On 1/13/2011 6:01 AM, John Geiger wrote:
> What am I to make of the reviews on eham.net of people who had RFI, then
> bought a TVI filter and that cleared it up? You can find quite a few like
> these in the product reviews section for RFI. Not trying to argue but it
> does seem like some people have been able to get a low pass filter to work
> for them. Maybe it will work for me.
Look at the dates of those reviews. The conditions I've outlined have
changed over the years, with the transition first from TV sets connected
to antennas, to TV sets connected to cable TV systems (CATV), and now to
digital transmission of TV. Each one of those transitions have made it
less likely that there will be TVI from harmonics or spurs from an ham
transmitter. The transition to CATV was gradual, occurring over a
couple of decades, while the transition to DTV was much more abrupt. I
would discount reviews from US hams older than about 2009 saying that a
low pass filter fixed a TVI problem, simply because of the DTV transition.
For example, until 2006, I lived in a residential neighborhood in the
middle of Chicago, with wood frame houses 15 ft from mine on either
side, and four apartment complexes and more houses across the alley.
Running legal power to low wire antennas, I never had a single TVI
complaint, even though CBS and NBC were on channels 2 and 5
respectively. Why? The fact that most of my neighbors received TV via
cable was very likely a factor. It also helped that I was using a well
designed amp (Ten Tec Titan 425) and decent antenna tuner. Both, by
their nature, tend to suppress harmonics. In 54 years of hamming, I've
never owned an outboard low pass filter, and the last TVI complaint I
had was around 1958. On the other hand, I've seem MANY RFI problems,
both from my own station, and those of other hams, that I've traced to
pin 1 problems, poor shielding of equipment, inadequate filtering of
victim equipment, and the use of un-twisted wiring for loudspeakers.
73, Jim Brown K9YC
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