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
"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 first.
"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 be dandy."
"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 spark.
"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
"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 systems?"
"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 to use."
Posted October 10, 2022
(updated from original post on 3/19/2015)
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
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
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.