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)?

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...:-)

General Electric used rectangular guide based filters for their 1.8
GHz microwave radio relay diplexer, a 1950 design. Efficient as a
multisection pair of narrowband bandpass filters for its time. Big,
though, was as long as four racks are wide.

At a PGMTT meeting in Santa Monica I once heard an informal
chat about three guys using air conditioning duct at high UHF in
lieu of buying special waveguide. It worked at low power. Think
about it...some standard ducting comes in 2:1 width:height ratios
just dandy for TE_1_0 mode. Heckuva lot cheaper than buying
specific waveguide at the same size...probably cheaper than used
waveguide (if such is available).

Len Anderson
retired (from regular hours) electronic engineer person