Reducing radio interference


It sounds like a crude form of Morse code on your single sideband transceiver. The interfering signal is strong enough to disrupt fax reception, render voice reception unintelligible and bring your onboard email to a halt. It seems to occur all across the bands, stronger on some frequencies, weaker on others. If when you turn off your 12-volt refrigeration the interference stops immediately, read on — this newsletter is for you.

The culprit is the electronic control module (ECM) on the compressor of your refrigeration unit. Danfoss manufactures the direct current hermetic compressors and electronic control modules for almost all of the world’s suppliers of small, direct-current refrigerators and freezers. Over the years they have changed the design of the electronic control module several times, adding better shielding, but to date they are still a long way from achieving an interference-free product. The ECM does a dandy job of driving the compressor and can also jam your SSB radio.

You could just switch off your refrigerator while using the radio, but if you forget to switch the refrigerator back on just once, well, need I say more?

A number of steps can be implemented to reduce, if not totally eliminate the interference. The first step is to inspect all of your radio grounds. The interfering energy must be diverted to ground and without a clean ground, success is not likely. Radio frequency energy flows along the surface of the conductor. Salt water rapidly corrodes copper, forming copper oxides that create a high impedance path to radio frequency energy. Replace any copper ground foils that appear to be corroded and thoroughly clean the connections. Coat the cleaned connections with silicone grease.

Conducted and radiated interference
The second step is to track down and identify the various paths of interference, of which there are two types: conducted and radiated. While the interference is created within the ECM itself, it is the connecting wiring and even the refrigerant tubing that acts as radiating antennas. Conducted interference travels from the ECM to your radio via the power wiring, and radiated interference travels through the air entering your radio via the antenna. To complicate matters even more, radiated interference can enter the various wires and cables connected to your radio. Most probably you will have to treat both conducted and radiated interference to achieve an acceptable reduction in interference.

A most effective tool to identify radiating items is a “sniffer” made from a length of RG-58U 50-ohm coaxial cable, the 1/4-inch (6 mm) diameter coax. The sniffer is actually a small directional antenna. The American Radio Relay League’s RFI Book shows how; you will need a length of coax sufficient to reach from your radio antenna connector to the compressor and to the battery bank, plus an extra foot or so (330 mm). Install a coax connector (typically a PL-259) that will fit your radio’s antenna connector on one end of the coax. On the other end remove 1.5 inches (36 mm) of the outer insulation jacket and braided shield, and then remove one inch (25 mm) of inner insulation from the end of the center conductor. At about 12 inches (305 mm) from the end of the coax, score the outer insulation and pull it back about 1/4 inch (6 mm) to expose the braided shield. Holding the end of the coax, form a coil of three turns about two inches (50 mm) in diameter. Wrap and solder the inner conductor at the end of the coax to the exposed shield. Use vinyl electrical tape or heat shrink insulation to cover the joint and to hold the coil together.

Using the sniffer
Prop open your freezer lid or lower the thermostat setting to make the compressor run constantly. Turn on your SSB radio and tune around for a strong interference signal. Disconnect the antenna coax from the radio. A significant reduction in interference should be noted (with the antenna coax disconnected, the reception of radiated interference is greatly diminished, and any interference noted will be conducted interference, entering the radio via the power cables). Connect the sniffer to the antenna connector on the radio and pass the loop over the compressor, refrigerant tubing, thermostat wiring and power wiring, noting changes in the interference signal. The coil provides directional sensitivity, with the flat side of the coil being the most sensitive. Make a list of the offending items.

Close the freezer, reset the thermostat to its correct setting, reconnect the antenna coax and store the sniffer for later use. Review your list of offending items and apply the following fixes, beginning with the item that is the strongest source of interference. After each fix, turn on your radio and make a note of the change (hopefully a decrease) in the interfering signal.

Strip the outer braid or shield from a length of coax and use the braid to connect the chassis of the compressor unit to the ship’s ground. You should use marine grade RG-213/U coax since the shield or braid is tinned and will not corrode as quickly as bare copper. Use crimp terminals on the ends and keep the length to a minimum; i.e., do not coil up any excess.

Refrigerant tubing: Again, using the outer braid from coax, make four or five wraps around each tube and connect the other end to the compressor chassis at the ground connection made in item one. Again, keep the length to a minimum.

Power cables: Replace these with RG-213/U coax (the 1/2 inch or 10 mm diameter type.) The RG-213/U coax has a 0.7-mm diameter inner conductor, much larger than the more common RG-8/U coax, so it will carry more current with less voltage drop. Allow for enough braid on each end to be connected to ground and keep the braid over the center conductor as close to the ends as possible without causing short circuits. Use heat shrink insulation to prevent short circuits. Always twist power cables a minimum of one twist per twenty-five cable diameters to cancel out interfering currents.

Thermostat wiring: Chances are that any excess thermostat wiring has been formed into a coil and secured near the compressor, as per the installation instructions. Your tests with the sniffer will have revealed this to be a prime radiating antenna, so simply cut out the extra wire and rejoin the ends. Install ferrites (either snap-on or toroids) on each end.

Construct a Faraday shield over the compressor unit. Galvanized steel sheet of 0.22 inches (0.56 mm) minimum will serve nicely. Begin by cutting out two sides, the top, and one end. Provide openings for the refrigerant tubing, thermostat wiring and power cables. When assembling the box, allow for overlapping seams of 7/8 inch (23 mm) minimum. Soldered seams are the most effective, but pop rivets may be used. Space them no more than two inches (50 mm) apart. Also, 3M makes an adhesive-backed EMI tape that can be used to seal the seams. Sealing the seams is particularly important, so don’t just cover the seams with duct tape. Fit the box over the compressor unit and hold it in place with duct tape. The condenser end of the box is effectively shielded by the condenser and the bottom of the box is the metal chassis of the compressor/condenser unit. Seal around the wires and refrigerant tubing entering the faraday shield with “donuts” made from at least two layers of metal mesh (bronze screen wire is good for this purpose).

Bypass capacitors
Install bypass capacitors from the power cables to ground. Use 0.5 µf 25 v (0.5 microfarad 25 volt) disc ceramic capacitors in parallel with 220 µf 25 v NPO electrolytic capacitors. If you replaced your power cables with coax as in item 3, then simply solder the capacitors to the inner conductor and braid, keeping the capacitor leads as short as possible. Use heat shrink insulation on the capacitor leads to prevent short circuits. The bypass capacitors act as short circuits to interfering signals, but act as open circuits to direct current.

Install an alternator noise filter in the positive power lead to the compressor. Be sure to connect the ground terminal on the noise filter to the common ground point on the compressor chassis.

A last desperate step, which should only be attempted if your electronic skills are top notch, is to install a speed controller on your compressor. The idea is to shift the speed of the compressor and thus the frequency of the interfering signal away from your SSB radio operating frequency. The ECM determines the operating speed of the compressor by measuring the resistance in the thermostat circuit. By replacing the fixed resistor in the thermostat circuit with a potentiometer mounted where it can be reached from your radio operating position, it will be possible to “tune” the interference either below or above your operating frequency. Only a couple of hundred revolutions per minute change in the compressor operating speed will be required, and will have negligible effect on the operation of your refrigeration system. In the installation manual for your compressor locate the schematic for the compressor and ECM. On the schematic, determine the physical location of the speed resistor. A chart will show various values of resistance versus compressor speeds. For instance, if the chart shows 1,500 ohms resistance for 3,000 rpm, replace the 1,500-ohm resistor with a 1,200-ohm resistor and install a 500-ohm potentiometer in series in the thermostat circuit.

To treat conducted interference, install ferrites on the radio power cables. These can be the snap-on type, which should surround both positive and negative power cables, or the toroid type. The more turns through the ferrite, the better the interference suppression. Multiple ferrites may be required. Ferrites act like electronic tourniquets by absorbing the interfering energy and preventing it from continuing along the conductor. Use type 43, 75, or J ferrites. Ferrites may be purchased from and and

A follow-on item, not related to interference, would be to insert some air conditioner filter media around the sides and top of the Faraday shield at the condenser. This will trap dust and lint that would otherwise clog the fins in the condenser, leading to a drop in efficiency.

Despite your best efforts it may not be possible to completely eliminate the interfering signal, but at least it can be attenuated to an acceptable level.

Harry Hungate is an extra class ham radio licensee (N1UDE/ZL1HAH) and also holds the FCC GROL with radar endorsement. He and his wife Jane completed a circumnavigation aboard Cormorant, their Corbin 39 cutter.

By Ocean Navigator