Diplexer
However poorly designed diplexers may have differing impedance on various ports, so it should not simply be assumed that any such device is fully reciprocal unless it is stated or the return loss measured.
That isolation is sufficient for many applications, but it is insufficient to allow simultaneous reception and transmission on one antenna.
Diplexers designed for simultaneous reception and transmission have more stringent isolation requirements and are known as duplexers.
Diplexers are typically used with radio receivers or transmitters on different, widely separated, frequency bands.
Diplexing is used to prevent intermodulation and keep reflected power (VSWR) to a minimum for each input transmitter and frequency.
While diplexers can combine a relatively wide bandwidth, the major limitation comes with the antenna itself, which must be sufficiently wideband to accept all of the signals being passed through it, and transfer them to the air efficiently.
Typically with a multi-band antenna the frequencies in use will bear an odd harmonic relationship to each other to take advantage of natural harmonic resonances (such as 145/435 MHz), making a highly efficient multi-band antenna.
The number of transmitters which can share an antenna is restricted by the spacing of their frequency bands.
If it is not possible to build a second antenna for the second transmitter due to space constraints, then the diplexer is used permanently.
A realization of diplexers for long wave broadcasting stations may be difficult, as the ratio of bandwidth (9 kHz) to transmission frequency is high.
In this frequency range their realization is very difficult because of the very high voltages that occur in the huge tuned loading coils that are used in the antenna feed.
For the diplexer to work, the existing cable must be able to pass the satellite frequencies with little loss.
In this application, there would be a diplexer on the roof that joins the satellite dish feed and the TV antenna together into a single coaxial cable.
