PS Advisor March 2017 Issue

Eliminating Radio Interference from Fridge Compressors


I’m a marine installer and we’ve come across another boat with a radio-frequency (RF) issue coming from a Frigoboat refrigeration system. The boat started with two Danfoss compressors and when one was replaced recently, the SSB whines when the new compressor is running. We’ve run though the normal RF isolation procedures, but haven’t had too much luck yet. It seems like RF leakage might be a good topic to explore. What really works to solve it? What installation procedures are necessary?

Name withheld upon request

Danfoss variable-speed compressors
Danfoss variable-speed compressors did well in our June 2009 test of fridge kits.

Whether you’re a pro or an amateur, tracking down radio frequency hiss can drive you up a vented loop. Danfoss offers the following guidance for tracking down and eliminating the annoying hiss that its equipment can generate on some radio frequencies:

“If the noise source has several components, all of which may contribute interference, separate each potential source to observe its effect.

“The battery is an effective trap for the electrical noise reaching the power connections to the noise source. To make effective use of that trap, connect the noise source directly to the battery with the shortest length of wire possible. No other device should be connected to this run of wire. Twisting the power leads will reduce the ability of the power leads to act as an antenna.

“The most effective way to reduce emissions from the power leads is to use shielded cable. All accessory and control leads at the noise source should be examined. Reduce lengths to the minimum. Twist and apply grounded shields as needed.

“It can be helpful to ground the negative power lead to the frame of the noise source. Look for a screw, which can clamp a short copper strap from the negative power lead to the frame of the noise source.

“A filter may be applied close to the noise source. The current rating of this filter should be about twice the maximum current rating of the noise source. If little noise is radiated, a filter at the power connections of the effected equipment may be less costly.”

It sounds to us like you’ve already tried all of these remedies. We turned to two of our contributing editors, marine surveyor Capt. Frank Lanier, and technical editor Ralph Naranjo for any more tips.

Lanier, a former U.S. Coast Guard officer who used to install and trouble shoot electronics, offered this advice:

“Noise from refrigerators and other onboard equipment is a very common problem. Based on what I’ve learned and from my own and other systems, there is no surefire cure. You can add chokes and filters, but in many cases there’s not a whole lot that can be done. The simplest solution is to shut off the fridge when you are using the radio—this is what I do.”

Naranjo, who carried out our June 2009 comparison of conversion kits that turn your icebox into a freezer/fridge, offered a similar response:

“Frank is spot-on with his comments and the only sure cure is to shut off the reefer when operating the SSB. Much of the problem stems from an SSB’s need for a large surface area counterpoise or ground plane that acts as a radial when it comes to RF signal propagation.

“This use of the ship’s ground as a counterpoise links the vessel’s negative with the ground plane for the antenna and imposes an RF pulse on every wire linked to the boat’s ground/bond circuit. The Brits originally tried keeping the boat’s bonded systems separate from the ground, but that didn’t completely solve the RF interference issue and it also exacerbated galvanic corrosion issues. The American Boat and Yacht Council standards commingled the grounding/bonding circuit resulting in better safety and corrosion resistance, but also producing more RF radio hiss.”

Comments (7)

What most analyses fail to understand is the role of the magnetic component of the electromagnetic field. It is common practice to run Positive and Negative (aka 'ground') wires separately. This creates a 'magnetic loop' that radiates ElectroMagnetic Interference that generates a signal in antennas. Magnetic inductance is proportional to loop area, which can be quite large in a typical installation. The remedy is to minimize loop area these 4 ways: 1) minimize the lengths of all wires, but 2) run positive and negative supply lines together in the same runs, even if one of them could be made shorter, and 3) better yet, twist positive and negative supply lines together (this neutralizes much of the remaining magnetic field) and 4) use coaxial cable, which has almost zero loop area by design.
There is much misunderstanding about how shielding functions. The idea that a shield somehow 'encloses noise' is only partly correct, and a shielded cable can still radiate if the currents in the interior conductor(s) and the shield are not equal and opposite. This means that a shield can make things better or worse, depending on how it is connected. Ordinary conductive shielding does very little to block magnetic fields, but it can reduce loop area if used to deliver power and if wiring is arranged so that supply current flows in the center conductor and 'out' the shield. In other words, avoid providing multiple 'grounds' to the same equipment.

Posted by: Optoeng | March 23, 2019 10:47 AM    Report this comment

The fact that installations with dipole antennas are also susceptible to EMI from switchmode electronics does not prove Naranjo wrong about the counterpoise. There are many EMI coupling paths, and you have to eliminate all of them to have quiet installation.

Posted by: Optoeng | June 24, 2017 8:23 AM    Report this comment

The fact that installations with dipole antennas are also susceptible to EMI from switchmode electronics does not prove Naranjo wrong about the counterpoise. There are many EMI coupling paths, and you have to eliminate all of them to have quiet installation.

Posted by: Optoeng | June 24, 2017 8:23 AM    Report this comment

Having just acquired an ICOM M802 (haven't installed it yet), this subject is new to me, forgive my ignorance. What about using the KISS-SSB counterpoise? It is self-contained, so decouples the counterpoise from the ship's bonding/grounding system. If the KISS-SSB is kept away from any nearby RF radiators, would that mitigate or even possibly eliminate SSB RF interference?

Posted by: 365Lusso | June 20, 2017 2:54 PM    Report this comment

In comment #2
You elude to possible using a pi type filter to eliminate 60 cycle noise. Great idea. It would be better if you described the inductor and capacitors and their configuration.
I look forward to your response.

Posted by: Sailaway1! | June 18, 2017 7:32 PM    Report this comment

Any kind of switching power supply will cause RF noise and should be shielded. This includes the compressor as discussed above, and also the current drivers on led circuits. The pulse width modulator current drivers create RF noise for the same reason, short rise and fall edges on the pulses.
We need products that are tested for RF noise.
Practical Sailor, have you done testing of products for RF noise?

Posted by: sailing Jack | June 18, 2017 11:08 AM    Report this comment

Good article! However, I disagree with the comment of Naranjo. The counterpoise of an SSB system has nothing to do with the noise generated by fridge compressors. The fridge noise is still a problem when using a genuine dipole antenna, without the need for a counterpoise. Most fridge compressors run internally on AC, and the conversion from DC to AC is what generates the noise. The more abrubt the polarity of the power is changed with each cycle, the more efficient the power is converted, but at the same time the sharp transitions have harmonics far into the HF spectrum. This still wouldn't be a problem if the manufacturers would take care that these harmonics can't escape. However, compressors are mostly used by people without sensitive HF-receivers nearby so the manufacturers save some money by not incorporating adequate filtering. Ironically, most boat owners with HF radio's would rather spend a bit more money for a "boat edition" of the fridge compressor with more adequate filtering. Maybe we can persuade the manufacturers to offer this if we just recognize the problem as being the fridge compressor rather than the "counterpoise". Anyway, except for following the common sense route with short twisted power leads one can filter the power leads directly adjacent the fridge, by looping the power leads through proper ferrite torroids (not the clamp on styles) and use of capacitors before and after the torroids.

Posted by: Lucish | March 26, 2017 1:25 PM    Report this comment

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