All You Want To Know About CATV Interference  By:  Chuck Manetta  N4YJC

Part #4:  Who's Is To Blame
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There is a lot of misunderstanding between the cable subscriber, 
the cable-company service personnel and the amateurs. The 
subscriber often feels that the amateur is to blame. After all, 
if the amateur wasn't operating, there wouldn't be a problem. The 
amateur often feels that the problem is always the responsibility 
of the cable company, and that it can always be fixed if the 
cable system is better shielded. The cable-company repair 
personnel are often stuck in the middle. Sometimes they tell the 
subscriber that the ham must be transmitting signals outside of 
the ham bands, thus causing the problem (he really ought to know 
better!), or, just as bad for him, believes that the interference 
problem is always due to some defect in the cable system.

In reality, any, or all, of the parties may have some 
responsibility. Let's take a close look at what is really going 
on!

Although it is not usually related to cable TVI to channel 18 or 
channel 24, I must point out that the amateur is, by law, 
required to ensure that any spurious emissions from his or her 
station do not cause interference to other services. Th ham must, 
and will, make any necessary adjustments to the station equipment 
to ensure that it is in compliance with FCC regulations.

In the case of channel 18, or channel 24, the cable company makes 
use of amateur frequencies for these channels. The problem is 
being caused by the amateur's fundamental signal. Any leakage, 
anywhere in the system, can allow that signal to get inside the 
cable. Once this happens, the interference cannot be filtered 
out.

If the leak is in the cable company's wiring, it is their 
responsibility to fix it. Keep in mind that they are not legally 
bound to fix leakage INTO the cable, only leakage OUT of the 
cable, but most operators will take whatever steps are necessary 
to ensure that their customers enjoy top-notch service.

If the channel-18 video carrier can be readily heard on the test 
2-meter receiver, the cable company will probably need to locate 
and repair a leak. If this carrier is nearly inaudible, it may be 
best to try some of the following cures first:

There are several other forms of "leaks" that are not the 
responsibility of the cable company, although they are usually 
willing to help if they can. Some cable ready TVs and VCRs, and 
even some set-top converters, can be affected by the strong 
amateur signal present on the OUTSIDE of the coaxial shield (this 
signal is a common-mode signal). Many a cable operator has spend 
days changing perfectly good wiring when the real problem was 
common-mode interference. They, the ham or the subscriber can put 
a common-mode choke on the incoming cable line. In many cases, 
this will now allow for interference-free viewing.

The house AC wiring may also pick up a fair amount of signal. A 
common-mode choke, sometimes in conjunction with an AC-line 
filter, may help here. In extreme cases, it may be necessary to 
install both types of filters in the TV, the VCR and the set-top 
converter.

If the cable system doesn't leak (as evidenced by your 2-meter 
receiver) and the application of the common-mode filters and ac-
line filters do not effect any improvement, you may be dealing 
with a case of direct pickup by the television set circuitry. 
There is (almost) nothing the cable company or the amateur can do 
in this case. Contact the manufacturer of the TV for assistance.

I say almost, because after the common-mode and ac-line filters 
are installed, it may now be possible to get good reception by 
using a set-top converter or VCR to tune in the desired channel. 
Keep in mind, when you are using an external tuner, the TV will 
not be tuned to the amateur frequency, so it may not be 
susceptible. Most set-top converters and modern VCRs are pretty 
well shielded, so after you solve the common-mode problem, they 
may now function just fine. The result is a happy subscriber, 
amateur and cable repair person.

In summary, all parties concerned are responsible for conducting 
themselves in a courteous and neighborly manner. The amateur is 
responsible for the proper operation of the amateur equipment, 
the cable operator is responsible for leakage within the cable 
wiring, and the manufacturers of the TVs and VCRs are responsible 
for ensuring that their equipment will continue to function 
properly near strong radio transmitters. Remember, if you have a 
hole in your roof, you don't blame the rain when your furniture 
gets wet! 73, 

Ed Hare, KA1CV, Senior ARRL Laboratory Engineer.