Got radios (plural)? If you have more than 1 radio and want an outside antenna you can share the antenna with multiple radios. There are 2 ways to do this, the cheap way and the right way.
The cheap way is to just put a passive splitter or “T” adapter. While this will work for 2 radios listening to strong local signals, it will not work well for anything else. It also opens you up to interference between the two radios.
The right way is to use a multicoupler. A multicoupler has special circuitry to isolate the radios from each other so they don’t cause interference. It also reduces loss and maintains proper impedance matches. Active (powered) multicouplers include a small amplifier that overcomes the insertion loss caused by the multicoupler itself so that the signal appearing at the radio is the same as it would be if the antenna were connected directly to it.
The Stridsberg line of Active Multicouplers are well known as the best way to allow many radios to share a single antenna. With the isolation and amplification provided by it there is little or no interference between the scanners. You can connect up to 8 receivers to one antenna with no loss or interference.
Stridsberg multicouplers come in 4 or 8 port versions. They use BNC connectors for the antenna input and radio outputs. ScannerMaster also sells BNC cable jumpers to connect the multicoupler to the scanner.