Hello Ray,
Thanks for this info as it makes more sense to me and just re-confirms my findings. I might go with a multi-layer PCB to kind of mimic the solenoid as much as possible and round my corners. I need a rectangular shape antenna so I can maximize the field coverage as the tag moves from one side of the antenna to the other. (Parallel motion) Antenna size is about 150mmX75mm.
Problem is that when trying to maximize the number of turns of the antenna the inductance increase a lot like you stated previously. Going with a parallel or series resonance I have to keep my inductance value below around 1.5 to 2uH or else I can’t match the output impedance of the 4W amplifier (50 ohm antenna). Are there other topologies I can approach so that using an inductor value higher than 2uH won’t be an issue? I would like to increase the turns as much as possible. Just having 2 full turns at 150mmX75mm is already ~1.5uH.
I would like to go series resonance but that means have to re-calculate the 4W amplifier output impedance to be lower, closer to 10 ohms or less. If I’m not mistaken the output impedance of the amplifier from TI EVM is more or less 50ohms? Since the low pass and band pass filters were designed to be optimized at 50 ohms.
I have also noticed that even If I completely tuned an antenna at 50ohms my RFID carrier frequency (13.56MHz) slightly distorts but if I load a pure 50ohm resistor the output has minimal distortion.
I also designed a 6 turn spiral antenna at 100mmX100mm where it is impossible to tune the circuit to 13.56MHz at 50 ohms using parallel resonance topology. Therefore I just place a cap value to try to remove as much as possible any reactance and to my surprise this was the best result for reading distances.
Vlad