|Mobile Radio Noise Elimination
Engine noise in the mobile
radio environment is as old as radios in cars, and it still plagues users
today. With cars becoming more sophisticated and on-board computers become
standard fashion for modern autos, the noise production potential of a
car's electrical system is steadily increasing. But don't lose hope - you
can still enjoy a trouble-free mobile radio installation. Here's how:
First, install the radio
equipment in the most ergonomic location convenient to the intended radio
operator, mounting it securely with fasteners that will not allow the equipment
to tear away during rough traveling. Second, always lay a direct wire line
with a fuse protector and connect the radio equipment directly to the car's
battery. This element of system design is important because the battery
acts as a noise filter through which other auto-produced noises cannot
easily pass to reach the DC input connections of the radio chassis. It
also provides better voltage regulation, overvoltage
protection, DC purity and prevents overload of the car's accessory
line. Don't forget or bypass the fuse in the +12V (red) line, and place
the fuse close to the battery. If the line should fault to ground otherwise
between the battery and the fuse a fire could, and probably will, erupt.
Try to place antennas on
the vehicle away from the engine, where most of the noise is produced during
travel. And always try to use antennas with mounts that make direct physical
and electrical connection of coaxial cable shields to the car body. This
keeps the integrity of the shields high.
Next, try the radio equipment
with the engine running and see if noise is noticeable. If it persists,
perform this simple test. Disconnect the antenna and see if the noise continues.
If it does, then all or part of the noise is still coming through the DC
line and a mobile regulator/filter may be needed for further isolation.
If the noise disappears with the antenna disconnected then the DC line
is substantially clean and the noise is being received as an RF-generated
signal from the vehicle's engine or electric system. If the noise changes
in speed or pitch with engine speed changes it's likely the result of the
most common noise - electrical charge and cylinder detonation. This type
of problem can only be resolved in the engine itself, but don't let that
stop you from pursuing a final solution.
First, install resistor-type
spark plugs and possibly resistor-type anti-noise spark plug wires in the
engine. This is good practice for all engines and not expensive. See if
the noise is resolved before going further. If not, see if you can
round up an oscilloscope. The scope is a commonly available and excellent
tool for running down noises. Place the scope lead on the 12V side of the
distributor or other 12V wiring around the engine block, or on the output
of the alternator or generator and observe the pattern. A typical cylinder
detonation appears to be a large thin line straight up (the initial spark
detonation) and then a trailing line outward to the zero line (burning
of remaining fuel until push - out during the exhaust stroke). Anywhere
a +12V line can be found in the ignition system that shows noise spikes,
attach a bypass capacitor on the order of 200-500 Mfd. at about 50 volts
from that point to chassis ground and then observe the reading again.
Using clip leads for the test allows rapid testing. Make sure that the
car's engine hood is electric ally bonded to the car chassis and that all
connections are tight.
Since all cars are somewhat
different it's difficult to provide a straight path for all troubleshooting
in vehicles, but a scope and a handful of capacitors can do wonderful things
with some careful experimentation.