August 1964 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Automotive ignition noise issues are rarely experienced these
days since not many people even listen to broadcast radio anymore,
and those that do tend to prefer FM stations. Portable music
devices rule the world both in and out of the car, with Bluetooth
or phono jack connections to the dashboard stereo making it
easy to bring your own entertainment and use just the amplifier
portion of the box. Back before such conveniences existed,
magneto, point, and condenser ignition systems wreaked havoc
with radio reception. AM was particularly vulnerable because
the noise was introduced inband and could not be readily filtered
out. FM helped matters, but even then it was not uncommon to
detect a background crackle in the audio that changed in frequency
with the engine rotation speed; noise on the DC distribution
systems was the culprit. Electronic ignition systems went a
long way toward eradicating the problem. Nearly complete computerization
of the entire vehicle control and signaling installation has
created a whole new source of high frequency noise, but it usually
is not noticeable to normal radio and digital data devices.
Ham radio operators still have to contend with aspects of both
ignition and digital noise. The lack of continuous, welded frames
and body sections makes installations difficult for a lot of
hobbyists, but that of course has not prevented determined operators
from finding ingenious solutions. Where there's a will, there's
a way, as the saying goes.There are still lot of people around
the world who drive older vehicles and appreciate articles like
this one by John Frye's fictional electronics repair shop owner,
Mac, and his sidekick technician, Barney. As is often the case
with both this series and his "Carl &
Jerry" series that ran in Popular Electronics,
real products and devices are referenced in the story.
Ignition Noise Problems
By John Frye
Noise suppression in mobile installations has become an urgent
problem - what with FM, CB, and v.h.f. receivers.
You know, Mac," Barney said to his employer, "automobile
noise suppression jobs are getting more numerous and tougher
every day. I'm not talking about broadcast-band car radios.
I'm talking about the FM receivers, the CB receivers, and the
commercial v.h.f. receivers so common in cars today. Believe
you me: there's a whale of a difference between 'de-noising'
an ordinary car radio and keeping the noise out of one of these
"I reckon we both know why," Mac answered. "Ignition noise
usually peaks somewhere between 25 and 100 megacycles. On top
of that, a broadcast receiver is ordinarily tuned to a nearby
station radiating hundreds or thousands of watts from an antenna
several hundred feet high. The CB receiver is trying to pull
in a signal from a transmitter running a measly three watts
into an antenna mounted, at best, on a rooftop.
"Incidentally, I dislike that term 'noise suppression.' It
sounds as though all you need do is prevent popping sounds issuing
from the speaker. Squelch or a.n.l. circuits will do that, especially
if you're content to receive only strong stations. In fact,
in an FM receiver with good limiters and a properly operating
FM detector, you won't hear quite heavy ignition interference;
yet that interference will be seriously degrading your reception
by loading up the a.g.c. circuit and reducing the receiver sensitivity.
A good way to check on the presence of such interference is
to use the diode probe of a signal tracer to pick off the signal
at the grid of the first limiter.
"It's very important to keep basic concepts clearly in mind
and to proceed in a methodical fashion in clearing interference.
Remember any electrical spark is a miniature broadband interference
transmitter. The higher the voltage producing the spark, the
stronger is the interfering signal. Since ignition voltage is
several hundred times greater than other voltages present in
the car, that's why ignition interference should be suppressed
"Wires leading to sparking electrodes serve both as radiating
antennas and linear tuned circuits peaking the interference.
Resistance in these wires lowers the 'Q' of the tuned circuits
they represent and damps out r.f. oscillations in them. The
r.f. oscillations can be short-circuited to ground through a
bypass capacitor of sufficiently low impedance at the interfering
frequency where this does not interfere with the ignition spark.
Lumped resistance inserted in the wire or bypass capacitors
will be most effective when placed as near as possible to the
interference-generating spark, for this effectively shortens
the radiating portion of the lead."
"You mentioned interfering with the ignition spark. Some
guys insist any kind of resistance noise suppression degrades
engine performance. What do you think?"
"What I think doesn't matter, but tests conducted by automobile
manufacturers have proved that where the ignition system is
in good shape, the addition of approved resistor-type spark
plugs, ignition cable, or suppressors does not affect engine
performance. Of course, if the system performance is already
marginal because of a weak coil, leaky capacitor, worn-out plugs,
or some similar condition, the addition of suppression resistance
may cause the engine to miss. In that case the answer is to
correct the ignition defect, not take off the suppressors."
"Am I right in thinking the fundamental steps in preventing
ignition interference are: (1) attenuate the pulses of interference
greatly through the use of resistor suppression and bypassing;
(2) bottle up the remaining interference inside the engine compartment
by judicious bonding, shielding, and bypassing of all routes
by which it can leak or be conducted outside; and (3) make sure
the only way any signal-including ignition interference - can
reach the receiver is by way of the antenna itself?"
"You've got the big picture! The first thing to do when starting
on a job is determine how many of these measures have already
been taken. Is the engine equipped with resistor-type spark
plugs? Champion precedes the plug number with an 'X.' Autolite
and A.C. resistor plugs have an 'R' in the plug number. Is resistance
ignition cable used? If so, what kind? General Motors resistance
cable is plainly marked 'GM Radio TVRS.' It has a resistance
of about 4000 ohms per foot. Other kinds will be marked 'HTLR'
(3000 to 7000 ohms per foot) and 'HTHR' (6000 to 12,000 ohms
per foot). This cable will not stand rough handling and should
never be cut to install a screw-on suppressor or to attach a
new end terminal. When resistance value exceeds 18,000 ohms
per foot, the cable should be replaced."
"How about combining resistor plugs, resistance cable, and
lumped-resistance suppressors? If one is good, three ought to
"That's like taking a whole bottle of aspirin to cure a headache!
Actually, resistor plugs are often used in conjunction with
resistance cable or suppressors in two-way radio installations.
The Law of Diminishing Returns prevents the improvement of adding
external resistance to resistance plugs from being breath-taking,
but it is usually noticeable. Increasing resistance, though,
makes it tougher for the ignition system to produce the proper
"A good ignition system in top shape may put out as much
as 30,000 volts across an open circuit. In operation, this voltage
is lowered to the amount actually needed to jump a spark across
the plug electrodes. This required voltage goes up under heavy
acceleration, and it also goes up if the plug is dirty or too
widely gapped. With a single type of suppression installed,
there is enough reserve to take care of ordinary variations,
but adding resistance to the high-tension system cuts down on
this reserve. The kind of driving also determines how much suppression
an ignition system can stand. Stop-start, long-idle driving
will sometimes foul plugs in an overly suppressed installation.
If you're worried about the car's performance, it's a good idea
to have the car checked with an electronic ignition analyzer,
such as the Heath job or the Champion 'Plug-Scope,' after the
suppression is in place."
"To avoid over-suppressing, a guy has to be on guard against
hidden built-in suppressors," Barney offered. "Some older cars
and trucks have 15,000-ohm suppressors built into the distributor
rotor arm or concealed in the cap.
"Right, and in concentrating on the high-tension circuits,
you must not overlook the importance of proper bypassing of
the coil primary. Ignition pulses from the secondary can 'kick
back' through the primary and spread out over the whole low-voltage
system. A.1-μf. coaxial bypass capacitor should have its
center lead inserted in the lead coming from the BAT connection
of the coil as close to that connection as possible. An ordinary
capacitor here will not work at v.h.f. and u.h.f. frequencies.
It has too much inductance. Only a coaxial capacitor presents
sufficiently low impedance to the high-frequency r.f. to do
an adequate job of bypassing. Both the capacitor case and the
coil case should have an excellent, paint-free electrical connection
to the firewall or block."
"How about bonding?"
"That's what confines the residual ignition interference
to the motor compartment. Heavy duty Belden 8662 bonding braid
should be used to bond all four corners of the motor, the steering
column, the firewall, the exhaust pipe, and the tail pipe to
the frame. Lighter Belden 668 can be used to bond metallic tubes
and rods to the firewall at the point where they pass through
this wall. Tooth-type washers that will bite through the paint
should be used to secure the ends of the bonding braid. In some
cases, it may be found necessary to bond both sides of the hood
and the trunk lid."
"How can we be sure any signal reaching the receiver comes
only by way of the antenna. I don't quite dig that."
"It's mostly a matter of proper installation. The case of
the receiver must make a good connection to the car frame or
body. The antenna lead shield must have good ground connections
at both the receiver end and the antenna end. A body- or fender-mounted
antenna can be mechanically solid and yet have a few ohms of
resistance between the grounding lug and the body because of
paint or undercoating. In the case of a bumper-mounted antenna,
the bumper itself may have to be bonded to the frame. A shielded
lead should bring power directly from the battery to the receiver
- ideally by way of a feed through coaxial capacitor through
the firewall. It's a good idea to install a 0.5-μf. capacitor
between the hot battery connection and ground at the receiver
if one is not already present. When the receiver is not picking
up any signal through the battery lead or through a poorly grounded
case or antenna lead, disconnecting the antenna will reduce
the ignition noise to zero."
"That about takes care of the three main steps I mentioned.
What if you still have some ignition noise?"
"You probably will be able to hear a little, especially after
you have cleaned up voltage-regulator, generator, instrument,
and other kinds of masking noise and when you are not tuned
to a station. But it will disappear when even a weak station
is tuned in. If you want to get the ultimate in noise suppression
and are willing to pay for it, you must go to complete shielding
of the ignition system as the aircraft and military people do.
That means completely shielding the coil, the distributor, all
high-tension wiring, and the spark plugs themselves. The shielding
must be as complete and free of r.f. leaks as that used in keeping
a transmitter from causing TVI."
"Can't you buy this sort of shielding and install it yourself?"
"Yes, the Hallett Mfg. Co. in Los Angeles manufactures two
lines of shielding. The 'Signal-Saver' line provides complete
customized shielding for practically any motor. In ordering,
you give the make, model, and year of the car, plus the displacement
of the engine; and the shielding sent you is tailored to fit
your particular motor.
"The same company manufactures the 'Eliminoise' universal
noise suppression kit which is marketed through the E. F. Johnson
Company. With this kit, which sells for considerably less than
the customized line, you have to do more fitting and trying;
but step-by-step instructions are sent to help you."
"You've been reeling off facts and figures pretty recklessly,"
Barney observed shrewdly. "I somehow get the feeling you didn't
learn all this on your own."
"Never said I did," Mac defended himself. "I learned long
ago to supplement my own experience and observation with expert
help whenever possible; so I've written spark-plug companies,
capacitor manufacturers, automobile makers, ignition-component
manufacturers, and shielding manufacturers for any facts they
had turned up in their research. All were most helpful. Champion
Spark Plug puts out a free booklet entitled 'Giving Two-way
Radio Its Voice' that is full of helpful tips. Both Cornell-Dubilier
and Sprague have special catalogue sheets describing their capacitors
specifically designed for noise suppression. Delco-Radio has
good material on the subject, and Hallett Mfg. Co. has promotional
material and installation manuals that show you just how their
shielding is installed. Well we better get to work, but what
say we continue the discussion by talking about stopping noise
from non-ignition sources in the near future?"
"Fine with me," Barney said; "but before we leave ignition
noise, I've got a question: do you think transistorized ignition
systems are going to give more noise trouble than conventional
"While I could get no auto manufacturer to stick his neck
out on this subject, I got the impression they are anticipating
no special problems with the transistorized systems they expect
Posted March 19, 2015