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Re: [RFI] Low pass filter opinions

To: "John Geiger" <aa5jg@fidmail.com>, rfi@contesting.com
Subject: Re: [RFI] Low pass filter opinions
From: "Dale Svetanoff" <svetanoff@earthlink.net>
Reply-to: svetanoff@earthlink.net
Date: Tue, 11 Jan 2011 23:19:48 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>

Having read your initial post yesterday (on the 10th), I'd like to make a
few comments regarding the situation.  

First, what is the location of your antenna to either your neighbor's TV
set or to her TV antenna (or cable/satellite box)?

Second, if the problem really is RF fundamental overload, then a low pass
filter will NOT help the problem.  (Fundamental overload occurs when the
fundamental RF signal is enough to disturb the victim device [her TV or
related equipment].  Harmonics may or may not have any effect.)  The best
suggestion is, in fact. to try reducing power.  You mention 25 watts,
reduced from your normal 100 watt level.  If 25 watts results in proper TV
operation, but more than that causes problems, then you know that you are
fighting a 6 dB reduction issue, which is not too bad.  However, if you
have problems down to even lower levels, such as 10 watts or 5 watts, then
you have 10 dB or 13 dB problems.  In short, you need to find the threshold
of pain for that TV.

Remember, curing these types of problems involves working the issues of
Source emission frequencies and power level (your rig), Source proximity to
the victim (NOT your rig, but your antenna relative to her antenna and/or
TV and related equipment), and Threshold of disturbance for the victim. 
The problem will go away if you reduce power enough, move your antennas
away far enough from her house and/or equipment, and provide shielding,
choking, and by-passing on all I/O connections to the TV.  However, is that
realistic?  Probably not, so work the one or two issues that you can
control and see what happens.  Keep us posted.

Final note: if your rig is a modern solid state one, the use of addditonal
low pass filters is almost meaningless.  Study your schematics or tech
specs for the rig.  Since modern solid state HF rigs mostly use broadband
PA stages, thay employ low pass and/or bandpass filter networks as integral
components of their design in order to meet FCC spur and harmonic specs. 
The only places I use low pass filters these days is on my classic tube
rigs and after a linear tube-type amp.  After all, you can not filter the
fundamental and when a rig has harmonics down by 60 dB or more, keep in
mind that 60 dB down from 100 watts (which is +50 dBm into 50 ohms) is -10
dBm, which is 0.1 milliwatt!

I suspect that either your antenna (or part of it) is much too close to her
equipment, or her TV and related equipment has a problem that either
requires a lot of work to fix or that should be checked for bad connections
(at all cable fittings).  Good luck.

73, Dale


> [Original Message]
> From: John Geiger <aa5jg@fidmail.com>
> To: <rfi@contesting.com>
> Date: 1/11/2011 7:49:23
> Subject: [RFI] Low pass filter opinions
> What have list members found to be the best low pass filters to prevent
TVI?  I currently have a Drake TV-1000 in line but that allows RF up to
52mhz, and actually above that because I have used it successfully on
52.525mhz for 6m FM.  Today I borrowed a Kenwood LF-30A from a friend. I
can't find too many specs on it other than it has 90db of attenuation above
90mhz.  It didn't make any difference compared to my Drake.  I was thinking
that it might do better since it should have a lower cutoff frequency.
> I see that Drake also made a TV3300 filter that is rated at 80db above
41mhz.  And the ICE filters get good reviews on eham.net.  So, any opinions
as to what low pass filters work best?
> 73s John AA5JG
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