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Tracking down the source
of TVI (television interference) or any kind of RFI (radio frequency interference)
can be a challenge. In the days when most people received their audio and visual
entertainment (radio and TV) via over-the-air broadcasts*, any form of interference
was usually picked up by multiple people, who promptly reported the issue to
the power company or the FCC. Arcing power lines, transformers, and switches
were prime generators of RFI, particularly during wet and high wind conditions.
However, much more prevalent was interference from industrial and domestic equipment.
Neon (see
The Neon Interference Problem) and fluorescent light, heating pads
(see
Hazel: Stop Rockin' Our Reception episode), ignition systems (automotive
and oil burners), diathermy (heat therapy popular in the day), industrial RF
heating, kitchen appliances, and tool motors were among the likeliest offenders.
Of course amateur radio operators were guilty of spewing out interfering EM
crap, but contrary to popular belief, they were (are) one of the least offenders
of all groups. RF Cafe visitor Denny Condron (K0LGI) provided a short article
entitled, "Tracking Down
a Mystery Signal," describing his effort to locate a particularly annoying
interference instance with a local repeater station.
* If you ask what other kind of entertainment was there?, the answer is records
and magnetic tapes (maybe a few wire and wax cylinders, too, in 1954). See also
Interference Analysis (Radio-Craft),
Mac's Service Shop: Radio Interference,
Canada Puts Limit on R.F. Interference (Electronics World),
Television Interference (TVI)
and the Novice (QST).
Mac's Radio Service Shop: The TVI Sleuth
 By John T. Frye
Barney considered returning late from lunch his own peculiar prerogative;
therefore he was considerably annoyed when Mac, his employer, did not show up
until the youth had been back at the service bench a full hour.
"Have something especially good for dessert?" he inquired sarcastically as
the older man entered the service department.
"Nope," Mac replied with a teasing grin. "I simply ran into Joe Smith, radio
communications man for a public service company, in the restaurant, and he wanted
to show me the new mobile unit he has just built up for running down radio and
television interference charged to their lines. I've been riding around with
him while he demonstrated it."
"What's it like?" Barney capitulated, deciding to give up being mad in order
to satisfy his curiosity.
"The whole installation is housed in a metal-body station wagon that has
had ignition noise reduced to a minimum by the use of resistor-type spark plugs,
suppressors, heavy bonding to the frame, and a reduced gap in the distributor.
A heavy-duty 800 watt alternator is driven off the engine. The ten-volt a.c.
output of this is rectified and used to charge the battery. The same output
is stepped up by a transformer to 117 volts for powering the receiving equipment.
In addition, for operating the equipment when the car's engine is turned off,
there is a motor generator that puts out 250 watts at 117 volts a.c. Both of
these power sources are voltage regulated.
"What kind of noise-detecting equipment does he use?"
"First, there's a broadcast receiver with the a.v.c. disabled and with a
meter on the output. Next is a communications receiver that tunes from 540 kilocycles
to 109 megacycles and can receive both AM and FM signals. Then there are two
television receivers with signal strength meters connected to their video detectors.
Finally there is a calibrated field strength meter with inputs for either twin-lead
or coaxial line. In addition, of course, any special service receiver, such
as police, taxicab, airways, etc., can be set in for use if needed. He also
has two-way communication with company radio stations."
"What antennas does he use on the receivers?"
"A whip is used for general coverage reception and for picking up strong
TV interference. There is a rotating mounting on the rear of the car on which
he can place any of his single-channel yagi antennas for determining the direction
from which noise is coming on a particular channel."
"Is that all Joe does, ride around and listen for noise?"
"Not by a long shot. As radio communications man, he has to take care of
the company's microwave, mobile, carrier telephone, carrier relay, and carrier
telemetering equipment. In addition, however, he is on call by any district
in which the company operates if the district people are not able to clear up
a case of interference by themselves. Since Joe has a full ten years of experience
at this interference-sleuthing business, he is darned good at it and gets all
the really tough cases. He also runs a complete noise check on any new lines
or other equipment as soon as it is put into service. Then he makes periodic
noise surveys over the transmission lines and substations in this half of the
state. All the time he is driving around - and he drives about 4000 miles a
month - the noise-detecting equipment in that station wagon is running; and
if he notices anything unusual, he reports it, together with the pole number
where it seems to peak, to the local office of the company."
"He must be a busy cuss," Barney remarked. "How does the company receive
interference complaints?"
"A few come through the Public Service Commission. More of them are turned
in to the various district offices of the company. Quite often the first report
of noise comes from company employees using the carrier telephone for talking
from one plant to another over the transmission lines. Since the receivers pick
the signal right off the wire, many unusual noisy conditions are immediately
noticed in these receivers."
"Is power line noise all he runs down?"
"I should say not!" Mac chuckled.
"While it is true that he only goes out on a complaint when someone thinks
a power line is causing the trouble, he finds the line guilty only one time
in ten. That checks closely with a national survey that placed line noise ninth
on the list of interference producers. The other nine times Joe finds the interference
coming from an oscillating booster, TV set oscillator, neon sign, thermostat,
heating pad, oil burner, ignition system, old-fashioned lamp bulb, diathermy,
food mixer, blower motor, germicidal ozone or fluorescent lamp, light bulb loose
in its socket, defective switch, or what have you."
"Does he always pinpoint the real cause of trouble?"
"No. When he is sure the company lines are not at fault, he usually stops
right there. There are good reasons for not identifying sources of interference
outside the company property. In the first place, many people owning interference-producing
equipment do not take kindly to having this fact spotlighted by the power company.
To do so is to run the risk of gaining ill will with a service primarily intended
to promote and maintain good will. Secondly, if the power company cleared up
every case of interference, no matter what the cause, it would soon become a
clearing house for all complaints. It could not possibly take care of all the
complaints that would then pour in, nor is there any valid reason why it should
try. Quite often. however, it is necessary to find out what is causing the interference
to prove the lines are not.
"A good example of that," Mac went on, "is contained in an incident that
happened one Sunday morning a few weeks ago. Before noon on that day the power
company logged between three and four hundred complaints of interference on
channel 6. The district manager called Joe and asked him to see if he could
find the trouble. One glance at the TV set in the station wagon was all that
was needed to know the interference was not coming from a power line, for it
displayed typical r.f. cross-hatching. Nevertheless, since three-hundred-odd
persons believed it might be line noise, Joe set out to prove it was not. This
was the routine.
"First he looked at a city map and discovered that the outline of the area
from which complaints were coming strongly resembled the response pattern of
a yagi antenna pointing north with the intersection of the major and minor lobes
on the south side of town. Next he took a couple of bearings on the interference
with his channel 6 yagi and noted these lines also intersected on the south
side. Connecting the signal strength meter to the whip antenna, he drove to
that vicinity. By driving back and forth in a criss-crossing pattern while watching
the meter, he soon spotted a house that gave a maximum signal of 500 microvolts
directly in front of it. No one was home at the house; so Joe briefly opened
the line switch at the meter. The interference promptly disappeared.
"This information was relayed to the police, and they managed to find the
owner of the house and summon him home. Joe went inside with him and found his
TV set turned off, but a one-tube booster on top of the set had been left on.
When the TV set was turned on, the booster stopped oscillating; but it promptly
started again when the set was turned off. That booster was putting out twenty
microvolts of signal strength two miles away!"
"That's one for the book!" Barney exclaimed.
"Here's another Joe calls The Case of the Methodical Maid," Mac said. "This
case of interference looked like diathermy and came on promptly at eight o'clock
each evening and stopped just as promptly at nine. The chief complainant was
an elderly doctor. The best Joe could do was pin the interference source down
to the single block in which the doctor lived. Signal strength was about the
same anywhere inside the block, and the yagi antenna would not give a conclusive
directional reading. One evening Joe was parked in the alley of that block,
elbows stuck through the spokes of the steering wheel, chin resting on the palms
of his hands, staring, vacantly up into space while the TV set behind him gave
out with the characteristic rasping sound of the interference. He had just decided
the only thing left to do was to call out a crew of men and cut off the drops
to the houses one at a time, when a little rectangle of light in his line of
vision winked out. At the same instant the noise stopped, and a glance at the
TV screen revealed the broad band of interference was gone.
"In a matter of seconds Joe was ringing the doorbell of the house in which
the light had been turned off - the house of the doctor, no less! It was quickly
revealed the window from which the light had disappeared was in the maid's room
on the third floor. Subsequent investigation revealed that her room was lighted
by an ancient tungsten-filament bulb that was the best little interference transmitter
you ever saw.
"The thing that threw Joe was the precise timing of the interference. He
had been thinking all the time of some mechanical device that was turned on
and off with a time clock, or something like that. Talking with the neighbors
revealed that the doctor always ate dinner at exactly the same time. The maid
cleared off the dishes and retired to her room at exactly eight o'clock. Then
she read for precisely an hour before turning off the light and going to sleep."
"Hey, this is more fun than ghost stories," Barney exclaimed. "Tell me another!"
"Okay," Mac agreed, "but it'll have to be a short-short. We've got to get
to work. One day a report came in of interference along a brand new 230,000
volt line that had just been erected. Incidentally, Joe tells me the higher
the voltage of the transmission line, the less noise you have. It is a lot harder
to keep 12,000 and 33,000 volt lines quiet than it is 60,000, 132,000, and 230,000
volt jobs. Anyway, he did not believe the complaint could be justified for he
had driven the full length of the line when it was first activated, and there
was nary a sound. In fact, he had parked directly under this line and pulled
in a TV station forty miles away with just his truck-mounted yagi, and there
was not the first trace of interference.
"When he checked the line this time, however, he found there was bad interference
peaked on channel 6. Using his field strength meter, he was able to drive right
to the cause of the noise: a piece of bailing wire tossed over one of the wires.
When that wire was removed and measured, it was found to be exactly one-half
wavelength long on channel 6."
"Is it unusual for line interference to be peaked on one TV channel?"
"That's right. Power line interference is of the random noise type and normally
covers a broad band of frequencies. Occasionally this band will extend from
the broadcast band through the TV channels. In a very few cases it will peak
up on the higher frequencies and not be heard on the broadcast band. In the
great majority of instances, however, it will be quite strong on frequencies
up to about three or four megacycles but cannot be detected at all above thirty
megacycles.
"Joe made several other points," Mac said as he ticked them off on his fingers.
"For one thing, he wished people could learn interference that shows up on just
one channel is practically never from a power line. Neither is interference
that produces cross-hatching of the picture or a reversal of the black and white
portions. That comes from some r.f. producing gadget, such as an oscillating
booster. Secondly, he wants people to know power companies are sincerely glad
to know of any unusual noise along their lines, for such noise is often a warning
something is going wrong. If they find out what it is promptly, this may save
them thousands of dollars in equipment. People who believe power companies do
not seriously hunt noise sources are just not being realistic.
"Third, Joe says no transmission line can be made absolutely noise free any
more than a gasoline motor can be so completely muffled it cannot be heard running
in a quiet room. A good line will not cause any interference to a TV set operating
within a few yards of its poles if that set has any kind of a normal signal;
but when you try to receive a TV signal of five or ten microvolts right under
the line, you will probably see, mixed in with the thick snowflakes produced
by the set's own noise, a few sparkles of light that represent noise from the
line. Power companies are engaged in constant research and rebuilding to make
their lines more and more nearly noise free, but they do not hope to achieve
100% success in this effort.
"Finally, it must be understood that while power companies are doing everything
they can to kill their own snakes, they simply cannot use their special noise-locating
equipment to run down general complaints of interference not connected with
their systems. That is a job for service organizations, TV viewer groups, radio
clubs, city governments, or whoever else is willing and able to take on a chore
that can be thankless and frustrating, but never dull - at least that's what
Joe says!"
Posted March 15, 2021
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
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