I am experimeting with 2.4 GHz pyramidal hornantennas like this one:

http://seattlewireless.net/index.cgi/CardboardHorn

But I ask myself: what is the optimal hight of the waveguide part?

Of course the hight of the waveguide has to be more than lambda/4 in
order for the lambda/4 antenna rod to fit in it. But how much bigger
should it be?

Jeppe