March 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
When I read this installment
of Mac's Service Shop, in Popular Electronics magazine, the first thing that
came to mind was my own experience with television interference (TVI) when I was a kid.
In that case, the transmitter of my radio control system for a model airplane was the
culprit. The frequencies and channels are almost exactly the same as reported in this
infodrama. In the 1970s, citizen band (CB) radios operated in the 27 MHz realm,
as did my R/C transmitter. During summer vacation from junior high and high school, I
would run my model airplanes up down the street in front of my house, getting up just
enough speed to lift off and then immediately chopping the throttle and landing. As soon
as the transmitter was switched on, nearby housewives would lean out their doors and
yell at me for screwing up "As the World Turns" or "All My Children." They were not particularly
happy with noise from the engine, either. I eventually adjusted my operation times to
avoid the afternoon soap opera prime time.
Here is a web page showing the
OS Digitron DP-3, 3-channel R/C system.
Mac's Service Shop: TVI from the Victim's Viewpoint
By John T. Frye, W9EGV, KHD4167
Mac could hear Matilda's voice, shrill with anger,
even before he opened the door.
"I tell you, Barney," she was saying to the Number Two Technician of Mac's Service
Shop, "I'm not going to have my lovely Englebert Humperdinck program ruined by any blabbermouth
ham operating off his frequency."
"Now don't lose your cool, Matilda," Barney said soothingly. "How do you know it was
an amateur interfering with your program? Are you sure it wasn't a CB station? Did you
hear any call letters?"
"I certainly did," she answered, fishing a slip of paper from her purse, "and I wrote
them down. They were KHD4167, and he was talking to a KLK something or other."
"It was a CB station," Barney stated triumphantly. "CB stations have three letters
followed by four numerals. U.S. amateur stations have one or two letters followed by
a numeral and then two or three more letters."
"What's all this about?" Mac asked, shrugging off his overcoat. "Has someone dared
clobber our Girl Friday's dreamboat program?"
But she refused to be teased into a good mood. "I'm going to do something about this,"
she warned. "I can't expect Barney to help me because he is a ham, but I thought I could
depend on you, Mac."
"Now just a cotton-picking minute!" Barney exploded. "Just because I'm a ham doesn't
mean I go along with interference to radio or TV reception that is the fault of the transmitting
station. I want to help you, too, but you've got to quit sizzling. Did you talk to the
CB operator? He probably has no idea he is interfering with your reception."
"That's right, Matilda," Mac chimed in.
"Barney and I are on your side, but we need more facts. Come to think of it, Barney,
this is probably a good time to continue that review of interference problems we started
a couple of months ago. You'll remember then we talked about interference to radios,
PA systems, electronic organs, etc. Suppose now we talk about television interference,
or TVI, while Matilda listens for clues that may help with her particular problem."
"I'm all ears." Barney agreed.
Television Interference. "The basic difference between radio interference
and TV interference lies in the receiving frequencies involved," Mac began, lighting
his pipe. "The radio broadcast band lies below the frequencies on which amateur and CB
stations operate, while the TV channels lie above the frequencies of most amateur and
CB stations. Also most radio receivers have an i-f frequency of about 455 kHz, while
TV i-f frequencies are likely to be near 41 MHz, with older sets having i-f's from 21
to 27 MHz."
"I think I get it," Barney said slowly. "Radio interference almost always results
from a lack of selectivity in the receiver or its propensity to respond to spurious signals,
such as true images or harmonic images. But TV interference often comes from a transmitter
putting out appreciable amounts of power in the form of harmonics, or multiples of the
fundamental frequency, that fall in a TV channel. In such a case, nothing done at the
receiver will help. The harmonies must be attenuated at the transmitter itself."
"That's right. While the fundamental frequency of a ham station at 21 MHz might possibly
get into the i-f of an old TV set because it was not properly shielded, the third harmonic
of such a signal will fall in Channel 3, 60-66 MHz, and the fourth harmonic in Channel
6. 82-88 MHz. A 28-MHz ham station may have a second harmonic in Channel 2, 54-60 MHz,
and a third in Channel 6. A CB station operating in the 27-MHz band can have a second
harmonic in Channel 2 and a third in Channel 5, 76-82 MHz. These harmonics constitute
actual signals appearing in the TV spectrum, and there's no way a receiver can differentiate
between them and a duly transmitted TV signal. Other higher order harmonics fall in the
upper vhf, channels 7-13, but ordinarily they are too weak to cause trouble except to
nearby receivers picking up weak telecast signals."
"Does an unwanted signal cause the same amount of interference, no matter where it
falls in a 6-MHz TV channel?"
"No. The worst interference results when the interfering signal falls near the picture
carrier, 1.25 MHz from the low edge of the channel; the color subcarrier, 4.83 MHz from
the low end; or the sound carrier, 0.25 MHz from the high end. In the first case, interference
blacks out the picture entirely, makes a negative of it by reversing the light and dark
areas, or produces bars and cross-hatching in the picture through beats between the harmonic
signal and the picture carrier. In the second case, there's breakup of the color. A harmonic
falling near the audio carrier interferes with the telecast sound. The degree of interference
usually depends on how near the interfering signal is to one of these sensitive frequencies."
"Then TV interference is almost always the fault of the transmitting equipment!" Matilda
"Whoa now! I didn't say that. I said such a possibility exists with TVI but it is
not present with radio interference. A 'clean' transmitted signal with spurious radiations
attenuated far below FCC requirements can still cause interference in a TV receiver by
overloading stages in the front end, by cross-modulation, or by means of harmonics generated
outside the transmitter by some non-linear device. In all these cases; the fault is not
in the transmitter.
"Front-end overload is very common when the TV receiving antenna is quite near the
transmitting antenna or in the beam of the latter. When an r-f stage is overloaded, it
operates nonlinearly and generates harmonics that are difficult to differentiate from
transmitted harmonics. However, a properly installed high-pass filter at the TV tuner
will greatly attenuate all signals below 54 MHz and usually cure front-end overload,
but it will not affect transmitted harmonics. On the other hand, a low-pass filter installed
on the transmitter output will attenuate harmonics that affect TV reception. So will
proper bypassing, shielding, r-f choking, etc., but we can't go into measures taken to
prevent the radiation of harmonics by ham or CB transmitters. There are books devoted
to that subject."
"Don't forget a strong nearby signal can get into the audio amplifier of a TV set
just as it does with a radio, PA amplifier, or any other audio amplifying device," Barney
suggested. "A high-pass filter mayor may not help this. That business of an external
non-linear device producing harmonics is a rough one. The device can be any oxidized
joint between two pieces of metal, such as downspouting, electrical wiring shielding,
a metal clothes line, joints in a metal tower, a bad lightning arrestor, or what have
you. Often the condition is intermittent, to add to the confusion. The Radio Amateur's
Handbook published by the ARRL is probably the best down-to-earth source of information
on the cause and cure of all kinds of TVI."
"All very interesting - I think - but it doesn't solve my problem," Matilda said impatiently.
"Was the interference only with the audio or was the picture messed up, too?" Mac
asked. "Did you try other channels? If so, were they OK?"
"The picture was not disturbed, only Humperdinck's lovely singing. Other channels
were fine, but only Channel 5 carried Humperdinck."
Mac did a little figuring and then made a telephone call. "Now we're getting somewhere,"
he announced as he returned to the desk. "CB Channel 23 on 27.255 MHz has a third harmonic
on 81.765 MHz, only 15 kHz away from the sound carrier frequency of Channel 5. A CB friend
I know tells me the call letters you heard belong to a man living directly across the
alley behind you. Here is his name. The local CB club has a TVI committee, as does the
local ham club, and they will call you shortly and set up a time when they can run some
tests and see where the fault lies: in his transmitter, your receiver, or neither. Then
they will do what they can to correct it."
"What if his transmitter is at fault but he'll not do anything about it?"
A Last Resort. "In that unlikely event you can, as a last resort,
send a complaint to the FCC in Washington, D. C. giving the name, address, and call letters
of the person causing the described interference. If he is not licensed, they will send
letters to him and to you. He will be given your name and address and the nature of your
complaint and will be requested to contact you for the running of tests. He will be requested
then to make a complete report, covering items as his transmitting equipment and your
receiving equipment, the tests he ran and the results, the measures he took to correct
the condition and the success he had, your attitude, and the distance you live from the
TV station being interfered with.
"If you are the only person in your neighborhood experiencing interference or if you
are trying to receive a TV station beyond normal good-reception distance, the FCC probably
will consider your complaint unreasonable and do nothing about it. On the other hand,
if the transmitting station is causing widespread interference to the reception of good
signals, the owner may be mandated to observe silence during prime viewing time until
the condition is corrected Note, however, that lodging a complaint does not mean the
offending station is going to be automatically 'taken off the air.'"
"It might in one case," Barney offered. "If that station is operating illegally -
is not properly licensed, has an antenna higher than the legal limit, or is running more
power than permitted - the operator may be subject to a stiff fine. If he uses profane
or obscene language over the air, this is a federal offense and the fine may be as much
as $10,000, accompanied by imprisonment. A station operating illegally certainly does
not want the FCC monitoring truck parked in his alley."
"It's' far better to settle the complaint at the local level," Mac pointed out. "A
complaint sent the FCC usually means neighbors become enemies; yet they still must cooperate
to clear up the trouble."
"One more thing," Barney interrupted. "Yes?" Matilda questioned, her pencil poised.
"Don't call CB operators 'hams.' They are two entirely different kinds of cats!"
Posted December 13, 2017
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of none other than John T.
Frye, creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine
(which itself started as simply Radio News), and then changed its name to Mac's
Service Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his
eager, if not somewhat naive, technician assistant. 'Lessons' are taught in story format
with dialogs between Mac and Barney.