An Interference Filter for a Security Alarm System.
John,
I have a security alarm system that plugs into the telephone line and my radio transmitter interferes with it.
It plugs into the telephone system using an 8 conductor RJ-45 modular plug. Do you have a filter for this problem or would you consider designing one.
Thanks.
Dick from New York
From John Browne ki6ky@ix.netcom.com
Dick,
This is regarding our discussion of RF filters for security alarm systems on the telephone.
I found 8 pin RJ 45 connectors in one of our catalogs. However my attempts to make an 8 line filter using 8 conductors and a large toroid were not entirely successful. I could not get the amount of attenuation that I consider adequate.
Filtering each line with a single choke would be too labor intensive given the hand winding of our chokes.
The best way to attack this problem would be to open up the alarm box and install RF bypass capacitors (.001 or .01 UFD) across from each sensor input to ground.
Thanks again for the inquiry.
Best regards,
John Browne
John,
If you would consider a multiple filter arrangement, custom made, I would be more than happy to pay for it; the attenuation might not be as great and the bypass idea is good but if i fool with that system i am going to have problems with the company.
what do you think?
for example, we could have a relatively large "package" maybe a pair of them, shrink-wrapped which would each have half the number of wires...they would have a pigtail lead with a socket for the input and obviously a lead with the RJ 45 on the output side with, for example, a 6 inch lead which would then plug into the system.
thank for your thughts on this.
Dick
John,
I just thought I'd ask if you have had any success with the attenuation issue.
I need something!
Thanks and I am sure we can do business; I just gotta get some help.
Many thanks
Dick,
I've been pretty busy lately but I haven't forgotten about you.
I have to order some 8 pin connectors, before I can do anything.
Hopefully I can get to it soon.
Best regards,
John
That's wonderful news, John.
please look at my potential order as if i would commission an artist to do a painting. it might be a "one-off" and i will happily compensate you for your expertise. if it is successful, which i believe it will be, you will have something great in your future product line.
Dick,
Here is a picture of the prototype alarm RFI filter, during construction. It is almost finished.
John
John,
The prototype RF filter for the alarm system arrived today in good shape, Thanks.
It is obvious that a lot of thought and work went into the prototype.
i have done some initial testing...the filter is working extremely well. all functions of the alarm are working normally and a test was held with central station monitoring to verify.
i tried RF. I used a KW output, something i could not do before on 40 m. in fact i was getting RF into it at 300 watts. now, there is nothing.
your prototype is a success!
from what I can see with empirical testing, all RF has been filtered away from the box and telephone system.
nice going....you have a real product there....i may be guessing but in my opinion, i am not alone, there are many people with complex security systems who are being troubled by RF.
thanks.
if I can add any more, please let me know. this is a remarkable achievement.
Dick,
Thank you for the glowing report and timely payment (your check arrived today).
I had some reservations because the attenuation is not as good as our RF-1 telephone/modem filters.
Our standard filters attenuate approx. 100 times or more (40 to 45 dB)at 7 mHz and 14 mHz but the
best I could get with the alarm filter is about a 50 times (approx. 30dB) attenuation even though it uses the same chokes as the RF-1. Having 4 chokes sitting parallel in the box must degrade the performance somewhat. Anyway, I'm glad that it turned out to be adequate for your situation.
Thanks again.
73.......................de...............ki6ky............sk........
John
This is possible, John as there may be some inductive coupling. however, 30db is a lot and if figured vs. the increase in RF from a linear amp, this is adequate. i really needed 20 db to be safe. a 100 watt transceiver didn't overload anything. at a KW (which is all i usually every run unless going into my beam which is far away and on 20 m) on 40 with a wire antenna in fairly close proximity, that 10db increase in power in the immediate field area would break through into the system...at 30db using your unit, there is nothing evident at all.
now that you know this can be done, you may end up experimenting and building for the markets a similar filter which, with internal isolation or some technique, will give you the additional 10-15 db. of course, erring on the side of caution, i promise i will be your first customer.
have a wonderful weekend and thank you once again.
73
Dick