Design feedhorn to give an attenuation of 10 to 12 dB towards the rim of the dish.

Andy,

For parabolic dishes you want to design your feedhorn in such a way that it has an attenuation of 10 to 12 dB towards the rim of the dish. What that means in practical terms is dependent on the f/D ratio of the dish.

My “goto” for advice is https://www.w1ghz.org/antbook/chap6-3.pdf

Wolfgang