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What Is a Diplexer?

Andrew Kirmayer
Andrew Kirmayer

A diplexer is a three-port combiner splitter device that inputs a signal on one end and directs it through two outputs to two different lines. Without drawing any power from outside the system, it also combines two signals into one single output. The two incoming frequencies must be different enough for the device to work properly, such as video and audio signals combined for use with television sets. Lines feeding into the diplexer can come from rooftop aerials, for example, or one path can come from a satellite dish and one from an aerial. Output from closed circuit television cameras can also be fed into one of the inputs on the device.

When using a diplexer, it is important that the two signals differ in frequency so that other filtering components can process them. The unit performs functions similar to systems that are used for multiplexing. Instead of many different signals being split from one place, only two are processed to meet the functional needs of the system. These signals are fed through one channel, often a coaxial cable, and are processed by diplexers on each end. The signals are routed to different receivers according to matching frequencies.

Woman holding a disc
Woman holding a disc

Diplexers can be used as power dividers to feed power to direct current (DC) devices. They are often used in communication systems to transmit radio signals through an antenna that could otherwise only handle a limited number of signals. Depending on the diversity of frequencies, Ultra High Frequency (UHF) and Very High Frequency (VHF) transmitters use a diplexer to combine signals into one channel. Diplexers can also be used as a backup in a two-antenna system, in the case of maintenance or the failure of one receiving antenna.

Often used in industrial settings and with home televisions, a diplexer is also suited for splitting audio and data in Digital Subscriber Line (DSL) telephone systems. Compatible with DC devices, it can be used as a bias tee for electronic components, to reduce signal loss in radio frequencies when combining DC signals with other inputs. Another benefit of the diplexer is that this loss is less than that from a typical splitter combiner. With diplexers, the loss can be up to 1.0 decibel (dB), but the losses are sometimes up to 4 dB otherwise. They are simpler than a triplexer or other multiplexing device, but integrate into communication systems with other components like amplifiers, taps, and splitters.

Discussion Comments


@hamje32 - You have a very unorthodox approach to splitting your Internet signals. Most people would have gone with a WiFi setup from the start.

Anyway, what you’ve described does make sense. I think the splitter will only work if there are two separate signals. The article mentions splitting the audio and data portions of a DSL connection as one example. I guess splitting the data portion won’t work.

I don’t know much about IP addresses but you’re right, the ISP would have some way of figuring out what you’re trying to do and blocking it.


Before you buy one of these diplexer units you should understand what they can and cannot do.

In my case, I had two cable modems and two computers, and I wanted to split my cable Internet connection among both machines.

So I bought a diplexer and split my coaxial cable connection. I now had two outputs for my coaxial cables instead of one. I ran a coaxial cable to each modem, which was connected to one of the computers.

There, I thought to myself. I’ve split my Internet cable connection to two computers. It should work right, so I thought.

Alas, it didn’t. The cable signal detects two separate IP addresses, so it only allows one signal to go through. The other computer didn’t get an Internet connection.

I called the cable company and they told me to get a second account. There was no way I was going to do that, so I finally bought a wireless router and split my signals that way.

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