[RFI] Alarm Systems and EMI/EMC

Sun May 10 07:14:26 PDT 2009

I knew there was a good reason not to install a doorbell in my house.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

> -----Original Message-----
> From: Frank Haas KB4T [mailto:kb4t at arrl.net]
> Sent: Sunday, May 10, 2009 14:06
> To: rfi at contesting.com
> Subject: [RFI] Alarm Systems and EMI/EMC
> I am an Interference Investigator for a large electric utility in
> Florida. This thread reminded me about an interesting case I
> investigated a couple of years ago.
> The customer reported that key fobs/remotes for cars parked in his
> driveway and his wireless security alarm keypad mounted on the side of
> the garage door had stopped working recently at his home.
> I Know that most automotive key fobs/remotes usually operated in the
> 315 MHz range. I didn't know anything about wireless security alarm
> keypads.
> I visited the customer's premises and parked my Interference Hunting
> truck in his driveway. I tuned one of my wide band receivers across
> the spectrum between 300 and 320 MHz and noted a very strong
> continuous carrier at 314 MHz. Even my key fob had trouble working
> while I was parked there. I broke out a yagi and a receiver and
> started walking around the house to see where the lines crossed.
> (Bearings of the strongest signals taken along the 4 walls of the
> home's exterior.) The lines crossed near the garage. Checking inside
> the home it was very difficult to pinpoint a specific location.
> Turning off computers and related network equipment didn't stop the
> carrier.
> Since DFing the signal proved to be inconclusive, we started shutting
> off breakers to power down different parts of the house. The carrier
> was still present even after all the breakers (except the Main
> Breaker) had been turned off. Turning off the Main Breaker made the
> carrier disappear. The only conclusion I could draw from that was the
> I asked the customer to disconnect one power lead from the door bell
> transformer secured to the garage ceiling and we turned the Main
> Breaker back on. No Carrier. I asked the customer to show me his door
> bell system. The door bell system was mounted in a difficult to reach
> spot at the ceiling where the wall to the garage met the wall for the
> living room. Because of its location and difficult accessibility I
> could not read the manufacturer's label on the doorbell's case. The
> customer told me that the doorbell was a programmable electronic music
> type which was now going to be replaced.
> The wireless keypad secured to the side of the garage door frame
> outside tested perfectly with the doorbell system disabled. While the
> customer operated the keypad, I monitored my handheld receiver and
> could hear the pulses from the keypad at 316 MHz. The customer's
> automotive key fobs all worked perfectly as well.
> The doorbell system's microprocessor clock oscillator was producing a
> very strong continuous carrier on 314.4 MHz that drifted plus or minus
> about 500 kHz. The doorbell had two pushbuttons connected to it whose
> wiring served as long wire antennas (which made DFing/pinpointing the
> exact location of the source difficult. The doorbell's errant
> "transmissions" literally wiped out that portion of the spectrum used
> by a wide variety of automotive key fobs and other consumer electronic
> devices.
> Reading the recent thread on wireless alarm systems reminded me of
> this interesting case. Doorbell transformers are notorious producers
> of HF/VHF spectrum-blanketing buzzing interference. Beware the EVIL
> doorbell!
> 73,
> Frank N. Haas KB4T
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