Microwave waveguide and transmission line system
4.2 COMPONENTS
4.2.4 Microwave passive components
Microwave passive components are used to connect the radio frequency generators and amplifiers inside the transmitter, the transmitter to the antenna, the antenna to the receiver, and the radio frequency signals inside the receiver. Other microwave components are used to direct and sample the signals for monitoring.
4.2.4.1 Circulators
Circulators are used to route the transmitter pulses to the antenna and the returning echo signals to the receiver. These are ferrite components and need cooling for the higher powers. The symbol for a circulator is shown in Figure 4.10 where signals entering one arm leave by the next arm.
They are specified by:
• The loss from one arm to the next, typically 0.5 dB;
• Voltage standing wave ratio;
• The power that does not leave by the next arm which may be typically 20 dB down (1/100) compared to the input power;
• Peak power;
• Mean power.
Circulators are realized as waveguide, strip, or microstrip components. Typically a four-port circulator is used and arm 4 would be connected to the transmitter, arm 1 to the antenna, and arm 2 to the receiver. Arm 3 is connected to a dummy load for the case that there is a short circuit or other blockage in the antenna waveguide and the transmitter pulse is reflected with full power. The TR tube or receiver protector in the waveguide to the receiver will fire to reflect the transmitter pulse back into the circulator to be passed into the dummy load. The dummy load must be capable of dissipating the full transmitter power until protective circuitry switches off the transmitter. The dummy load prevents the transmitter pulse entering the transmitter to cause damage.
4.2.4.2 Isolators
Isolators are used to absorb reflected energy in order to improve the voltage standing wave ratio (VSWR). These either absorb the returning power internally or are circulators which route the returning power to a separately cooled load. They are specified by:
• The forward loss and voltage standing wave ratio (VSWR);
• The loss for returning energy;
• Peak forward power;
• Mean forward power.
4.2.4.3 Transmit-receive tubes
Semiconductor limiters have not yet taken over from gas-filled tubes. Gas-filled tubes have a limited life and need replacing regularly when the losses increase. They are specified by:
• The loss when the tube is extinguished, which must be added to the receiver input losses;
• The amount of the energy spike passed to the receiver when the tube starts to fire;
• The attenuation when the tube has fired;
• The mean and peak power handling (reflection) capacity;
• The time taken to extinguish which determines the minimum range of the radar (see Section 1.6);
• Voltage standing wave ratio.
Though circulators have taken over the main burden of routing the transmitter pulse away from the receiver, transmit- receive tubes still have to block the transmitter pulse in the case of a fault in the antenna. If the tube does not fire, the first receiver stages will be destroyed.
4.2.4.4 Diplexers, triplexers, and so on
A number of transmitter-receivers using different frequencies may be connected to a single antenna. Two transmitter- receiver groups working on different frequencies allow two statistically independent looks at an aircraft, for example, each antenna scan, which increases the probability of detection and false alarms.
These components are specified mainly by:
• Losses and voltage standing wave ratio; • Mean power;
• Peak power; • Amount of power that leaks into the other channel.
Two types of diplexers are those that combine the two channels into one waveguide before the rotary joint and those that require two high power rotary joints and combine the signals in orthogonal polarizations in a square waveguide on their way to the horn in the antenna. Polarization diversity additionally gives a look at a different polarization. The radar transmitters normally transmit one after the other and do not overlap in order to hold the peak power requirement to that of one transmitter alone. The mean power is the sum of the mean powers of the transmitters.
Great care must be taken to keep the losses as small as possible as they can nullify the effects of the “diversity gain”, or changing the effective scattering model from Swerling case I to Swerling case III (see Chapter 12).
T-R Power divider to give a single cosecant squared transmitted
beam
Rotary joints
Receiving beam 1 to receiver 1 Receiving beam 2 to receiver 2 Receiving beam 3 to receiver 3 Receiving beam 4 to receiver 4 Stack of
horns
T-R T-R T-R
From the transmitter
Radio frequency amplifiers T-R tubes
Figure 4.11 The arrangement of duplexers and rotary joints in a stacked beam radar.
4.2.4.5 Rotary joints
Rotary joints connect a rotating antenna to a fixed transmitter and receiver system. Before rotary joints became common, the transmitter and receivers were rotated with the antenna and this made maintenance more difficult. If the rotation is only occasional and less than 370 degrees, then (spiral) flexible cables may be used. Rotary joints often have a number of channels, each of which is specified mainly by:
• Frequency range;
• Losses and voltage standing wave ratio: average and variation about the average or wow;
• Peak power;
• Mean power.
The number of channels in the rotary joint depends on the radar. Figure 4.11 shows the arrangement for a three- dimensional radar with four receiving beams. The output from the transmitter is carried by the high-power rotating joint to the power divider for the formation of a single cosecant squared transmitter beam. The returning echoes are passed to the receivers by low-power joints. To these the rotating joints of the secondary radar channels must be added (see Figure 4.4).
4.2.4.6 Switches
Switches are used to switch the energy carried by the waveguide from the working direction to dummy loads or other components for maintenance. Normally, the switching takes place when the transmitter is switched off or muted by the removal of the radio frequency drive and a common form of waveguide switch is shown in Figure 4.12. The rotor consists of two waveguide bends to connect, on the left of Figure 4.12, port 1 with port 4 and port 2 with port 3. The rotor may be turned by hand or by electric motor but must be locked in one of the two positions before it starts to carry radio frequency power.
1
2
3
4
1
2
3
4
Position 1 Position 2
Rotor
8g/4 8g/4
A B
A º B B º A
Figure 4.12 The two positions of a waveguide switch.
Figure 4.13 Directional couplers for monitoring forward and returning power.
A typical switch for two transmitters would connect transmitter A (port 4) to the antenna (port 1) and transmitter B (port 2) to the dummy load (port 3). The waveguide switch may be switched so that transmitter may be tested at full power with the dummy load, and in the other position it is connected to the antenna.
Switches are specified mainly by:
• Losses and voltage standing wave ratio; • Mean power;
• Isolation in the unconnected ports; • Switching time.
• Peak power;
High-speed synchronous switches using quartz blades have been used to connect two pulse transmitters to a single antenna.
4.2.4.7 Dummy loads
Dummy loads are load resistors in coaxial or waveguide form. They are dimensioned to absorb the expected radio frequency power under both normal and, for a short time, under fault conditions. This short time is long enough to allow the fault detectors to switch off the transmitter. Dummy loads are specified mainly by:
• Peak power;
• Mean power;
• Continuous and short time power ratings;
• Voltage standing wave ratio.
A number of water cooled loads are equipped with thermometers and water flow meters to measure the power dissipated.
4.2.4.8 Directional couplers for injecting signals or measurement
Directional couplers take a sample energy from the main waveguide for transmitter and standing wave monitoring, or they feed test signals into the receiver waveguides and two couplers as shown in Figure 4.13. They are especially useful for monitoring the voltage standing wave ratio.
A
B C
D
Hybrid transformer
38/4
8/4 8/4
8/4 A
C B
D Rat race
hybrid
A C
B
D
Horns for monopulse feed
Sum (E) channel Difference
()) channel
Figure 4.14 Hybrid transformers and the rat race waveguide hybrid.
Figure 4.15 The hybrid or magic T hybrid and its use for monopulse sum and difference channels.
These couplers are specified mainly by:
• Attenuation between the signal in the waveguide and the coupled signal in the same direction;
• Attenuation between the signal in the waveguide and the coupled signal in the reverse direction;
• Leakage between these two signals, or directivity;
• Peak power;
• Mean power.
Figure 4.13 shows two-hole couplers that have a rather narrow frequency range. Greater bandwidth can be obtained using more holes with diameters given by Pascal’s triangle, namely 1,2,1 or 1, 3, 3, 1 and so on [8, p. 286]. The coupling holes are often circular for coupling factors of 1% (20 dB) or less, and the coupling depends on the sixth power of the diameter. For greater amounts of coupling, rectangular holes are used leading to the 3 dB hybrid couplers in Figure 4.1(c). There are other forms with angled or crossed waveguides.
4.2.4.9 Hybrid couplers
Hybrid transformers are used in communications circuits to divide a signal equally or reassemble them. In Figure 4.14 the input at A is divided equally between B and D, and the difference from any reflections from B and D exits at C. The hybrid transformers are used in a two-wire repeater to router the signals to the amplifiers and prevent positive feedback leading to oscillation. The loads on the C terminals dissipate any unbalanced signals.
A form of waveguide hybrid, the rat race, is shown on the right of Figure 4.14. Again there is no coupling between A and C additionally between B and D.
The size of the rat race depends on wavelength, and a waveguide hybrid with a wider bandwidth is the hybrid or magic T shown in Figure 4.15 [3, p. 291]. The magic T is more compact and is often used with horns as a feed in monopulse antennas with the sum channel connected via the transmit-receive switch to the transmitter and the sum receiver. The difference output from the magic T is only used for reception and is fed to the difference receiver.
4.2.4.10 Tuners
In the past, waveguide tuners reflected the conjugate of the mismatch impedance into the waveguide, and voltage standing wave ratios of less than 1.1:1 were specified. Broadband radars and the advent of less frequency dependent components, circulators, and “clean” waveguide runs allowed the tuners to be eliminated. The maximum voltage standing wave ratio requirement was relaxed to 2:1. This gives a forward loss of 0.51152 dB, the returned power is 9.54242 dB down (about 1/9) compared with the forward power, but the voltage in the waveguide is doubled which divides the maximum power that can be carried by the waveguide by four.