NATIONAL RADIO ASTRONOMY OBSERVATORYGREEN BANK, WEST VIRGINIA

ELECTRONICS DIVISION TECHNICAL NOTE NO 134

Title: SMALL-APERTURE SCALAR HORN AS A FEED FOR THE7-FEED, 6-CM RECEIVER ON THE 300-FT TELESCOPE

Author(s): S. Srikanth

Date: January 22, 1986

DISTRIBUTION:

GB CV TU VLAGB Library CV Library Library Downtown VLA LibraryR. Norrod H. Hvatum Library Mountain P. NapierR. Weimer M. Balister J. Payne J. CampbellD. Schiebel S. Weinreb R. Freund W. BrundageE . Childers C. Burgess J. LambS. Srikanth S-K. PanC. Brockway A. R. KerrJ. Coe P. SiegelG. Behrens L. D'AddarioR. MauzyR. BradleyR. LacasseR. FisherF. CrewsB . Peery

2

SMALL-APERTURE SCALAR HORN AS FEED FOR THE7-FEED, 6-CM RECEIVER ON THE 300-FT TELESCOPE

S. Srikanth

Introduction

The 7-beam, 14-channel (7 feeds x 2 polarizations) 6-cm

receiver was proposed by J. J. Condon [1] for the purpose of

making radio maps of the entire sky visible from Green Bank.

This receiver, making use of NRAO cooled FET amplifiers, is being

put together by G. Behrens and I. Jeffries and is scheduled to go

on the 300-ft telescope in June 1986. The operating range of the

receiver is 4.6 to 5.1 GHz. The proposed layout of the feeds is

shown in Figure 1. The beam spacing is approximately 3 HPBW at

the center frequency. Acceptable levels of beam distortion and

loss of aperture efficiency warrant tight packing of the feeds

and set the limitation on the maximum dimension of the feed at

4.25". The following two feeds were considered for this system.

Dual-Mode, Small-Aperture Feed

A dual-mode, small-aperture feed with an aperture diameter of

1.31 A was designed [2], [3] and fabricated. In this feed, TEll

and TM11 modes are excited at the circular aperture where the

relative phases and amplitudes of the two modes are adjusted to

cancel the electric field at the aperture boundary. This tapered

distribution over the aperture provides nearly circularly symmetric

radiation patterns. Far-field pattern measurements made on the

fabricated feed revealed that this feed did not have the required

3

bandwidth. Due to the differential dispersion between the modes

the amplitude and phase patterns on either side of the design

frequency started to lobe, rendering this feed unsuitable.

Hybrid-Mode Scalar Horn

The second alternative was a hybrid-mode scalar horn (wide-

flare-angle corrugated horn) where the two modes TE„ and TM„

are locked together in the HE„ mode configuration The hybrid-

mode condition is achieved by imposing an anisotropic boundary

condition on the excitation structure leading to the aperture.

These feeds exhibit radiation pattern symmetry resulting in high

gain and low spillover. They radiate with very low cross-polari-

zation. A bandwidth as high as 1 7:1 is attainable with the

scalar horn. Scalar horns have been found to perform efficiently

even when they are small in terms of wavelengths [4], [5], [6].

Experimental Results

A scalar horn with a flare half-angle of 450 shown in Figure

2 was designed and fabricated. The aperture diameter is approximately

1.31X at the center frequency of 4.85 GHz. The input section of

the horn is 1.768" diameter. The corrugations of this horn are

perpendicular to the wall of the horn and have uniform slot depth

of little more than a quarter wavelength at the lower frequency

end. The widths of the slot and ridge are 0.12 X and 0.03 X,

respectively. Because of the size of the aperture, only two

corrugations could be accommodated. The location of the first

corrugation has been chosen for a good match between the TE„

4

mode in the circular waveguide and the TM„ mode in the corrugated

circular section. The various dimensions are shown in Figure 3.

Co-polar and cross-polar field patterns in the two principal

planes were measured. The results are shown in Figures 5 and 6.

At the edge of the dish (300-ft) the field levels varied between

-11 dB and -12.5 dB in the E-plane and between -14.5 and -16 dB

in the H-plane. As can be seen from the plots, cross-polarization

levels were never greater than -38 dB with respect to the boresight

co-polarization levels in the two planes. The patterns in the

two planes are almost identical down to about -9 dB. The phase

centers in the two planes are found to be coincident.

A tuning washer has been introduced into the input section

of the horn for improving the impedance match. The return loss

measured is shown in Figure 4 and is found to be excellent.

The various efficiencies and noise temperatures contributed

by the feed on the 300-ft telescope are shown in Table 1. The

aperture efficiency varies between 69% and 75% while the spillover

noise temperature varies between 6.5°K and 10°K. It can be

concluded that scalar horns need not be electrically large for

reasonably good performance.

REFERENCES

[1] J. J. Condon, "Multiple-Beam Receiver Proposal," Memorandumto R. J. Havlen, September 28, 1982.

[2] R. C. Johnson and H. Jasik, Antenna Engineering Handbook,Second Edition, pp. 15-19 to 15-25.

[3] R. H. Turrin, "Dual Mode Small-Aperture Antennas," IEEETransactions on Antennas and Propagation, Vol. AP-15,pp. 307-308, March 1967.

[4] P. J. B. Clarricoats and A. D. Olver, "Design of Cylindricaland Conical Corrugated Horns," Chapter 5, Corrugated Horns for Microwave Antennas, 1984.

[5] P. J. B. Claricoats and P. K. Saha, "Propagation and RadiationBehaviour of Corrugated Feeds, Part 2: Corrugated -Conical - Horn Feed," Proc. Inst. Elect. Engrs. Vci.Mpp. 1177-1186, September 1971.

[6] A. J. Estin, C. F. Stubenrauch, A. G. Repjar and A. C. Newell,"Optimized Wavelength-Sized Scalar Horns as AntennaRadiation Standards," IEEE Trans. on Inst. & Meas. Vol. IM-31, No 1, pp. 53-56, March 1982.

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FIGURE 1. Layout of the 6-cm feeds.

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FIGURE 3. 6-cm Scalar Horn.

8

(All dimensions in inches.)

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