September 7, 2015: I had gotten a bit stuck a few weeks ago. I need a flexible design to fit inside a tire tube, but I knew a run of flex circuits was going to be too expensive. Then I realized that my size is really only driven by the antenna. I can't make it much smaller or the power transfer efficiency will plunge. The antenna is really the main thing I need to make flexible. I realized I could wind my own antenna coils from wire, or even better still, buy flexible antennas. I found 13.56 MHz flex antennas for a low cost and I bought some. Then I shrunk the rest of my layout to about 15 mm x 16 mm, and I ordered that circuit board. It should be here sometime in the next week, and I should be able to assemble some sensors and start tuning!

A word about range. Unfortunately, it didn't quite pan out that making an antenna that was closer to the target inductance would improve range. I hit the target inductance pretty much dead on at about 2.5 uH. The tuning cap shrunk from 200 pF, to something like 10 pF. I assumed this meant that more power would be transferred to the 35 pF capacitance in the RF430, and that I would get more range. That doesn't appear to be the case. In fact, TI forum posts on the RF430 seem to indicate that the best range I can hope to achieve with a cell phone will be about 4 cm. You can achieve much greater range with a more powerful RFID reader. However, a major point of my project is to read tire pressure with a cell phone. 4 cm will be adequate, as long as the metal wheel and metal tire belts don't attenuate my signal too much...