[TowerTalk] Narrow Band Filters

[Jim Brown]

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