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Re: [RFI] Low pass filter opinions

To: rfi@contesting.com
Subject: Re: [RFI] Low pass filter opinions
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 12 Jan 2011 11:27:50 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 1/12/2011 9:22 AM, Scott Holisky wrote:
> Lets not throw out those LPF yet. I live a few miles from the local
> broadcast towers here in the Twin Cities. The ERP from the three towers is
> ~30 Megawatts, give or take.

I was similarly located in Chicago for nearly 20 years. For a research 
project on RFI to audio systems, I did both calculations and 
measurements (HP Spectrum Analyzer). At close in distances, field 
strength is considerably less than a simple ERP calculation suggests, 
because all those TX antennas have a LOT of gain in the vertical plane, 
with VERY narrow beamwidth. Indeed, the greatest field strengths were 
from low band VHF and FM stations, all of which were using one bay or 
two-bay antennas and a lot less ERP.
>   In the days of analog TV you could tune across
> the bands and hear the occasional sync buzz of a TV station.

I'd bet that most, if not all, of what you heard was low band VHF (Ch 
2-6). BTW -- video buzz is a better word than sync buzz -- the 
repetition rate of NTSC video is 59.97 Hz, video is 75% of that 
waveform, sync is only 25%, and you can (could) clearly hear the 
character of the buzz vary as the video content changes.

> I have used a LPF (the Drake version) for years. It eliminated the mixing or
> overload in my radio's front end.

While that's certainly a good solution, but the fact that it's needed 
indicates it's a poor radio. If I had that sort of problem with a radio 
today, I'd sell it and buy a better one. :)

I strongly agree with others who say that an outboard LPF is rarely 
needed in today's world.  Virtually all GOOD rigs and amps have very 
good bandpass filters built in, and for all the reasons noted, VERY 
little TVI is caused by harmonics of the TX unless something is badly 
broken or badly mistuned. Antenna tuners also tend to minimize VHF/UHF 
harmonics, and HF antennas don't radiate VHF/UHF very well. 174 MHz is 
the lowest harmonic we need to worry about, because there are VERY few 
US TV stations below Ch 7 (174-180 MHz). Most that were on Ch 2-6 (54-88 
MHz) have moved to Ch 7 or above. Study 
http://www.fcc.gov/dtv/markets/   to see who's on what RF channel in 
your part of the country, or use 
http://www.fcc.gov/fcc-bin/audio/tvq.html to search channel by channel. 
(To avoid confusing non-techie viewers, stations that moved from one 
channel to another continue to be identified by their old analog 
designations.  For example, a station that was on Ch 9 in analog days is 
still identified as Ch 9, even though they might actually be 
transmitting on Ch 22.)

Rather, the primary causes are poor shield connections in the RF system 
(rooftop antenna, cable TV, and interconnections of those systems) and 
pin 1 problems in equipment connected to the TV (like stereo rigs and 
video recorders of various sorts), and an LPF does NOTHING to solve 
these problems. Money would be far better spent on a good choke to keep 
RF off the feedline, getting antennas a bit higher (thus further from 
the victim equipment, and on chokes to kill RF current on wires 
connected to the victim equipment.

There's an RFI tutorial on my website.  

73, Jim Brown K9YC
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