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October 1953 QST
Television interference (TVI) was a major concern for amateur radio operators back in the heyday of broadcast TV. Other than radio broadcasts, magazines, and newspapers, it was the only other major form of media available; there was no Internet. Even the lowest priced TV sets represented a significant portion of a typical family's disposable income. There was no government handout program that provided every household with a television set and antenna. Consequently, people were very irritated by nearby electrical or electronic equipment that dared to interfere with their reception - and rightly so. Ham radio operators broadcast on bands that were fairly well separate from the TV channel frequencies; however, harmonics and intermodulation products often fell in the TV bands, and that caused real problems with the public perception of amateur radio. Articles like this were aimed at helping people tame the TVI and make peace with their neighbors.
A while back, I wrote about how my radio control transmitter, broadcasting its fundamental at 27.195 MHz, generated 2nd and 3rd harmonics that wreaked havoc with my neighbors' daytime soap operas on VHF channels when I would run my model airplane up and down the street in front of the house.
TVI and the Novice
Understanding the Problem
By Lewis G. McCoy,* W1ICP
In the writer's capacity as an ARRL Technical Assistant it has been one of his jobs to handle correspondence with amateurs having TVI problems. A recent letter was particularly interesting because it voiced a newcomer's fears of TVI. He wrote: "I am interested in becoming a ham operator ... however, I am quite disturbed about all the remarks being made about TVI. I am located in a small town about 60 miles from the one and only TV station we can receive. The TV station is in the process of increasing its power from 17,000 watts to 100,000; do you think this will help?" He went on: "I am completely encircled by TV antennas and also have been warned already by the wife and kids that I must not interfere with their favorite programs. My question is this: Can a beginner in ham radio do all the necessary shielding, trapping, etc., that must be done to prevent TVI?"
Here is a typical case of a newcomer voicing his fears of the nastiest problems in amateur radio today. Actually, there is no need for the beginner to be frightened by TVI. There are plenty of experienced people around to offer help and guidance. Many cities in the country have TVI committees, made up of amateurs, ready to offer their assistance. If you happen to live in an area that has no committee, there are probably amateurs in the local club who have had experience with interference and will be willing to help. In addition, your ARRL Headquarters staff will be glad to help by answering your questions.
The cure for TVI is not as difficult as it may seem, and it certainly doesn't require any great technical skill to whip the problem. In this article we hope to acquaint the newcomer with the subject and try to give him a clear approach to the answer.
What Is TVI?
There are many sources of TVI ... industrial heating equipment, electric devices that have sparking contacts, diathermy, short-wave stations, and several others. Unfortunately, even though the ham causes a very small percentage of the interference, he somehow manages to get blamed for much of it. However, extensive public education on the part of the amateur has gone a long way toward correcting this misunderstanding.
Forms of TVI
When you enter into a discussion about TVI with your neighbor, you must be equipped with the necessary knowledge to know what you're talking about. It makes the job a whole lot easier.
There are two basic forms of interference the amateur is concerned with. First, there is the interference that comes from his station, because of faults in his transmitter; and second, interference due to faulty design of the TV receiver. Obviously, one form of interference is the responsibility of the ham while the other is not. Let's for a moment discuss TVI that is caused by the transmitter.
When an amateur turns on his transmitter he is interested in transmitting his signal on only one frequency - the "fundamental" frequency - and no others. However, it is one of the characteristics of radio transmitters that they generate energy at multiples of the fundamental frequency. These multiple frequencies are called "harmonics," and when radiated are classed as "spurious" - that is, unnecessary and unwanted - emissions. It is these harmonics that can cause TVI when they happen to fall in a TV channel.
To obtain a clear picture of harmonics and their relation to TV, let's for a moment look at Fig. 1. In Fig. 1A, we see the amateur bands d the TV channels represented along a horizontal. line. Rising above the line, at 3700 kc., is a vertical line representing the fundamental signal. At twice the frequency, 7400 kc., we find another vertical line which represents the second harmonic. Each harmonic is shown in this manner as we go higher in frequency. In a rough way, the length of each line represents the amplitude of the harmonic, so it is apparent as we go higher that the harmonics get weaker. (Although we only show harmonics through Channel 6, they will also extend up into the higher channels, 7 through 13. However, we seldom need to concern ourselves with these higher channels because the harmonics become so weak that it is a rare case when harmonics from 80 or 40 meters cause interference.)
Now, looking at Fig. 1B, we see what happens to the harmonics when the transmitter is in the 7-Mc. band. Observe that the harmonics in the TV range are considerably stronger. And .if we look at Fig. 1C, with the transmitter operating in the 21-Mc. band, we find we have really strong harmonics falling in the TV Channels 3 and 6. It becomes very apparent that the lower the "order," or number, of the harmonic, the stronger it will be. In other words, harmonics from an 80-meter signal are less likely to interfere than those from a 40-meter, or 15-meter one. If we can visualize a TV signal in Channel 3 having a strength equal to one-half inch on this scale, we'll see some startling results. On 80 meters, the harmonic in Channel 3 is a high one and is considerably weaker than the TV signal. The same is true of the 40-meter harmonic. However, on 15 meters, the third harmonic now falls in Channel 3 and it will be practically as strong as the TV signal. Harmonics as low as the third and fourth can be tough to handle, even in an area where there is a strong TV signal.
This brings up another point when considering harmonic interference - the strength of the TV signal being received. It is quite possible to have a harmonic in a given TV channel and not cause TVI simply because the TV signal is strong enough to override the harmonic. So it is apparent that the stronger the TV signal the less the problem we have with TVI. Probably the answer is for the ham to move next door to the TV station! Unfortunately, that isn't always feasible, so the next best thing is to try to get the strongest possible TV signal to work with. It can be pointed out to your neighbor that if he has a good antenna system, he'll not only get less interference, he'll also have a much clearer picture to watch. When you talk to him about a better antenna, pass along the information that such things as spark-plug interference also will be a great deal weaker, and in many cases disappear. This is particularly true in weak-signal areas.
We could spend considerable time in this article explaining how to suppress harmonics, but the reader would profit more by studying the chapter on BCI and TVI in The Radio Amateur's Handbook. All phases of interference and its cure are treated in more detail than would be possible in this space. It can be pointed out, however, that usually only a minimum of shielding and filtering in a transmitter is necessary to eliminate harmonic interference from 80- and 40-meter operation. In the case of 15-meter work, good shielding, lead filtering, and a low-pass filter at the transmitter output will be needed to do a good job. An antenna coupler frequently will provide all the harmonic attenuation needed to clean up interference. In some cases, a low-pass filter may be needed in addition.
Fig. 2 illustrates the difference between an antenna coupler and a low-pass filter. Fig. 2A shows a 21-Mc. fundamental and the harmonics that fall in Channels 3 and 6, as they might be if the transmitter were fed directly to the antenna. Fig. 2B shows what happens when an antenna coupler is used between the transmitter and the antenna. You will note that the harmonics are sharply reduced in strength. Aside from TV, harmonics can get you into trouble if they should interfere with any other service, and an antenna coupler reduces the strength of all harmonics, whether they fall in the TV range or not. Now, in Fig. 2C, we show what happens when you employ a low-pass filter between the rig and the antenna. In this case, all the signals lower than the filter's cut-off frequency of 45 Mc. are passed through without attenuation. Those harmonics above the cut-off frequency are sharply attenuated.
Many newcomers hear the term "low-pass filter" and don't know exactly what is meant. A low-pass filter is simply a combination of coils and condensers designed to pass all frequencies below one known as the "cut-off" frequency and to reject all frequencies above it. To make it even clearer, let's assume we have a transmitter operating on 21,100 kc. in the Novice band. The third harmonic of the signal falls at 63,300 kc., smack in Channel 3. With a properly designed low-pass filter installed at the output of the transmitter, our 21,100-kc. signal will be passed to the antenna without attenuation. However, because the filter is designed to have a cut-off frequency at, say, 45,000 kc., the third harmonic will be attenuated.
Some amateurs have the mistaken idea that a low-pass filter is a "cure-all" for TVI. It is true that a filter will help attenuate harmonics, but for the filter to function properly, good shielding and lead filtering are also necessary. Otherwise, some of the harmonic energy may get to the antenna without passing through the filter.
Fundamental Interference - Receiver Overloading
We've discussed interference that is the responsibility of the ham, now let's talk about the other common form of TVI, TV receiver overload, or "fundamental" interference.
When an amateur station and a TV receiver are close to each other - usually less than 100 yards1 - it is possible for the receiver to be interfered with by r.f. from the ham's fundamental signal. Even though the amateur is transmitting a signal on his fundamental frequency and no other, interference is likely to occur if the receiver selectivity is inadequate. The amateur is not responsible for poor receiver design and is not to blame for this type of interference, although it sometimes is a little difficult to convince the TV owner of that fact.
It must be remembered that the average TV owner hasn't the vaguest idea of how a TV receiver works. He knows that he has paid a lot of money for his set and feels that he shouldn't get interference. He also thinks that he only gets interference when the amateur is operating his station. Before getting into any discussions with the TV owner, you must be positive that you are not radiating harmonics and that the interference is the receiver's fault. About the only sure method of determining whether or not your rig is clean is to make a test with your own TV set, if you happen to have one. If not, try to borrow a receiver to make a test.
Assuming you have a TV set, you are now ready to determine what kind of TVI you have, if any. If interference is present only on channels having a direct harmonic relationship with your fundamental signal, then the trouble might be purely harmonic. You would know for sure that interference was fundamental overloading if you had trouble on channels that did not have harmonic relationship, but harmonics from the Novice 40-meter band hit every channel. This is also true of 80 meters. In the case of 15 meters, the only harmonics to interfere would be the 3rd in Channel 3 and the 4th in Channel 6. In such a spot you could look for fundamental overloading on Channels 2, 4, and 5. Observing interference on channels where there is no station won't be of any help as there is no sure-fire method of getting conclusive results.
The best method of making sure the receiver won't be troubled by fundamental overloading is to use a high-pass filter.
We've already discussed what a low-pass filter is and how it works when used on a transmitter. A high-pass filter is designed to pass all TV channels without attenuation while attenuating any frequencies lower than its cut-off frequency, usually around 40 Mc. Thus, a high-pass filter will keep your fundamental signal out of the TV receiver.
For a high-pass filter to work properly, it should be installed as close as possible to the antenna input of the TV receiver. By close, we don't mean on the back of the set. In most TV receivers there is a short length of 300-ohm line that connects the external antenna terminals to the tuner. This length of line is long enough to pick up considerable r.f., so to avoid pick-up of this kind be sure the filter is mounted right at the tuner. Before purchasing a high-pass filter, check to find if the TV receiver you have already has one built in. Many of the late model sets have built-in filters.
If you interfere with a receiver which has a high-pass filter, you'd better look for harmonic trouble in your rig. Experience has it that when interference is present on a receiver with a properly installed filter, the trouble is usually harmonics.
Once you are sure that your rig is clean - and your own TV set is the best witness you have - then it is time to talk to your neighbor about his troubles. It should be pointed out to the TV owner that you're not having trouble with your set, so it might be possible his set is not properly filtered. It should also be pointed out that the amateur is in no way responsible for interference in poorly designed TV sets, and that the TV owner would be well-advised to have a reliable serviceman install a high-pass filter. Experience has proven that it is much better for all concerned for the ham to adopt a "hands off" attitude with respect to the neighbor's set. What the amateur can do is explain to the serviceman exactly why the TV set is being interfered with an how it can be cured.
Most TV receiver manufacturers have a policy of reimbursing the serviceman for the cost of the high-pass filter installation. If there is a TVI committee in your area, the complaint should be referred to the committee. Not only do its members have the necessary "know-how," in addition, the FCC will approve the committee's recommendations for a filter installation to the receiver manufacturer.
Ask an aspiring amateur fifteen years ago what his biggest problem was, and he would probably reply, "Getting a license. After that I will be all set." Ask the same question today and you might hear, "Well, getting a license isn't much of a problem, but the threat of TVI worries me a lot." While one can't disregard the threat of TVI these days, there is no need to let it make your life miserable, as W1ICP points out in this article.
In places where a TVI committee is not functioning, a serviceman may be reluctant to install a filter free of charge because he feels he won't be reimbursed by the manufacturer. In such a case, it may be necessary to persuade the set owner to write and explain the situation to the manufacturer. Another method of getting results would be for the amateur involved to write to his regional FCC office. The FCC should be informed of all the steps the amateur has taken and it should be pointed out that the interference is no doubt due to fundamental overload.
As we said earlier, the BCI-TVI chapter in The Radio Amateur's Handbook should be carefully studied. Also, ARRL Headquarters has many printed helps on TVI that are available free of charge. These helps include such items as sample letters to the TV owner showing him his responsibility, samples of newspaper publicity, information on forming TVI committees and several other pieces of printed matter designed to help the ham.
In addition, Phil Rand, W1DBM has published an excellent book on TVI.2 Spend a few evenings reading this material. You'll be well equipped to handle the problem of TVI when you encounter it.
* Technical Assistant, QST.
1 It is possible to have this type of interference at longer distances but usually only on receivers manufactured prior to 1949.
2 Send request and 25 cents in coin to cover cost of mailing to: Miss Anne Smith, Remington Rand, Inc., 315 Fourth Ave., New York 10, N. Y.
Posted October 10, 2012