The fact that they are spec'd by fairly wide ranges was the info I was hoping for. I called a friend today whose system switched from the 150 band to a 400 MHz trunked system a few years ago to see if they had any of the old cavities laying around. He said that they had just finished cleaning house last week and all the old junk went to the dumpster, but he would do some diving to see what was still there.
Regards,
Don
At 12:03 PM 6/22/2005 -0400, you wrote:
Don,
> 1. Cavities are roughly tuned by their
> physical dimensions, and then tweaked by rods, plates,
> etc., eh?
The length of the cavity (more importantly the inner
conductor of the cavity) sets the frequency. Usually
cavities are designed to cover a RANGE of frequencies...
the tuning rod coming out the top adjusts the length of the
center conductor inside the cavity. The tuning range
varies depending on manufacturer and model... some typical
ranges are 136-150, 150-174, 136-174... and many others.
You might want to check some of the articles referenced on
the following page for more on cavity and duplexer theory:
http://www.repeater-builder.com/rbtip/ant-sys-index.html
There is a section on Cavities, Duplexers, etc. part way
down that page.
> 3. How awful a compromise would it be to use available
> cavities from a 2 meter system at railroad frequencies
> (160-161 MHz) ?
If the cavities will tune that high, they probably work just
fine. If the are now on the 2 meter band (below 148 MHz)
they may or may not tune at 160 MHz... it depends on
whether the center conductor can be adjusted that far by
means of the tuning rod.
If you are actually referring to a duplexer (several
interconnected cavities to allow use of a single antenna
for transmitting and receiving) there may be other factors
to consider. The cavities themselves may tune where you
want them, but the length of the coaxial cables
interconnecting the cavities may need to be optimized for
that frequency to get best performance.
The cavities may have more than one adjustment... if they
are bandpass-bandreject cavities, there will be one
adjustment for the length of the center conductor (usually
a big knob / tuning rod) and another adjustment of some
sort to set the frequency of the notch relative to the pass
frequency.
If you can tell us what specific cavities (or duplexer) you
are looking at, some of us can probably provide more
specific information.
Paul
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