Terry,
Toroids in TVI applications are largely, IMO, a big waste of
time and money. There are much more effective ways to
accomplish the same thing, and at the same time improve
lightning immunity.
There are three main ways RF causes a problem:
Direct into internal wiring, and this is often into the IF
system.
RF differential between power lines and antenna leads. The
current path is of course through the TV wiring and the root
cause is common mode on the antenna or power line with the
opposite connection serving as the termination. In effect
you have a big push-pull system with the feedline acting
like one leg and the power mains acting like the other.
Very high frequency RF direct into tuners, normally
something just out of the TV channels although some
harmonics can wind up right on the TV channels. It could be
fundemental overload, but that's unlikely on a cable system
unless there is a real defect in a shield connection
someplace. Quad shield IMO is a waste of money. Any good
quality single shield would be adequate. The largest
problems by far with ingress is really poor cable or a bad
or poorly laid out connection on a shield.
The IF system is normally in the 40-45 MHz range with color
and video amps below the sound IF which is 4.5MHz. You could
be getting IF ingress after the bandpass filtering, but that
would show on all channels.
If you have a large amount of differential between the power
line (or other wires) and the antenna cable, by far the most
effective way to stop the problem is a common old lightning
protection outlet strip with F connectors. Make sure the
strip grounds the CATV shield to the power mains safety
ground. You might have to add bypass caps from each side of
the mains to the safety ground. Just be sure they are UL/CSA
power line rated caps. A regular .01uF 1kV disc is NOT safe.
Route the power line and the CATV cable to the set held
together in a tight bundle after passing through that outlet
strip. This closes the loop and is hundreds of times more
effective than common mode chokes. It also adds significant
lightning protection. Then, if you want to have the bead
fairy visit, have her sprinkle the beads on the power mains
or antenna feed side of that grounding strip.
If the TV has IF blow through or a weak front end, it would
take an exceptional filter to knock 50 MHz out. Think about
it this way. To not tilt the response over a channel, the
filter must pass all frequencies above ~55 MHz virtually
flat . Now, just 10% lower in frequency, you want
significant attenuation. That just isn't going to happen
unless the filter has special lossly coupled high Q traps
specifically to suck out 50 MHz.
Better to stop the ingress by finding and cleaning up
whatever is letting the signal into the closed system, and
it probably isn't the cable shield unless the cable is
absolute junk. Most likely it is a bad shield connection or
poorly designed or defective hardware. (I think you said
this is a CATV system.)
My suggestion to you is to weed back through the system by
looking at the six meter signal on the TV and yanking wires
off. First pull the coax at the TV. Then, if you still see a
very strong signal on channel 2, you know it is inside the
TV. You can work back through the system that way. Leave the
coax on and disconnect at the wall plate. Work back through.
It is abnormal to see anything on channel two with a clean
six meter signal unless the TV is really getting whacked
hard or has a nasty problem inside. The TV might have
something as simple as a broken shield connection or a poor
shield termination at the input connector.
73 Tom
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