October 1953 QST
Table of Contents
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
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?"
Fig. 1 - At A above, we have a graphic illustration of an 30-meter fundamental signal
shown by the dotted line, and to the right the harmonics, gradually getting weaker as the frequency
increases. At B, we see the increase in harmonic strength in the TV band when the fundamental is moved
up to 40 meters, while at C, the low-order harmonics of the 15-meter fundamental are much greater in
amplitude than those from either 30 or 40 meters. In actual practice, the harmonics may not have the
relative strengths shown, but this drawing gives a rough idea of what takes place.
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
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.
Fig. 2 - What happens with two methods of attenuating harmonics. At A, we have just
the transmitter and antenna. When an antenna coupler is used (B), the amplitudes of all harmonics are
sharply reduced with a low-pass filter (C). Harmonics above the filter's cut-off frequency are greatly
attenuated hut those harmonics below the cut-off frequency are not affected.
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
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
Here is a case where a TV set doesn't have enough rejection to keep out a fundamental
short-wave signal. As can be seen, the TV set is rather unhappy about the whole thing.
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
The best method of making sure the receiver won't be troubled by fundamental overloading is to use
a high-pass filter.
Everybody's smiling again; the high-pass filter keeps out the fundamental signal
and hence - no interference.
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