🚀 go-pugleaf

josemj1@aplmail.jhuapl.edu (Marshall Jose) writes:

>> I need a VERY sharp notch filter (or actually 3, if sharp enough) to reject
>> FM broadcast stations from an antenna signal. The requirements are tough:
>> a. VERY sharp - I need to measure spectra of broadband noiselike signals and
>> the present LC filters make a far too visible hole.
>> b. No other holes (or even ripples) in the pass band - 57 to 180MHz.
>> c. Low loss.

>In regard to the "open-stub" notch that someone else suggested, I suppose
>it is something inexpensively verified, although my calculations told me
>that the notch width will be too broad for the application. It will remove
>a TV channel but will obliterate a large portion of the FM broadcast band

Depends on the Q of the transmission line.  If very high quality coax,
hardline or even better, air hardline is used for the stub, it will be 
quite narrow.  My worry would be in making it too narrow for the 
fairly wideband FM signal.  For 2-way work, where the bandwidth is
much less, a piece of 1" CATV coax works just fine to notch out offending
intereference.

John
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