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Re: Topband: CNN notch filter recommendations

To: Kevin KL7KY <>,
Subject: Re: Topband: CNN notch filter recommendations
From: "Richard (Rick) Karlquist" <>
Date: Sun, 14 May 2017 11:10:26 -0700
List-post: <">>
I've done a lot of filter design.  I am assuming that your requirement
is to remove a single station, and that you don't have any requirement
to pass the rest of the BCB band.  The question is: which is more
difficult:  a single frequency notch or a traditional high pass
filter that filters out the whole BCB.  To get 50 dB in a notch,
you will need to have multiple resonators in cascade.  You have to
watch the insertion loss since you are putting 100W through it.
So by the time you get done, it is not clear that the single
frequency notch is easier.

That's filter theory.  But in the real world, there are various
off the shelf high pass filters available, and even construction
articles if you want to build your own.  A 50 dB notch filter, OTOH,
will not be off the shelf or even have construction articles.
There might a software program somewhere to design it, but you
still need to know what you are doing.

A particular problem with notch filters is that many designs are
based on a notch within a bandpass.  IOW, you only need to
pass frequencies near the notch.  In your case, you need to
pass frequencies way above the notch.  Now you are even farther
off the beaten track.  This is probably only for the filter

The commercially made filters have rejection that varies with
the frequency of the station.  You didn't say what the frequency
of the station is.  If it is fairly far down in the band, nearly
any filter than can handle the power should work.  If it is on
1690 (we have one of those here), you need the top of the line filter. Watch out of specsmanship games on the data sheets in that case.
Some filters may talk about rejection at 1500 kHz.

Rick N6RK

On 5/14/2017 9:55 AM, Kevin KL7KY wrote:
Hi all,Has anyone had "success" notching out a single AM bcb station that 
+50dB?I see a lot of bcb filters that are commercially made, but wanted some real world 
feedback from someone that has dealt with it.I'm looking for something that I can leave 
in-line between the rig and the amp - that will handle at least 200w.Any input would be 
appreciated.ThanksKevin KL7KY

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