Interference from Left Field
March 1970 Popular Electronics Article
1970, the airwaves were really getting crowded. Lots of high power
commercial and military gear was online, and the radio listening
public was setting new record highs every year. As such, many new
sources for radio interference were being discovered, and sometimes
the problems caused went well beyond just a little noise being superimposed
on top of Neil Diamond's newly released
Cracklin' Rosie or the lads from Liverpool's
The Long and Winding Road. Often, the interference was
overwhelmingly annoying. The FCC was being flooded with complaints.
Digital computers were creating a whole new type of electronic havoc,
and leaky cable television cables and amplifiers caused all kinds
of headaches to over-the-air sets. Rusty bolts and chain link fences
in the vicinity of high power radio and TV towers - and even radar
installations - manifested themselves as detectors by virtue of
their nonlinear nature. I remember when people at Robins AFB, in
Georgia, would sometimes complain to our radar shop because their
radios would blip once every four seconds as the search radar antenna
swept past their radios. There was also the time while working as
an electrician in my pre-USAF days that I remember picking up a
nearby AM station on my audible continuity tester while ohming out
some cables in a roof-top air conditioner unit on commercial building
I was wiring.
March 1970 Popular Electronics
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Interference from Left Field
by Edward Arnold
In a south Florida town recently, a newcomer to the community complained
that all she got on the "local" TV channels was interference; yet
she received distant stations - more than 100 miles away - without
Who would have suspected that a computer could act as a
Upon investigation, it was determined that a distribution
amplifier for a local cable-TV (CATV) system was leaking signals
right into the young lady's antenna. On the local channels, she
received TV direct from the stations mixed with the signals from
the amplifier. For distant stations on other channels, she received
only the signals from the CATV amplifier and no local interference.
In this day of super-power transmitters and super-sensitive
receivers, incidents such as this are becoming too common, unfortunately,
to even be reported. But what is worse is that now we're getting
interference in the oddest and goofiest ways-you might call it interference
from left field.
The problem is simply that devices that
weren't intended to be transmitters are transmitting and devices
that weren't intended to be receivers are receiving all sorts of
Even Computers Are Culprits.
Who would ever have suspected, for instance, that a computer
could act as a receiver? Yet one radar installation kept having
its core memory erased for no apparent reason until someone noticed
that a radar antenna nearby was swinging its beam toward the computer
just when the cores were erased.
And if a computer can act
as a receiver, who's to say that it can't transmit also?
At one telemetry station a while back, the operators kept getting
garbled outputs until radio frequency interference (RFI) specialists
determined that computer equipment at the station was radiating
a signal that was being received by the station's huge telemetry
In this computer-happy world, we probably are in for
a lot of such computer-generated interference; the square-wave signals
in digital circuits have a wide range of harmonics.
On the home side, of course, digital circuits are uncommon, but
new devices like cable TV have their share of interference problems.
CATV firms have found (the hard way) that 300-ohm twin-lead in a
customer's house can radiate the CATV signals so they now require
use of coaxial cable exclusively.
Site radars would be turned on only to direct missiles toward
That Old Bugaboo
Whether on the home front, in industry,
or in the military, we still get interference from an old enemy:
corrosion in metal joints. The corrosion sometimes creates a non-linear
resistor which acts as a mixer of electromagnetic signals. When
the signals are very strong, the joint may even re-radiate the mixed
Typical of such occurrences is the case of the
radio that would suddenly receive several stations at the same time
when anyone walked across the floor. It was found that metal heating
ducts in the floor had poor joints which acted as mixers and re-radiators
when sufficient pressure was applied from above.
same thing happened to a Chicago engineer back in the days of trolley
cars. As he was waiting in his car at a traffic light, a trolley
car pulled up beside him; whereupon his car radio was overcome with
cross-modulation. His theory was that the poor connection between
the trolley wheels and the ground rail at that intersection created
a nonlinear resistance.
While such events are normally humorous
and of no particular consequence, when they happen on board some
of the U.S. Navy's ships loaded with tons of high-power transmitters
and very sensitive receivers, they can be almost as dangerous as
they are exasperating.
corrosion naturally is an everyday problem on ships, but if you
have dozens of different antennas in a limited, crowded space, salt
corrosion can create almost unbearable RFI. Although the re-radiated
signals from the nonlinear joints or contacts may be too weak to
be transmitted more than a few feet, on a ship it may take only
a few feet to get into somebody else's receiver.
ships, the abundant metal surfaces, chains, and doors sometimes
are of a size or length that is related to the wavelength of high-power
transmitters on board. More than one sailor has started to open
a hatch only to find that the door is hot with RFI.
kind of RFI problem for the military is predicted when the Nike
X anti-ballistic missile weapon system goes into use, according
to a story in "Electronic News" (March 25, 1968). The site radars
for this ABM system threaten to knock out virtually all other radio
communications in the vicinity. Hopefully most of us would not complain
about this RFI since the site radars would be turned on only to
direct missiles toward incoming warheads. Surely the most impatient
TV viewers can tolerate TVI for such a good cause.
RFI problems such as this, the Department of Defense has set up
an Electromagnetic Compatibility Analysis Center. Its job is to
tell the military specifically what types of interference they can
expect if they change locations or shift frequencies.
Enter the Forest Ranger.
The military will have to discuss electronics with, of all
people, the U.S. Forestry Service.
If a military
organization decides to run from its RFI problems and place its
transmitter on a high mountain top on government land, it will have
to discuss electronics with, of all people, the U.S. Forestry Service,
probably the last group in the world you would expect to have an
The reason for their involvement,
however, is quite simple: government land on mountains is often
administered by the Service. Mountains obviously are just the place
for line-of-sight microwave transmitters, etc. Put all those transmitters
on a mountain top and you have a possible RFI problem!
should we say an "EMC" problem? In keeping with the times, some
engineers are now referring to RFI problems as electromagnetic compatibility
(EMC) problems to give recognition to the fact that not all emanations
are at radio frequencies but can occur throughout most of the spectrum.
Regardless of the name, EMC problems do not often have easy
(inexpensive, that is) solutions. To cure these problems requires
proper circuit design as well as bonding, grounding, and shielding
which can be expensive and often create their own problems. Bonding
straps, for instance, may be resonant at certain frequencies and
create their own RFI.
We are bound to have problems with
electromagnetic smog as long as, in the words (in "IEEE Transactions
on EMC," October 1964) of Rexford Daniels, president of Interference
Consultants, "we try to fit a microvolt civilization into a millivolt
January 31, 2014