NATIONAL RADIO ASTRONOMY OBSERVATORY GREEN BANK, WEST VIRGINIA ELECTRONICS DIVISION TECHNICAL NOTE NO. 134

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1 NATIONAL RADIO ASTRONOMY OBSERVATORY GREEN BANK, WEST VIRGINIA ELECTRONICS DIVISION TECHNICAL NOTE NO. 134 Title: SMALL-APERTURE SCALAR HORN AS A FEED FOR THE 7-FEED, 6-CM RECEIVER ON THE 300-FT TELESCOPE Author(s): S. Srikath Date: Jauary 22, 1986 DISTRIBUTION: GB CV TU VLA GB Library CV Library Library Dowtow VLA Library R. Norrod H. Hvatura Library Moutai P. Napier R. Weimer M. Balister J. Paye J. Campbell D. Schiebel S. Weireb R. Freud W. Brudage E. Childers C. Burgess J. Lamb S. Srikath S-l K. Pa C. Brockway A. R. Kerr J. Coe P. Siegel G. Behres L. D*Addario R. Mauzy R. Bradley R. Lacasse R. Fisher F. Crews

2 SMALL-APERTURE SCALAR HORN AS FEED FOR THE 7-FEED, 6-CM RECEIVER ON THE 300-FT TELESCOPE S. Srikath Itroductio The 7-beam, 14-chael (7 feeds x 2 polarizatios) 6-cra receiver was proposed by J. J. Codo [1] for the purpose of makig radio maps of the etire sky visible from Gree Bak. This receiver, makig use of NRAO cooled FET amplifiers, is beig put together by G. Behres ad I. Jeffries ad is scheduled to go o the 300-ft telescope i Jue The operatig rage of the receiver is 4.6 to 5.1 GHz. The proposed layout of the feeds is show i Figure 1. The beam spacig is approximately 3 HPBW at the ceter frequecy. Acceptable levels of beam distortio ad loss of aperture efficiecy warrat tight packig of the feeds ad set the limitatio o the maximum dimesio of the feed at 4.25". The followig two feeds were cosidered for this system. Dual-Mode, Small-Aperture Feed A dual-mode, small-aperture feed with a aperture diameter of 1.31 X was desiged [2], [3] ad fabricated. I this feed, TE ad TM^ modes are excited at the circular aperture where the relative phases ad amplitudes of the two modes are adjusted to cacel the electric field at the aperture boudary. This tapered distributio over the aperture provides early circularly symmetric radiatio patters. Far-field patter measuremets made o the fabricated feed revealed that this feed did ot have the required

3 badwidth. Due to the differetial dispersio betwee the modes the amplitude ad phase patters o either side of the desig frequecy started to lobe, rederig this feed usuitable. Hybrid-Mode Scalar Hor The secod alterative was a hybrid-mode scalar hor (wideflare-agle corrugated hor) where the two modes TE 1 1 ad TM 11 are locked together i the HEj! mode cofiguratio. The hybridmode coditio is achieved by imposig a aisotropic boudary coditio o the excitatio structure leadig to the aperture. These feeds exhibit radiatio patter symmetry resultig i high gai ad low spillover. They radiate with very low cross-polari zatio. A badwidth as high as 1.7:1 is attaiable with the scalar hor. Scalar hors have bee foud to perform efficietly eve whe they are small i terms of wavelegths [4], [5], [6]. Experimetal Results A scalar hor with a flare half-agle of 45 show i Figure 2 was desiged ad fabricated. The aperture diameter is approximately 1.31X at the ceter frequecy of 4.85 GHz. The iput sectio of the hor is 1.768" diameter. The corrugatios of this hor are perpedicular to the wall of the hor ad have uiform slot depth of little more tha a quarter wavelegth at the lower frequecy ed. The widths of the slot ad ridge are 0.12 X ad 0.03 A, respectively. Because of the size of the aperture, oly two corrugatios could be accommodated. The locatio of the first corrugatio has bee chose for a good match betwee the TE l l

4 mode i the circular waveguide ad the TM^ mode i the corrugated circular sectio. The various dimesios are show i Figure 3. Co-polar ad cross-polar field patters i the two pricipal plaes were measured. The results are show i Figures 5 ad 6. At the edge of the dish (300-ft) the field levels varied betwee -11 db ad db i the E-plae ad betwee ad -16 db i the H-plae. As ca be see from the plots, cross-polarizatio levels were ever greater tha -38 db with respect to the boresight co-polarizatio levels i the two plaes. The patters i the two plaes are almost idetical dow to about -9 db. The phase ceters i the two plaes are foud to be coicidet. A tuig washer has bee itroduced ito the iput sectio of the hor for improvig the impedace match. The retur loss measured is show i Figure 4 ad is foud to be excellet. The various efficiecies ad oise temperatures cotributed by the feed o the 300-ft telescope are show i Table 1. The aperture efficiecy varies betwee 69$ ad 75% while the spillover oise temperature varies betwee K ad 10 o K. It ca be cocluded that scalar hors eed ot be electrically large for reasoably good performace.

5 REFERENCES [1] J. J. Codo, "Multiple-Beam Receiver Proposal," Memoradum to R. J. Havle, September 28, [2] R. C. Johso ad H. Jasik, Atea Egieerig Hadbook, Secod Editio, pp to [3] R. H. Turri, "Dual Mode Small-Aperture Ateas," IEEE Trasactios o Ateas ad Propagatio, Vol. AP-15, pp , March [4] P. J. B. Clarricoats ad A. D. Olver, "Desig of Cylidrical ad Coical Corrugated Hors," Chapter 5, Corrugated Hors for Microwave Ateas, [5] P. J. B. Claricoats ad P. K. Saha, "Propagatio ad Radiatio Behaviour of Corrugated Feeds, Part 2: Corrugated Coical - Hor Feed," Proc. Ist. Elect. Egrs., )M. T^ pp , September [6] A. J. Esti,C. F. Stuber auch, A. G. Repj ar ad A. C. Newell, "Optimized Wavelegth-Sized Scalar Hors as Atea Radiatio Stadards," IEEE Tras, o Ist. & Meas., Vol. IM-31, No. 1, pp , March 1982.

6 TABLE 1 H-PLANE E-PLANE f GHz Reflec tio Taper Spill over Aperture Noise Tempera ture 0 K Taper Spill over Aperture Noise Tempera ture 0 K ON

7 FIGURE 1. Layout of the 6-cm feeds, FIGURE 2. Small-aperture scalar hor with flare setiagle of 45*

8 FIGURE 3. 6-cm Scalar Hor. (All dimesios i iches.) FIGURE 4. Retur Loss.

9 (a) 4.6 GHz (b) 4.85 GHz FIGURE 5 Co-polar ad cross-polar patters: H-Plae Note: X-polar patter up + 20 db.. LL ' MINUS. i. AZIMUTH ANGLE (DEC.) i I I Li 1 I 1 1 i i ii i i I I L (c) 5.1 GHz JO , PLUS

10 (a) 4.6 GHz (b) 4.85 GHz FEED_1 SJL. r. i S s - '- \ / ' 9 ": \ \ - 12-^ s / - 15-^ N \ a - 18-; -c - icflcftrt 2. i, : i A "A ZX ~ " u N V Vi _ c -24 l\ XK " t ( \ J \ ft - 33-^ Mill - 36-^ \ 0E-PLANE PLOT No. U M-KLANt PiATF: u'*? REMARKS: 'V l/' J^ I p K 1 v a A / \ \ 2 \\ < wi A i i\ i -J 1 ; \\ 1 J 1 1 f 1 i 1 I i \ / f f l\ f 1 J f /! 1 / I FIGURE 6 Co-polar ad cross-polar patter^: E-Plae Note: X-polar patter up + 20 db i 1 Ml : 1 1, 1 i 1 I 1 I 1 I 1 l 1 I 1.i_LiJ C <0 (c) 5.1 GHz