July 1960 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Interference takes on many
forms and can be caused by nearly an infinite number of sources. In 1960 when this
article appeared in Radio-Electronics magazine, most manmade interference (QRM)
originated in electrical machinery, faulty contacts in switches and lighting, improperly
shielded and/or filtered RF generators, and arcing transmission lines. Natural interference
(QRN) has not fundamentally changed since the same process continue to occur - lightning,
meteoric ionization of the atmosphere, solar particles, etc. It was nearly all caused
by analog processes. Nowadays, ambient noise both conducted and induced is as likely
- or even more likely - to come from digital circuits. The base level (noise floor)
of electromagnetic (EM) signals in a typical urban or suburban environment is significantly
higher than it would have been in 1960. Fortunately, the same group of engineers
and scientists that generated the condition have also figured out ways to mitigate
the issues - for the most part. Some forms of increased EM interference will never
be able to be compensated, like light pollution that degrades the quality of astronomical
observation. It extends outside the visible spectrum into infrared and ultraviolet.
In fact the situation is so bad that projects are underway to build giant telescopes
on the far side of the moon in order to shield detectors from Earth-sourced interference.
Interference - Causes, Remedies and Location
Fig. 1 - Waveform of a: ideal and b: practical switch operation.
Problems with TV sets and radios can generally be solved by finding out what
causes the interference as the first step in eliminating it
By Forrest H. Frantz, Sr.
Electrical interference is annoying to the broadcast listener, exasperating for
the shortwave listener and frustrating to hams.
Natural electrical interference cannot be controlled; man-made interference can.
The effects can often be eliminated at the receiver, but in some cases interference
must be eliminated at the source.
To cope effectively with electrical interference, it is important to understand
how it's generated and transmitted. When the type is recognized, the source can
be found and eliminated.
The causes of man-made interference fall into four general categories:
3. Radio-frequency leakage
4. Harmonic generation
Switching noise occurs at electrical switches and commutators. Typical offenders
are: light switches; thermostats on electric irons, heating pads and home heating
systems; switches and flashers on advertising signs; commutators on electric motors,
industrial equipment and home appliances, and poor electrical connections.
Fig. 2 - Current in a glow discharge tube goes through several
high-frequency oscillations each cycle.
Fig. 3 - High-voltage corona discharge nor-madly occurs during
Fig. 4 - Line filters for a: ac and ac-dc and b: ac only receivers.
Fig. 5 - Shielded and twisted-pair ins reduce noise.
Fig. 6 - a: parallel-resonant wavetrap in series with antenna,
b: series-resonant trap across antenna-ground terminals.
Fig. 7 - Series-resonant wave trap may be connected directly
across transmission line.
Fig. 8 a: R-C combination suppresses arcing, b: rf chokes can
be used on device where timing is important.
Fig. 9 - Resistors damp oscillation in neon tubes.
This interference is caused by arcing when an electrical circuit is opened or
closed. Fig. 1-a shows the idealized waveform for switch operation, with its characteristic
steep rise and fall. Fig. 1-b shows a practical waveform. The fall time is increased
and current decay is irregular. The arc radiates a complex signal which contains
An intermittent break in a poor electrical connection can be extremely harassing
because arcs are continuously struck and extinguished in a haphazard way. Arcing
is a type of electrical discharge that occurs at relatively high currents (generally
100 ma or more).
Glow discharges such as those associated with neon and fluorescent lamps occur
at intermediate currents (generally a few microamperes to several milliamperes).
Glow discharge tubes are ordinarily ac-operated. The ac voltage acts as a switch
for the discharge. The resulting interference looks like the waveforms of Fig. 2.
The voltage increases from zero to the glow striking voltage. The tube fires and
the current drops in an oscillatory manner till the voltage reaches the extinguishing
value. Again a series of harmonic-rich oscillations occur as the current drops to
zero. The process is repeated during the negative half-cycle. A total of four oscillatory
discharges occurs during each voltage cycle, or 240 times a second with a 60-cycle
Corona discharges usually occur at extremely high voltages and are characterized
by extremely low currents. Interference picked up on automobile radios while passing
near high-voltage lines is due to corona discharge. This type of discharge occurs
during the voltage peaks of the ac cycle. Fig. 3 shows the waveform. This interference
may contain frequencies from the audio range up to several hundred megacycles.
Radio-frequency (rf) leakage interference is generally caused by inadequate shielding
of rf generators such as induction heaters, diathermy machines and high-voltage
power supplies which employ rf oscillators. Occasionally, it's an interfering radio
station. Interference of this type differs from the others-it is tunable.
Harmonic interference is caused by generators of nonsinusoidal waveforms. Sawtooth
and square waves contain many harmonics of the fundamental frequency. Thus, a 10-kc
sawtooth signal might contain troublesome harmonics at frequencies as high as a
Remedies at the Receiver
A receiver picks up signals with power levels of considerably less than a billionth
of a watt. Although interference may reach a receiver by conduction through the
power lines, most interference is picked up by the receiver antenna. Interference
conducted to the receiver through the power lines may be radiated within the set
and picked up by the receiver input circuit.
Conduction of interference to the set via the power lines may generally be eliminated
by connecting a capacitor of 0.01 to 0.05 μf across the ac lines for ac-dc receivers
(Fig. 4-a). The arrangement in Fig. 4-b, with a capacitor from each side of the
line to ground, may be employed for ac receivers. A 0.05-μf capacitor has a reactance
of 3.18 ohms at 1 megacycle and 0.318 ohm at 10 mc. This is a near short circuit
to rf signals.
Interference introduced via the antenna is another matter. If the receiver uses
a loop antenna, it may be possible to take advantage of the directional characteristics
of the set. The other avenue open is to locate the noise and eliminate or suppress
it at the source.
The external antenna in this age of high-sensitivity receivers is frequently
a piece of wire strung inconspicuously around the room. If the wire is close to
a household power line or within several feet of a fluorescent lamp (and it often
is), interference may be reduced by relocating the antenna wire or resorting to
an outside antenna with a shielded or twisted-pair lead-in. The shield or unused
wire in the case of a twisted pair should be connected to the receiver ground (Fig.
An outside antenna can become a source of noise in itself if the lead-in is not
soldered to the antenna proper or if the lead-in is permitted to touch rain gutters
or other metal objects.
Tunable rf signals may be reduced with a wavetrap, Two types of wave-traps and
their, connection modes are shown in Fig. 6. The inductance-capacitance combination
is· the same as you'd use in a receiver to tune to the interfering station. Set
the wavetrap capacitor for minimum signal pickup. The trap of Fig. 6-a inserts a
large impedance between antenna and receiver at the interference frequency. The
trap of Fig. 6-b places a low impedance across the antenna and ground terminals
at the interference frequency.
In a television receiver, interference signals below 50 mc may feed through the
front end of the receiver and be amplified by the broad band if stages. High-pass
filters which attenuate signals up to about 50 mc are available commercially. This
type of filter is connected between the lead-in and the TV set's antenna terminals.
The filter must be chosen to match the impedance of the transmission line-300 ohms
for ribbon, 72 ohms for coax.
If TV interference is tunable, a wavetrap may be employed. A single series-resonant
trap connected across the transmission line terminals as shown in Fig. 7 is preferable.
(This arrangement will also work on FM receivers.) Symptoms of interference that
a filter or a trap might cure are noise streaks, sound bars and herringbone patterns
on the picture. TV picture pulling and FM receiver cross-talk between adjacent stations
can often be eliminated with a wavetrap tuned to suppress the stronger station.
Locating the Noise
If the suggested remedies applied at the receiver do not eliminate interference,
you might as well get out your deerstalker and turn into a noise sleuth.
If the noise is the result of non-sinusoidal signal generation, the source is
usually a piece of electronic equipment. Television receivers are primary suspects.
Find out if your own TV set is causing your interference by observing if the noise
exists with the set on and disappears with it off. Discharge interference will be
apparent at relatively high frequencies. Sources of thermostat arc noises and appliance
interference created in your home are usually obvious, since you know when these
devices are being operated.
Assuming nothing this obvious, turn on a battery-operated portable radio and
tune it off station. Pull the main fuse or switch on your house wiring panel. If
the interference disappears or drops significantly, the noise source is probably
connected to. the electrical circuit in your home. Now you can turn the main switch
on and turn the other circuits off one at a time till the interference stops. From
this point on, it's a matter of locating the offending appliance, lamp or defective
A battery portable radio may be useful in locating the source of the noise by
actually homing in on it. For this type of work, a battery receiver with a highly
directional antenna and a signal-level meter such as the Heathkit DF-1 radio direction
finder is invaluable. If the noise is not being generated on your premises, scout
the neighborhood with your battery-operated portable.
Remedies at the Source
Remedies at the source depend, of course, on the type of interference. Noise
due to. motor commutator arcing may usually be suppressed with a 1-μf 600-volt
capacitor connected across the line terminals at the motor. A 0.1-μf 600-volt
capacitor is more practical for portable tool and vacuum cleaner motors. The motor
frame should be grounded as a safety as well as a noise precaution, A 0.02-μf capacitor
connected from each side of the line to the grounded frame may reduce the interference
further. In extreme cases, a commercial filter (a capacitor-inductor combination)
may have to be used.
Contact arcing on switches, relays and thermostats where currents of less than
3 amperes are involved may be reduced with a 0.1-μf capacitor connected across
the contacts, To limit charge and discharge currents, a 100-ohm 1.2-watt resistor
should be connected in series with the capacitor (Fig. 8-a). The voltage rating
of the capacitor depends on the circuit inductance. If the contacts control a device
for which timing is important, the R-C combination of Fig. 8-3, cannot be used.
Instead, two 1-millihenry rf chokes of adequate current rating for the load may
be connected as shown in Fig. 8-b.
To suppress fluorescent fixture interference, connect a 0.05μf capacitor across
the line and connect a 0.02-μf capacitor from each side of the line to the metal
frame of the fixture. The filter should be mounted inside the grounded fixture frame.
Interference from neon signs may be minimized by removing protruding points in the
wiring which might allow auxiliary discharges and by cleaning switch contacts periodically.
High-frequency oscillations may be damped by placing a 47,000-ohm resistor in series
with each of the transformer secondary leads to the tubing (Fig. 9).
Interference from automobile ignition systems to home receivers is rarely ever
bothersome. But the ignition system will introduce noise to the automobile radio
if it isn't suppressed. Ordinarily a resistive suppressor in the ignition coil lead
to the distributor, and a 0.5- or 1-μf capacitor from the ammeter to ground will
suppress ignition noise. This, of course, assumes a properly shielded antenna lead
with a good shield connection to the auto frame. A 0.5- or 1-μf capacitor should
be connected across the generator to suppress generator noise. In severe cases,
particularly in older cars, additional measures may be required.
Posted July 19, 2023