That’s an active antenna circuit used for receiving purposes in instances where a normal antenna would be impossible to accommodate in a physical sense. Such an antenna is sometimes called an antenna booster. For that purpose we are using a field effect transistor as the amplifying device. The JFET transistor configuration will be a source follower. There’s a 10M bias resistor from gate to common at the input to a 2N5484 JFET and rely on that 2N5484 RF stage input impedance to be high enough compared to the antenna impedance. This RF stage immediately following the antenna has an input impedance high enough so that the signal voltage on the antenna is essentially transferred to the RFID reader input without antenna tuning.