I used gutter pipe (3 by 5 or so) in one band and built one out of brass
@
2.5 Ghz worked like a champ.
Slots were 1/4 wave (freespace) and seperated by 1/2 wave (speed of
waveguide, not freespace)

How do you figure the speed in waveguide rather then free space?

Matt


Feed was 1/4 wave sticking into the rect pipe, in the middle.
Round pipe should work just fine.