TUNING OF PCB ANTENNA FOR NFC CIRCUITS #17102016

For testing the antenna, we first made a loop antenna as per reference to ISO/IEC 7810 ID-1 outline as per the figure shown below with dimension given.

o   Then an experimental setup was made as shown below:

o   The signal generator is connected across the antenna we had designed and then this PCB antenna board was placed on the loop antenna, which was connected to oscilloscope to view the frequency, and the voltage induced in the loop antenna (with 50Ω input resistance).

o   While measuring the inductance of the antenna alone using LCR meter, we observed it to be 3.6 micro henry at 1mhz. Since our LCR meter can only measure up to 1uH, we used hit and trial method using convergence of the resonant frequency by calculating the tuning capacitor and inductor each time with the frequency that was seen on oscilloscope.

o   The initial tests and results made showed the result as:

Antenna + 30pf(NFC ic equivalent) capacitor in parallel	At 26mhz, induced voltage = 150mv	At 15.8mV, induced voltage = 50mv	At 43.8mhz, induced voltage = 80mv
Antenna + 30pf(NFC ic equivalent) +10pf(tuning capacitor) in parallel	At 27.6mhz, induced voltage = 474mv

o  From the above, result we figured out that the tuning of capacitance leads to increase in the induced voltage as well resulting the increased efficiency. But the problem was the antenna was still tuned to 27.6Mhz, that is around next order of our required frequency 13.56mhz. i.e double than the required.

o   In above picture, we can see the voltage induced in loop antenna from our designed antenna when initially only some random tuning capacitor of 40pf was placed and the frequency was not tuned yet.

o   So to tune the antenna to the 13.56mhz, we first calculated the inductance at the frequency at which the highest voltage was induced and by placing value of capacitance as the used capacitor value. For this  

      formula was used.

o   For initial tuned frequency = 27.6mHz, tuning capacitor = 40pf, resultant calculated inductance was 0.83131uH.

o   So, in next iteration we used frequency = 13.56mhz, inductance = 0.831831uH, the calculated capacitor was 165.71pf.

o   So, next time we put this capacitor in the circuit. In continuing this iteration for several run, we got the calculated capacitor as 400pf and actual used capacitor as 380pf to get the resonant frequency at 13.56mhz and the maximum voltage induced.

o   Below is the maximum induced voltage at our required 13.56 mhz signal. But the efficiency is still low so which seems to be enough to get signal but not power up the circuit. (further study should be done on knowing about if this induced signal is enough or not as well as increase the efficiency)

    Induced signal on loop antenna         input signal (13.56mhz @ 5V p-p)

o   We also checked our designed antenna as receiver and the previously available 13.56mhz signal generator. The setup and response is shown in figure below.

o   Here we can see almost 3V was induced in our designed antenna.

We need to further study about antenna design, antenna tuning and antenna testing (use of network analyzer, use of antenna design software etc) and implement in our next version board.