[RFI] Alarm Systems and EMI/EMC

Dennis Vernacchia n6ki73 at gmail.com
Sat May 9 20:36:55 PDT 2009


here is  a more detailed answer from my amigo John WB6IQS who has a lot of
practical experience in the EMI/EMC field.

73, Dennis N6KI

Security Systems:
They are all over the place actually.  Honeywell uses 345 MHz, GE uses 319.5
and Linear has numerous frequencies but most of them now are on 315 MHz.
There are also some narrow band FM European types that are imported into the
US that operate on 433.92 MHz.

The security receivers are generally superhets now, fairly broad in
frequency bandwidth (+/- 125 KHz) and they typically use pulse position
encoded AM modulation.  Older systems have super-regenerative receivers that
are much more susceptible to outside interference.

Garage door operators:
These are all over the place.   372.5 MHz for Genie, 390 MHz for Chamberlain
(old frequency), 315 - 318 MHz is commonly used now for Chamberlain and
Linear products.

Other systems had 288, 310, 318 MHz all the way up to 433.92 MHz.

Most garage door operators are also AM pulse modulation and some of the
cheap systems still use super-regenerative receivers.  These receivers
are about 5 MHz wide in bandwidth for only -3dB down.   Really broad as a
barn door.

Sorry that there is no easy answer but "that's the way it is".

Ham HF or VHF operation is unlikely to cause false alarms.   For HF
frequencies we are far enough away that the only problem might be that their
EMI microprocessor noise may interfere with our receivers.  For VHF/UHF
operations we may jam them for a short while, but they will normally reject
our FM signals since they only demodulate AM pulse coded signals.

I have heard of a number of instances where very high power HF stations
caused false alarms to the panels, but that was due to so much RF saturating
the remote signal wires that the ICs sensed a push button or sensor signal
where there really was no signal.  Using some ferrite cores and wrapping
the sensor wires around the core at the alarm panel fixed these problems.

The new microprocessors are getting very fast, have more EMI output and
are more sensitive to outside EMI.   Every time that Microchip does a die
shrink to make their processors faster, cheaper and better we get bit in the

John Kuivinen, WB6IQS
EMI/EMC Engineer
Linear LLC
Carlsbad, CA.

On Sat, May 9, 2009 at 7:52 PM, Jim Brown <jim at audiosystemsgroup.com> wrote:

> On Sat, 09 May 2009 12:28:05 -0700, Michael Tope wrote:
> >Anyone know any home security equipment dealers who are hams or who
> >know anything about EMI/EMC?
> No experience with these guys, but security systems are notorious
> for RFI susceptibility.  FWIW -- that 300 MHz range is commonly used
> for garage doors and remote control of A/V systems. I don't know of
> instances of RFI to the RF functions, but expect problems with  RFI
> to other circuitry. The usual fixes are twisted pair wiring, caps
> across wiring from dry contacts, and chokes to kill common mode.
> 73,
> Jim K9YC
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