Alternator Whine, Ground Loops, Noise
Found this article while surfing internet, I
think it is very much informative.
"I want to revisit this problem, because there
seems to be a lot of poor advice floating around on these pages.
Let's start out with a few basic facts, but keep in mind this is NOT
an alternator primer. If you need or want more data, the internet is
your best friend.
The average alternator's output is between 13.9
and 14.2 VDC. It might be less if there is a problem with the
alternator. In some cases it may be a little higher, but voltages
over 14.6 VDC should be considered abnormal.
Continuous output and peak current ratings vary
quite a bit. The requisite amperage ratings selected by OEMs are
largely based on content. That is to say, how many features like
rear window defrosters, premium sound systems, electric windows, and
heated seats any given vehicle is equipped with. Heavy duty and
high-end vehicles usually have larger ones as do those with
extra-cost trailer towing packages.
Nowadays, the smallest OEM ones are rated about
90 amps peak, and the larger OEM ones about 150 amps peak. There are
a few exceptions, but the highest rated OEM units are about 225 amps
peak. The reason I use the term peak is this; very few OEM
alternators will deliver their rated output continuously, and
contrary to popular belief, there isn't any standard rule for peak
Almost all alternator stators (the non-rotating
part) are wired in a wye configuration (as shown), and the rest are
wired in a delta configuration (primarily Ford products). Rotating
within the stator is the field. The field current and/or voltage is
varied by the regulator so the output voltage is constant,
regardless of the load, up to their peak amperage rating. There are
several different regulation strategies employed. Some simply use a
pass transistor, others use pulse width modulation, and some almost
Depending on the engine type (diesel or gas),
alternators are driven from two to five times engine speed, up to a
maximum of about 16,000 rpm. As a general rule, the output frequency
of an OEM alternator is equal to the engine rpm. That is to say,
1,000 rpm equals 1KHz. Their efficiency is about 90%. Thus, an
alternator rated at 130 amps, with an output of 14 vdc, will have an
input of around 2 KW, and will require about 3 HP to drive.
\In a never-ending quest to reduce weight, and
improve efficiency, most new-generation OEM alternators are double
wound, and use twelve diodes instead of six. This not only reduces
size and weight, the lower mass of the rotating field allows the
alternator to be driven faster, which improves low rpm power output.
It also doubles the ripple frequency.
As long as the diodes are doing their job, the
output ripple is nearly nonexistent, as the battery is acting like a
very large capacitor. When they don't do their job, the result is
what we commonly call alternator whine. To be sure, there are other
causes which will be discussed later.
While alternator whine can be a bane for us
amateurs, as long as the alternator delivers its rated output,
dealers don't care, and typically will not replace noisy ones under
warranty. So this leads those who are plagued to seek other avenues
of relief. For example, using RG8 as a power cord, or twisting the
factory power cords of their transceivers. Doing so is junk science.
Let's visit this in more depth.
First, any technique we use to shunt alternator
whine to ground must present a low impedance at the frequency we're
trying to suppress (less than 8 kilohertz typically). Further, it
must be of lower impedance than the circuit it is attached to. In
the case of vehicle DC wiring, that's seldom higher than a few
tenths of an ohm.
An average power cord is ten foot long. A ten
foot piece of RG8 has 250 pF of capacitance. At 8 kHz, 250 pF has a
reactance of about 1,500 ohms. In terms of suppression, this amount
Twisted or not, a 10 foot power cord made from
two number 10 conductors will have about 2 pF of capacitance per
foot. Ten feet of it is an insignificant reactance even at 80 kHz!
What's more, those who support twisting the power cord as a fix for
alternator whine, and a host of other maladies, ignore some basic
facts. Twisting works to reduce noise pickup only if both inputs and
outputs are balanced, and neither end is grounded. That's not the
Brute force filters offer some help, but there is
a big downside too, and that's voltage drop. Radio Shack used to
sell one that was rated at 20 amps. Inside its tubular construction
is 20 feet of what appears to be number 16 Thermalese wire wound
around a laminated steel core about 3/8 of an inch square, and and 2
inches long. A 1 uF coaxial capacitor completes the package. The
input and output are size 10. The voltage drop at 20 amps is almost
2 volts. At 8 kHz, the suppression is less than 2 dB.
In some cases, a 1 Farad cap, like those used in
mobile sound systems will suppress alternator whine if they're
placed near the radio end of the power cord. However, they have a
lot of drawbacks, not the least of which is their propensity to
explode if dead shorted.
The best place to cure alternator whine is at the
source. If you think it is a leaky diode causing your problem, use
an O scope to look at the alternator output directly at the output
terminal. If it is a diode, you'll easily see it. The fix is
As alluded to above, there are another situations
which can cause what ripple there is to invade the circuitry of your
transceiver. One of those is a ground loop. Ground loops occur when
there is a differential in current flow between the positive and
negative power leads feeding the radio. This is typically caused by
incorrect wiring techniques. Poor bonding of body on frame vehicles,
and poor coax connections can also cause the problem.
Another problem altogether, which is often
incorrectly identified as alternator whine, is the switching
transients from the alternator's regulator. While diode induced
whine directly varies with engine speed, regulator whine normally
does not. It will appear louder at low rpms, and when there is a
high amperage load. Since it is radiated RF energy, removing the
antenna will cause it to go away. The only fix is to replace the
Distracters will surely point out that they fixed
their alternator whine with one of the aforementioned anecdotal
remedies. If that is indeed the case, then the original wiring was