A self-synchronizing time-division duplex, TDD, antenna system (14; 50; 104; 154) comprising: an antenna (18; 58, 60) configured to communicate a transmit signal and a receive signal; and an antenna circuit (20; 52) coupled to the antenna, and for coupling to a user communication system via at least one transmission line cable (16; 62, 64; 106, 108; 156, 158), the antenna circuit comprising: a transmission line measurement circuit (22; 68; 250) configured to determine signal loss between the user communication system and the antenna circuit through the at least one transmission line cable; an amplitude adjustment circuit (24; 78, 80; 370; 400; 450; 500; 550; 600) configured to adjust amplitude of at least one of the transmit and receive signals based on the determined signal loss; a transmit detection circuit (26; 90, 92) configured to monitor signal power of the transmit signal obtained from the user communication system via the at least one transmission line cable; and a controller (28; 76; 300) configured to switch the amplitude adjustment circuit from a receive mode to a transmit mode in response to the monitored signal power exceeding a predetermined threshold, wherein in the receive mode the amplitude adjustment circuit is configured to apply a receive amplitude adjustment along a signal path to the receive signal, and in the transmit mode the amplitude adjustment circuit is configured to apply a transmit amplitude adjustment along the signal path to the transmit signal, wherein the transmission line measurement circuit comprises: a calibration signal generator configured to provide a calibration signal on the at least one transmission line cable from the antenna circuit to the user communication system; and a signal monitor configured to receive the calibration signal retransmitted back from the user communication system as a returned signal, and configured to detect a characteristic of the returned signal and to determine the signal loss based on the characteristic of the returned signal. an antenna (18; 58, 60) configured to communicate a transmit signal and a receive signal; and an antenna circuit (20; 52) coupled to the antenna, and for coupling to a user communication system via at least one transmission line cable (16; 62, 64; 106, 108; 156, 158), the antenna circuit comprising: a transmission line measurement circuit (22; 68; 250) configured to determine signal loss between the user communication system and the antenna circuit through the at least one transmission line cable; an amplitude adjustment circuit (24; 78, 80; 370; 400; 450; 500; 550; 600) configured to adjust amplitude of at least one of the transmit and receive signals based on the determined signal loss; a transmit detection circuit (26; 90, 92) configured to monitor signal power of the transmit signal obtained from the user communication system via the at least one transmission line cable; and a controller (28; 76; 300) configured to switch the amplitude adjustment circuit from a receive mode to a transmit mode in response to the monitored signal power exceeding a predetermined threshold, wherein in the receive mode the amplitude adjustment circuit is configured to apply a receive amplitude adjustment along a signal path to the receive signal, and in the transmit mode the amplitude adjustment circuit is configured to apply a transmit amplitude adjustment along the signal path to the transmit signal, wherein the transmission line measurement circuit comprises: a calibration signal generator configured to provide a calibration signal on the at least one transmission line cable from the antenna circuit to the user communication system; and a signal monitor configured to receive the calibration signal retransmitted back from the user communication system as a returned signal, and configured to detect a characteristic of the returned signal and to determine the signal loss based on the characteristic of the returned signal. a transmission line measurement circuit (22; 68; 250) configured to determine signal loss between the user communication system and the antenna circuit through the at least one transmission line cable; an amplitude adjustment circuit (24; 78, 80; 370; 400; 450; 500; 550; 600) configured to adjust amplitude of at least one of the transmit and receive signals based on the determined signal loss; a transmit detection circuit (26; 90, 92) configured to monitor signal power of the transmit signal obtained from the user communication system via the at least one transmission line cable; and a calibration signal generator configured to provide a calibration signal on the at least one transmission line cable from the antenna circuit to the user communication system; and a signal monitor configured to receive the calibration signal retransmitted back from the user communication system as a returned signal, and configured to detect a characteristic of the returned signal and to determine the signal loss based on the characteristic of the returned signal.