About duplexers

A duplexer is a device used in radio and telecommunications systems to split and combine the transmitter and receiver frequencies onto a single antenna.

How a duplexer works. When a transmitting device transmits a signal at a certain frequency, it must be separated from the receiving signal, which may be in the same range. This is done by using filters that pass only certain frequencies. A duplexer is two filters that pass the receiver and transmitter frequencies, respectively. The filter of the transmitter path passes its signal with minimal losses to the antenna and suppresses the signal at the reception frequency. The filter of the receiver path, on the contrary, delays the signal at the frequency of the transmitter, protects the receiver and does not interfere with its operation. At the same time, the receiving path filter passes the reception signal from the antenna with minimal loss. For the normal operation of the repeater, the duplexer must provide a separation of signal levels between reception and transmission of at least 70 dB.

Duplexers differ in design depending on the frequency range. For the VHF and UHF ranges – structures on volumetric resonators. For high-frequency devices, duplexers on quartz or ceramic filters are used.

The duplexer has three ports (connectors) – low frequency (LOW), high frequency (HIGT) and antenna.

It does not matter on which frequency the reception will be, and on which repeater transmission – on the upper or on the lower. It is important to match the ports of the lower and upper frequency settings.

For VHF and UHF radio communication systems, duplexers are made in a compact version (“mobile”, on 1-2 volume resonators) and on 5 and 8″ size resonators.

The defining parameters of the duplexer are:

Frequency band/subband. VHF and UHF duplexers are different products that differ in the geometric parameters of the elements. The 400-470 range is covered by one duplexer in a compact design. The range of 136-174 MHz is more often divided into two sub-ranges.

Duplex distribution (distribution of high and low frequencies, distribution of reception and transmission of the repeater). This parameter is set during the manufacture of the duplexer and cannot be changed. As a rule, when distributing frequencies in the VHF range, a spacing of 4.5-5 MHz is used, in UHF – 10 MHz. With such distances, a mobile duplexer with resonators from 1″ is sufficient. Duplexers in a compact design for VHF, respectively, for 4.5-6 MHz, 9-13 MHz, 24 MHz transmission (duplex pairs are organized on police frequencies 148/172 MHz).

When the duplex spacing is less than 4.5 MHz, duplexers of higher Q are used on 5 and 8″ volume resonators.

The maximum permissible power of the transmitter. Versions with 25, 50, 100 W and more transmitter power are usually produced. They differ constructively. The photo below shows an example of a 100W VHF duplexer on 2″ volume resonators.

As a rule, the specified parameter for the model is the power that the duplexer is designed for at 100% duty cycle of the repeater. If the duplexer does not work at the capacity calculated for it, it starts to heat up, which can lead to mechanical deformation and deviation from the setting parameters. An 80 W repeater can work with a 25 W duplexer if the transmission intensity is low. But with intensive work and with insufficient cooling of the equipment, the parameters may be disturbed.

A band of transmission frequencies is actually a band in which repeater frequencies can be changed without reconfiguring the duplexer. The same duplexer can be made with a sharp diagram, and then the suppression at the level of 100 dB will be in the range of +- 200 kHz from the set frequency, and it can be made with a canopy when the suppression of 80 dB (which is usually enough) will be in the range of + – 500 kHz. By default, we do it with sharp and more effective suppression. Graphs of parameters at LOW and HIGT frequencies are attached to duplexers, from which it is possible to estimate how far it is possible to move along the range without losing the quality of communication. That is, unless otherwise specified when ordering, duplexers are made with a band of 400-500 kHz.

But sometimes there is a need to quickly change repeater frequencies in the field, and then it is convenient to have a wide band. Also, a wide duplexer band may be needed in RF racks of multi-channel base stations when the distance between channels is large enough.

In the graphs below, there are two options for configuring the receive arm filter of the same duplexer with different bandwidths. The left curve is the graph of the SWR, the right is the amplitude-frequency characteristic (frequency response) of the filter.

In the first version on graph 1, the suppression at the transmitter frequency is at the level of -100 dB and the SWR is less than 1.25. At the same time, the rejection band at the level of -70 dB is 0.9 MHz.

graph 1

In the second option (graph 2), the rejection band at the level of -70 dB is closer to 1.3 MHz, and at the central frequency it is less than -80 dB.

graph 2

Some features of operation of duplexers

  • The duplexer, like any resonator RF device, has parameter dependence on geometry. Therefore, he is afraid of mechanical deformations and temperature changes, which lead to changes in geometry and, accordingly, settings. Impacts, falls, damage to adjustment screws, strong heating can lead to significant deterioration of communication.
  • As mentioned above, the lower nominal frequency is connected to the LOW arm, and the higher one to the HIGT arm, regardless of which frequency the repeater receives and transmits. If you get confused and the transmitter will not work at the transmission frequency of the duplexer, something will fail – either the transmitter or the duplexer. In any case, there will be no connection.

As an example. A base station with a duplexer, after some time of operation, the communication range drops sharply. In fact, the protection of the repeater worked and the power of the transmitter decreased. The reason may be in the duplexer. When using a faulty or out-of-band duplexer, the output power of the transmitter does not reach the antenna and significantly reduces the sensitivity of the receiving path of the repeater. Depending on how much and in which direction the frequencies of the repeater and duplexer do not match, it can be:

  • Loss of power in the duplexer. At normal setting, depending on the duplex spread, the losses are 1-1.3 dB at SWR≤1.3. That is, with a working power of the repeater of 50 W, the output of the duplexer will be 35-40 W. If the setting is not normal, the SWR increases and the losses increase. Losses of 3dB – at the output of a 25W duplexer and so on.

    • Under certain conditions, it is possible to burn the input of the repeater receiver if it does not enter the protection.
    • When setting up the rejection, the output power of the duplexer may be within the normal range, and the sensitivity of the receiver will drop, which will significantly affect the communication range.
  • Jumpers for connecting a duplexer with a repeater. If jumpers with a resistance of 50 Ohm are connected to HF devices matched to 50 Ohm, then their length does not affect anything, except for losses in the cable. Therefore, the minimum sufficient length is chosen for ease of installation.      

Losses in a jumper with a length of 0.5-1 m at VHF\UHF frequencies are not significant, so it is enough to use an RG58 or similar cable. There is no point in making jumpers from 3/8” or thicker cable, because they will be less convenient for installation.

  • Duplexers on volume resonators of 5 and 8 inches are several times more expensive than mobile duplexers. Therefore, if it is possible to choose a duplex pair of frequencies with a spacing of not 2-3 MHz, but from 4.5 on VHF and from 9 on UHF, then it is worth choosing exactly that.