Matt wrote:
Can I make a slot antenna out of round pipe rather then rectangular?

I assume you are referring to a circular waveguide with slot radiators
cut into the surface. The usual arrangement is to use a rectangular
waveguide with dimensions chosen so that only one possible propagation
mode exists for the wavelength in use. This makes it fairly simple to
determine suitable locations and dimensions of the radiating slots for
the desired current distribution.

The problem with circular waveguide is that it does not constrain the
polarization to any particular orientation. Even if the wave is
launched in a known orientation, the presence of the slots may very well
induce a 'twist' in the propagating wave which will affect the
excitation of the slots in a way which may be extremely difficult to
predict, or even measure.

One remedy is to insert features inside the circular waveguide which
constrain the propagating wave to the desired polarization direction.
Specific methods can be found in the microwave literature but I think
you will find fabrication of rectangular waveguide by far the easiest
solution if you're only going to construct a few units.

bart
wb6hqk



I am having a hard time finding inexpensive rectangular tubing for working
in 33cm band. Figure I need 2" x 8" and its not cheap. I can get 7" pipe
much cheaper.

Matt