Re: antenna design
It will be very difficult to construct the antenna on pcb at this freq. The antenna should be part of resonant circuit tuned at 125khz. BTW, for RFID, as far as I know, freq. is 125khz, not 135. The antenna should have the approximate area of a credit card, and the number of turns should be around 30. Wire diameter : 0.3-0.5mm. As I said before, the antenna should be a part of a resonant circuit, and in order to achieve maximum reading distance (U can obtain 12-15cm), you will have to tune the antenna by inserting/cutting turns untill you obtain a maximum voltage at its ends (>50Vpk-pk).
/pisoiu