RFID antenna response time

For the UHF RFID tag, what the response time of the reader?

For example, if the switch turns on at t=0s, what's the fast time that the reader can response?

It depends on the tag and it will be different from one tag type to another. There is a short time while the conducted power charges the tag power supply then some response time before it can apply it's data load to the field. You would have to ask the tag manufacturer how quickly it responded in the presence of a new field. The reader itself should start looking for modulation as soon as the carrier is field is turned on.

Brian.

As for the standard RFID protocols, much of the timing is fixed by the specification.