FW: [RFI] Motion Detector RFI?

Hare, Ed, W1RFI Hare, Ed, W1RFI" <w1rfi@arrl.org
Thu, 2 Dec 1999 13:43:10 -0500

From: Hare, Ed, W1RFI 
Sent: Tuesday, November 30, 1999 9:27 AM
To: 'Scott Neader KA9FOX'; rfi@contesting.com
Subject: RE: [RFI] Motion Detector RFI?

Hi, Scott,

I would treat the motion detector problem as any other case of RF getting
INTO a device connected to the power lines.  Chances are good that RF is
getting picked up on the wiring, coupled into the device and activating the
circuitry.  The old adage that components don't pick up RF -- wires do --

In most cases, the RF gets picked up on the line mostly in the common mode
(ie, both wires act as if they were a single wire).  A common mode choke can
often help.

To make a common-mode choke for HF, I recommend a 240-43 ferrite core
(Palomar, Amidon, etc). 

Unfortunately, this will need to be put on the electrical wiring, right
where it connects to the sensor light.  Normally, one needs at least a few
turns of wire to form an effective common-mode choke -- difficult to do with
existing electrical wiring (usually just long engough to get the job done),
even with a 240 size core.  It is very tempting to try a few large ferrite
"beads," but a common-mode choke is much more effective if it has
inductance.  At upper HF, several beads in series on the electrical wiring
may help in moderate cases of interference.

Electrical wiring is not well balanced at RF (the neutral line is grounded
near the service entry, and on the power-distribution system; the hot line
is not), so you may also have some differential-mode pickup (this will be an
RF signal that is INSIDE the wiring, just like the desired signal in a
transmission line).  In that case, a brute-force type filter may help, again
installed at the light.

In almost all cases, it is NOT a good idea to start tearing into a
neighbor's electrical wiring to install these cures.  For a number of good
reasons, most of the work I described above should be done by a qualified
electrician. The FCC's material about RFI clarifies that interference to
non-radio equipment is not the fault of the transmitter. 

It is not real likely, but the manufacturer of the light MAY have some info.
Do you know who made the light?

For sources of ferrite cores and ac filters, see the search engine at:

http://www.arrl.org/tis/tisfind.html.  Scroll down to the bottom of the
screen and use the pick list for categories.

73 from ARRL HQ,
Ed Hare, W1RFI
225 Main St
Newington, CT  06111
Visit the ARRL Website at http://www.arrl.org/. Check out the ARRL 
members-only website at http://www.arrl.org/members/.  Have your 
ARRL membership number (from your QST mailing label) available the 
first time you check in. For answers to many technical questions, 
consult the ARRL Technical Information Service page on the ARRL Web 
site at http://www.arrl.org/tis.

> -----Original Message-----
> From: Scott Neader KA9FOX [mailto:ka9fox@QTH.com]
> Sent: Tuesday, November 30, 1999 12:33 AM
> To: rfi@contesting.com
> Subject: [RFI] Motion Detector RFI?
> I've been a subscriber since January 1999 and have kept all 
> the postings as 
> excellent reference material.  I searched these postings, 
> plus the ARRL 
> site, and came up with nothing as to "motion detector" RFI fixes.
> During the CQWW CW contest, a neighbor called to say that 
> when the antenna 
> is pointed his way, I am constantly turning on his motion 
> detecting flood 
> lights.  This was happening on 40 meters with 1.5kw output to 
> a 2 ele at 75 
> feet.  We have small city lots here, so he is fairly close to 
> me I guess 
> (maybe 150 feet NE of the tower??).  I don't know about other 
> bands yet (I 
> did a single band contest effort)
> Anyway, to the question... any advice for troubleshooting 
> motion detector RFI?
> Thanks for the assistance!
> 73 - Scott KA9FOX
> tower:  http://www.qth.com/ka9fox/tower
> shack: http://www.qth.com/ka9fox/ka9fox.jpg 
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