We are designing a project at the moment that will need to read 134.2 KHz RFID devices using the TI chipset.
Having got off first base using the evaluation kit, we are now into prototyping and it appears (I may be wrong) that TI do not supply the 80mm disk antenna other than as a part of the evaluation kit.
Furthermore the antennae they DO supply appear to cost from $100 upwards.
So it looks like we may have to wind our own.
We have some instructions from TI that suggest we use 15 turns of 0.2mm enamelled copper wire at a diameter of 75mm. This suggests that we will end up with a 47 microHenry coil, with a Q factor of 17 or 18.
However, if we use any of the inductance calculation formulae that we find by Googling, we cannot get 47uH as an answer. We get an answer around 40uH. (The reason we are doing the calculation is that we are trying to find alternative coil dimensions that are better suited to our application, but have the same inductance, series resistance and Q factor.)
Can anybody explain to a humble software engineer why we may be getting this discrepancy?
Thanks in advance.