13.56MHz shielding re-radation nightmares
13.56MHz shielding re-radation nightmares
(OP)
I am working on an RF based access control system. Small 13.56MHz transmitters are detected by a receiver/controller to grant or deny access to a doorway.
The problem is the 13.56MHz signal couples easily to metal surfaces and other wiring and re-radiates near the receiver triggering many false alarms. Site surveys and location of receivers can solve many re-radiation problems but system interconnect wiring also conducts this signal causing false alarms.
I have tried shielded wiring only to find the signal travels well on the shield also. I have grounded the shield to earth ground, circuit ground, single ended and on both ends to no avail. The only thing that has worked so far is to install ferrites on the offending wiring, at certain distances from the reciever. As the ferrites are a fairly expensinve solution I am fishing for any tips or alternatives to what I have tried so far.
Thanks,
Mike S
The problem is the 13.56MHz signal couples easily to metal surfaces and other wiring and re-radiates near the receiver triggering many false alarms. Site surveys and location of receivers can solve many re-radiation problems but system interconnect wiring also conducts this signal causing false alarms.
I have tried shielded wiring only to find the signal travels well on the shield also. I have grounded the shield to earth ground, circuit ground, single ended and on both ends to no avail. The only thing that has worked so far is to install ferrites on the offending wiring, at certain distances from the reciever. As the ferrites are a fairly expensinve solution I am fishing for any tips or alternatives to what I have tried so far.
Thanks,
Mike S





RE: 13.56MHz shielding re-radation nightmares
Or use IR instead of HF
<nbucska@pcperipherals.com>
RE: 13.56MHz shielding re-radation nightmares
HF is a bad choice anyway - try tuning a SW radio to that frequency (including plus or minus your bandwidth) and see how much other junk is there.
RE: 13.56MHz shielding re-radation nightmares
RE: 13.56MHz shielding re-radation nightmares
The suggestion is to put a button on the transmitters so that if someone wants the door open then they can push a button on the remote control. At that point it doesn't matter how the signals get coupled in - so long as it is reliable.
You've set yourself up a difficult problem if you're trying to create a 'proximity detection system' in the HF bands. If I knew your digital code, then I could open your door from halfway around the world on a good day.
The only solution that I can imagine is to try to use multiple drectional antennas (ferrite loopsticks) and "AND" the data so that someone must be in a particular location to trigger the door. Even that wouldn't be perfectly reliable (probably not even acceptable)
It would be cheaper to purchase a commercial 'access control' system.
RE: 13.56MHz shielding re-radation nightmares
<nbucska@pcperipherals.com>
RE: 13.56MHz shielding re-radation nightmares
to keep the RF from crawling all over your wires, and shields, you may have to resort to twin ax (shield (balanced) twinline with a braided shield(96% coverage). Additionally you may have to use external capacitive chokes. These work as "band" stop filters for any external rf sneaking down the lines.
Baluns may be in order if you only have "single-ended" outputs from your sources...