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Looking at solar systems Hams mention that solar often brings RFI into the system, what part of the system in most likely to introduce RFI detectable on the ham bands?

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In the April 2016 issue of the ARRL's QST magazine article, "Can Home Solar Power and Ham Radio Coexist?," K1KP tracks down and attenuates RFI generated by his multi-kW rooftop solar installation. Using sensing loops and antennas connected to an all-band HF transceiver, Tony detected RFI from every part of the installation, though radiation from some of the sources dropped rapidly with distance.

The net of his analysis: RFI depends on the design and manufacture of the active system components (inverters, "optimizers") as well as the lengths and shielding of the passive components (wires). K1KP concluded that you may have to spend as much as 10% of the system cost and a considerable amount of time to identify and mitigate RFI using ferrite chokes.

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The panels themselves are essentially photodiodes, producing DC when exposed to light, and so generating no RFI. A very simple system consisting just of panels and a battery could generate no RFI at all.

However such a system system is not very efficient. There is an optimal impedance which the panels should see which optimizes the product of voltage and current and thus power they produce, but this varies depending on the light hitting the panels and temperature. Charge controller are thus essentially switch-mode power supplies (SMPS) which transform the load to this optimal impedance. Like the SMPS in LED drivers and cell phone chargers the controller will spew RFI from the high frequency square-wave switching, but as the power handled by a charge controller can be much greater, so too can be the RFI.

A quiet solar system thus depends on good PCB layout to minimize radiation, sufficient shielding around the controller to block radiation, and finally sufficient filtering to isolate the switching transients from being conducted on the input or output wiring.

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