[sdiy] Preventing Radio Ham Interference

Magnus Danielson magnus at rubidium.dyndns.org
Mon Nov 28 17:39:23 CET 2016

Hi Tony,

On 2016-11-28 17:14, 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?

Well, considering that you have a 2 m transmitter, it will kind of help.

I would assume that you have a problem when he does SSB DX rather than 
normal FM local. If he is cooperating, then you can coordinate so he can 
do test-transmissions in both modes. SSB is one of the AM side-bands, 
with the lower side-band and carrier removed. FM usually detects poorly, 
and only produces a DC effect as the carrier amplitude is stable and 
only frequency modulate around. The diode action and lowpass favours AM 
and SSB.

I've had the same issue with one of my neighbors.

The first line of defense is to make the equipment a worse receiver. 
RF-chokes and in particular with supporting capacitance to load them to 
ground is recommended. Typically it will induce common mode, so that is 
why a common mode RF-choke helps. Try to direct the RF power into ground 
(through capacitance) and object to it's conduction into your circuit 
(through inductance).

144 MHz is relatively "easy" is caps and inductors doesn't have to be 
very large to "act".

73 de Magnus SA0MAD

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