[RFI] Alarm Systems and EMI/EMC

Frank Haas KB4T kb4t at arrl.net
Sun May 10 07:05:45 PDT 2009

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

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

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

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


Frank N. Haas KB4T

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