Above all, and this cannot be stressed strongly enough, be polite, courteous and as civil as possible when you are dealing with your neighbors! This same cooperative, friendly attitude should be used when communicating with your local cable company. There are all kinds of "big" problems that can be caused by undiplomatically just telling someone "It's your problem!" And there may be no practical way to cure any specific problem! It may be that the only possible cure is for the local cable company move their premium channels to another frequency. And I have been told by other amateurs that this is exactly what the cable company in their areas have done.
Now for a little theory!
There really are only three ways an unwanted signal can interfere with wanted television signal:
Radiated Interference - This type of interference is usually the fundamental frequency and is usually radiated directly out of your antenna! But this method can also include signals radiated through your shielding, off of your shielding, off of your ground wire or anything else that could directly radiate your signal.
Conducted Interference - This interference is propagated directly via a conductor such as A/C wiring or a common ground system. There are two different types of conducted interference, differential and common mode. I will explain these shortly.
Induced Interference - Induced interference starts out as radiated interference and is picked up or received by some internal or external part of the affected television or its associated wiring. This type of interference is usually common mode.
Now to explain in more detail the distinct differences between differential mode and common mode interference. In any electrical circuit there must be two paths, the forward path and the return path.
In the differential mode signal, it is conducted via a two wire pair, such as your A/C power cord or your antenna lead in. In this case the circuit is created between the two wires in the system. In one wire and out the other! It is called differential because under normal circumstances these two wires are out of phase with each other. This circuit occurs without the necessity of an earth ground.
With the common mode signal, the unwanted signal can either overpower the normal out of phase signal and make the multi-wire appearto be a single hased long wire antenna with the return path being through the internal circuitry (IE: Capacitors) of the television to the earth ground, thus completing the circuit. Or, the signal can ride the outer coaxial shield and find the earth ground through the same internal circuitry to chassis ground.
It is important to make the distinction between the two, because leakage into or from the cable system, a differential-mode phenomenon, is clearly a cable company responsibility, but a common-mode signal conducted only via the coaxial shielding clearly is not.
Sidebar by Ed Hare, KA1CV, Senior ARRL Laboratory Engineer Who is to blame?
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. The 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.
(Continued in Part 4)
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