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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


Well, actually, you don't want the stub to be a full wavelength long-
actually a quarter wavelength. Also, it should be open ended, not shorted.
An quarter wave stub with one end open looks like a short at the other end
at its resonant frequency.

To design this filter, you need to know the free space wavelength (1.96
meters) as you determined. Divide by four to get a free space quarter
wave. Now you need to know the velocity of propagation (velocity factor)
of the coax you're using. RG-58 (not foam but solid polyethylene) has a
velocity factor of about 66% (.66). Foam RG-58 is about 79%. Anyway, 
multiply the free-space quarter wavelength you just came up with by the
velocity factor of the cable you're using and this is the correct length.

A convenient formula for VHF lengths is: L (inches)= 2952/F(MHz) X V

where L is the length of the stub, F is the Frequency in MHz and V is
the Velocity Factor of the cable. 

For your example, L=12.8 inches, using RG-58 cable. Build it that way,
leaving the free end open, and it should work. What most guys do is
build it long (also remember the connector adds length) and trim it about
1/8 or 1/4 inch at a time while listening to the offending signal. The
notch is very deep and will be very apparent. Just remember to make small
cuts, because after you go too far, you get to start over!

The length of the antenna feedline is not important, also, theoretically
it shouldn't matter where you place the filter- in reality, you want
to put it near the receiver so that stray pickup by the cable after the 
filter won't be significant.

As with everything, there is a down side to this type of filter. Though
the notch depth is good, the filter is somewhat broad and may (will)
also attenuate nearby frequencies to which you may want to listen. 
The better the cable quality you use, the higher the Q of the tuned
circuit, and the narrower the notch.

BTW, you can add more sections to the filter by adding more Tees and
more open stubs- probably the only thing that will happen though is
the notch will be broader still. 

Hope this helps.        73     Dave KD5RO