A Practical Guide to Locating Sources of AM and HF interference by Garry Cratt

The introduction of switch mode power supplies, light dimmers, flat screen TV sets, computers and their monitors, printers, modems, USB hubs etc and energy saving light bulbs has bought with it a range of interference problems for users of AM and HF shortwave bands.

RFI or radio frequency interference as it is formally known normally manifests itself on the AM broadcast and shortwave bands as harmonics of the fundamental interfering frequency.

In the “pre-digital” days, such battery chargers where “linear” in operation, typically containing a transformer, rectifier and filter capacitors. However the price of copper (used in the transformer) and the ability of switch mode designs to accept a wide AC voltage input range (typically 90-270V AC), has led to the almost universal adoption of this technology for battery chargers. No longer are there 110V and 240V AC variants of each battery charger, one design fits all!!

A primary source of these interfering harmonics, are such switch mode power supplies. Commonly known as “AC adaptors” or “plugpacks”, these are the little power supplies used as battery charges for smartphones, radio receivers themselves, battery chargers etc. Although these devices operate at 200KHz, the internal oscillator or more accurately the “chopper” circuit can generate harmonics well into the HF bands.

Normally built into a plastic case, the device is free to radiate, save for a few internal components designed to minimise interference. Assuming you are experiencing such interference, the first step is to locate the source.

First, ensure the receiver is being operated off its internal batteries, and tune to part of the AM or HF band where you can hear the interference but no radio signal is present. Turn off all room lights and see if there is any reduction in interference. Energy saving light bulbs contain electronic circuitry in the base of the light globe, which can cause interference, just as the electronic ballast can do in a fluorescent light. Dimmers are also a source of interference as well as fan controllers. All these need to be turned off.

It is prudent to only try one thing at a time, so turn the lights back on, and then systematically unplug every smart-phone battery charger in your house. A smart-phone charger generating interference on the AM band can be heard several rooms away, either by being radiated back down the house wiring, or by being radiated in free space.

The next potential culprits to eliminate are TV sets. Unplug each one from the wall (turning the power point off is OK) and see if there is any change in interference.

If you have progressed through the above steps without results, the next step is to turn off the power to your house at the fusebox. Whilst this normally means resetting all the clocks, microwave ovens etc. if will conclusively prove if the interference is coming from your house.

Next up: Learn how the Tecsun Shortwave Outdoor Antenna can help significantly enhance reception of signals in the medium wave (AM) and shortwave antenna bands coving 0.5-30 MHz.