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

On Fri, 17 Dec 2004 09:26:51 -0600, "Matt"
wrote:

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.

Hi Matt,

What you've "heard" is vague at best. You really need another source
of information, or at least more sources. Try searching the
googlegroups archive of this news group.

In your particular situation (you will eventually have to convert all
this to coax or wire) you either probe excite a waveguide or you loop
excite it.

73's
Richard Clark, KB7QHC

"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?

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

The 2" by 8" sounds wrong for gernic waveguide. it is usally in about a 1:2
ratio
It may be more like 4" by 8" for 900 Mhz.
The field max is in the middle of the waveguide, and the probe end needs to
be there.
ARRL had an article on the rectangular waveguide antenna.
The beam on this antenna if mounted horozontally is very narrow in the
horozontal but wide in the vertical. Fan shaped beam.

"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

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