Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

13.56MHz shielding re-radation nightmares

13.56MHz shielding re-radation nightmares

13.56MHz shielding re-radation nightmares

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.

Mike S

RE: 13.56MHz shielding re-radation nightmares

I would suggest more complex digital code -- instead of relying on the attenuation of the carrier
Or use IR instead of HF


RE: 13.56MHz shielding re-radation nightmares

Are the transmitters running all the time?  If so, why not add a 'push to open door' button?

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

The committment to RF has already been made, but we have thought of opto-isolation (IR) to bring the offending wires down to the controller (it is an expensive option).  Transmitters output a 13.56MHz on/off keyed (data is the carrier) ID signal with a random delay.  The purpose of the system is to prevent Alzhiemers patients from exiting the care facility and getting into harms way while alowing normal access by staff and visitors.  A push button is used on the outside, but using one on the other side would defeat the purpose of the system.  We seem to have good noise immunity, it is the re-radiation of our own transmitter signals that fakes the receiver into believing a patient (transmitter) is near a monitored door.

RE: 13.56MHz shielding re-radation nightmares

No, no.  The suggestion about the push button relates to the immediately previous question about the transmitters running all the time.  The description of your problem seems to indicate that these little transmitter are running all the time (continuous duty cycle), and you're trying to detect if they are close to the door or not.  The coupling into the power lines and so forth is defeating the entire plan (not surprising).

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

Or this ID chips they use for cows, pets, kids, etc.


RE: 13.56MHz shielding re-radation nightmares

the good thing about 13.56 Mhz is that is the same freq used by RF Diathermy, RF Induction heaters, and other industrial users. of all the freq's you could have picked....

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...


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close