HF Common Mode Choke

On HF and low VHF Common Mode can be a problem. This can cause excessive RF in the shack. Leading to problems with radio equipment and in severe cases, RF shock !

Some common symptoms :-

microphone buzzes.

SWR can be high.

RF can flow though PC interfaces and cause problems, such as keyboard and mice not working or monitors going black. The PC may reset and in bad cases, be damaged.

Excess background noise.

Modern equipment that’s micro-controlled. Can change mode, PWR, reset etc.

Common mode current is not just a problem for RF but for our purposes, it can be prevented. In some circumstances they can even lower the noise floor. which is picked up on the outside of the coax and fed into the radio equipment.

HF Common Mode choke

HF Common Mode choke

The above is a simple Common mode choke for 5 Mhz to 200 Mhz.  It is made from RG58, 50 ohm coax and some ferrites. The ferrites are ” type 31 ” these particular ones are large enough for 5 loops of RG58 to be fed through them. I have used 4 ferrites on either side. Once you have them in place, secure them with self amalgamating tape. This same design can be found on my 40 Meter Off centre fed Dipole page. here :

Alternate design

The above design is ok for permanent installations. Its not that great for portable use or field days. So I make a slightly different design.


for portable and field day use, I have squeezed the choke into a weather proof box. It uses the same design as above, its just fixed and less prone to damage. The toroid’s are fixed in place with silicone sealant.


Each end of the choke has SO239 plugs, this makes connecting it easier.


The joints are sealed with silicone sealant, for when the weather is against us.

That’s it, its very easy but it can make a massive difference ! Its best to place it outside of your operating building.










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