Rule of thumb for beam tilting for a reflector antenna with off-focus feed

[FMeyer]

Hi,

I'm looking for a rule of thumb for the beam tilting of a reflector antenna when the feed has an offset relative to the focus. So basically something like focal length+feed offset equals x° of beam tilting. I'm assume that the exact tilting depends on illumination etc., but for the first rough design it would be very useful to have a rule of thumb.

Thanks a lot in advance,

 

[IRstuff]

For small offsets, that should be pretty much it:

offset in mm divided by focal length in mm = angle in radians

TTFN
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[FMeyer]

Thanks a lot for answer