenna, while the lower acts as a trans-mitting antenna. Fig. 9 is a sketch of the radiating por-tion of the antenna. It consists of twelve bays or ele-ments, each composed of seven vertical slots cut sym-metrically in the circumference of 1- to 1/4-inch waveguide. These slots are approximately one-half wave-length long and are excited by pins on one edge project-ing radially into the guide. The elements are reversedevery half guide wavelength by alternately placing thepins on different sides of the slots. An additional set of

w30

I TO ZENITH-IM

-40 -30-20 -10 0 10 20 30* 40

Fig. 10-Elevation patterns of twelve-bay 3-centimeterantenna.

matching pins is placed midway between the radiatingelements for the purpose of increasing the bandwidthof the antenna. Not shown is a purely reactive terminat-ing plug at the end of the antenna. The TM(o,l) mode isexcited by a special converter which in addition acts asa transition from rectangular to round wave guide. Thismode is used, of course, because of its symmetric radialelectric field.Antennas using the principle just described have been

built in 3-, 6-, and 12-bay sizes. The azimuth patternof a typical 12-bay antenna is essentially a circle. Fig. 10

.. .

,i -z

(A 2-IU Yl_ _s -__

1.23.180 3.207 3.214 3.220

A(cM)Fig. 1 1-Standing-wave ratio of slotted Eo wave-guide

antennas.

It should be stated that the designer of a high-gainmicrowave omnidirectional antenna of the array typeshould have no difficulty in obtaining at least 90 percent of the maximum theoretical gain available from auniform line-current source of length L, where L equalsthe number of radiating elements times their spacing.This gain G is given by the expression

2LG-i

AIRBORNE ANTENNASThe antennas described thus far have been used,

principally, either in ground or ship installations. Forfast aircraft the problem is different. Here, an enormous

Fig. 12-Horizontally polarized half-slot antenna.

premium is placed on designs which lend themselves tostreamlining. This requires an antenna whic-h has onevery narrow dimension. Both horizontally and verticallypolarized streamlined antennas have been designed tomeet this requirement.

Fig. 12 shows one version of a scheme which will givesatisfactory performance for a horizontally polarizedomnidirectional antenna at 10 centimeters. It is an ex-perimental fact that a pair of slots centrally cut in theopposite sides of a thin wave guide, when excited 180 de-

.s

_-- ~12 ELEMENT-RECEIVER_--- 12 ELEMENT-TRANSMITTER_----

3 ELEMENT-RECEIVER _

0 q 10 0 q 4 0--F -Q-, C:p al_,.) Ci? C!.30 0 0 0. 0

.;-- -,-- -l-.-

I A- I

'I r oI

.

1-4qk-

R

VOll cr0

If 14

fV cl.

53.180 Cm

---3.195 CM3.219 Cm

477

Ll - - 4 /I

3.187 3.197

PROCEEDINGS OF THE I.R.E.

grees out of phase, has a rather uniform azimuth pat-tern. These slots may be excited either by probes ex-tending into the guides or by means of the slotted dipoleshown in Fig. 12. Another antenna similar to that shownin Fig. 12 has been carefully tested. It has a full-lengthslot, approximately 0.7 wavelength long, centrally ex-cited by means of the slotted dipole. The azimuthal pat-terns of these antennas are oval-shaped, with equalpower in the fore and aft directions which exceeds thepower at the sides by about 2.3 to 1. The elevationpattern for the full slot arrangement had a half width ofof 50 degrees, while the elevation pattern of the antennaof Fig. 12 had an 80-degree half width. For both of themthe standing'-wave ratio