[RFI] RFI suppression in alarm system

W5OT at aol.com W5OT at aol.com
Thu Oct 22 12:25:52 PDT 2009

Monitored Burglar/Fire Alarm System RFI  Solved
Most monitored systems have two keypads which are hard  wired to the 
central alarm unit.  The keypads are active, even when the system is in standby 
status.  I was experiencing ‘beeping” from the  keypads when transmitting on 
both the 160 and 80 meter bands.  RFI was not a problem at higher  
frequencies, probably because the filtering  built-in to the alarm unit was  
I tried several remedies including clamp-on toroids,  bypass capacitors, 
and single chokes which failed to prevent the RFI.   The RF chokes installed 
on the  keypad terminals at the central alarm unit seemed to offer the most 
promise, but  they only protected one band or the other.  I did some 
impedance measurement on the chokes using the Autek RF-1  Analyst and found that 
using a large value choke in series with a smaller value  choke exhibited a 
broadband high impedance range, sufficient to cover the  frequencies of 
interest.  This did  solve the RFI problem.
The chokes I used were Fastron series 23 with values of  680 microHenries 
in series with 47 microHenries. They are each about the size of  a ½ watt 
 A  “rule-of-thumb’ says that impedance of 1000 ohms or higher will 
usually provide  good RFI suppression.  Below is a  table of impedance 
measurements across the 160, 80, and 40 meter  bands.
Frequency, mHz           Impedance, ohms
1.5                                                              >2000
2.0                                                              >2000
2.5                                                              1860
3.0                                                              1520
3.5                                                              1370
4.0                                                              1196
7.0                                                              1462
7.5                                                              1250
Thus, high impedance is maintained over the frequencies  of interest and 
overall resistance was increased by about 8 ohms which did not  affect the 
operation of the keypads.  It is felt that combinations of chokes in series can 
be an effective way  of suppressing RFI in low current circuits where RFI 
is being experienced on  several frequencies.
Hope you find this info helpful.
73 de John,  K5SEE

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