Radio Science Bulletin 313 - June 2005 - URSI

Figure 17. TDR of the dual polar UCIRA-2 (Vertical).By measuring the antenna pattern of the UCIRA-2 onthe major planes, we found that the beamwidth isapproximately 10-12°. The design beamwidth is 20°greaterthan for a parabolic reflector, so it appears that we did notmeet this goal; however, the shape of the antenna is notquite as accurate as we would like. The fabric reflector isapproximately 4.3 cm (1.7 inches) deeper than desired andthe distance from the feed point to the reflector is not asaccurate as we would like. The reflector should be somewhatflatter than it is. This is in spite of several modifications thathave improved the shape to some extent. It is very difficultto compensate for the stretch in the fabric and variouseffects of the sewing operation.6. UCIRA-2 RF Measurements inDual Polarity ConfigurationNext, we provide the results of measurements madeon the UCIRA-2 in dual polarity mode. In Figure 17 we seethat the impedance jumps at the feed point where the 200 Ωtwinline connects to the 400 Ω feed arms. The impedanceshould jump from 50 Ω to 100 Ω in the TDR at this pointdue to the effect of the balun. However, in this case the jumpis not that great. This is perhaps due to losses in the twinlineFigure 18. Normalized impulse responses of the UCIRA-2.that reduce the reflected voltage, so the full effect of theimpedance mismatch is not seen in the TDR.We present the normalized impulse response for bothchannels of the UCIRA-2 in Figure 18. The shape of theimpulse response is not ideal and the FWHM is somewhathigh (108 ps for the vertical channel). This leads to reducedgain at high frequencies as seen in Figure 19. In Figure 20we show the theoretical gain for the UCIRA-2 in dualpolarity mode based on work by Scott Tyo [4, 5]. The gainof the vertical channel of the UCIRA-2 agrees reasonablywell with the predictions, but it falls off more at 10 GHz. Itis interesting that the reduced gain above about 5 GHz isdue to defocusing the reflector and not primarily due toantenna assembly problems as is sometimes the case. Ideallythe gain of the horizontal channel should be the same as thatof the vertical channel; however, as can be seen in Figure 19it is quite different. The differences are due to variations inthe antenna especially at the feed point. At the feed point,two of the wires from separate twinline feeds must cross tobe connected to the tips of the feed arms in the properconfiguration. This requires one wire to be in front of theother, so the wiring cannot be totally identical for the twofeeds.Figure 19. Realized gains of thedual polar UCIRA-2.46TheRadio Science Bulletin No 313 (June, 2005)