by Dave G0HVN

What is EMC?

In very simple terms if you are operating a transmitter on any band there is a possibility that it will cause interference to some nearby electronic devices. However it also works the other way around, nearby electronic devices can cause you interference. In particular this occurs more in the HF and low VHF spectrum more commonly, but can be present on higher frequency bands as well.

The general cause of interference to the amateur station is usually from switch mode power supplies (the sort that charges your phone). Issues I found:

  • Power supply to external computer hard disks,
  • Cable TV receiver power supply and also an HDMI splitter amplifiers used on my television to switch multiple inputs.

The symptoms are usually a high level of noise, the noise floor rises to about S9 making reception on some HF bands impossible without locating the source and eliminating it.

Practical steps you can take

When you start looking for sources of noise, the best approach is to run the receiver you are using on batteries, then turn off the mains supply to the house (not forgetting Burglar alarms have backup batteries in them so still supply power to the circuit board). They also can have very nice long runs of cables connecting the devices to the control unit, which act quite nicely as an antenna.

Once the power is off check to see if you still have the noise. If it has gone then it is more than likely coming from within your house. Turn off every piece of electronic equipment in the house including the boiler, fridges and freezers (don’t forget to turn them back on after you have finished testing!). Then restore the mains supply and turn everything back on in turn until the noise comes back. If you are lucky it is one device and you can address it with the same methods that I will explain below:

If you still have the noise when everything is turned off in your house then it may be coming from a neighboring house or street lamp for instance. You can track it down with a portable radio and perhaps if you know your neighbors well you may be able to get them to turn the power off and do the same tests as above. If not then you can call in OFCOM to investigate.

A number of devices that are used for relaying home networks across the mains have now been made illegal if they are not approved devices you do have some protection from OFCOM. If this is the case a visit to the OFCOM website for advice.

One issue we have being radio amateurs is that you need to put transmitting antennas up. In the loft space they can’t be seen, so your local neighborhood does not know they are there. With visible antennas if people are experiencing interference they may well assume it is you. In which case ask them to log the time the interference starts and stops and you do the same by keeping a log of your transmission and compare the two. If you are lucky they won’t mach. If not then you have something to work with as it may be only a certain bands that are to blame.

Cases I have had in the past when I visiting a neighbor was bad connection to the TV aerial. Once this was cleaned and re made and some anti interference chokes (or braid breaker) added it was resolved.

Symptoms, Causes and Fixes

With Modern electronic devices cases of interference are less than they used to be but it still happens. Typical cases are HIFI systems whereby you break into them so they either buzz when you’re transmitting or sound like Donald Duck when they pick up your SSB transmission. The cause is usually pick-up on the audio cables that feed the signal in to the Amplifier. Equally speaker cables can be an excellent means of picking up interference. Effectively the cable going to the music amplifier act like a big antenna so the amplifier picks up the RF coming from your transmitter.   The same can apply to; computer speakers, play stations and televisions. Almost anything with cables.

This is not the end of the world in terms of amplifiers, all it takes in most cases is to change the speaker cables from flat twin feeder to balanced screened cable and to add clamp on ferrite chokes to all of the cables going in to and out from the Amplifier. Remember when earthing screened cables to speakers, earth them at the Amplifier end only other wise you will not have a screened cable but a very good RF receiving loop wrapped around the speaker cable.

RF Chokes

When adding chokes to stop RF breakthrough you need at least six turns of the cable through the choke or you can put six chokes on the cable (see picture below). Just keep adding chokes until it the interference stops, Note: they need to be as close to the Amplifier as possible. You can add six to each cable and then take them off one at a time until you reach the minimum you need to prevent the interference. Please also note that interference can vary from band to band, you may find one band causes no problems but others do. You may also find you need different number of chokes depending on the frequency band. I needed more chokes to stop 80m band interference Than I did to stop 20m band interference.

This is a photograph showing the addition of chokes to an amplifier, untidy but normally this would be hidden behind the amplifier. The cable on the left is the mains power supply, the two grey cables are the screened speaker cables the screen is grounded to the phono input earth post on the back of the amplifier. I placed three on the mains cable, three on each of the speaker cables and 6 on the audio input, two chokes at the sauce end and four at the amplifier end of the cable. The photograph on the right also shows the choked cables that feed in to the back of my computer monitor.

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I operate with antennas in my loft space so I have placed chokes on all of telephone, television, alarm and power cable that pass through the loft. I have also added coaxial chokes to my transmitting antennas but I will cover this in another document.

Since doing this to my amplifier I can operate full power on all bands and have no more issues. Before it used to sound like a car horn every time I transmitted, even at low power.

Ferrite chokes are available mail order on the Internet from many suppliers in many different sizes. Remember to pick a size that you can get a number of turns of cable in or use a minimum of six chokes for larger diameter cables.

The approach I used here with the amplifier can be used on all cables going in to any devices you are causing issues with. Also the same applies if it is noise that is caused by a device. I fixed my USB hard drive power supply issue by adding two ring chokes and winding the very small diameter cable through it about ten times stopped this interfering with the HF radio.

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Further Reading



I hope you have found this article useful and have years of fun in Amateur radio.

Dave G0HVN