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Re: [TowerTalk] Narrow Band Filters

To: "Tower Talk List" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Narrow Band Filters
From: "Jim Brown" <jim@audiosystemsgroup.com>
Reply-to: Jim Brown <jim@audiosystemsgroup.com>
Date: Mon, 05 Jul 2004 08:14:27 -0500
List-post: <mailto:towertalk@contesting.com>
On Mon, 5 Jul 2004 04:25:41 -0700, Michael Tope wrote:

>source, the noise cancellors like the MFJ unit are very
>easy to adjust. If fundamental overload from the
>co-band/cross-mode station is a problem 

I have no experience with the MFJ unit, but my ANC4 is a dog for overload from 
broadcast signals that aren't even very strong at my QTH (nothing within 7 
miles of 
me). I understand that newer production units have a BC filter, but that won't 
solve 
the basic overload problem on in-band signals. The ANC4 can be effective on 
some bands with stable local sources, but can be a PITA to keep tuned with 
different/multiple sources. And I think that fundamental overload would 
severely limit 
its usefullness on FD. I would like to find a better solution. Maybe the MFJ is 
better?

>One the other hand, long pieces of coax are even simpler.
>We do our field day on a mountain top, so we could
>conceivable put separate receive antennas down the
>hill (towards the northeast of course) to improve isolation.

Yes, but you're still stuck with that 1,000 ft diameter circle. 

>The co-band/adjacent-mode stations could share a single
>receive antenna (and coax) located at maximum distance
>from the TX antennas (a small tribander with a power
>divider for instance).

BTW, I've measured quarter wave open stubs made from very low loss RG-8, cut 
for 
both 80 meters and 6 meters, and they have far too low Q to be effective. K4GLM 
tried quarter wave stubs on FD, with the same result. 

I'm a big believer in the value of adding multiple small improvements to solve 
a 
problem. I agree that the design of these filters is non-trivial, and the 
comments re: 
circulating currents are probably dead on. But how about this -- a multistage 
bandpass filter for 80 intentionally designed to be as narrow as possible, 
tuned to 
3525, and a bandpass filter for 75 with the sharpest possible cutoff at 3750. 
If you 
could pick up 6 dB from each filter, that's a 12 dB reduction in the trash that 
each 
receiver sees. Even 3 dB from each filter is a 6 dB reduction.  Add that to the 
physical separation of antennas and the choice of rigs with less trash and 
you're on 
your way to solving the problem.  

75/80 are the easiest, of course, based on percentage bandwidth. But once 
you've 
found a solution there, it might  be possible  to translate it to 40 and maybe 
20.  

Jim Brown  K9YC


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