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From: cyamamot@nunki.usc.edu (Cliff Yamamoto)
Newsgroups: sci.electronics,rec.ham-radio
Subject: How do I make a notch filter with cable?
Keywords: filter, notch, cable, short, simple, hi-Q
Message-ID: <4436@merlin.usc.edu>
Date: 19 Jul 89 18:12:29 GMT
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Reply-To: cyamamot@nunki.usc.edu (Cliff Yamamoto)
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Organization: University of Southern California, Los Angeles, CA
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Greetings,

	I am trying to make a sharp notch filter to cut out an overbearing
paging transmitter that swamps my PRO-2004 scanner on the VHF bands.  I
*think* I've read somewhere before about how one can cut a piece of cable
to a certain length and short it at the end to kill the frequency of said
wavelength.  Something like this...

         |<------------feed line cable length important??--------->|

         -----------------------------------------------------------
Antenna  ------------------------------+---------------------------- Receiver
         ----------------------------+ | +--------------------------
                              ^      | | | \
                              |      | | |  \ 
         The length of this   |      | | |   \___location of BNC "T" connector
         "stub" is cut to the |      | | |       important??
         wavelength of the    |      | | |
         offending frequency  |      | | |
                              v      |_|_| <---Short center conductor to shield

The pager frequency is 152.4 Mhz.  So using the ol' ARRL handbook I get a
wavelength of 1.96 meters (length of the "stub").

Has anyone heard of this before?  I know it's cheap and dirty, but I'm not sure
if the location of the "T" along the feedline is important.  There's no mention
of this kind of filter in the handbook, but I'm sure I've seen it before.

Thanks in advance for your input.
Cliff Yamamoto