Slot Antenna
 

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

On Thu, 16 Dec 2004 10:42:48 -0600, "Matt"
wrote:
Can I make a slot antenna out of round pipe rather then rectangular?

Hi Matt,

Round pipe is quite common.

73's
Richard Clark, KB7QHC

Sure!

Matt,

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 doubt if you are interested in an exceptionally well-defined beam. So the
differences between a rectangular and a circular tube of roughly the same
perimeter will probably be of no great consequence.

( Hi Richard - so we meet again!
----
Acqured nom-de-plume, Punchinello.)

"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.

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


I doubt if you are interested in an exceptionally well-defined beam. So
the
differences between a rectangular and a circular tube of roughly the same
perimeter will probably be of no great consequence.

( Hi Richard - so we meet again!
----
Acqured nom-de-plume, Punchinello.)

"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.

"Matt" wrote in message
...
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?

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