From: Martin Griffith
Subject: Re: Another (FM) antenna question
Date: Sun, 17 Nov 2002 14:17:22 +0000 (UTC)
Organization: BT Openworld
NNTP-Posting-Date: Sun, 17 Nov 2002 14:17:22 +0000 (UTC)
X-Newsreader: Forte Free Agent 1.92/32.572
On Sun, 17 Nov 2002 07:11:02 GMT, "theother_bob"
>I have recently aquired an old (I'm guessing late 50's or early 60's)
>Packard Bell stereophonic console, model# RPC-17, Tuner 7TU-4. It's an old
>tube job, and we've been enjoying it, but it won't pick up our favorite
>station (93.3 MHz) because it's drowned out by the nearby 92.5MHz broadcast.
>Is it possible to either boost the desired frequency or filter out the
>undesirable? (without damaging the tuner)
>theother_bob at yahoo...
>P.S. How can I figure out when it was made?
Coaxial Stub Filters
A simple piece of coaxial cable cut to a quarter-wavelength can be
used as a filter.
You simply install a "tee" connector in your feedline, with the radio
connected to one port,
your antenna connected to the second port, and the coaxial cable
"filter" (or "stub") connected
to the third port. The trick is to cut the "stub" to exactly the
right length. What
you want to do is tune it to "short out" a specific frequency, such as
that of an offending pager.
I tuned one using an ICOM R7000 receiver, which has an excellent
S-meter. First, I cut the
intended filter somewhat longer than necessary. When calculating how
piece to start with, simply divide the frequency (in Mhz) you want to
"kill" into 2952.72,
then multiply by the velocity factor of the coaxial cable you are
using. (Most coax
will be in the range of, say, 0.66 to 0.75). Then ADD a few inches, to
be safe. For example,
if I want to "kill" a pager which operates at 152.2 Mhz, I divide
152.2 to get 19.4 inches. I multiply this by the velocity factor of
0.66 to get 12.8 inches.
The final filter will probably end up near this length once it has
been correctly cut to
size. Then, I ADD about two inches for safe measure.
Next, I tune the R7000 to 152.2 Mhz, where a pager is putting out a
30db over S9 signal.
I place a "tee" connector in the antenna line at the back of the
receiver, and attach
the stub coax to one port. Then, watching the S-meter, I begin to cut
the coax stub down.
As the stub gets shorter, the S-meter will begin to drop, showing that
indeed notching out the pager frequency. As you get nearer the exact
frequency you will
find that even a 1/16th of an inch will make a difference in signal
strength, so be
very careful and work slowly. If the S-meter starts going back up, you
went too far.
(OK, I did trim too much the first time I tried this, but the second
one I cut was better).
The final length came out to be 12.5 inches.
What you will end up with is a filter that will knock about 35 db off
that pager signal.
Back in the 2-meter ham band, you will also see some attenuation,
perhaps as much as
20 db, depending on the quality of the coax you use, but you will also
have cut down the
intermod from the pagers significantly.
Besides, it doesn't cost much, and it's a start at eliminating pager
intermod. You can
also use this type of filter to cut down other problem stations, such
as major FM
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