On Thu, 19 Feb 2015 13:22:56 -0800, Dave Platt wrote:
wrote:

If I have this correct, the "stub" is a 1/4 wave length of the offending
frequency. The 75 ohm coax is cut to that length and is shorted on the far
end. The other end is connected via a "T" connector.

If it's a quarter-wavelength long at frequency of interest, then
shorting the far end will cause it to behave very much like an open
circuit at the "T" at that frequency. A quarter-wavelength
transmission line results in the maximum transformation of the
impedance from one end to the other.

If what you're attempting to do is "short out" the frequency of that
one channel, then you'd want either a quarter-wavelength stub which is
open at the far end (and these can be tricky due to parasitic
capacitance, and could also radiate signals from other channels) or a
half-wavelength stub which is shorted at the end.

And, be aware that the _open_ quarter wave stub will also demonstrate
'zero' impeadance at all the odd 1/4 wave frequencies -- IOW, at 3X your
"200 to 300 MHz" target, and at 5X, etc.

Also, I don't know if the notch will be "deep" (high Q) enough to make
the digital data "un-usable" (can't decode). Another concern is the width of
the notch. If it is too wide, it may affect the adjacent channel.

The latter is not at all unlikely.

I have made 1/4 wave stubs of High Q out of 1/2" and 3/4" cable TV
hardline to eliminate harmonic interference in TVs from nearby
(non-TV) transmitters. WFM.

HTH
Jonesy
--
Marvin L Jones | W3DHJ | W3DHJ | http://W3DHJ.net/
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