In article , "Larry
King" writes:

"Avery Fineman" wrote in message
...
In article ,
"Larry
King" writes:

"J M Noeding" wrote in message
...
......
You seldom see such large dimensions, believe they had
waveguide for the over hoizon radio link system on 900MHz, but usually
WG is not practical below 3.3GHz .......

Nonsense, rectangular waveguide is used quite frequently in UHF
television
broadcast transmitting installations. Granted it's pretty good sized
stuff
to get the lower cutoff frequency down where it needs to be in the 450 -
800
MHz range (WR1150, WR1500, and WR1800 for example) .. at least 6" x 12"
outside dimensions. But it's definitely practical at those frequencies
which are far below 3.3 Ghz.

Rectangular waveguide below 1 GHz (which is above UHF TV)?

Sure, take a look at the URL in my original message from the Dielectric
webpages :-) And the Andrew Corp. extolls the virtues of its circular
waveguide product line for the same application.....

http://www.andrew.com/search/docviewer.aspx?docid=398

I'm not going to say it can't be done or even shouldn't. :-)

Rectangular guide at TE-sub-10 mode is convenient to use
in general applications. Except for the choke-joint flanges, it
is fairly easy to fabricate, even in the home workshop; think
raw PCB stock for stiffness and easy solderability or other
sheet metal joined with a copper-pipe-plumber's torch. The
launch probes are also mechanically easy on fabrication.

In circular waveguide, there's a different mode choice and
you can't fabricate that easily in the home workshop. Also
the I/O structures are a bit more difficult for the hobbyist.

I'm not going to drag out the Matthei for this but the above is
looking at practicality.

Andrew is a good company and has been making antennas for
a while. I got very intimate with their 10 foot dishes back in
1954...:-) But then they were using 1 5/8" rigid coax feed,
pressurized over 250-foot length of terminal to antenna.

Coaxial structures, yes, especially for the visual-aural diplexer.
I've walked INTO the channel 13 diplexer for KCOP on Mount Wilson
(serving Los Angeles with 50 KW into the antenna). Just open the
door in the corner formed by the visual and aural transmitters and
one comes upon a massive array of round structures...:-)

When I first read this, I though you meant that you were ***inside*** of the
diplexer [hopefully not while it was 'on the air'] :-) Then I reread it,
and understood what you meant. Yes those coaxial cavities can be quite
impressive.

KCOP does the 24/7 bit now as it did in 1958 when a buddy and I got
a tour of it. It was ON when we stepped into the collection of large-
scale plumbing. Not a problem. Two transmitters at right angles,
diplexer at the corner. At the time their older transmitter was on the
other side of the transmitter room, ready to use in case of main xmtr
catastrophe...forget how the antenna connection switchover was done
although it was explained (I'll blame the altitude...Mt. Wilson is about
5700 feet ASL...:-).

Most TV broadcasters don't mind interested viewers visiting their
stations if one calls ahead. Remote transmit sites don't get a lot of
activity. Mt. Wilson for Greater Los Angeles coverage is an exception
with over two dozen transmitters along with a solar observatory, a
couple hotel/motels, and the beginning of a national park area.

Len Anderson
retired (from regular hours) electronic engineer person