[TenTec] TVI

[Ken Brown]

Hi all,

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 
HF operating)

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.

DE N6KB