why isn't my RFID 1-bit transponder not resonanting?

Hi,
i am trying out 1 circuit that i found from purdue's website.. i couldn't resonant it between the reader circuit and the transponder circuit..it is supposed to resonant at 1MHz. my problem is that the transponder is not inducing voltage to the reader.. why?? any suggestion? thanx..

What are you using for L1 and L2 ? I would guess that they should both be fairly large loops. If you are using ferrite core inductors, you would not have much mutual inductance.

I used to try with a circuit like this one, But it operate on 125KHz. By the way, I suggest you to check about your inductant value and Capaitor, don't forget that both of it should resonate. and in practical they must be tuned.

for my L1 it is 10 cm in diameter and for L2 it is 5 cm in diameter. I am using magnetic wire for this inductor and i wound it on a PVC and nothing..

If I understand well the cct operates in the following manner:
You inject a 5Vp-p signal from the source on the reader's cct. Then the transponder resonates at this frequency, hence a dip in the voltage at the common junction of L1, C1, D1 should occur. This dip is translated by the rest of the ccircuit as "a correct transponder is in range".

I do suggest to check the resonance frequency of u r transponder first and if it is ok then chech u r receiver's. They both must be equal to the source's frequency.

To further experiment in order to correct the resonance, put a variable capacitor in parallel to C2 (transponder) and monitor the voltage dip after D1 (cathode). By turning the var capacitor you should get the maximum responce. Then this is the real resonance of the circuit. Finally measure the value of the total capacitance in order to know what is your new C2 value for a fixed capacitor in the future ccts you will make.
Regards
P.

This basic mutual coupling method is the basis of several RFID systems used over the past several decades only the commercial ones are more complex. One of these commercial systems uses a Wheatstone bridge as the reader. One of the four elements is the large area inductor. The frequency is swept and the bridge is unbalanced by the mutual coupling of the RFID card resonant circuit.