In this lecture, we are going to learn about the Duplexer in Radar use, how it can be used in various radar applications, the operation of the duplexer, and the types of duplexers with a very detailed discussion. so let’s start with the basics of duplexers in radar.
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Duplexer In Radar
- Whenever a single antenna is used for both transmitting and receiving, as in a radar system, problems arise. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another.
- The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the return (echo) pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, ELECTRONIC Switches must be used. Switching systems of this type are called duplexers.
- The duplexer enables a radar system to transmit powerful signals and still receive very weak radar echoes.
- The duplexer acts as a gate between the antenna and the receiver and transmitter. It keeps the intense signals from the transmitter from passing to the receiver and overloading it, and also ensures that weak signals coming in from the antenna go to the receiver.
- A pulse radar duplexer connects the transmitter to the antenna only when a pulse is being emitted. Between pulses, the duplexer disconnects the transmitter and connects the receiver to the antenna.
- If the receiver were connected to the antenna while the pulse was being transmitted, the high power level of the pulse would damage the receiver’s sensitive circuits.
- In continuous-wave radar, the receivers and transmitters operate at the same time. These systems have no duplexer. In this case, the receiver separates the signals by frequency alone. Because the receiver must listen for weak signals at the same time that the transmitter is operating, high-power continuous-wave radar systems use separate transmitting and receiving antennas.
Block Diagram of Duplexer
Operation of Duplexer
- In selecting a switch for this task, you must remember that the protection of the receiver input circuit is as important as the output power consideration. At frequencies where amplifiers may be used, amplifier tubes can be chosen to withstand immense input powers without damage. However, the input circuit of the receivers is easily damaged by largely applied signals and must be carefully protected.
- An effective radar duplexing system must meet the following four requirements:
- During the period of transmission, the switch must connect the antenna to the transmitter and disconnect it from the receiver.
- The receiver must be thoroughly isolated from the transmitter during the transmission of the high-power pulse to avoid damage to sensitive receiver components.
- After transmission, the switch must rapidly disconnect the transmitter and connect the receiver to the antenna. For targets close to the radar to be seen, the action of the switch must be extremely rapid.
- The switch should absorb an absolute minimum of power both during transmission and reception, Therefore, a radar duplexer is the microwave equivalent of a fast, low-loss, single pole, double throw switch. the devices developed for this purpose are similar to spark gaps in which high-current microwave discharge furnish low-impedance paths. A duplexer usually contains two switching tubes.
- Generally, Gas-Discharge devices are being used as duplexers but ferrites and solid-state varactor diode have also been used. It makes use of fast-acting radio frequency switches known as TR ( Transit-Receive) and ATR ( Anti-Trasit-Receiver) which are operated by energy from the transmitter, and protect the sensitive receiver from the damaging effect of the high energy of the transmitter.
- In a typical application, the transmitter power might be 106 watts and the maximum safe power at the receiver might be a few watts.
- The duplexer must provide isolation between Tx and Rx at approximately 60dB, with a negligible reduction in the transmitted power.
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Types of Duplexer
There are different types of duplexer have been used, a brief explanation of these duplexers are given below:
| Sr. No. | Types of Duplexer |
|---|---|
| 1. | Branch Type Duplexer |
| 2. | Balanced Duplexer |
| 3. | Phase Shift Duplexer |
| 4. | Ferrite Duplexer |
| 5. | Solid-State Duplexer |
1. Branch Type Duplexer:
- Branch type duplexer, as shown in Figure below consists of an ATR switch and an ATR switch, both of which are gas-discharge tubes. During the transmissions, both the TR and ATR tubes ionize and fire. In the fired condition the TR acts as a short circuit to protect the receiver by blocking the transmitter power.
- As the TR tube is located at a distance of \lambda/4from the transmission lines so it acts as a short circuit at the receiver end but as an open circuit at the transmission line and does not limit the flow of transmitter power.
- The purpose of TR is to protect the receiver from excess power by disconnecting the receiver from the transmitter during transmission.
- The ATR acts as an open circuit during the transmission, so it does not affect the flow of transmission.
- Both TR and the ATR are not fired.
- While receiving the reflected signals from the target. The open circuit of the ATR gap is positioned \lambda/4 away from the transmissions line and acts as a short circuit across the line.
- The short circuit is located a \lambda/4 from the branch line of a receiver then the transmitter is disconnected from the line and reflected received signal power is directed towards the receiver. So, during the reception ATR is not fired and the entire energy of the echo signal is directed toward the receiver.
2. Balanced Duplexer:
- The branch duplexer has got a main drawback in that its bandwidth is limited, so broadband operation cannot be possible.
- To have more broadband operation, a balanced duplexer arrangement is used. It is shown in the Figure below, a Balanced duplexer uses the principle of the hybrid junction.
- A hybrid junction is a combination of waveguides, and coaxial, or other transmission lines having four branches.
- According to the principle of hybrid junction if the energy entering from any one branch or arm is equally transferred into two arms of the remaining.
- A short-slot hybrid junction is used in the balanced duplexer which is basically a directions coupler with a coupling ratio of 3 dB. It consists of two waveguides which are coupled through the slots cut in the narrow wall of the waveguides.
- During transmissions, the whole energy should be directed toward the antenna and the receiver should be isolated at least by 30 dB. When the transmitter is ‘on’ then the whole transmitted power is divided into two equal paths into waveguides by the first short slot hybrid junction. Both the TR tubes are fired which makes the waveguide short-circuited and the energy is reflected toward the antenna arm. One of the properties of the short-slot hybrid is that each time the energy passes through the slot, its phase advances by 90°. The energy travel as shown in the above figure, in the direction indicated. If the energy leaks through the TR tubes then this energy is directed toward the dummy load, not to the receiver.
- During receptions, the reflected signal power should be reached the receiver with the minimum attenuations. On reception, the TR tubes are unfired and act as open circuits so that the whole energy should reach the receiver. First, the received energy is divided into two equal parts, and due to the 90° phase advance introduced by the slot, the energy is joined in the receiving arm. No energy will enter the dummy load.
- There is another form of balanced duplexer which uses four ATR tubes and two hybrid junctions. A balanced duplexer with ATR has higher power handling capacity and less bandwidth than the TR-balanced duplexer.
3. Phase Shift Duplexer:
- There is one more method to have a duplexer function, which is called phase shift duplexer. In this type of duplexer, two hybrid junctions and a \lambda/2 power-sensitive phase shifter are used as shown in Figure below. During transmissions, the whole power is given arm 1, where the hybrid junction divides to its into two equal parts between branches 3 and 4. A 90° phase shift is introduced by the hybrid junctions which will make the whole energy reach the antenna arm.
- During receptions, the received energy is received by the antenna arm 7 which is passed through a phase shifter. The purpose of the power-sensitive phase shifter is to change the phase by \lambda/2. Due to the phase of the received energy, the whole received energy will enter the receiver arm. The received energy will not go to the transmission arm due to the mismatches of the phase.
- The N2 power-sensitive phase shifter uses a hybrid junction and two gas-discharge tubes as shown in Figure to provide the \lambda/2 phase shift.
4. Ferrite Duplexer:
- There is some duplexer that uses the ferrite devices such as ferrites phase shifters, isolators, and circulators to perform duplexer actions. Instead of using a power-sensitive phase shifter in the phase shift duplexer. We may use a ferrite phase shifter.
- The ferrite duplexer uses a magic T, two non-reciprocal ferrite phase shifters, and a short slot hybrid junction to perform the duplexer operation as shown in Figure below.
- The phase shifter provides the 45° phase shift either it advances the phase or may restart the phase according to the directions of the energy flow.
- A TR may be used to protect the energy flow. A TR may be used to protect the receiver against leakage from the Transmitter or reflected energy of the transmitter from the antenna.
- The ferrite duplexer has a lot of advantages over gas discharge tubes. It has wide bandwidth and fast recovery time than the gas discharge tubes. But it has a large size and weight and it requires a magnetic field for operations.
5. Solid State Duplexer
- As of now, we have discussed the gas discharge tubes used in the duplexer. These duplexers have disadvantages such as long recovery time, radioactive priming, and large size.
- These tubes may be replaced by the solid-state duplexer which has the advantages of fast recovery time and is small in size.
- The only problem with the solid-state duplexers is that it has less power handling capacity. But if PIN diodes are used in solid-state duplexers, they can handle more power.
- Generally, all solid-state duplexer consists of a three-port ferrite circulator with a diode limiter at the receiver.
