I need to scan 134.2kHz PIT FDX tags from a range of 5cm to 10cm. So far, I have built custom reading circuit. It has the standard R+L+C in series driven by 134.2 kHz square wave and the output is taken across the capacitor and processed. I am only able to read the tag when it is at the center of the coil. I was wondering how I can increase the range:

  1. What all factors does the range depend on? (Current? if yes, how do I increase current? will decreasing impedance by changing L and C work?)
  2. Does range/SNR depend on Lcoil and Ltag. Do I have to do some matching to increase this?

Any help regarding this will be extremely appreciated. Thank you so much for your time!


LF RFID read range depends on both the strength of the magnetic charge field as well as the reader plus antenna sensitivity when listening. One of the challenges in LF RFID antenna design is that it is used to both transmit and receive.

The magnetic charge strength is related to the current flow through the loop. The power can be increased with lower resistance antenna wire or shorter length by using fewer turns. Decreasing the turns will decrease the inductance which also lowers the antenna Q, which may counteract the power gain from the lower resistance. The antenna will perform best with the correct ratio of inductance to resistance (Q).

FDX transponders use load modulation to send the 10 byte response so the challenge is to find the small changes in antenna voltage caused by the tag within ambient noise. Effective filtering will increase read range. Try winding the antenna in a figure 8 to form two loops with opposite flows to cancel noise.

A quiet reader power supply is necessary. If working indoors, turn off switching power supplies, florescent lights and other low frequency noise generators.

Impedance matching isn't effective when using centimeter diameter loops at kilometer wavelengths.


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