DaveC wrote:

Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
thumb-sized products) and an old 18-inch satellite TV dish.


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.