Repeater Duplexers and Cavity Filters: One Antenna, Two Jobs

Next time you stand near a repeater site, look at the tall metal cylinders connected by loops of cable in the equipment room. They are the cavity filters of a repeater duplexer, and without them the repeater could not work with a single antenna. A repeater receives your signal on one frequency and transmits it on another at the same moment, at a power of tens of watts, only a few hundred kilohertz away from the very weak signal it is trying to hear. The duplexer keeps the powerful transmitter out of the sensitive receiver. This guide explains the problem, the solution, the numbers that matter and the care needed in tuning, so that you can understand why repeaters are built as they are and why a poorly tuned one sounds deaf.

The Problem: a Loud Transmitter Beside a Quiet Receiver

A repeater station consists of a receiver, a transmitter and a controller. If the transmitter produces 50 watts, which is 47 dBm, and the receiver must detect signals of about minus 120 dBm, the difference is nearly 170 dB. If even a tiny fraction of the transmitter power gets into the receiver input, the receiver is overloaded, or its sensitivity falls. This effect is called desense, short for desensitisation. The transmitter is a problem in two ways. First, its main signal is so strong that it can saturate the receiver front end, even though it is on a different frequency. Second, any transmitter produces a faint noise on either side of its carrier, called sideband noise, and some of that noise falls on the receive frequency, where it adds to the noise floor and hides weak signals. The receiver-side aspects are discussed in our guide to receiver dynamic range, blocking and IMD.

One solution is to use two antennas, one for transmit and one for receive, separated vertically on a tower by enough distance to give isolation. The other is to use one antenna and a duplexer. The first approach avoids the loss of filters but needs a tall structure. The second is compact and common on mountaintop and rooftop sites where a single antenna is all there is room for. A short description of the device is in the article on the duplexer.

What a Duplexer Does

A duplexer has three ports: one to the antenna, one to the transmitter and one to the receiver. Inside, the path between the transmitter and the antenna is a filter that passes the transmit frequency and blocks the receive frequency, and the path between the antenna and the receiver does the opposite. The additional requirement is that the isolation between the transmitter port and the receiver port must be very high. For a typical amateur repeater this isolation is around 80 to 100 dB, and often more. This is not the same device as the diplexer that shares a cable between two distant bands, which is covered in a separate article; the two names look alike, but the task is different, because a duplexer must separate frequencies that are only a short distance apart.

To obtain this isolation, duplexers use cavity resonators. A cavity is a metal chamber, a tube for VHF and UHF, with an inner rod that resonates at the desired frequency. It has a very high Q, in the thousands, so it can be extremely selective. Two kinds are used: the notch cavity, which blocks a narrow range of frequencies, and the pass cavity, which allows only a narrow range to pass. A common design uses a combination known as notch and pass, in which each cavity has both functions at once, so the pair gives deep rejection at the unwanted frequency with a low loss at the wanted one.

Offset: the Distance That Sets the Size

The difference between the transmit and receive frequencies is the repeater offset. On the 2 metre band a common offset is 600 kHz, and on 70 centimetres it is 5 MHz in some regions and 7.6 MHz in others, depending on the band plan. The smaller the offset relative to the frequency, the harder the filtering. On 2 metres, 600 kHz is only about 0.4 percent of the frequency, so a cavity has to be very selective, and the duplexer is larger. On 70 centimetres, with a 5 MHz offset, the relative separation is similar, but cavities are physically smaller because the wavelength is shorter. The table shows how the demands vary.

You can see the same principle in the band plans: where possible, planners prefer a larger offset, because it makes the repeaters simpler to build. Our overview of VHF and UHF simplex and repeater operation describes the offsets from the user’s side, and the guide to CTCSS tones covers the access side.

Insertion Loss and Isolation

A duplexer is a compromise between isolation and loss. More cavities increase the isolation, but each cavity adds a little loss, typically 0.1 to 0.3 dB, so a four-cavity duplexer might lose 1 to 1.5 dB in each path. On transmit this wastes power, which is not very significant, and on receive it raises the noise figure of the system by the same amount, which is more important. For this reason a repeater often has a low-noise preamplifier ahead of the receiver, behind the duplexer, to compensate. Additional protection comes from isolators or circulators placed after the transmitter. These are ferrite devices that allow the signal to travel in only one direction, so that the energy reflected from the antenna does not reach the transmitter, and they help to reduce intermodulation.

The isolation needed depends on the quality of the transmitter. A clean transmitter with low sideband noise at the receive frequency requires less isolation than one that produces a wide noise skirt. This is why professional repeater transmitters are specified for their noise at an offset of 600 kHz, and why home-built stations made of cheap radios often need much better filtering than a purpose-built repeater, or may never reach the same performance. A related effect for receivers is the subject of our guide to receive filter bandwidth and the roofing filter.

Tuning: the Careful Part

A new duplexer, or one that has been moved, must be tuned. The cavities, each a kind of cavity resonator, have adjusting screws that change the length of the resonator, and each one must be set to the correct frequency, to within a few kilohertz. Temperature changes the dimensions of the metal, so a duplexer that was tuned in the warm shop may drift when installed on a cold mountain. Better cavities use invar rods, an alloy that expands very little, to keep the tuning stable.

The standard tuning method uses a signal generator and a detector, or today a vector network analyser, to sweep the response of each cavity. One cavity is tuned at a time, while the others are disconnected or terminated. The notch is set to the exact frequency to be rejected, and the pass response is set at the wanted frequency. After the individual cavities are tuned, the whole assembly is connected and the isolation between the transmit and receive ports is checked with the analyser. The cables between the cavities have defined lengths, usually specified by the maker, because their electrical length affects the phase at the junction. Do not replace them by other cables, even those that look the same. Always use a dummy load for the transmitter when tuning, and never apply power to a duplexer that is not tuned, because the reflected power can damage the transmitter.

Choosing and Buying a Duplexer

When a club plans a repeater, the duplexer is often the largest single cost, so choose it with care. First, confirm the band and the offset: a duplexer is built for one band and a defined transmit and receive spacing, and although the cavities can be retuned over a limited range, a unit for 600 kHz on 2 metres will not serve a 1 MHz repeater on 70 centimetres. Second, check the power rating. Commercial duplexers are usually specified for 100 watts or more, which is plenty for an amateur repeater, and the connectors, often N type or 7/16 DIN, must match the cables and the other equipment.

Third, ask for the isolation and insertion loss figures in the data sheet, and be careful with a second-hand unit whose condition is unknown. Fourth, plan for the whole chain: the cables between the cavities, the transmitter, the receiver with its preamplifier, the controller and the antenna. A second-hand duplexer from a commercial site is often an excellent buy, but it needs to be cleaned, checked for damage and retuned on the exact frequencies you will use. Allow a day for the work, and arrange the use of a network analyser, which most clubs can borrow. The result is a repeater that hears as well as it speaks, which is what the users expect.

Common Problems

When a repeater sounds deaf, the duplexer is the first suspect. A deaf repeater with a good receiver often means low isolation: someone has moved or knocked a cavity, a connector is loose or corroded, or the temperature has caused the tuning to drift. A symptom of desense is that the noise at the receiver rises when the transmitter is keyed, which can be seen by listening to the receiver audio with and without the carrier. Another is that the repeater hears well when no one talks and badly when it is transmitting. In such cases measure the isolation again and look for loose connections.

Corrosion is another problem. The cavities are made of metal and have many joints, and rain or condensation can enter the connectors. Use proper weatherproofing, as in our article on weatherproof coax connectors, and keep the equipment room dry. Check the cables for damage, particularly the short jumper cables between the cavities, whose condition determines the isolation. Finally, intermodulation in the antenna system, caused by rusty joints in a tower or by loose bolts, can produce a signal that falls on the receive frequency; this is known as the rusty bolt effect, and is cured by cleaning and tightening the metal contacts.

Is a Duplexer the Right Choice for a Hotspot or a Home Station?

A duplexer is only necessary where transmit and receive are at the same time on one antenna, as in a repeater. A personal hotspot works in simplex on a single frequency, as explained in our guide to the MMDVM hotspot, and does not need one, because it never transmits and receives at the same moment. A club that builds its first repeater, however, must decide early whether it will use a duplexer or two antennas. The decision depends on the site, the budget and the available height. A new duplexer is expensive, a used commercial unit is a good bargain if it is of the right band and offset, and a unit designed for a different offset can often be retuned within a limited range, but not far.

The Bottom Line

A repeater duplexer lets one antenna transmit and receive at the same time by placing high-Q cavity filters between the antenna, the transmitter and the receiver. It must give isolation of around 80 to 100 dB or more, because a 50 watt transmitter would otherwise overload a receiver that hears signals near minus 120 dBm. The smaller the offset, the more cavities are needed. Tune it carefully with an analyser, keep the jumper cables as specified, protect it from the weather and measure the isolation again if the repeater sounds deaf. For a simplex hotspot or a home station, you need none of this.

Frequently Asked Questions

What does a repeater duplexer do?

It allows a repeater to transmit and receive at once on one antenna by filtering the two paths so that the transmitter does not overload the receiver.

How much isolation does a duplexer need?

Typically 80 to 100 dB or more between the transmitter and receiver ports, depending on the power and the sideband noise of the transmitter.

What is the difference between a duplexer and a diplexer?

A diplexer separates widely spaced bands on one cable. A duplexer separates two frequencies that are very close together, as in a repeater, and needs large cavity filters.

Why is a 600 kHz offset on 2 metres hard to filter?

Because the separation is only about 0.4 percent of the frequency, so the cavities must be very selective, and the duplexer needs more of them.

What is desense?

A loss of receiver sensitivity caused by the transmitter, either by overload from its strong carrier or by its sideband noise on the receive frequency.

Why does a duplexer need retuning?

Temperature changes, movement and ageing shift the resonance of the cavities. A tuning error of a few kilohertz can reduce the isolation greatly.