Diplexer
A passive device that frequency-multiplexes two ports onto one.
A diplexer is a passive device used for frequency-domain multiplexing. It combines two ports—often labeled L and H—onto a third port, S, by ensuring the signals on L and H use separate frequency ranges. This allows both signals to share port S without interference. Typically, port L handles a single low-frequency band, and port H handles a higher-frequency band. The diplexer then uses a low-pass filter between L and S and a high-pass filter between H and S. Ideally, all low-band power transfers between L and S, and all high-band power transfers between H and S, with no signal leaking from L to H. In practice, some power is lost, and some signal may leak to the wrong port.
The device is normally reciprocal—it has no inherent input or output—but poorly designed models may have different impedances on different ports, so full reciprocity should not be assumed without verification. A diplexer differs from a passive combiner or splitter: its ports are frequency-selective, whereas a combiner’s are not. A combiner divides all power from port S equally between two ports, while a diplexer does not. More than two ports can be multiplexed onto one: a three-port to one-port device is a triplexer, and a four-port to one-port device is a quadplexer or quadruplexer.
A typical diplexer provides around 30 dB isolation between its L and H ports, which is enough for many uses but insufficient for simultaneous reception and transmission on one antenna. For example, a 1 kW transmitter would send 1 W to the receiver, potentially overloading it. Devices designed for simultaneous transmit and receive with stricter isolation are called duplexers.
Common uses include allowing two devices to share a single communications channel, such as a long coaxial cable, with diplexers at both ends. This works if the devices operate on different frequencies and is economical if the diplexers cost less than running a second cable. Diplexers are often used with radio receivers or transmitters on widely separated bands. For instance, a city radio tower might combine a police antenna at 460 MHz and a fire department antenna at 156 MHz onto one feedline, then separate them inside the building with another diplexer. Some diplexers support up to four antennas or radios on different bands. They are also common with multi-band antennas on towers, splitting bands like VHF and UHF inside the building.
- Isolation
- around 30 dB between L and H ports
- Typical frequency bands
- low band (e.g., 50–870 MHz) and high band (e.g., 950–1450 MHz) in residential satellite/TV applications
- Example police/fire frequencies
- 460 MHz (police) and 156 MHz (fire department)
- Example amateur radio harmonic relations
- 145/435 MHz
Lore & Background
The diplexer is a different device from a passive combiner or splitter; its ports are frequency selective, whereas a combiner's ports are not. A combiner equally divides power from the S port between its A and B ports, while a diplexer does not. A typical diplexer may have around 30 dB isolation between its L and H ports, which is sufficient for many applications but insufficient for simultaneous reception and transmission on one antenna—if a transmitter emits 1 kW, 1 W would appear at the receiver, potentially overloading it. Devices designed for simultaneous reception and transmission with more stringent isolation are known as duplexers.
Diplexers are commonly used to allow two different devices to share a common communications channel, such as a long coaxial cable, often with a diplexer at both ends. This is economical if the diplexers cost less than running a second cable. In industrial applications, diplexing prevents intermodulation and keeps reflected power (VSWR) to a minimum for each input transmitter and frequency. The number of transmitters that can share an antenna is restricted by the spacing of their frequency bands; transmitters with frequencies too close together cannot be combined successfully by a diplexer.
Diplexers are also used at medium wave broadcasting stations, though less commonly than at FM sites, because the wavelength varies more across the medium wave band. They may serve as a back-up device, for example allowing one antenna to broadcast two channels during maintenance. At long wave broadcasting sites, diplexers are normally not used, as these stations usually broadcast on only one frequency, and realization may be difficult due to the high ratio of bandwidth (9 kHz) to transmission frequency. Diplexers are not used at VLF transmitters because of the very high voltages in the tuned loading coils of the antenna feed.
Reader's Guide
The diplexer's significance lies in its ability to economically share a single communications channel—such as a coaxial cable or antenna—between two devices operating on different frequency bands. This is notable in residential settings, where a direct broadcast satellite TV dish and a terrestrial TV antenna can share one cable, eliminating the need for a second cable installation. In such applications, the diplexer on the roof joins the satellite feed and TV antenna, and a second diplexer inside splits the signals to the TV and satellite set-top box. The existing cable must pass satellite frequencies with little loss; older RG-59 cable may be inadequate, while RG-6 is typically used.
In radio communications, diplexers allow multiple services (e.g., police and fire departments) to share a single tower antenna and feedline, reducing infrastructure costs. They are also used with multi-band antennas, where the diplexer splits bands such as VHF and UHF inside a building. The major limitation is that the antenna must be sufficiently wideband to accept all signals efficiently. Multi-band antennas often use frequencies with odd harmonic relationships (e.g., 145/435 MHz) to take advantage of natural harmonic resonances. The diplexer's legacy is as a practical, passive solution for frequency-domain multiplexing in both broadcast and non-broadcast applications, including amateur radio.
Did You Know?
- A diplexer is a passive device that does not have a notion of input or output; it is normally reciprocal.
- Diplexers are different from combiners: combiner ports are not frequency selective and equally divide power.
- 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.
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