Transmit, receive or both?

One antenna, both directions

Antenna reciprocity: a passive antenna's transmit and receive patterns are identical, so the same antenna serves both.^[1]

What matters on transmit

• Match. "Mismatching of the antenna is one of the largest factors that reduce the total RF link budget." (TI AN058)^[2] A poor VSWR reflects power back toward the transmitter.
• Power rating. Every part in the chain has one, and it falls as frequency rises. Belden's RG-213 (8267), for example, is rated 748 W at 100 MHz, 326 W at 400 MHz and 205 W at 900 MHz.^[3] Tell us your transmitter power when you ask for a quote.
• Transmit and receive on one antenna. A duplexer lets a repeater's transmitter and receiver share an antenna; it is tuned at the factory to your exact frequency pair.

Receive-only systems

• GNSS (GPS, Galileo, GLONASS, BeiDou). Satellites broadcast; your antenna only receives, and the signal is tiny: the GPS minimum received L1 power is −158.5 dBW.^{[4, 5]} CompleTech's CAGNSSY++ yagi "covers all GPS, GLONASS, Galileo, BeiDou, NavIC, SBAS and QZSS frequencies" and lists "testing and detection of jamming and spoofing". (ComAnt catalog, p. 34)^[6]
• AIS. Ships transmit and receive on the same two frequencies, 161.975 and 162.025 MHz;^[7] a shore station that only listens needs only a receive antenna on those frequencies.