Loop Antenna Interference

[lixnig]

Hello, Everyone.

I'm not an antenna engineer in any sense, so I was hoping I could gain some knowledge from the experts here.

I work for a company that manufacturers and supplies LCDs. We are currently providing a small display which they are now considering placing a small loop antenna in very close proximity to (i.e. between panel and backlight). The antenna is a receiver-only for an RFID tag application operating around 13.5 MHz. They have asked whether we can foresee any problems with this, and at first my initial reaction was that there shouldn't be since LCDS especially of this size are not really huge sources of radiation, but I don't really have a way to quantify that reaction at the moment.

Could someone with some experience with these types of antennas share some thoughts with me regarding the susceptibility of these antennas to interference or anything in particular that I should keep in mind going forward on this?

Thanks.

 

[E Kafeman]

I have been involved in similar designs with antennas 800 MHz and upward. It is a lot of things to think about that can cause problems but a not unlikely problem is if RFID antenna RF power causes modulation of the display. It can be seen as reduced contrast during transmitting phase. Do not harm display but can be a bit disturbing.
As I understand is the loop antenna oriented that it also can interfere with other circuits in the phone, especially other loops such as long PCB traces and coils. Maybe is output TX level to low to cause any bigger problem but can be worth to at least keep an eye on.

Sorry, didn't read that it was RX only. Worst case of of noise radiating from a LCD display in this frequency range that I have seen was for a display which had graphical driver placed on the thin film PCB , between display and main-board, resulting in poor grounding and a lot of local generated EMI with peak harmonics around 60 MHz.

 

[biff44]

many lcd's have a metal backing, and as such would greatly impact RF energy going thru it! Find out exactly what they mean by an "lcd".

 

[lixnig]

I am one of the engineers for the company that is supplying the LCDs. This particular model does not have a metal back plate. It is plastic.

 

[FvM]

The question isn't clear. You mean, the tag antenna is placed under the LCD, or the reader antenna?
In both ecases, the resonant cicruit will be slightly de-tuned, but it can be compensated of course.

 

[vfone]

Usually receiver desense and noise or spurious generated by LCD goes hand in hand.
Here is about two separate things:
Antenna type placed near an interferer source. And loop antenna from this point of view has one of the worst behavior. Just think that all the EMI or noise probes actually are loops.
Second, is about LCD behavior in terms of generating noise or spurious that can desense the receiver. I've seen all kind of noise (narrow or broad band), or spurious as clocks, pulses, carriers, generated by LCDs, especially by their flex cables, but not only.
You have to use a sensitive probe before and find the right spot near an LCD where to place an antenna.

 

[lixnig]

Honestly, I'm not exactly clear on what it is they want to do with this either. Based on my knowledge of what they are developing, I can not see a good use for what it is they are trying to accomplish. I have not met with them to discuss it in detail. I was just trying to get some insights so I had a better idea of what I should look further into before our meeting with them.

Thank you all for you help.