antenna design for small rfid tag needed !

antenna as radiator

Greetings,

I'm writing my MSc thesis in microelectronics and need to design chip for remote measurements. Such chip would communicate wit base station just like RFID tags do (13,56 MHz), but communication range is small (1-2 cm) and the size of the chip should not exceed 5x5mm (including antenna!!). As I'm starting with the thesis, I should first check if design of such small antenna is possible. Will spiral, planar antenna do ? I will use a few layers of metalization and plan to design something similar to Maxtor's Coil-on-Chip, but a bit larger (they have 2,5x2,5mm chip). How should I design such antenna (can I use MWO or Ansoft Designer) ? Will such small antenna induce enough energy to power my chip (chip will have an analog part for resistance measurements and digital part for creating packets and sending them). I will really appreciate any design hints, links to similar designs, CAD software links. I repeat - the antenna must not exceed 5x5 mm !!

Regards and best wishes
Zbigniew
zbigniew.piatek@poczta.fm

A coil of many turns such as on a multilayer PC board might work. The reader should have a coil of a similar size for best mutual coupling. The reader output power can be very large because the antenna is small and will not radiate very well out to the 30 m measurement range specified in most government regulations for EMI testing.

Two more thing - in Maxtor's coil every turn has (as far as I remember) 14 um width and 4 um spacing between turns. I can make similar coil, as my target technology will be probably AMS 0,35 um.

As far as I know - planar coils (antennas) are not omnidirectional. Can I use dipol antenna ? Will it work in such application (i.e. will it induce enough energy) ?

Best wisheds,
Zbigniew

Hi Zbigniew,

take into account that the distance is small in relation with wavelength, so the antennas are in near field (induction region) rather in far field (radiation region). The coupling properties are quite different.
If you use coils, you should think in mutual coupling rather than in radiation pattern. The power transfer can be very good with low radiation, as flatulent said.

Regards

Z

Re: Re: antenna design for small rfid tag needed !

Hello Zorro,

You're perfectly right - this is near field. But the prolem is that most literature cover topics of far field region antennas. I haven't found aby good tutorial for an antenna of my type. I guess I will have to (in this case) look for mutual inductance (M) rather that only for coil's inductance (L). Maybe You have some advice ?

And the second thing - my transmitter probably will be used in other applications (with other antennas - let's call them extenders). But in such case this would far field. How should I design such antenna then ? Shouldn't I start my design with higher frequencies (such as 900 MHz) and design one antenna for all apllications ? But will I induce enough energy then ? Or maybe I should still target 13,56 MHz and spiral, multi-layered coil ? I have lot's of similar questions, but I don't want to go too deep into physics and fields now - first I need general hints what to choose and if it works in my case.

Regards,
Zbigniew

Hi Zbigniew,

Yes, mutual inductance is what is important using inductive coupling. There are formulas that gives the self and mutual inductance of coils. One simple free program that gives values for two coaxial coils is TWOCOILS, from www.g4fgq.com/page3.html , but you are interested in coplanar coils. Look at some handbook for formulas.
The other possibilty you can consider is to use capacitive instead of inductive coupling.

For the second thing (antennas in far field ) the problem is that the size of the antennas would be too small compared with wavelength, and for this reason the efficiency of the antenna as radiator would be very low. Look at this:



Regards

Z