It is possible to connect two antennas to a single radio via a properly specified (operating frequency and power handling capability) two-way, 50 ohm RF power divider. One example of this type of power divider is offered by Pasternack. Note that using a device like this will induce additional losses into the system (3 to 4 dB, typically).
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About UHF / VHF Radios
Campbell Scientific's narrow-band RF systems consist of a radio modem and a low-powered transceiver at the remote station(s), and a transceiver connected to a modem or base station at the computer site. Up to 254 stations can be interrogated over a single UHF or VHF frequency. Stations with compatible radio and radio modem can serve as repeaters to extend the line-of-sight transmission of the base station. Transmission distance is typically up to 25 miles between stations (line-of-sight).
An FCC-approved frequency assignment is required before radios and antennas can be ordered. Within a typical RF telemetry network, field sites use a directional "Yagi" antenna, repeater sites use omnidirectional antennas, and the base station may use either an omnidirectional or Yagi antenna depending on the geographic distribution of the network.
A typical field station contains a datalogger and is located at the measurement site. Repeater stations act as a communication relay between stations that cannot communicate due to distance or obstacles. A repeater station can be upgraded to a field station with the addition of a datalogger and sensors.