Industrial Applications of diplexer

The chief advantage of a diplexer is that it allows two different devices to share a common communications channel. Typically the shared channel is a long piece of coaxial cable. Rather than run two separate cables, a single cable with diplexers at each end is used. The plan is economical if the diplexers cost less than running the second cable.

Diplexers might be used to connect two receivers inside a building to two antennas that are some distance away using a single cable. Alternatively, two transmitters might connect to two antennas.

A diplexer may be used as a form of duplexer, which is a device to enable bi-directional (duplex) communication over a single path. In this usage the high and low frequency signals are travelling in opposite directions at the shared port of the diplexer.

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.

One of the most massive diplexers in use is atop the Empire State Building in New York, where over a dozen FM radio stations transmit through one four-panel antenna. Another such setup is on a tower in Miami Gardens, serving the Miami and Fort Lauderdale media market.

Many other large UHF-/VHF-transmitters use diplexers. The number of transmitters which can share an antenna is restricted by the spacing of their frequency bands. Transmitters whose frequencies are too close together cannot be combined successfully by a diplexer.

Diplexers are also used at medium wave broadcasting stations. However their use is not that common in this frequency range because the corresponding wavelength varies much more across the medium wave band than across the FM band and so it is more practicable to use a separate antenna for each frequency: medium wave transmission sites usually broadcast only on one to four frequencies, while FM-broadcasting sites often uses four and more frequencies.