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[AMPS] Band Pass Filtering

To: <amps@contesting.com>
Subject: [AMPS] Band Pass Filtering
From: k8cc@ix.netcom.com (David A. Pruett)
Date: Thu, 29 Apr 1999 00:24:01 -0400
Where the filter needs to be located depends on what you're expecting it to
do.

ICE, Dunestar, TopTen, K4VX, W3NQN, et al have designed and built filters
for HF applications which are used after the typical 100W transceiver, but
before the KW final.  There are several reasons for this:

Goal #1 is to protect the receiver from transmitters on other bands in
multi-transmitter contest setups.

Goal #2 is to clean up the broad-band, low-level junk that comes out of the
typical modern synthesized transceiver which can impair nearby receivers on
adjacent bands, again in multi-transmitter contest setups.

Reality #1 is that building a filter to handle 1500W is hard, 250W is much
easier.

The history of the ICE amateur product line started with the 160M-10M
single-band bandpass filters that fit the scenario listed above.  I've own
a set of these for probably eight years.  Only one out the six meet ICE's
own insertion loss spec.  The 80M unit is blown up for reasons unknown.  I
also have a 40M-10M set from the 1996 WRTC contest.  These meet spec and
have not failed.

Its been my experience that ICE's ratings are very marginal.  I've known
competent contesters who have blown up 200W ICE filters driving amps like
Ten-Tec Titans with less than 100W.

Operator error can raise havoc too.  I have an 80M W3NQN filter that a
guest op blew up at my station.  I know just how he did it - a full dwarp
(but barefoot) FT-1000 at 200W output into an 80M dipole in a RTTY contest
- the problem being the dipole was tuned for 3850 and the contest was on
3580.  The FT-1000 internal tuner was quite happy tuning up into whatever
load reflected thru the 'NQN filter, however the filter did not last long
with the >5:1 load it was seeing.

Jon's plan of putting the filter after his brick would seem reasonable when
you look at the ratings.  However, as I said I think the ICE ratings are
marginal.  Also, 66% duty cycle (1 minute on, 30 seconds off) might not
produce the same effects as 66% duty cycle SSB.

But again, what is the point?  All the comments about how dirty a VHF brick
is overlooks the fact that putting the filter after the brick won't help
the real problem.  People complain about bricks due to IMD, which creates
in-band trash.  Putting a bandpass filter after the brick will not solve
the problem unless its REAL narrow - but then you can't QSY :-)

If Jon is trying to protect his receiver, put the filter between the radio
and the brick.  If you're trying to "clean up" the brick, I'd skip the
filter and get your money back.

73,

Dave/K8CC


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