ASK THE EXPERTS: Ham Radio Interference

Snap-around ferrite bead.
Snap-around ferrite bead.
Snap-around ferrite bead.

I was recently asked by a local ham radio club to speak about renewable energy, since I have designed and built several RE systems, both for myself and for others. This group is especially curious about how much radio frequency interference (RFI) there is with components of a solar-electric system, especially inverters.

John Faughn • KD0CAC

I’m a ham (KC2FSW) and host a two-meter repeater (147.045, with the same call letters) that is powered by a 150-watt off-grid PV system. Early on, the first issue I had was interference from a pulse-width-modulation (PWM) type charge controller. We were not able to run that controller in PWM mode, due to a high level of RFI. I changed repeater radios around that time and the controller did not affect the new radio. Unfortunately, the new radio had a tendency to burn out controllers—four of them, all different types from various manufacturers. From what we could gather, it was stray radio frequency (RF) coming in from the repeater antenna via the solar-electric modules that was burning out the charge controller front ends. Bob Gudgel, a fellow ham (and electronics designer) from MidNite Solar, sent me a Classic controller to see if we could blow that up, but it’s still running after three years—and there have been no RFI issues with this controller.

As for inverter RFI, some are worse than others. The OutBack inverters tend to interfere at the lower HF frequencies, but rarely on the two-meter or higher bands. The interference from inverters on the AM broadcast band is legendary in the RE business. Feedback from other hams suggests that torite cores on the inverter and radio DC cables work well. They need to be large enough to fit around both the negative and positive conductors. These can be found at electronics supply houses. Capacitors are also used to filter out RFI.

The best defense against RFI is distance—the farther your radio is from the inverter, the better!

Our county’s emergency management office and the National Weather Service both love my repeater because it never goes down in an outage. This repeater has seen frequent use over the past 10 years by these agencies to pass weather, health, and welfare traffic. 73!

Roy Butler • Four Winds Renewable Energy, Arkport, New York

Comments (1)

James Lynn Skinner's picture

Roy, I am a ham also and am building up a larger solar array, do you have any observations of RFI from MPPT charge controllers? My current array is 4 ea 100 watt panels using a PWM controller. I'm
thinking of adding 4 more 100 watt panels and using a MPPT controller

Tnx Jim K6BPT

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