> Jim: MFJ makes a filter for the 259 and etc.
>
> I believe that it must be a series resonate circuit across
> the input,
> tunable to the interfering stations freq. Z=0.
The MFJ filter PARALLEL resonates across the input. Z is
thousands of ohms.
In parallel with that parallel resonant circuit is a series
tuned circuit in the BC band. This has a residual inductive
reactance on 160 and higher. To cancel it you just tweak the
capacitance that sets SWR to minimum into a dummy load.
Now you are left with a series to ground trap that shorts
the BC station RF to ground and looks like 1-4K ohms pure
resistance on the measurement frequency.
See how simple the design is? ....and it doesn't modify the
impedance of the system very much.
The limitations are the Q of the tuning capacitor and
(mostly) Q of the inductors. With better inductors it would
an almost perfect filter. The cost and size prohibits using
really good inductors.
73 Tom
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