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In addition to the discussion[s] on focal point, the received signal
gain from the 18 inch antenna is down approximately -14 dB from the 12 GHz nominal frequency for the satellite tv signals. There will be a further reduction from the off center feed [It's been 15 years since I ran the calculations and I'm too rusty at age 66 to do them now grin]. From old experience, I would expect somewhere around a 10 +/- ? degree beamwidth at 12 GHz and somewhere around 30 to 40 +/- ? degree beamwidth at 2.4 GHZ. So, a fairly broad pattern that you describe does not seem unreasonable. However, I invite a microwave engineer with more recent experience to comment and I'll freely accept a better analysis and conclusions. For a circular aperture--a good approximation-- the gain equation approximates to : 10 log (10 D(L)^2) where D(L) is the diameter of the aperture in wavelengths. If the dish is 0.45 meters, we get the following for the gains (assuming (naively) 100% aperture efficiency): Freq gain 12 GHz 35 dB 2.45 GHz 21 dB Thus Dave's accurate answer. Going off-axis lowers the aperture efficiency anywhere from 1-3 dB from ideal, although it usually is only 1 dB or so down from the losses of vignetting from the (centered) feed. So you're probably seeing about 18+ or so dB of gain. The FWHM of the beam is approximately 57/D(L) degrees. So at 2.4 GHz this is about 16 degrees to the 3 dB points. Possibly broadened by the feed arrangement and dish curvature , but the 30-40 degrees sounds a little high Dave. Not a microwave engineer; will a radio astronomer do:-)? 73, Chip N1IR |