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TUCoPS :: Radio :: engnoise.txt

Dealing with Mobile noise in your radio

Dealing With Mobile Noise:

This file will deal with mobile noise problems, especially the commonly-found
mobile noise from electric fuel pumps on modern cars. The general
troubleshooting procedure for finding and eliminating these noises will be
discussed, along with the general theory behind it all.

Step 1: Determining the noise source

     a. Spark plug noise (A popping noise heard only when the engine is
running, the popping gets faster when you rev up the motor).

     b. Alternator noise (A whining noise in the reciever, which gets
higher-pitched when you rev up the motor).

     c. Electric fuel pump noise (heard anytime the key is in the RUN position
, whether the motor is running or not. Does not change with engine RPM.
Resembles a steady 'buzzing' noise).

Step 2: Determining how it enters the radio

     Here is how we determine how the noise gets into the radio. First, we must
produce the noise, which means starting the vehicle up and/or turning on the
device that produces the noise in the reciever, with the reciever on. We then
proceed to disconnect the coax from the radio. If the noise d
isappears or is greatly reduced, then we know that it is coming in through the
coax. If the noise stays the same after disconnecting the coax, then it is
coming in through the power leads of the radio. This will tell us just what
course to follow in getting rid of the noise. (It is not unheard o
f for it to be coming in both ways, but we would STILL see some modification of
the noise upon dropping the coax, even in this case).

A Discussion of "GROUND"

     We all know that 'GROUND' refers to the body of the car. However, to an RF
signal, 'GROUND' can be something else. We have to understand the shield wire
(outer braid) of the coax and what it is supposed to do. It has two functions.
The first is to keep your RF signal 'bottled up' in the coa
x until it reaches the antenna, where it is radiated. This is during transmit.
However, the braid of the coax has a very important job to do during recieve,
as well. That function is to KEEP NOISE FROM ENTERING THE COAX. It is only able
to perform these two functions if it is GROUNDED. Now, 'GR
OUND' to a coax shield will often only exist for 2-3 feet up the braid of the
coax. Why ? Well, for RF purposes, the RF wants to take the path of least
resistance. we have to realize that a capacitive coupling exists between the
braid and center conductor of the coax, and that if that coupling o
ffers less resistance to the RF than the actual DC resistance of the braid,
then the RF (noise) is going to want to couple to the center lead! Also, we
have what is known as the 'skin effect', which tends to make the braid look
resistive to RF in the DC sense. That is why it is EXTEREMELY IMPORT
part of the discussion...........


     I have never advocated the use of magnetic mount antennas. There is one
good reason. There is no actual ground connection for the braid of the coax at
the antenna. (Look at the last few sentences of the previous discussion).
This will tend to let noise into the coax, since the braid is only 'grounded'
at the radio! This tends to increase noise pick-up tremendously.
I suppose that they have their cosmetic uses, but they are inherently lacking
in this respect. In the last 30 years or so, I have not been installing them on
mobiles. If you absolutely MUST use one of these, and you have a noise problem,
then we probably don`t have anything further to talk about
 here, especially if you have determined the noise is coming in through the
coax. You are going to live with the noise and have a 'pretty vehicle'.
     On the other hand, if you are serious about being able to hear something
on the radio beside the spark plugs, alternator, and/or fuel pump noise, then
read on.
     There have been countless times that I have changed one of these
'mag-mount' antennas to a conventional body-mount type, and had ALL THE NOISE
SUDDENLY DISAPPEAR. I don`t think I ever changed one that had a noise problem
and not had at least a substantial reduction in noise. Nuff said.


     If you are using a conventional body-mount antenna, the following
procedures should yield results. You will need an ohm-meter or small test light
that checks continuity. First, disconnect the coax from the radio. Now, apply
one lead to the shell of the coax connector, and the other lead to
body ground. If you have a connection, then we know that the ground at the
antenna end of the coax is good. if not, then you will have to find out why you
don`t have a ground, and fix it. In the case of trunk-mounted antennas, this
usually means scraping the paint under the trunk lid away where
the set screws clamp on. Or, maybe you have a bad coax connector that is not
making contact with the shield. Take the ohm-meter or light and apply a lead to
the shell of the coax connector at each end of the coax, with the coax
disconnected at both ends. If you don`t have braid continuity, then
you will have to put on new connectors till you do. If the coax checks out
good, then it is in the antenna mount. In any case, GET A GROUND READING BEFORE
     The next thing you will do is determine if the radio chassis is grounded
to the car. Simply apply one lead to a ground, and apply the other to the
chassis of the radio (screw heads on the back of the radio usually work good
for this). Got a reading ? If not, you are going to have to mount t
he radio in such a way that the radio chassis is in contact with ground. Why?
Because that is the only way the coax gets a ground connection, is when you
screw it into the back of the grounded radio !!! (You did MOUNT the radio,
didn`t you ?)
     If you don`t WANT to mount the radio, you CAN run a SHORT ground wire from
the radio chassis to a good ground. This will accomplish the same thing. By
SHORT, I mean NO MORE THAN TWO FEET. Any longer will tend to make your troubles
WORSE, not better. I will usually put an alligator clip on t
he end of the wire, if I am constantly switching the radio from vehicle to
     If you had bad grounds, and you have fixed them, you probably have seen a
substantial noise reduction, unless you have the 'fuel pump' type noise, which
is covered later in this file. Not to mention the fact that you will usually
'get out' better!


     This is a different type of situation. The first thing to do is to move
the radio`s power lead so that it is connected to the fuse block`s 'radio'
fuse. Why ? Because the factory has already intalled a noise filter on that
terminal of the fuse block ! I have done this time and time again wi
th good results. If this doesn`t solve your problem, then there is more.........
     You can buy an excellent noise filter for your radio at Radio Shack. Just
tell `em that you want the high-current 'noise eliminator'. Follow the
directions that come with it. It installs in the power (positive) lead of your
rig, and will filter out either spark plug or alternator noise like
 MAGIC. This is good in stubborn cases.
     If you don`t have deep pockets, then here is a VERY old trick I have used
successfully over the years. Get an old piece of RG-8 coax long enough to reach
right from the back of your rig, through the firewall, and right to the battery
in your vehicle. On the radio end, strip back about 3 or
4 inches of the outer jacket, and un-braid the shield braid. twist this up into
a wire. Now, just strip about 1/2 inch off the center conductor insulation, and
hook this to the positive lead of the rig (you probably should trim the leads
back on the rig so they are about 6 inches). Tape this up
well. Now, hook the braid to the negative lead of the rig. Strip back about 10
inches of it down to the braid at the battery end, and unbraid the braid. Once
un-braided, twist this so that it makes a wire. hook this to the NEGATIVE
terminal of the battery. Strip back obout 3 inches of the center
 conducter, and hook this to the battery POSITIVE lead. Once you are back in
the vehicle, run a short wire from the braid wire to the chassis of the rig.
You now have a shielded power lead. However, I have found that this will only
work in about 50 percent of the cases. In some cases, it will ac
tually INCREASE the noise because of the extra length of the ground lead, while
in others it will wipe the noise right out. If you`re broke ( and I sometimes
have been) you can give this a try.......... And don`t forget what I said
before about grounding the chassis of the rig.


     Now let`s try to understand how something like an electric fuel pump can
make life so miserable for the mobiler. The pump is fed 13.8 VDC from the
system, whenever the key is on. But it is not the DC that causes the trouble,
but what the pump is DOING with the DC! The pump is switching the
DC on and off hundreds of times a minute, which generates what is known to
engineers as 'switching hash'. This switching hash is made out of square waves,
which are rich in harmonics. Some of these harmonics appear in the CB band.
This is the 'buzzing noise' that is driving you up the wall!
     The standard cure for this is cheap, and hasn`t changed for 50 years.
Obtain a .05 MFD disc ceramic capacitor of about 250 volt rating from radio
shack. Now you have some work to do. You must install this little gem AS CLOSE
TO THE FUEL PUMP AS POSSIBLE to get the best out of it. What you w
ant to do is, you want to hook one lead of the capacitor to the wire that
powers the pump. The other lead of the capacitor will go right to body ground.
This will 'short out' the switching hash to ground, while having no effect at
all on the DC supply to the pump. Usually, the wires from the pum
p surface right under the back seat, and that is close enough to get the job
done. The floor is right there for a ground. Make sure you wrap up the
connection to the pump DC line, though. Put the seat back in, power up the
radio, and enjoy the sudden silence.................

     Note: I have no objection to you posting this message to any newsgroup,
user group, ect, ect, as long as you do not charge anything for it. This is
information that should be available to anyone. I just wanted people to enjoy
their mobile radio experience. The info in this letter has been o
btained from some 30-odd years of mobil operation and installing them for other
people. Enjoy !


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