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How an Antenna Diplexer Allows for Multiple Broadcasts on One Transmitter

March 2, 2017

A diplexer, which is a form of duplexer, is a three-port frequency-dependent device used either for transmitting or receiving purposes.

It can take different frequency bands and multiplex and de-multiplex two ports and put them onto one port on a single coaxial cable. Or a diplexer can route signals based on frequency that have a sufficient wideband and able to and transfer them to the air efficiently. A diplexer essentially combines signals onto one download or perhaps link those being passed through it onto radio transmitters.

This passive device can also split two signals apart from being on the same receive path like audio and data in Digital Subscriber Line (DSL) telephone systems or combine two signals into one single output on the same transmit path. It’s also capable of forcing constant impedance to occupy a single broadcast frequency or enable one transmitter used by a broadcast station to operate and transmit multiple frequencies at the same time off one RF antenna.

In addition, an antenna diplexer is an electronic component that can also be used to transmit and receive simultaneously using the same antenna – or it can enable a single antenna to be used while preventing the output of the other or else. They allow signals such as from an antenna and a satellite TV dish to share the same feed. Even more, it can separate signals according to the frequency they use.

On the downside, diplexers are not for signals that have overlapping frequency ranges. And if transmitters have frequencies too close together they cannot be combined successfully by a diplexer.

Typically, diplexers are used for high frequency satellite signals and low frequency CATV (cable television) / OTA (over-the-air) antenna signals. They can be also be found as part of a front end module for a mobile telecommunication system.

Diplexers are essential when trying to overcome the problem of the ban on building new transmitting towers in some areas. It is very hard nowadays to get permits to build new transmitting towers in many places as there are laws that say one can no longer build towers. Federal Aviation Administration (FAA) and Federal Communications Commission (FCC) rules related to tower erection restrictions and local zoning regulations drive many of these restrictions.

Although there are exceptions for businesses categorized as “public utilities” to build even in zones where it is explicitly prohibited, in reality constructing new towers is nowadays almost impossible.

Overcoming the problem is possible only by utilizing an existing tower as much as possible. Agreement must be found with owners of established transmitting sites. This is where diplexers come in to play – by allowing two frequency bands to be transmitted together without interferences.

Diplexing is a suitable solution for medium wave broadcasting stations and to allow more than one transmitter such as UHF-/VHF from a single pre-built tower to be used for broadcast television reception.

It is said that diplexing AM (Amplitude Modulation) transmitters is easier than diplexing FM (Frequency Modulation) transmitters due to the fact that not all radiators will work for that type of transmissions. Antennas, in fact, must be broadbanded to allow the broad range of FM frequencies to be transmitted.

Distance is also a factor as it affects FM transmission more than AM ones.

Due to the special attention FM duplexing requires, costs might be a concern if the antenna was not already built for this type of transmission. Other factors that need to be evaluated include:

  • Are tower antennas are non-directional or directional?
  • What’s the antenna efficiency?
  • Does the tow have the needed orientation?

Examples of tower mounted diplexers (TMDs) are abundant.

An example of a TMD is the Sutro Tower of San Francisco, a 977-foot steel tower used by 11 television stations, four FM radio stations, as well as satellite companies, cable providers and for wireless communication services. Even more iconic is the Empire State Building in New York that hosts a very busy diplexer that appeared way back in March of 1952 where ABC and CBS FM stations operated from a common antenna.

The signals were diplexed into a single set of dipoles on the latter stations’ antenna near the bottom of the tower.

It is clear how diplexers are essential in allowing new transmissions even though there are building prohibitions against new towers. Thanks to these devices, stations are able to prevent intermodulation and keep the voltage component of a standing wave (VSWR) to the lowest amount for each input transmitter and frequency even in presence of tower building blocks.

Consequently, new transmissions can still be allowed.

Despite this, diplexers appear to meet both the wishes of residents who don’t want any more towers built and able to serve the interests of businesses needing to have transmit/receive antennas for telecommunications. With this device and purpose, towers can be used to provide the locals with communication systems capabilities and companies the ability to have a transmission medium to conduct business.

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