Half Duplex (HDX) PIT tags have the same look and size as the FDX-B PIT tags but are programmed differently. These tags and readers communicate much like walkie-talkies. The reader sends a pulse that charges the HDX Tag; then the reader turns off and the tag uses the stored power to send back the unique number. This technology allows for larger antennas with simpler designs. Unlike FDX antennas, they can be placed directly in the water without an air gab. The drawback is that HDX tags are slower than the FDX tags, which reduces the read time. They operate at a frequency of 134.2 kHz for optional read range.
HDX PIT tags are available from UID in bulk, or in sterile needle.