have more stringent isolation requirements and are known as duplexers. Common uses A diplexer allows two different devices to share a common communications channel. Typically, the channel is a long coaxial cable, and a diplexer is often used at both ends of the coaxial cable. The plan is feasible if the two devices operate on different frequencies. The plan is economical if the diplexers cost less than running a second cable. Diplexers are typically used with radio receivers or transmitters on different, widely separated, frequency bands. A single city radio tower might have a police department antenna on 460 MHz and a fire department antenna on 156 MHz. A diplexer at the top combines the two antenna signals to the single coaxial feedline, and a second identical diplexer inside the building separates the feedline signals to the two dispatch radios. Some diplexers support as many as four antennas or radios that work on different radio bands. Diplexers are also commonly used where a multi-band antenna is used on a tower, with a common feedline. The diplexer will split the two bands inside the building (such as VHF and UHF systems combined with a diplexer onto a common antenna). Industrial applications 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. Other times tuned traps will be used, which is less efficient and generally not a technique used at VHF/UHF. 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