Explaination for antenna coil and magnetic field

[ckzai]

this Thursday is my final year project presentation, my final project are low cost extended range RFID skimmer and i using a rectangular coil (magnet wire) that larger than a original RFID reader antenna coil to extend the range.
i would like to ask a few question related to RFID reader antenna coil

1)if i using a rectangular coil for RFID reader antenna, is this the calculation for rectangular coil


2) how do i related the antenna size and number of turn of the antenna coil with magnetic field?
isn't the more magnetic field on the antenna coil the more longer reading range? correct me please

3)what effect the RFID reader antenna coil is larger than the tag antenna coil?
isn't the antenna unable to receive the feedback signal from tag because of the coupling problem ?

or something else i need to know before the presentation

i really need your help for the question above. thank you

 

[FvM]

1. You'll find dozens of empirical formulas for estimation of rectangular coil inductance, in so far none of it deserves the title "the calculation". I would compare with results of EM solvers to judge about their correctness.

2. RFID standards are prescribing a certain range of magnetic field strength at the target. The coil and reader circuit has to be designed accordingly. Magnetic field is essentially proportional to N*I. If I is given, more windings mean higher field strength. If the coil voltage is limited by the reader circuit, more turns also reduce the coil current. If you refer to 13.56 MHz RFID, coils larger than 15 or 20 cm are usually build as single turn.

3. Yes, the necessity to receive the tag's load modulation sets an effective limit to the coil size.

 

[ninju]

Use wheeler's formula. I found it in both Terman (1943 book) and in Thomas Lee(2000s book). Its an empirical formula, but its reasonably good.