rfid antenna design tutorial

Hello all,

I'm not expert in RFID antenna design, especially magnetic coupling.
But that's my today (and for the rest of the month) duty ...

I have a few books, about RFID theory (physics, economy, and so on...) but not so many information about layout design.
For example, here's a few question I'd really appreciate you help me answer.
Thanks a lot if you can help me !

In general, the RFID antenna should resonate at the desired frequency, but I don't know if have, first, draw the antenna (in my case spiral rectangular) "on the fly" then, judging from it inductance, build the resonance with an appropriated capacitor !?
Or is there rules of design?
I found a formula for inductance and mutual inductance calculation, but nothing about capacitive effects, and consequences on the self-resonant frequency.
And I'm not even sure metal losses are taken in consideration in simulation. Just depending on the conductivity, and other substrate parameters, or do I miss something ?

I use Agilent ADS Momentum for simulation, that might not be the perfect tool (don't have a clue if I can extract R, L, C parameters from a design with this...), but neverming, I have to face it :)
Is there any tutorial/design guide for RFIC spiral antennas ? I'm working like some lot others at 13,56 MHz

Hope you can give me a hand on this, this will be very appreciated !
Best regards

Chromatik

Added after 58 seconds:

Well, I designed a rectangular spiral antenna, thanks to models existing in Momentum.
The idea is to make a kind of "transformer", so I first draw a spiral I feed with a source, then the other port to ground. I'll do the same for the other coil...

Now a little problem is... what kind of port should I use?
I guess Internal would be the best suitable, because I'd like to take into account the EM coupling effects (thanks to the helpfile), but are they referenced to a ground, or a floating ground, and what is the ground level ?? Is it possible to change it ?

Do you have any idea, advice, complains about it ?
I'd really appreciate your help.

Best Regards

Well, my question were maybe too wide, or not so much precise, or written in a poor english (I admit)...
... but hey! It might be useful for some many other people !

Nevermind

I remember there is an excellent report on designing 13.56MHz loop antennas with Sonnet, titled "Electromagnetic Analysis Speeds RFID Design", you can google it... Hope it helps.

Thanks guys !
In fact I already had these documents, but I read them again and found a few interesting extra-stuff :)