[RFI] Re:Home Security System RFI

[Bud Hippisley, K2KIR]

At 12:39 AM 2002-11-04 , WB2JUF wrote:

>I think I’ve determined that its coming from my home alarm system.  
>How do I tackle this?  I’ve had the alarm co. here and they insist it’s not
>the alarm system.  I’m pretty sure it is.

A few years ago I had the same problem.  But through trial and error I found a technician at the security system office (it was ADT in Syracuse) who told me the process to follow, and which wires to remove from the control unit, to:  
      a.)  remove the battery backup power from the digital board;
      b.)  remove all the remote sensor wires from the digital board.

When doing this, I needed to notify them, because those disconnects cause an alarm in the central office (obviously).  They were actually quite cooperative about it.  In fact, rather than mess around trying to "quiet" their circuit board, I reached agreement with them that on major contest weekends when I needed really quiet low-band DXing conditions, I could call them and notify them that I was going to be "out of service" for 24 or 48 or ?? hours. 

It should be no surprise that the security system creates RFI -- it has a digital control board connected to remote sensors through very, very long runs of plain old unshielded twisted pair wires that tend to be routed all sorts of crazy ways through your house, often running very near your AC wiring. 

In general, there are at least two common sources of RFI in your own home that can continue to cause problems even when the AC mains are off or disconnected.  As you have discovered, the home alarm system is one.  But any UPS (uninterruptable power supply) is also a candidate -- at least until its internal battery runs out of gas.  

As an aside, it is important to buy a UPS designated for home environments, as I found out the expensive way.  My first TrippLite turns out to have been spec'd for commercial/office usage, and it puts out a broadband buzz from 1.5 MHz to 3.6 MHz.  My latest TrippLite seems to be clean; it's spec'd for home use.  (FCC Type A versus Type B, but I don't remember which is which.)

By the way, a few years ago I tracked down an unexpected source of low-band RFI that I had unwittingly brought into my own house -- a Canon multi-function fax/copier/scanner !!!  (It created the noise even when it was just sitting idle, waiting for incoming faxes.)  Of course, the problem went away when the power went out, but so did most of my low-band DXing....

One other source of noise that took me a while to figure out:  most newer garage door openers have an "electric eye" circuit that senses for obstacles (little children, especially) about a foot above the garage floor and stops or reverses the downward motion of the door if such an obstacle is detected.  On my 3-year old openers, whenever the garage door is open (not just in motion, but open and not moving), the circuitry throws major hash into the AM broadcast band and of course 160.   Again, the wiring to the remote sensors is unshielded twisted pair. 

Bud, K2KIR