🚀 go-pugleaf

wtm@uhura.neoucom.edu (Bill Mayhew) writes:

>A very effective way to trap out an annoying signal is to build a
>1/4 wavelength stub transmission line.  The stub can be connected
>directly across the antenna terminals of the receiver.  The far end
>of the stub should be shorted.  The stub will reflect energy over a
>distance of 1/2 wavelength.  This means that you'll be adding back
>energy 180 degrees out of phase at the antenna terminals (at one
>very specific frequency).  A tuned stub, if carefully constucted
>has a very high Q factor.  

A 1/4 wave stub shorted at the free end will look like an "open"
(at the frequency at which it's 1/4 wave electrically), not a
short.  But even if you open the free end to yield a short at the 
resonant freq., I question whether you will be able to get a high
enough Q with a reasonable (e.g., RG-8 or smaller) transmission 
line resonator for the (200kHz/100MHz) problem.  It'll still look 
too much like a short 200kHz away from resonance to be much good.  
A helical resonator shunted by an appropriate L or C to yield a 
transmission pole on the desired station and zero on the undesired 
might do the trick.  See ham publications about repeaters:  they
have a similar problem at 150MHz with 600kHz spacing, where they 
need (well) in excess of 60dB attenuation of one of the pair and 
want less than 2 or 3 dB attenuation of the other.