Charger Radiated Emissions

I usually have a respectable HF noise floor at home, for which I count myself lucky in today’s sea of cheaply made switching power supplies. However, we were expecting some severe winter weather a couple of weeks ago (it never materialized, but that’s beside the point) and I noticed that my noise floor had gone up from around an S1-2 to an S5-7, and it was consistent. I did a bit of sleuthing, and it turns out that it was my own doing. I had plugged in a Baofeng UV-5R, and a Radioddity GD-77 to charge in preparation for the potential power outages we were facing with the incoming winter weather.

At first, I blamed noise conducted from the chargers leaking back towards my radios since a lot of these cheap chargers have minimal to no mains input filtering. Thinking more about this, I do have a very aggressive filter on my mains supply, and the power supply I use for my radios also has an aggressive mains input filter included, so it was more likely that what I was seeing was radiated noise, transmitted via the DC power leads which supply the charging power to the radio charger bases, and received via my antenna.

I have extoled the virtues of clip-on ferrites in the past, many times, and this is yet another application for them.

Baofeng UV-5R charger (left). Radioddity GD-77 charger (right).

As you can see, I managed to get three turns of each charger cable through a clip on ferrite. It is important to get as many turns as you can. See “Number of turns of wire” section at this link. One turn is each pass through the ferrite – not each loop over. It is also important to install the ferrite close to the body of the charger so that the majority of the cable is filtered.

I used a low-frequency near field pickup coil to examine the emissions on the cables before ferrite installation and after ferrite installation. The difference was quite clear.

Blue = No ferrite. Red = Ferrite installed, 1 turn. Green = Ferrite installed, 3 turns.
Frequency x-axis is 509.6 kHz per division.

I realize I should have extended the upper frequency limit to include the whole HF spectrum, but the frequency response of the pickup coil rolls off after about 4 MHz, so there was little point. I may revisit this in the future, but the improvement is significant not only up to 5 MHz in the lab, but it in the real world also, since my HF noise floor returned to normal S1-2 after ferrite installation.

This is not to rag on switching power supplies, for they are great even for radio use, if they are well-designed. Cheap, small chargers and small switching supplies are missing much-needed filtering due to the need to keep the small size, and most importantly – to keep the cost down.

These cheap radios may well come with an FCC sticker, and even an FCC ID number, but I’m willing to bet that the power supplies / chargers do not!

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AD5GG works in the real world as an RF design engineer. Occasionally, he posts articles on this very site. Sometimes they're even worth reading. Please feel free to comment on posts using the comment system at the end of each post, and use the discussion forum.

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Peter L
5 years ago

40 dB in the middle of the “Standard Broadcast Band” – nice! Should be able to do some DXing now. 🙂

2 years ago

The number of turns of wire link is invalid

Questions and Comments are welcome!x