Diplexers and Triplexers: Share One Coax Between Bands

Running a separate cable for every antenna is expensive, heavy and sometimes impossible, especially in a mobile installation, on a balcony or on a long run up a tower. A diplexer solves part of that problem: it allows two bands that are far apart in frequency, such as HF and VHF, to share a single coaxial cable. At the radio end the cable is split again so that each radio sees only its own band. A triplexer does the same for three bands. The device is simple and passive, but it has limits that are often misunderstood, and it is regularly confused with the duplexer of a repeater. This guide explains how it works, what the specifications mean and when it is a good idea.

How a Diplexer Works

A diplexer is a pair of filters joined at a common port. One is a low-pass filter, which passes the low band and blocks the high band, and the other is a high-pass filter that does the opposite. The common port connects to the coax, the low port to the HF radio or antenna, and the high port to the VHF or UHF radio or antenna. Because each filter presents a high impedance to the frequencies it blocks, the two paths do not load each other, and a signal on either band finds its way to the right port. A short description of the principle is in the article on the diplexer.

The frequencies where the filters change from passing to blocking are called the crossover. A typical amateur diplexer for HF and VHF has a crossover somewhere between 50 and 100 MHz, for instance to pass 1.8 to 54 MHz on one port and 144 to 450 MHz on the other, with a gap in the middle. The design guarantees that the bands you use are well inside the pass regions. A device for 6 metres and below on one port and 2 metres upward on the other will not work properly if you try to use 70 MHz on a port intended for another range, so check the specification before you buy.

Where a Diplexer Is Useful

There are several typical situations. In a vehicle, a dual-band mobile antenna for 2 metres and 70 centimetres sometimes already contains an internal diplexer. In another case, a car may have an HF whip and a VHF antenna on one mount, and a single coax is the only practical way through the bodywork. A satellite station often needs 144 MHz and 432 MHz on the same coax to the antenna mast, with a diplexer at each end, or a triplexer when a third band is added. On a tower where the cable run is long and an additional piece of coax is costly, a diplexer allows an HF antenna and a VHF antenna to share the feedline, saving cost and weight.

Diplexers are also used at receive-only sites, for example to combine a wideband receive antenna for scanning with a transmitting antenna. And in a portable station, one coax to a mast that holds both a VHF Yagi and an HF antenna simplifies the setup. In each case the logic is the same: if the bands are far apart, a filter pair can separate them cleanly, and if the bands are close, it cannot.

Reading the Specifications

Four numbers describe a diplexer. Insertion loss is the power lost in the filter in each path, typically 0.1 to 0.5 dB for a good device, and it adds to the loss of the coax. Isolation is the attenuation between the two ports, normally 40 to 60 dB or better, and it determines how much of the transmit power on one band leaks into the receiver on the other. Return loss, or SWR, indicates how well the device keeps a 50 ohm match within each pass region. And power handling gives the maximum power the filters can take, which depends on the capacitors and inductors used. The table shows realistic ranges.

The isolation figure deserves special attention. If you transmit 100 watts on HF into one port, even a 50 dB isolation lets 1 milliwatt reach the other port. A VHF receiver that is connected there can be overloaded by this, since a good VHF receiver is sensitive to a signal measured in picowatts. If the two radios operate at the same time, a stronger isolation or an additional filter may be needed. Our guide to the dynamic range of a receiver explains why strong signals on another band can still degrade reception.

The Mismatch Trap

The most common mistake is to assume that the diplexer makes both antennas work from one cable regardless of their quality. It does not. The device presents a 50 ohm load to the transmitter only if the antenna on the corresponding port is also a 50 ohm load for that band. An HF antenna that is matched on 14 MHz has an unpredictable impedance on 144 MHz, but this does not matter, because the filter on the HF port blocks the VHF signals before they reach the antenna. Similarly, the VHF antenna is not heard on HF. The filter therefore also protects each antenna from the other band, but only when each antenna is connected to the proper port.

If one port is left open or connected to a poor antenna, the filter will reflect the signals of that band. For example, when you transmit on a band whose antenna is disconnected, the power is reflected back to the transmitter, and a high SWR results. Treat each port like an antenna connector: never transmit into a port with nothing connected to it. The same care applies when you reuse an old diplexer without knowing its crossover. The coax itself should also be of good quality. See our comparison of RG-58, RG-8X and LMR-400 coax for the loss figures that add to the diplexer losses.

Diplexers at Both Ends

A diplexer must be used as a pair. One unit at the antenna end combines the two antennas onto the shared coax, and another unit at the radio end separates the signals again. Use the common port toward the cable in both places. If you use only one diplexer, you will have two signals on a single cable, which is fine, but then you will need to connect both radios to the same port, which defeats the point. Where the cable run is long, loss on the higher band is greater than on the lower, so it is wise to choose a cable that handles the higher band with acceptable loss. For VHF and UHF this usually means a low-loss cable, such as the larger LMR-type coax.

The location of the device at the antenna end matters in bad weather. A diplexer on a mast must be weatherproof, with sealed connectors, because water in a filter coil will detune it. Our article on weatherproofing coax connectors describes the tape, the mastic and the drip loop that keep an outdoor connection dry. Mount the device so that the connectors point down, and grease the threads lightly.

Diplexer, Triplexer, Duplexer: Do Not Mix Them Up

The names are similar and the devices are really different. A diplexer or triplexer splits widely separated bands onto one cable. A duplexer, in the repeater world, allows one antenna to transmit and receive at the same time on two frequencies that are close together, for example 600 kHz apart on 2 metres. This requires large cavity filters with very high isolation, usually 80 dB or more, and a diplexer cannot do it. Repeater basics are in our article on VHF and UHF repeater operation.

The reverse also holds: a repeater duplexer cannot combine HF and VHF. When you read an advertisement, check the frequency ranges and the stated isolation, and you will see which device it is. A triplexer extends the diplexer to three bands, for instance HF, 2 metres and 70 centimetres. Its filters are more complex, the loss is slightly higher, and the filter at the middle band must be a band-pass type, so it is important that the stated ranges match your bands.

Typical Set-Ups

Three examples show how the device is used in practice. In the first, a mobile station has an HF antenna on the rear bumper and a dual-band VHF antenna on the roof. Both are connected to one diplexer under the dashboard, with a short cable from each antenna. The HF radio and the VHF radio sit side by side, and only the antenna lines need the filters. This is a case where you may not need a long common cable at all, and the diplexer then simply makes the shared mounting point possible.

In the second, a satellite station has a cross-polarised pair of Yagis for 144 and 432 MHz. A single coax to each antenna would double the cable, so a diplexer at the mast separates the two bands, and a second at the shack end joins them again to the radio. Because satellite signals are weak, the insertion loss and the isolation both count, and a good-quality unit is worth its price.

In the third, a portable HF and VHF station on a telescopic mast uses one cable to the base of the mast, where the diplexer separates the two bands to a vertical wire for HF and a small VHF whip. This avoids a second long cable, and the setup time is shorter. Whatever your case, mark the ports on the device with a permanent marker, because it is easy to mix up the low and the high port in the dark.

Testing After Installation

Measure the SWR on each band at the radio end of the cable. It should be about the same as that of the antenna measured on its own, plus a small increase due to cable and filter loss. If the SWR is higher on one band than before, check the connectors, then the port assignment. Next, transmit briefly on one band at low power while listening on the other, to see whether any of the signal leaks into the receiver, which would show as a sudden rise of the noise or a loss of sensitivity. A simple test with a dummy load on the unused port gives you an idea of how much of the power goes where it should.

Diplexers and Preamplifiers

Some stations place a preamplifier at the antenna for the high band. The amplifier needs DC power, which is usually sent up the coax. Check whether the diplexer passes DC on the high port, since many do not, and whether the common port passes DC to the amplifier. If DC is blocked, the preamplifier will not work, and you will need a separate power line. The article on the mast-mounted preamplifier covers sequencing and the effects on the noise figure. An amplifier on a shared line should also never see the HF transmit power, which is another reason to keep the diplexer ahead of it.

Building or Buying

A good commercial diplexer costs little compared with a second cable run, and the specifications are published. A home-built version is possible with simple LC filters, and many amateurs have built them for a mobile installation, but the design needs careful measurement. Without an analyser, you cannot be sure about the crossover and the isolation. If you build, test with a network analyser such as the NanoVNA, as in our guide to calibration and sweeps, and check the loss and the SWR on each band, then check the isolation by measuring the leak from one port to the other.

The Bottom Line

A diplexer joins two widely separated bands on one coax with a low-pass and a high-pass filter, and a triplexer does it for three. Look at the crossover, the insertion loss, the isolation and the power rating, use a pair of them, one at each end of the cable, and never transmit into a port with no antenna. It cannot combine bands that lie close together, it does not improve a poor antenna, and it is not a repeater duplexer. Used correctly, it saves cable and weight and keeps both bands neat on a single run.

Frequently Asked Questions

What is a diplexer in ham radio?

It is a passive device with a low-pass and a high-pass filter that lets two bands, such as HF and VHF, share one coaxial cable.

What is the difference between a diplexer and a duplexer?

A diplexer separates widely spaced bands. A duplexer lets a repeater transmit and receive on one antenna on two nearby frequencies and needs large cavity filters.

Can I use one diplexer or do I need two?

Normally two: one at the antenna end to combine, and one at the radio end to separate again.

Can a diplexer combine 40 m and 20 m?

No. The bands are too close together for a simple filter pair to separate them with good isolation. Use an antenna that covers both or a different method.

How much loss does a diplexer add?

A good one adds about 0.1 to 0.5 dB on each path, which should be added to the loss of the cable itself.

Does a diplexer pass DC for a preamplifier?

Some do and some do not. Check the specification, because a blocked DC path stops a mast-mounted preamplifier from working.