HF RFID tag (antenna)
I am designing something like HF RFID tag (13.56M), my target right now is to collect the transmitted energy, rectify the tag antenna output and generate a DC level to supply my forthcoming circuit
I have no idea how to deal with the coil antenna design, theoritically i studied some in depth courses in my faculty hence the concpets are not a problem, the problem is that i can not use this info for practical implementation
So, would you please guide me with this?
First, i need a good simulator
second, i need a general list to the parameters of interest
third, it would be sooooo great if there were any example cuz unfortunately i can not find any on the web
Thanks
A important design part is to tune the antenna impedance so it effectively can transfer energy from antenna to receiver and rectifier. For that must your receiver/rectifier impedance be known. Proposals how to tune and impedance match similar coil antennas can be found at a lot of HAM-sites or ARRL.
It is possible to simulate the antenna but antenna impedance must be calculated for a certain distance (near field) for best result.
Check homepages of RFID chip vendors as they usually have some application examples, including antenna.
Ok, I can assume a parallel RC for the load right now with reasonable values
So we can view the circuit like : atenna->matching->Load
I am confused which to start with ? matching or antenna ?
I think i have to design the antenna then caculate its impedence and match the load to what i have calculated, right ?
If so, which parameters and equations i need to consider while designing the antenna?
My initial guess, the antenna can be modeled with RLC resonant circuit (series or parallel) so i should design for a certain L achieving the required coupling and this is L, then caculate the ohmic losses and this is R (perhaps i might add another R to get a certain impedence, e.g 50ohm), distributed C should also be caculated hence i can add another C to tune the antenna to 13.56M
I think this is what i am supposed to do, but i do not know how to practically achieve this? the first step is the big problem ? how to design for a certain L?
there are a lot of things here like geometry, number of turns, dimensions, etc
which drives me crazy !
So what do you think i should do ?
By the way, i thinks ADS can help with this, so is there any example of designing a microstrip antenna with ADS ?
can you provide me with URL to any of those RFID vendors that provide examples?
In most cases do practical limitations limit coil maximum size. Make the coil that big, with wide cu traces. It will most likely anyway be a small coil compared to wavelength so inductance can be found with simple coil formulas. Traces must be wide to reduce ohmic losses. Forget about calculate stray capacitanse at this level. Use a network analyzer to tune for resonance with a variable capacitor, and read resulting resistance (a few Ohm as best).
You now know what the impedance transformation that is needed for 50 Ohm input. Correction network can usally be done by just add one aditional capacitor.
Texas is one of the bigger RFID vendors and this can be a good start for antenna design/tuning: http://focus.ti.com/lit/an/sloa135/sloa135.pdf
The said TI application note is dedicated to reader "antenna" design. For 13.56 MHz you have rather coils than antennas, because the dimensions are small compared to wavelength and the transmission can be calculated as inductive coupling by pure magnetostatic formulas. For tags, the geometry is mostly prefined, e.g. an ID1 credit card format, all you have to decide is the optimal number of turns. Tag IC vendors are mostly suggesting a circuit for standard designs.
You didn't specify a particular range of RFID applications, e.g. proximity (ISO/IEC 14443 and similar) or vincinity (ISO/IEC 15693). But you'll find many general informations in RFID tutorials on the net. The official ISO standards have to be purchased, but you may want to refer to the respective drafts, see Standing Document 1 (SD1): WG8 Projects