Module 18—Radar Principles
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(spark gaps) connected in a microwave circuit with three terminal transmission lines, one each for the transmitter, receiver, and antenna. As shown in views A and B of figure 2-12, these circuits may be connected in parallel or in series. Both systems will be discussed in detail in this section. One tube is called the TRANSMIT-RECEIVER TUBE, or TR TUBE; the other is called the ANTITRANSMIT- RECEIVE TUBE, or ATR TUBE. The TR tube has the primary function of disconnecting the receiver, and the ATR tube of disconnecting the transmitter.
Figure 2-12.—Duplexer systems.
The overall action of the TR and ATR circuits depends upon the impedance characteristics of the
quarter-wavelength section of transmission line. A quarter-wavelength, or an odd multiple of the quarter- wavelength, transmission line presents opposite impedance values at the ends; one end of the line appears as a short and the other end appears as an open.
The type of spark gap used as a TR tube may vary. It may be one that is simply formed by two electrodes placed across the transmission line; or it may be one enclosed in an evacuated glass envelope with special features to improve operation. The requirements of the spark gap are (1) high impedance prior to the arc and (2) very low impedance during arc time. At the end of the transmitted pulse the arc
should be extinguished as rapidly as possible. Extinguishing the arc stops any loss caused by the arc and permits signals from nearby targets to reach the receiver.
The simple gap formed in air has a resistance during conduction of from 30 to 50 ohms. This is usually too high for use with any but an open-wire transmission line. The time required for the air surrounding the gap to completely deionize after the pulse voltage has been removed is about 10 microseconds. During this time the gap acts as an increasing resistance across the transmission line to which it is connected. However, in a TR system using an air gap, the echo signals reaching the receiver beyond the gap will be permitted to increase to half their proper magnitude 3 microseconds after the pulse voltage has been removed. This interval is known as RECOVERY TIME.
Tr tubes are usually conventional spark gaps enclosed in partially evacuated, sealed glass envelopes, as shown in figure 2-13. The arc is formed as electrons are conducted through the ionized gas or vapor. You may lower the magnitude of voltage necessary to break down a gap by reducing the pressure of the gas that surrounds the electrodes. Optimum pressure achieves the most efficient TR operation. You can reduce the recovery time, or DEIONIZATION TIME, of the gap by introducing water vapor into the TR tube. A TR tube containing water vapor at a pressure of 1 millimeter of mercury will recover in 0.5 microseconds. It is important for a TR tube to have a short recovery time to reduce the range at which targets near the radar can be detected. If, for example, echo signals reflected from nearby objects return to the radar before the TR tube has recovered, those signals will be unable to enter the receiver.
Figure 2-13.—TR tube with keep-alive electrode.
Tr tubes used at microwave frequencies are built to fit into, and become a part of, a resonant cavity. You may increase the speed with which the gap breaks down after the transmitter fires by placing a voltage across the gap electrodes. This potential is known as KEEP-ALIVE VOLTAGE and ranges from 100 volts to 1,000 volts. A glow discharge is maintained between the electrodes. (The term GLOW DISCHARGE refers to the discharge of electricity through a gas-filled electron tube. This is distinguished by a cathode glow and voltage drop much higher than the gas-ionization voltage in the cathode vicinity.) This action provides for rapid ionization when the transmitter pulse arrives.
Failure of the TR tube is primarily caused by two factors. The first and most common cause of failure is the gradual buildup of metal particles that have been dislodged from the electrodes. Such metal bits become spattered on the inside of the glass envelope. These particles act as small, conducting areas and tend to lower the Q of the resonant cavity and dissipate power. If the tube continues in use for too long a
period in this condition, the particles will form a detuning wall within the cavity and eventually prevent the tube from functioning. A second cause of failure is the absorption of gas within the enclosure by the metal electrodes. This results in a gradual reduction of pressure within the tube to a point where gap breakdown becomes very difficult. The final result is that extremely strong signals (from the transmitter) are coupled to the receiver. Because both types of failures develop gradually, the TR tube periodically must be checked carefully to determine performance level.
Q27. What type of switches are used as duplexers?
Q28. What tube in a duplexer has the primary function of disconnecting the receiver?
Q29. How may the TR tube ionization speed be increased?
The ATR tube is usually a simpler device than a TR tube. An ATR tube might use a pure inert gas, such as argon, because recovery time generally is not a vital factor. Furthermore, a priming agent, such as keep- alive voltage, is not needed. The absence of either a chemically active gas or a keep-alive voltage results in ATR tubes having longer useful lives than TR tubes.
Tr and ATR tubes may contain radioactive material. Handle with care to avoid breakage and possible contamination.
There are two basic tr-atr duplexer configurations. They are the parallel-connected and the series- connected duplexer systems. The following paragraphs describe the operation of both systems.
Parallel Connected Duplexer Operation
First, let’s consider a PARALLEL-CONNECTED DUPLEXER system, as shown in figure 2-14. The TR spark gap shown in figure 2-14 is located in the receiver coupling line one-quarter wavelength from the T-junction. A half-wavelength, closed-end section of transmission line, called a STUB, is shunted across the main transmission line. An ATR spark gap is located in this line one-quarter wavelength from the main transmission line and one-quarter wavelength from the closed end of the stub. As shown in the figure, antenna impedance, line impedance, and transmitter output impedance, when transmitting, are all equal. The action of the circuit during transmission is shown in figure 2-15.
Figure 2-14.—Parallel-connected duplexer showing distance and impedance.
Figure 2-15.—Parallel-connected duplexer during transmission.
During the transmitting pulse, an arc appears across both spark gaps and causes the TR and ATR circuits to act as shorted (closed-end) quarter-wave stubs. The circuits then reflect an open circuit to the TR and ATR circuit connections to the main transmission line. None of the transmitted energy can pass through these reflected opens into the ATR stub or into the receiver. Therefore, all of the transmitted energy is directed to the antenna.
During reception, as shown in figure 2-16, the amplitude of the received echo is not sufficient to cause an arc across either spark gap. Under this condition, the ATR circuit now acts as a half-wave transmission line terminated in a short-circuit. This is reflected as an open circuit at the receiver T-junction, three-quarter wavelengths away. The received echo sees an open circuit in the direction of the transmitter. However, the receiver input impedance is matched to the transmission line impedance so that the entire received signal will go to the receiver with a minimum amount of loss.
Figure 2-16.—Parallel-connected duplexer during reception.
Series-Connected Duplexer Operation
In the SERIES-CONNECTED DUPLEXER SYSTEM, shown in figure 2-17, the TR spark gap is located one-half wavelength from the receiver T-junction. The ATR spark gap is located one-half wavelength from the transmission line and three-quarters wavelength from the receiver T-junction. During transmission, the TR and ATR gaps fire in the series-connected duplexer system, as shown in figure 2-18. This causes a short-circuit to be reflected at the series connection to the main transmission line one- half wavelength away. The transmitted pulse "sees" a low impedance path in the direction of the antenna and does not go into the ATR stub or the receiver.
Figure 2-17.—Series-connected duplexer showing distance and impedance.
Figure 2-18.—Series-connected duplexer during transmission.
During reception, neither spark gap is fired, as shown in figure 2-19. The ATR acts as a half-wave stub terminated in an open. This open is reflected as a short-circuit at the T-junction three-quarters of a wavelength away. Consequently, the received signal sees a low impedance path to the receiver, and none of the received signal is lost in the transmitting circuit.
Figure 2-19.—Series-connected duplexer during reception.
Duplexers are constructed in many forms, such as RESONANT-CAVITY COAXIAL SYSTEMS, WAVEGUIDE SYSTEMS, and HYBRID RINGS. Since waveguide and hybrid-ring duplexers are most common in radar systems, those will be discussed in this section.
WAVEGUIDE DUPLEXERS usually consist of TR tubes and ATR tubes housed in a resonant cavity and attached to a waveguide system in some manner. Resonant-cavity TR tubes may be applied to waveguides, either directly or indirectly, to obtain switching action. The indirect method uses a coaxial line system, and then couples the coaxial line into the waveguide that feeds the antenna. If large losses are incurred by the use of a coaxial line, the resonant cavity can be coupled directly to the waveguide. Figure 2-20 shows a direct method of cavity TR switching in a waveguide system. The waveguide terminates in the antenna at one end and in a shorting plate at the other. The magnetron uses a voltage probe to excite the waveguide. The transmitted pulse travels up the guide and moves into the TR box through a slot. The cavity builds up a strong electric field across the gap, breaks it down, and detunes the cavity. This action effectively seals the opening and passes the pulse energy to the antenna.
Figure 2-20.—Waveguide duplexer with cavity TR tube.
The signals received during the resting time travel down the guide to the magnetron and the shorting end plate where they are reflected. The slot coupling the waveguide to the cavity is located at a point where the standing-wave magnetic field produced by reflections in the waveguide is maximum. The maximum magnetic field, therefore, energizes the cavity. The echo signals are not strong enough to cause an arc, and the cavity field is undisturbed by the gap. Therefore, the cavity field couples rf energy into the receiver coaxial line and provides maximum energy transfer.
The cavity TR switch can also be applied to branch lines of the waveguide, as shown in figure 2-21. The magnetron is coupled to the guide by a voltage probe to produce proper excitation.
Figure 2-21.—Branched waveguide duplexer.
Maximum use of the received signals is ensured by an ATR tube. The transmitted pulse travels from the magnetron to the ATR branch where part of the energy is diverted into the gap. A slot (S) is placed across the waveguide one-half wavelength from the main guide, and passes the rf energy through it and into the cavity. The cavity builds up the electric field that breaks down the gap, detunes the cavity, and, as a result, effectively closes the slot. One-half wavelength away, this action effectively closes the entrance to the ATR branch and limits the amount of energy entering the ATR branch to a small value.
Most of the energy is, therefore, directed down the guide to the antenna. Upon reaching the receiver branch, the same effect is produced by the TR tube in the receiver line. Because the energy entering both openings is effectively limited by the gaps, maximum energy is transferred between the magnetron and the antenna.
During the resting time, the ATR spark gap is not broken down by the received signals. The received signal sets up standing waves within the cavity that cause it to resonate. At resonance, the low impedance of the ATR cavity is reflected as a high impedance at the entrance to the transmitter waveguide (three- quarter wavelength away). This ensures that the maximum received signal will enter the receiver branch.
Hybrid Ring Duplexer
The HYBRID RING is used as a duplexer in high-power radar systems. It is very effective in isolating the receiver during transmission. A simplified version of the hybrid-ring duplexer is shown in views A and B of figure 2-22. The operation of the duplexer, in terms of the E field distribution during transmission and reception, is illustrated in views C and D. The H lines, though present, have been omitted to simplify the explanation.
Figure 2-22.—Hybrid-ring duplexer.
During transmission the E field from the transmitter enters arm 3 and divides into two fields 180 degrees out of phase. One field moves clockwise around the ring and the other moves counterclockwise. The two fields must be 180 degrees out of phase at the entrance of an arm to propagate any energy down the arm. The field moving clockwise from arm 3 ionizes the TR tube in arm 4, and the energy is blocked from the receiver. The TR tube reflects a high impedance equivalent to an open circuit. This high impedance prevents any energy from entering the receiver - even though the two fields are out of phase at the entrance to arm 4. The field moving counterclockwise from arm 3 ionizes the TR tube in arm 2, which reflects a short circuit back to the ring junction. No energy is sent to the receiver, however, because the fields arriving at arm 2 are in phase. The clockwise and counterclockwise fields arrive at arm 1 out of phase by 180 degrees. They are then propagated through the arm to the antenna.
During reception, the relatively weak field from the antenna enters arm 1 and divides at the junction into two out-of-phase components. Neither field is sufficient to fire the TR tubes in arms 2 and 4; since the fields arrive at these arms out of phase, energy is propagated to the receiver. The energy arriving at arm 3 is in phase and will not be coupled to the transmitter. Since the operation of the arms of a hybrid ring is the same as the operation of E-type waveguide T-junctions, you may find it helpful to review NEETS, Module 11, Microwave Principles.
Q30. The actions of the TR and ATR circuits depend on the impedance characteristics of what length of transmission line?
Q31. During which of the transmit or receive cycles are both the TR and ATR tubes of a parallel- connected duplexer ionized (arcing)?
Q32. In a series-connected duplexer, what tube (tr or atr), if any, fires during the receive cycle?
Q33. To propagate energy down an arm of a hybrid ring duplexer, the two fields at the junction of the arm and the ring must have what phase relationship?
The energy that a distant object reflects back to the antenna in a radar system is a very small fraction of the original transmitted energy. The echoes return as pulses of rf energy of the same nature as those sent out by the transmitter. However, the power of a return pulse is measured in fractions of microwatts instead of in kilowatts, and the voltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radar receiver collects those pulses and provides a visual display of object information.
Information about the position of the object is present visually when the reception of an echo causes the movement or appearance of a spot of light on a cathode-ray tube (crt). The crt requires a signal on the order of at least several volts for proper operation and will not respond to the high frequencies within a return pulse. Therefore, a receiver amplifier and detector must be used that are capable of producing a visible indication on the cathode-ray tube under the following conditions: (1) when the input signal to the amplifier is in the form of pulses of extremely high-frequency, (2) the amplitude of the pulses is in the microvolt range, and (3) the pulses last for only a few microseconds.
The radar receiver evolved directly from the simple radio receiver. The radar receiver operates on exactly the same principles as the radio receiver. However, the overall requirements and limitations of a radar receiver differ somewhat from those of a radio receiver because of the higher frequencies involved and the greater sensitivity desired.
In studying the radar receiver, we will first examine the overall requirements of a radar receiver. Second, we will examine a typical radar receiver that satisfies these requirements. Finally, we will discuss the individual components of the receiver.
RADAR RECEIVER REQUIREMENTS
The following characteristics determine the design requirements of an effective radar receiver:
Introduction to Matter, Energy, and Direct Current,
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