The AM Forum

Help!  A neighbor has complained that he hears my transmissions
on his Brookstone "sound maker" (white noise generator plus other recorded sounds).
The rf is clearly coming into the device thru the power connection, since when the
device is disconnected from the ac line and run off batteries the interference stops.
I tried winding several turns of the cord from the wall-wart thru a clamp-on ferrite
choke, but this didn't make any difference. Also tried a brute-force emi filter between
the ac line and the wall-wart, with no effect. Ideas?  I am also apparently getting into
his baby monitor, and chokes did not do anything there, either.  The monitor operates
at 49 MHz, but don't believe I have a seventh harmonic strong enough to travel several
blocks (I am on 40 M).  My guess is that my sig just blasts right thru whatever mickeymouse
tuned circuit is used in this thing.

I'm bummed!

Line cord is acting like an antenna and the RF is being rectified by the power supply diodes.   In the case of the baby monitor, many times the line cord is the antenna for the unit.

In the case of the noise generator, a cure might be to replace the zip wire from the wall wart to the noise generator with shielded wire.  You can do a quick and dirty check by wrapping some aluminum foil around the cord and seeing if it helps at all.

In the case of the baby monitor, I'd just go buy several models, try 'em all, and return all the ones that exhibit RF susceptibility.  20 bux or so is cheap to keep the neighbors happy.

Radio Shack used to sell wireless intercoms that used carrier current.  I have a set of those and I have no RF susceptibility problems with them, even at full strap.  They have a key-down lock that would work well for a baby monitor.

I have had some success using the line filters available at the surplus places that are normally found as the AC line filters in power supplies, etc. with audio devices (stereos & surround sound rigs) I place the filters right on the speaker leads at the amplifier. This has always worked quite well. Both of the devices you are talking about are cheap pieces of swill. You may need to tear these apart to try and make the voices in their head go away. Tell her that the whole world can hear everything going on in their home due to the baby monitor transmitter running all the time!
:)

My guess is that the wall wart is a low voltage AC supply, and that the interference is coming through as a differential signal across the low voltage AC output of the wall wart. Try putting a 0.1 uF capacitor across the wall wart's output (e.g. make up a small box that the wall wart's output connector can plug into and which has an output cable that plugs into the appliance. Put the capacitor close to the appliance.

Good luck!

Stu

Thanks Guys!  I think Stu is correct about the differential mode, explaining why
the common mode chokes didn't have any effect.  The baby monitor has separate
antenna (doesn't use line cord as ant.), so I believe the rf is getting into the
unit via antenna...this could be a more difficult problem.  BUT, you've given me
some good ideas to try.  I will post again after trying them!
Thanks again.

At&T wall phones were good radio receivers too.
The baby monitor is fundamental overload and John has a good approach. The carrier current intercoms are good too. The baby monitor is just a one-way device anyway.

Fred

  Trying to solve these kinds of issues is a good thing to do. Sometimes however it is not easy, or the neighbor gets pretty upset with threats, etc. Then it is time to make sure they know your ham callsign, and also give them the phone number of the FCC.

   Also if the part being interfered with is compliant with FCC part 15 that means that the owner legally approved with part 15 terms whenever they bought it. This means they MUST accept any interference from a licensed and legal radio transmitter.

   Ironically my big brother is a lawyer, and a diabetic who uses an insulin pump. That pump is compliant with FCC part 15 rules. Maybe this is a stretch, but does this mean that If I key a 2m HT near him and his pump goes to full flow killing him, arn't both I and the pump maker free from liability? ;-)

HNY
Jim
WD5JKO

While the pump is FCC part 15, part of the approval by the FDA for the device is a detailed review of the failure modes of the device, the effects on the patient for each, and mitigations the manufacturer has taken to prevent the severity or occurance rate. Design of the insulin pump includes circuitry that prevents a full-tilt injection of the insulin. At worst, keying up your 2M HT next to the pump will cause it to stop and/or produce an audio alarm.

If keying up the radio did cause a catastophic failure of the device, the device manufacturer is responsible for investigating the root cause of that failure, and developing any corrective actions to prevent its occurance again in the future.

FDA regulations at work.