[TowerTalk] Narrow Band Filters

[Jim Lux]

I was thinking about the same thing, after trying to get a few PSK31 Qs in
Sunday morning on 14070 while Marty N6VI was hammering away on  20m CW in
the next tent.  Likewise when the GOTA operation happened to be on the same
band as the main operation, even second harmonic was a problem between 20
and 10 (although we didn't bother to figure out if it was 2nd harmonic of
the Tx or some IM problem in the receiver) Here are my initial thoughts:

Making a suitably narrow band filter shouldn't be a challenge, except:
1) most LC filters are quite sensitive to their terminating impedances.  If
the antenna you're hooked up to isn't a nice 50 ohm resistive (and none
are), you might need some way to tweak the filter to tune it after
installation.
2) You do need filters on the TX as well.   Even transmitters with very good
phase noise will have significant power a few tens of kHz away that needs to
be notched/filtered out.
3) The usual field day operation requires some amount of tuning around, so
the very narrow band high-q approaches used, in, e.g. 2m repeaters won't
work.
4) On receive, it might be possible to use an interference cancelling
approach (sort of like the MFJ (and other) widgets that use a secondary
antenna.  I'm not sure about bandwidth, though.  Maybe some sort of true
time delay using coax would work.
5) An active canceller/filter would be the hot ticket, but dynamic range
might (WILL) be a problem.  It would give you the tunability, etc. though.

Unfortunately, field day isn't a great opportunity to make detailed
measurements, and we didn't shlep a spectrum analyzer up there to really
look at the problem.
Jim, W6RMK

----- Original Message -----
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: "Tower Talk List" <towertalk at contesting.com>
Sent: Saturday, July 03, 2004 8:26 PM
Subject: [TowerTalk] Narrow Band Filters


> I'm thinking about building some narrow band filters (as opposed to a
bandpass filter that
> covers an entire band) for use on Field Day. The object is to minimize cro
sstalk between CW
> and sideband stations on the same band. Has anyone tried this?  Is it
practical to get enough Q
> to pick up enough rejection to make it worth the trouble?  Certainly 80/75
would be the most
> practical, since it is the greatest percentage bandwidth. Is anyone making
a commercial
> product?
>
> Jim K9YC
>
>
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