Re: Waveguide (was "size a major consideration...") [Telecom]

On Mon, 4 May 2009 15:26:43 -0400 (EDT), kludge@xxxxxxxxx (Scott
Dorsey) wrote:
I have seen 450MHz waveguide used for early satellite antenna systems.
It wasn't big enough to walk inside, but you could climb through it.
--scott

For a really big waveguide, consider a highway tunnel. AM radio, at
frequencies around 1 MHz, cannot penetrate the tunnel. But FM radio,
at about 100 MHz, can penetrate. A 100 MHz signal has a wavelength of
3 meters, which is 10 feet. The cut-off frequency of a waveguide is
that at which the longest dimension of the opening is 1/2 wavelength.
For a 100 MHz signal, 1/2 wavelength is 5 feet, and any highway
vehicle tunnel is larger that that. I have even picked up FM radio in
the parking garage under Boston Common in Boston, MA.

***** Moderator's Note *****

.... which would mean waveguide for 450MHz would be about 13 inches
wide, so that's not likely to be economical. The 23 cm band could be
done with ~4.5 inches, but the price of copper is too high for that
unless someone knows a way to do it with less expensive material.

Bill Horne
Temporary Moderator

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