Slot antennas began life as slots in curved surfaces, eg., aircraft
fuselages and wings. Most structural alloys will do fine. Rectangular
tubes are unusual. They are often unsightly protusions.

The maths is more complicated with curved surfaces.

But if you just copy somebody else's slot, and scale dimensions
according
to
frequency, the maths reduces to simple A*B/C schoolboy arithmetic.

I have not found any round rather then square designs on the net. Do you
know of any? If I found one I could scale it down to 33cm. I have not
found any 33cm slotted designs period. I have found a calculator for
rectangular waveguide though which I ran for 33cm. Not sure how I would
adapt its results to round though.

Matt

It should be close, yoy may have to move the slots closer of further apart
by the velocity factor inside the waveguide (going from rectangular to
round, round could be faster so slots may be further apart, but not by
much)
How many slots, how many wavelengths long?

I am wanting 10 slots, 5 per side. I think 7" inch aluminun pipe for
waveguide in 33cm band.

Another question. I have heard for injection into the waveguide you should
use a 1/4 wave stub. I also heard its not supposed to farther then half way
into the waveguide. If I were to use 2" x "8 waveguide how would I do that
since the 1/4 wave stub is 3.2 inches and the waveguide is only 2 inches
wide?

Matt