[sdiy] Preventing Radio Ham Interference

[Tim Ressel]

I spent 3 years at HP doing EMC testing on new products so I know a 
thing or two about interference. There are 3 paths the EMI can get into 
your equipment: the audio cables, the power cables, and directly through 
the chassis. The cables act like antennas and conduct the RF into the 
circuitry, where it gets turned into audio signals.

Audio cables should respond well to clip-on ferrites. Digikey 
240-2245-ND is an example of an inexpensive ferrite to try. Place the 
ferrites close to the input end of the cable. If you're lucky there is 
one piece of equipment that is sensitive and a few ferrites will fix it 
right up.

Power cables can also benefit from ferrites. Try this after you've done 
all the audio cables. Place the ferrites close to the chassis.

Direct into the chassis is a tougher animal. Metal cases should be 
grounded to form a Faraday shield. Plastic cases are a real pain. The 
original Midi Verb had an EMC issue and the wound up painting the inside 
of the case with conductive paint. If that is the point of the 
interference then you've got a battle on your hands.

Jarno mentioned a copper screen to block the signal. Its not a bad idea, 
but it doesn't have to be copper. You can try aluminum foil as an 
experiment. You'll need to ground it somehow. If it works you can turn 
it into an art piece.

Another technique I've seen is to use a handheld transmitter to pinpoint 
the trouble area. See if your neighbor has a 2M handheld and would be 
willing to do some testing. A six pack might grease the skids. Key the 
transmitter and wave it around the equipment and cables while listening 
for interference.

Good luck!

--TimR


On 11/28/2016 8:14 AM, Oakley Sound wrote:
> Hi all,
>
> My next door neighbour, a keen radio ham, has erected a new 144MHz 
> directional beam antenna. Unfortunately, for both of us, when it's 
> pointing towards my house and transmitting, it interferes with my 
> audio set-up in both my living room and my little music area. I get on 
> very well with my neighbour and we've briefly worked together on this 
> so we know what particular combination of output power, frequency and 
> antenna choice causes the problem. This antenna mast is around 5m away 
> from the wall where my audio gear is situated. When he uses his other 
> non directional antennas there is no problem.
>
> Anything more than 20W and the RF signal has an affect. Generally, 
> with my more modern gear it is not a problem, but my vintage gear (eg. 
> my hi-fi and older synths) are badly affected. More annoyingly, it's 
> also the modular and my ten year old Event active monitors.
>
> When transmission is ongoing I hear what seems to be a loud buzz at 
> 100Hz and harmonics. It's as if the mains is being gated into the 
> audio pathway during transmission. The higher the RF output power the 
> louder the buzz. The Event monitors pick up the noise even with the 
> audio cables unplugged and the mains additionally filtered. Bizarrely 
> it also makes the modular and my old SY-1 go out tune but I can't 
> detect any drop in power supply voltage so I think it's directly 
> affecting the VCO circuitry.
>
> I think I may have solved it with ferrites on the loudspeaker cables 
> on the hi-fi but the music area is still a problem.
>
> At the other end of my house I haven't got an issue but moving all my 
> gear there isn't really practical - although I'm thinking about it.
>
> Any ideas? Would some sort of earthed wire mesh fitted to the wall 
> directly between him and me create a suitable RF shadow?
>
> Tony
>
> www.oakleysound.com
>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com