Antenna choice for the ptFlex is a critical item and should be carefully considered. The recommended configuration for High Altitude Ballooning is a half-wave dipole such as the one available at Custom Digital Services. Other antennas can be used, as long as they are 50 ohms of impedance at the 144MHz design frequency, and can be attached to the SMA antenna connector.
It should be noted that if a handheld whip-style antenna is used such as those commonly found on handheld radios, it is necessary to install a counterpoise wire onto the shield side of the SMA coax connector. This counterpoise should be 49cm (19.25") and should extend to the opposite direction of the whip antenna. Failure to provide a counterpoise will reduce radio reception range, and may result in a lost tracker unit.