Determining RFI problems in mobile

Troubleshooting Your Power Supply

If you continue to experience noise in your receiver when the coax has been disconnected from the antenna connection, your source more than likely RFI coming through the radio’s power supply. The best way confirm this is to disconnect the radio from the car’s 12-volt power and supply it from an external dc power supply. If the noise disappears, then you have confirmed this, and it’s time to investigate exactly how your radio is drawing power from the electrical system.

See how you have treated each of the following potential RFI sources in your installation:

Connections To The Battery

It is always advisable to draw your power directly from the battery, connecting through the firewall to the battery terminals using good heavy gauge wire suited for this purpose and available at most ham outlets and electrical supply houses. When connecting directly to the battery you achieve three benefits:

You prevent the possibility that your negative connection may be “floating” above RF ground. Using convenient body sheet metal connections inside the passenger compartment may allow RFI to be introduced into the receiver and should be avoided.

You will benefit from the tendency of the natural capacitance of the battery itself to aid in the suppression of RFI, which may be conducted along the wiring harness.

You avoid piggybacking a 20-amp supply into your fuse block. Running the rig at full power on a tap on the air conditioner circuit, for example, is not advised and will regularly blow fuses and create hazardous stress on a line that was not designed for this purpose. Even if you have a free spot on the block open you still won’t be able to benefit from the preceding two items described above.

A nice touch involves replacing the bolts fastening the terminal clamps to the battery with longer ones. Attach ring-tongue lugs to the power supply wires and slip them on to the additional bolt lengths and secure with appropriate flat and lock washers and double nuts to prevent loosening due to vehicle vibration. The extra bolt length will also afford room to supply your FM rig or other “always-on” accessories. Cover any part of the exposed positive bolt to avoid sparks during under-hood work. As an aside here, when your cabling is mounted in this fashion you create an excellent facility for attaching jumper cables to boost your car when necessary…just remember the radio is now connected directly to it and disconnect the power lead from the radio before you boost!

When routing the power line through the firewall keep it away from heat sources, moving parts and, of course, any other harness wiring which may represent an RFI source which, through inductive coupling, may allow noise into your power supply. If possible, drilling a dedicated hole specifically for passing through your power line is the most ideal option. Place it as far away from other wiring as possible, and prepare the hole with a grommet to avoid abrasion of the insulation. After placing the wire, weather-seal the hole with silicone caulk.

One very important point: Both the negative and positive leads from the battery should be fused. Many mobile hams do not appreciate that, should the battery become disconnected from it’s engine block ground during service, the car’s electrical system could seek a path to ground through your radio, both destroying your rig and presenting a high-current risk to your safety. Although the risk may be a rare one, it is nevertheless cheap insurance and a snap to apply.

Alternator and Regulator Interference

Almost all of us know alternator-induced noise when we hear it. It appears as a high-pitched whine whose tone increases and decreases in step with the speed of the engine and when lights or other heavy current draws are switched on. Generally most alternator whine is grounded through the natural filtering capacitance present in the battery. If you jumped right to this section, you might want to roll the page up and read the previous section in which this phenomenon is addressed.

Alternator whine, when viewed on an oscilloscope, is actually AC ripple in the rectified DC output and can be received through the vehicle’s wiring. If you still have objectionable noise of this nature present in the receiver check your battery for loose connections or corrosion. Also check, if you can, along the length of the harness from the alternator to the battery as there may be loose connections along the way there as well. Sometimes when digging this deep it is helpful to take the car to the auto center and ask the technician to run the length of the cabling with you with the option of putting the car up on the lift, if need be. In especially noisy cases you may even wish to check the yellow pages for the location of an electric motor refurbishing firm who can give your noisy beast a checkup.

Generally the manufacturer includes a capacitor within the alternator itself for noise suppression. Another capacitor in parallel will provide for more suppression. A commercially available RF noise suppression capacitor is fine to perform this task.

Filtering Power Supply Leads

Finally, many commercially-made noise suppression filters are available at most radio and auto supply stores. These are designed to fit in the power leads between the battery and radio. They are inexpensive and easy to install and actually work quite well.

This article is by KB2J originally available here