A half duplex RFID reader generates short magnetic pulses that wirelessly charge a capacitor inside an HDX tag. When the charge field turns off, the tag uses the stored power to send the tag number back to the reader without interference from the reader.
The charge capacitor limits how small a PIT tag can be made. Currently the smallest HDX tag is 12.0 mm x 2.15 mm.
HDX uses Frequency Shift Keying (FM) to send 1's and 0's on two different frequencies.
Antennas for HDX readers are simple loops of insulated wire that can be placed directly in the water. Some antennas use plastic tubing or garden hose for protection.
HDX uses charge pulses to operate which decreases the power versus a continuous field. The power level can be further lowered by slowing down the scan rate.
The largest known HDX antenna is 190'/60 meters wide and 18"/50 cm tall made from a single loop of wire.
HDX sends 1's and 0's on two different frequencies and is not affected by amplitude noise.