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In article <4436@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes:
> 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

These filters work, though they tend to be a bit broad. Use of very good
coax helps keep the Q high and thus narrow the notch. However, you don't
want to use a free space wavelength, as you calculated. You need to add
in the velocity of propagation (velocity factor) which will make the
actual length somewhat shorter. Also, you needn't use a full wave of cable
- a half-wave or any multiple thereof will work. Also, you can use an
open quarter-wave stub just as effectively. If you're using RG-58 (solid
dielectric, not foam), the velocity factor is 66% (.66). For a half-wave
shorted stub, the formula is: L(inches)= 5904 / Freq in MHz X VF where
L is the length of the stub in inches (convenient at VHF) and VF is the 
Velocity Factor of your cable. For a quarter-wave open stub, it's 
2952 / F (MHz) X VF.

In your case then, using RG-58, you want a cable length of about 25.6 inches.
Or, 12.8 in the case of the quarter- wave stub. The connector adds a
little length. Best results are to start an inch or so too long, listen
to the offending signal, and trim the end (the reason why I like the open
stub better- you don't have to keep remaking the connection on the end)
in small (1/8 or 1/4 inch) steps until you observe the drop  in the 
signal. It will be quite apparent. Remember, if you go too far and cut
too much off, you get to start over with a new piece of cable.

In theory, it doesn't matter where in the feedline you place the filter,
but in reality, you want it as near to the receiver as possible to 
minimize stray pickup of the offending signal after the filter.

The only real down side to this circuit is that signals a couple of
MHz away will also be significantly attenuated. This may or may not
matter.

Hope this helps.  

73                   Dave KD5RO