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