Duplexer or Diplexer, that is the Question

Duplexers and Diplexers are very similar in name and somewhat in function and therefore are often the subject of heated debate. They perform similar functions but are not the same thing. Duplexers are used to separate two frequencies on the same band that are fairly close together, and Diplexers are used to separate two different bands for use on a single feed line and antenna system. Duplexers are more complex to build and tune and are usually larger physically than Diplexers. Duplexers require more selective circuits often utilizing physically large metal high-Q resonators to achieve the desired results. On the other hand Diplexers are much smaller physically since they are usually made up of simple low-pass and high-pass filter circuits designed to separate frequencies several megacycles apart and in different frequency bands. Highly selective circuits are not required to separate frequencies that are spread as far apart as two separate bands hence the smaller and simpler construction of Diplexers.
A Diplexer, for instance, separates 2 meters from 70 cm when used on the same coaxial cable. A Diplexer will enable you to use two antennas over the same coaxial cable and will allow you to receive on one band while simultaneously transmitting on the other band; if it is connected on the antenna side of the circuit. Conversely it may be configured on the radio side of the circuit to allow two transceivers operating on different frequency bands to share one transmission cable and antenna system. Simple "pass" filters present less than 0.2 dB insertion losses, are physically relatively small, and can typically separate the two bands of interest by 60 to 70 dB or more.
On the other hand duplexers are most often used at repeater sites to connect the transmitter and receiver of a repeater station to a common feed line and antenna. When properly tuned, a duplexer will allow simeoultaneous operation of transmitters and receivers sharing an antenna system as close as a few hundred kilocycles apart on the same frequency band. Duplexers are also connected together to form Combiners which allow different stations operating on the same band at different frequencies to share a common feed line and antenna system. Duplexers can have very large cavity filters which can be sensitive to temperature changes and become "detuned" from expansion and contraction of the metals with which they are made. They sometimes require controlled environments because of this and the fact that their circuits are very sensitive high-Q circuits which must be held to high tolerances to operate efficiently. Duplexers, depending on their function and design, can have up to several dB of insertion loss and provide only 50 to 60 dB of separation between adjacent frequencies on their band of operation. Larger cavities may be required for higher power operation and more cavities may be added for more selectivity and separation if necessary.
I am sure that many will disagree with my assessment of these differences since there are occasions when a combination of the two are used in transmitter and antenna system at some sites further fogging the distinction between the two. I hope that this discussion has shed some light on the actual differences between Duplexers and Diplexers, there usage, and will encourage you to look further into this interesting and commonly used circuitry.

73 de KF4BI

Ref. HB9ABX on the internet, TxRx Systems data sheets, dB Products data sheets, and others
 

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