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
adequate..
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
resistor.
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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