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.
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim Brown K9YC
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