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A diplexer is a passive device that implements frequency-domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and H can coexist on port S without interfering with each other.

Electronic component basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Frequency-division multiplexing

In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal. This allows a single transmission medium such as a cable or optical fiber to be shared by multiple independent signals. Another use is to carry separate serial bits or segments of a higher rate signal in parallel.

Port (circuit theory) pair of terminals connecting an electrical network or circuit to an external circuit, a point of entry or exit for electrical energy

In electrical circuit theory, a port is a pair of terminals connecting an electrical network or circuit to an external circuit, a point of entry or exit for electrical energy. A port consists of two nodes (terminals) connected to an outside circuit, that meets the port condition; the currents flowing into the two nodes must be equal and opposite.


Typically, the signal on port L will occupy a single low frequency band and the signal on port H will occupy a higher frequency band. In that situation, the diplexer consists of a lowpass filter connecting ports L and S and high pass filter connecting ports H and S. Ideally, all the lowband signal power on port L is transferred to the S port and vice versa. All the highband signal power on port H is transferred to port S and vice versa. Ideally, the separation of the signals is complete. None of the low band signal is transferred from the L port to the H port. In the real world, some power will be lost, and some signal power will leak to the wrong port.

Television diplexer consisting of a high-pass filter (left) and a low-pass filter (right). The antenna cable is connected on the back to the screw terminals to the left of center. Hilofilter.agr.jpg
Television diplexer consisting of a high-pass filter (left) and a low-pass filter (right). The antenna cable is connected on the back to the screw terminals to the left of center.

The diplexer, being a passive device, is normally reciprocal: the device itself doesn't have a notion of input or output. 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.

The diplexer is a different device than a passive combiner or splitter. The ports of a diplexer are frequency selective; the ports of a combiner are not. There is also a power "loss" difference - a combiner takes all the power delivered to the S port and equally divides it between the A and B ports. A diplexer does not.

Power dividers and directional couplers

Power dividers and directional couplers are passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmission line to a port enabling the signal to be used in another circuit. An essential feature of directional couplers is that they only couple power flowing in one direction. Power entering the output port is coupled to the isolated port but not to the coupled port. A directional coupler designed to split power equally between two ports is called a hybrid coupler.

A diplexer frequency multiplexes two ports onto one port, but more than two ports may be multiplexed. A three-port to one-port multiplexer is known as a triplexer, and a four-port to one-port multiplexer is a quadplexer or quadruplexer.

A typical diplexer may have around 30 dB isolation between its L and H ports. That isolation is sufficient for many applications, but it is insufficient to allow simultaneous reception and transmission on one antenna. If the transmitter emits 1 kW, then 1 W of that signal would appear at the receiver; that 1 W may be enough power to overload the receiver. Diplexers designed for simultaneous reception and transmission have more stringent isolation requirements and are known as duplexers.

A duplexer is an electronic device that allows bi-directional (duplex) communication over a single path. In radar and radio communications systems, it isolates the receiver from the transmitter while permitting them to share a common antenna. Most radio repeater systems include a duplexer. Duplexers can be based on frequency, polarization, or timing.

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.


Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency components will form additional components at frequencies that are not just at harmonic frequencies of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at sums and differences of multiples of those frequencies.

Bandwidth (signal processing) difference between the upper and lower frequencies in a continuous set of frequencies

Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to passband bandwidth or baseband bandwidth. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a band-pass filter, a communication channel, or a signal spectrum. Baseband bandwidth applies to a low-pass filter or baseband signal; the bandwidth is equal to its upper cutoff frequency.

In communications, a system is wideband when the message bandwidth significantly exceeds the coherence bandwidth of the channel. Some communication links have such a high data rate that they are forced to use a wide bandwidth; other links may have relatively low data rates, but deliberately use a wider bandwidth than "necessary" for that data rate in order to gain other advantages; see spread spectrum.

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 frequency: medium wave transmission sites usually broadcast only on one to four frequencies, while FM-broadcasting sites often uses four and more frequencies.

Diplexers may be used as a back-up device. An example is maintenance work at one antenna of a medium wave transmission site that has two antennas transmitting on two frequencies. Then the other antenna can be used for broadcasting both channels. If it is not possible to build a second antenna for the second transmitter due to space constraints, then the diplexer is used permanently.

At long wave broadcasting sites diplexers are normally not used since these stations usually broadcast on only one frequency. A realization of diplexers for long wave broadcasting stations may be difficult, as the ratio of bandwidth (9 kHz) to transmission frequency is high.

Diplexers are not used at VLF transmitters. 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.

Diplexers are also used for non-broadcast applications such as amateur radio.

Broadcasting distribution of audio and video content to a dispersed audience via any audio or visual mass communications medium

Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum, in a one-to-many model. Broadcasting began with AM radio, which came into popular use around 1920 with the spread of vacuum tube radio transmitters and receivers. Before this, all forms of electronic communication were one-to-one, with the message intended for a single recipient. The term broadcasting evolved from its use as the agricultural method of sowing seeds in a field by casting them broadly about. It was later adopted for describing the widespread distribution of information by printed materials or by telegraph. Examples applying it to "one-to-many" radio transmissions of an individual station to multiple listeners appeared as early as 1898.

Amateur radio use of designated radio frequency spectra for purposes of non-commercial exchange of messages

Amateur radio, also known as ham radio, describes the use of radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communication. The term "amateur" is used to specify "a duly authorised person interested in radioelectric practice with a purely personal aim and without pecuniary interest;" and to differentiate it from commercial broadcasting, public safety, or professional two-way radio services.


Diplexers are also used in the home to allow a direct broadcast satellite TV dish antenna and a terrestrial TV antenna (local broadcast channels) to share one coaxial cable. The dish antenna occupies the high frequencies (typically 950 to 1450 MHz), and the TV antenna uses lower television channel frequencies (typically 50 to 870 MHz). In addition, the satellite also gets a DC to low frequency band to power the dish's block converter and select the dish antenna polarization (e.g., voltage signaling or DiSEqC). The diplexer is useful in homes that are already wired with one cable, because it eliminates the need to install a second cable. For the diplexer to work, the existing cable must be able to pass the satellite frequencies with little loss. Older TV installations may use a solid dielectric RG-59 cable, and that cable may be inadequate. [1] RG-6 cable is typically used for satellite feed lines.

Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna commonly referred to as a satellite dish and a low-noise block downconverter.

Satellite dish antenna for TV and radio reception

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

Coaxial cable A type of electrical cable with an inner conductor surrounded by concentric insulating layer and conducting shield

Coaxial cable, or coax, is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880.

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. That cable would then run from the roof into the house. At a convenient point, a second diplexer would split the two signals apart; one signal would go to the TV set and the other to the IRD of the DBS set-top box. These usually have an antenna input and a diplexer, so that the antenna signal is also distributed along with the satellite.

More modern installations confront several issues. There are often multiple satellite dishes that need to feed several receivers or even multichannel receivers. See, for example, single cable distribution.

Diplexers were also used to combine UHF TV and VHF TV and FM signals onto one downlead, which can then be split back into its component parts as required.

See also

Related Research Articles

Cable television television content transmitted via signals on coaxial cable

Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fiber-optic cables. This contrasts with broadcast television, in which the television signal is transmitted over the air by radio waves and received by a television antenna attached to the television; or satellite television, in which the television signal is transmitted by a communications satellite orbiting the Earth and received by a satellite dish on the roof. FM radio programming, high-speed Internet, telephone services, and similar non-television services may also be provided through these cables. Analog television was standard in the 20th century, but since the 2000s, cable systems have been upgraded to digital cable operation.

Set-top box information appliance device

A set-top box (STB) or set-top unit (STU) is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of signal, turning the source signal into content in a form that then be displayed on the television screen or other display device. They are used in cable television, satellite television, and over-the-air television systems, as well as other uses.

Feed horn small horn antenna used to convey radio waves between a transmitter and/or receiver and a parabolic reflector

In parabolic antennas such as satellite dishes, a feed horn is a small horn antenna used to convey radio waves between the transmitter and/or receiver and the parabolic reflector. In transmitting antennas, it is connected to the transmitter and converts the radio frequency alternating current from the transmitter to radio waves and feeds them to the rest of the antenna, which focuses them into a beam. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which converts them to a tiny radio frequency voltage which is amplified by the receiver. Feed horns are used mainly at microwave (SHF) and higher frequencies.

Very high frequency class of radio waves

Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

Ultra high frequency radio waves

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

Low-noise block downconverter

A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low-noise amplifier (LNA).

Amateur television

Amateur television (ATV) is the transmission of broadcast quality video and audio over the wide range of frequencies of radio waves allocated for radio amateur (Ham) use. ATV is used for non-commercial experimentation, pleasure, and public service events. Ham TV stations were on the air in many cities before commercial television stations came on the air. Various transmission standards are used, these include the broadcast transmission standards of NTSC in North America and Japan, and PAL or SECAM elsewhere, utilizing the full refresh rates of those standards. ATV includes the study of building of such transmitters and receivers, and the study of radio propagation of signals travelling between transmitting and receiving stations.

F connector

The F connector is a coaxial RF connector commonly used for "over the air" terrestrial television, cable television and universally for satellite television and cable modems, usually with RG-6/U cable or, in older installations, with RG-59/U cable.

Hybrid fiber-coaxial (HFC) is a telecommunications industry term for a broadband network that combines optical fiber and coaxial cable. It has been commonly employed globally by cable television operators since the early 1990s.

A link budget is an accounting of all of the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains and feedline and other losses. Randomly varying channel gains such as fading are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping.

Television antenna

A television antenna, or TV aerial, is an antenna specifically designed for the reception of over-the-air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries. Television antennas are manufactured in two different types: "indoor" antennas, to be located on top of or next to the television set, and "outdoor" antennas, mounted on a mast on top of the owner's house. They can also be mounted in a loft or attic, where the dry conditions and increased elevation are advantageous for reception and antenna longevity. Outdoor antennas are more expensive and difficult to install, but are necessary for adequate reception in fringe areas far from television stations. The most common types of indoor antennas are the dipole and loop antennas, and for outdoor antennas the yagi, log periodic, and for UHF channels the multi-bay reflective array antenna.

A land mobile radio system (LMRS), also called public land mobile radio or private land mobile radio, is a person-to-person voice communication system consisting of two-way radio transceivers which can be mobile, installed in vehicles, or portable (walkie-talkies). Public land mobile radio systems are made for use exclusively by public safety organizations such as police, fire, and ambulance services, and other governmental organizations, and use special frequencies reserved for these services. Private land mobile radio systems are designed for private commercial use, by firms such as taxis or delivery services. Most systems are half-duplex, with multiple radios sharing a single radio channel, so only one radio can transmit at a time. The transceiver is normally in receiving mode so the user can hear other radios on the channel; when a user wants to talk he presses a push to talk button on his microphone, which turns on his transmitter. They use channels in the VHF or UHF bands giving them a limited range, usually 3 to 20 miles depending on terrain, although repeaters installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use AM or FM modulation, while some recent systems use digital modulation allowing them to transmit data as well as sound.

Amateur radio repeater

An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. Many repeaters are located on hilltops or on tall buildings as the higher location increases their coverage area, sometimes referred to as the radio horizon, or "footprint". Amateur radio repeaters are similar in concept to those used by public safety entities, businesses, government, military, and more. Amateur radio repeaters may even use commercially packaged repeater systems that have been adjusted to operate within amateur radio frequency bands, but more often amateur repeaters are assembled from receivers, transmitters, controllers, power supplies, antennas, and other components, from various sources.

Radio repeater

A radio repeater is a combination of a radio receiver and a radio transmitter that receives a signal and retransmits it, so that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of each other, to communicate. Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio.

Single cable distribution

Single cable distribution is a satellite TV technology that enables the delivery of broadcast programming to multiple users over a single coaxial cable, and eliminates the numerous cables required to support consumer electronics devices such as twin-tuner Digital Video Recorders (DVRs) and high end receivers.

Antenna amplifier

In electronics, an antenna amplifier is a device that amplifies an antenna signal, usually into an output with the same impedance as the input impedance. Typically 75 ohm for coaxial cable and 300 ohm for twin-lead cable.

Polarization-division multiplexing

Polarization-division multiplexing (PDM) is a physical layer method for multiplexing signals carried on electromagnetic waves, allowing two channels of information to be transmitted on the same carrier frequency by using waves of two orthogonal polarization states. It is used in microwave links such as satellite television downlinks to double the bandwidth by using two orthogonally polarized feed antennas in satellite dishes. It is also used in fiber optic communication by transmitting separate left and right circularly polarized light beams through the same optical fiber.

In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.


  1. Legacy satellite receivers instructed the LNB to send only one polarization (half the possible channels). Modern receivers have dual channels, so they may need both polarizations at the same time. A DishPro LNB "stacks" the two polarizations (sends both polarizations down the same cable; one polarization is sent in a higher (stacked) band). Consequently, the LNB signal occupies a wider bandwidth, 950 to 2150 MHz. RG-59 has significant loss at the higher frequencies.