Resolving the IC-705 USB interference problem

Since getting my Icom IC-705 I’ve had a constant issue with interference on the HF bands from the USB connection on the radio when the AH-705 remote auto ATU is connected.

If I use the radio without the AH-705 inline there is no issue at all but, as soon as the AH-705 is connected to the radio there is some 9 S points of computer data noise across all the HF bands rendering them unusable.

Reading online this appears to be a common issue and so I set about trying to find a resolution.

The noise is most likely being induced onto the outer braid of the screen on the cables and so I decided to look into making a common mode choke.

Visiting the HAM Goodies website I ordered a couple of FT240-31 Ferrite Toroids as I’d read this particular mix is ideal for making common mode chokes.

I wrapped a long USB cable around the toroid and used some tape to hold the cable in place, it’s surprising how much length is lost by simply wrapping around what looks like a very small toroid.

Firing up the radio the noise was reduced considerably but, it was still there and so I decided to do the same to the coaxial cable between the AH-705 and the IC-705.

Once I’d wrapped the coax around the toroid and reconnected the coax between the radio and ATU I was pleased to see that the computer data noise was now gone on all bands except for the 10m band. For some reason the 10m band interference is still as strong as it was without the toroids in place.

I’m not too sure why this is. I tried more and less turns on the toroid to no avail and so eventually I decided to just live with it for the time being.

At least now I can use 160m -> 12m without any interference whatsoever, a massive improvement on what I had before.

I need to do some more research on the different mixes of the ferrite toroids and perhaps trying a different mix will resolve the problem on the 10m band too.

More soon …