I have not carefully read every post in the recent 95% coax shield/TVI
discussion, so perhaps someone has already stated the following. Even
so, perhaps it bears repeating.
TVI causes can be divided into two basic categories. The first one would
be the case in which the amateur radio equipment generates a spurious
signal within the spectrum the television is supposed to receive. The
second category would be when the television responds to signals it is
not supposed to respond to.
These days the first category is a lot less common. Commercially
produced amateur radio gear must comply to standards that insure the
spurious emissions are very low, and most of us are running commercial
gear most of the time. Since our gear is mostly very clean, low pass
filters on our gear will seldom cure a TVI problem. (I am talking about
The second category is much more common these days. There are a lot of
ways that a clean HF amateur signal can get into a television system and
cause interference. This can be due to the television itself responding
to HF amateur frequency signals (which they should not do) or it could
be due to harmonic or intermodulation distortion in TV cable or antenna
system components, generating spurious signals inside the spectrum that
TVs are supposed to respond to. If there is a distortion problem
creating an interfering signal inside the TV channel spectrum, in a
system that feeds the TV but not the TV itself, a high pass filter at
the TV will not fix it. Generation of interfering signals due to
distortion of our clean signals can also occur in other electronic
devices that are not part of the TV system or our stations.
Changes made in our antenna and feed line systems can have an affect on
TVI caused by both categories, because the overall radiation pattern of
our system will change the amplitude of signals we generate, both
desired and spurious, at the part of the TV system that it gets into.
Most of the time though, if our transmitted signal is clean, it is not
really going to matter much whether we radiate only from our "intended
antenna" or both from our intended antenna and our "accidental antennas"
(which would be leaky coax, unbalanced currents in coax or parallel
transmission line, rf flowing in other wires in the station, etcetera).
If the TV system responds to signals it is not supposed to respond to,
we can probably generate a strong enough field from our intentional
antenna to cause a problem, even if we don't radiate a femtowatt of RF
from anywhere else.
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