"Matt" wrote in message
...
Can I make a slot antenna out of round pipe rather then rectangular?

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


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

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.

"Matt" wrote in message
...
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?


It is in several books, called lambda sub-g
you could probably google it, starting with waveguide transmission.
I can't remember, but I think is is about 75 to 85%
It is determined by the dimentions of the waveguide, (same with coax, and
material slows it down further)
lambda is wavelength in free space.



Matt


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

zaashy wrote:
"Matt" wrote in message
...

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



It is in several books, called lambda sub-g
you could probably google it, starting with waveguide transmission.
I can't remember, but I think is is about 75 to 85%
It is determined by the dimentions of the waveguide, (same with coax, and
material slows it down further)
lambda is wavelength in free space.

The phase velocity in a hollow, air filled waveguide is always faster
than the speed of light, so the velocity factor is always 100%. The
amount faster depends on the operating frequency relative to the
waveguide's cutoff frequency.

The velocity factor in a hollow, air-filled guide =

1 / sqrt(1 - (fc/f)^2)

where fc = cutoff frequency
f = operating frequency

Too bad you can't get information through a waveguide that fast. It goes
at the group velocity, which of course is always slower than the speed
of light. Sigh.

Roy Lewallen, W7EL