I'd say common mode and differential mode are equally likely. A ferrite choke
on the coax is the right fix for common mode. A bandpass filter (or bandstop
filter) is the right fix for differential mode. The transformer someone
mentioned is NOT likely to be a good fix.
See my RFI tutorial for specific recommendations for a ferrite choke for 6M.
Use the graphs of choking impedance to obtain at least 1,000 ohms at 50 MHz.
Depending on the severity of the interference, a higher impedance may be
required. You need to put the choke right in front of the gear that is letting
the RF into the system. That MIGHT be the distro amp, or it might be the TV
set, or it might be something else around the TV set.
Also study the material in my tutorial about "the pin 1 problem." It is the
MOST common coupling mechanism for RFI.
The FILTER should go in front of the distro amp. Tin Lee Electronics, near
Toronto, is a good source of filters. I used two of their filters when I lived
in Chicago. They worked. I also had ferrite chokes on the cables.
One easy way to check for common mode or differential mode is to connect ONLY
the shield of the coax to the distro amp and see if you have the RFI. If you
do, it's common mode. If you don't, it's differential. It's also possible that
you have both differential and common mode coupling.
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