CHAPTER 10 :

1. CHAPTER 10 : RADAR TRANSMITTER

2. Radar Transmitters • Produces the short duration high-power RF pulses. • Produces the required mean RF power and the required peak power. • Suitable RF bandwidth. • A high RF stability to meet signal processing requirements. • Easily modulated to meet waveform design requirements. • Efficient, reliable and easy to maintain.

3. Radar Transmitters Microwave Sources :

4. Radar Transmitters Microwave Sources :

5. Radar Transmitters Microwave Sources :

6. Radar Transmitters Microwave Sources :

7. Radar Transmitters Microwave Sources :

8. Radar Transmitters Microwave Sources : • One main type of transmitters is the Keyed-Oscillator type. • In this transmitter one stage or tube, usually a Magnetron produces the RF pulse. • The oscillator tube is keyed by a high-power DC pulse of energy generated by a separate unit called the Modulator. • This transmitting system is called POT (Power Oscillator Transmitter). • Radar units fitted with a POT are either Non Coherent or Pseudo Coherent.  • Power Amplifier Transmitters (PAT) is used in many recently developed radar sets. • In this system the transmitting pulse is caused with a small performance in a Waveform Generator. • It is taken to the necessary power with an amplifier following (Amplitron, Klystron or Solid State Amplifier).

9. Radar Transmitters Microwave Sources : • Radar units fitted with an pat are Fully Coherent in the majority of cases. • A special case of the pat is the Active Antenna. • Even every antenna element or every antenna-group is equipped with an own amplifier here. • Solid-state transmit/receive modules appear attractive for constructing Phased Array Radar systems. • However, Microwave Tube Technology continues to offer substantial advantages in power output over solid-state technology

10. Radar Transmitters Transmitters to Antenna Connections :

11. Radar Transmitters Transmitters to Antenna Connections :

12. Radar Transmitters • A keyed oscillator transmitter of the historically Russian radar set P-37 (NATO-designator: ‘’bar lock”). • The picture shows the typical transmitter system that uses a Magnetron oscillator and a waveguide transmission line. • The magnetron at the middle of the figure is connected to the waveguide by a coaxial connector. • High-power magnetrons, however, are usually coupled directly to the waveguide. • Beside the magnetron with its magnets you can see the modulator with its Thyratron. • The impulse-transformer and the pulse-forming network with the charging diode and the high-voltage transformer are in the lower bay of this rack. TRANSMITTER OF P-37

13. Radar Transmitters Transmitter technologies are summarized in the following table:

14. Radar Transmitters Pseudo (semi) Coherent Radar : Pseudo-coherent Radar sets are sometimes called: ‘’coherent-on-receive’’ The principle of a pseudo-coherent radar

15. Radar Transmitters Disadvantages of The Pseudo Coherent Radar • The pseudo-coherent radar is a retired one today, but some older (or low-cost) radar sets are still operational. • The phase locking process is not as accurate as a Fully Coherent System, which reduces the MTI improvement. • Cannot be applied to frequency agile radar. Frequency change in a Magnetron relies on the mechanical tuning. • It cannot easily accommodate changes in the PRF, pulse width( can be performed at low level) and perform FM mod. • Second time around echoes are returns from large fixed clutter areas located a long distance from the radar.

16. Radar Transmitters Radar Modulator (Thyratron Modulator) Thyratron Modulator

17. Radar Transmitters Radar Modulator (Thyratron Modulator) • A special modulator is needed to produce this impulse of high voltage. • The Hydrogen Thyratron modulator is the most common radar modulator. • It employs a Pulse Forming Network (PFN) is charged up slowly to a high value of voltage. • The network is discharged rapidly through a pulse transformer by the thyratron keyed tube to develop an output pulse. • The shape and duration of the pulse are determined by the electrical characteristics of the PFN and of the pulse transformer. • As circuit for storing energy the thyratron modulator uses essentially a short section of artificial transmission line which is known as the PFN. • Via the charging path this PFN is charged on the double voltage of the high voltage power supply with help of the magnetic field of the charging impedance. • Simultaneously this charging impedance limits the charging current. • The charging diode prevents that the PFN discharge himself about the intrinsic resistance of the power supply again.

18. Radar Transmitters Thyratron • A typical thyratron is a gas-filled tube for radar modulator. • A switch to turn a pulse ON and OFF at the transmitter in response to a control signal. • The anode is completely shielded from the cathode by the grid. • Unlike most other thyratron, the positive grid-control characteristic ensures stable operation. • Because of the very high anode voltage the anode is attached most on the upper end of the glass bulb.

19. Radar Transmitters Thyratron Tgi2-400/16 Thyratron • The function of thyratron is to act as an electronic switch which requires A positive trigger of only 150 volts. • The thyratron requires a sharp leading edge for a trigger pulse and depends on a sudden drop in anode voltage (controlled by the pulse- forming network) to terminate the pulse and cut off the tube. • The RC combination acts as a DC shield and protect the grid of the thyratron. • This trigger pulse initiates the ionization of the complete thyratron by the charging voltage. • This ionization allows conduction from the charged pulse-forming network through pulse transformer. • The output pulse is then applied to an oscillating device, such as a magnetron. Thyratron with a Shell of Ceramic The Thyratron Symbol

20. Radar Transmitters Charging Path • The charge path: primary of pulse transformer, DC power supply and the charging impedance. • The thyratron (as the modulator switching device) is an open circuit in the time between the trigger pulses. • Once the power supply is switched on , the current flows through the charging diode and the charging impedance, charges the condensers of PFN. • The coils of the PFN are not yet functional. However, the induction of the charging impedance offers a great inductive resistance to the current and builds up a strong magnetic field. • The charging of the condensers follows an exponential function (line drawing green). The self- induction of the charging impedance overlaps for this

21. Radar Transmitters Diagram of Charging Currents • If the condensers are charged with the power supplies voltage, decreases the current and the magnetic field breaks down. • The breaking down magnetic field causes an additional induction of a voltage. • This one continues the charging of the condensers up to the double voltage of the power supply. • Now the condensers would discharged (ice blue curve) about the power supplies resistance, but the charging diode cut off this current direction and the energy remains stored therefore in the condensers

22. Radar Transmitters Discharging Path: Diagram of Discharging Currents • When a positive trigger pulse is applied, the tube ionizes causing the PFN to discharge through the thyratron and the primary of the pulse transformer. • The Tyratron is fired and grounds the pulse line at the charging coil and the charging diode effectively. • Therefore, a current flows for the duration pw through the pulse transformer primary coil to ground and from ground through the thyratron, which is now conducting to the other side of PFN. • The high voltage pulse for the transmitting tube can be taken on the secondary coil of the pulse transformer. • Because of the inductive properties of the PFN, the positive discharge voltage has a tendency to swing negative. • If the oscillator and pulse transformer circuit impedance is properly matched to the line impedance, the voltage pulse that appears across the transformer primary equals one-half the voltage to which the line was initially charged. Discharging Currents Path

23. Radar Transmitters Magnetron Tube: Magnetron МИ 29Г of the old Russian Radar ‘’Bar Lock” • In 1921 Albert Wallace Hull invented the magnetron as a microwave tube. • During World War II it was developed by John Randall and Henry Boot to a powerful microwave generator. • Magnetrons function as self-excited microwave oscillators. • Crossed electron and magnetic fields are used in the magnetron to produce the high-power output. • These multi-cavity devices may be used in radar transmitters as either pulsed or CW oscillators. • Frequencies ranging from approximately 0.6 to 30 GHz. • The relatively simple construction has the disadvantage, that the Magnetron usually can work only on a constructively fixed frequency.

24. Radar Transmitters Physical Construction of a Magnetron: • The magnetron is classed as a diode because it has no grid. • The anode of a magnetron is fabricated into a cylindrical solid copper block. • The cathode and filament are at the center of the tube and are supported by the filament leads. • The filament leads are large and rigid enough to keep the cathode and filament structure fixed in position. • The cathode is indirectly heated and is constructed of a high-emission material. • The 8 up to 20 cylindrical holes around its circumference are resonant cavities. • The cavities control the output frequency. • A narrow slot runs from each cavity into the central portion of the tube dividing the inner structure into as many segments as there are cavities Resonant Cavity Anode Cathode Filament Leads Pickup Loop Cutaway view of a magnetron

25. Radar Transmitters • The open space between the plate and the cathode is called the interaction space. • In this space the electric and magnetic fields interact to exert force upon the electrons. • The magnetic field is usually provided by a strong, permanent magnet mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode. • The output lead is usually a probe or loop extending into one of the tuned cavities and coupled into a waveguide or coaxial line. • slot- type • vane- type • rising sun- type • hole-and-slot- type the form of plate of magnetron

26. Radar Transmitters Basic Magnetron Operation: • Electronic events at a magnetron can be subdivided into four phases: • Production and acceleration of an electron beam • Velocity-modulation of the electron beam • Forming of a ‘’space-charge wheel” • Dispense energy to the AC field Production and Acceleration of an Electron Beam: • When no magnetic field exists, heating the cathode results in a uniform and direct movement of the field from the cathode to the plate (the blue path). • The permanent magnetic field bends the electron path. • If the electron flow reaches the plate, so a large amount of plate current is flowing. • If the strength of the magnetic field is increased, the path of the electron will have a sharper bend. • Likewise, if the velocity of the electron increases, the field around it increases and the path will bend more sharply. • However, when the critical field value is reached, as shown in the figure as a red path, the electrons are deflected away from the plate and the plate current then drops quickly to a very small value. • When the field strength is made still greater, the plate current drops to zero. The electron path under the influence of different strength of the magnetic field

27. Radar Transmitters Velocity-modulation of Electron Beam : • The electric field in the magnetron oscillator is a product of AC and DC fields (AC field is only shown). • The DC field extends radially from adjacent anode segments to the cathode. • The AC fields, extending between adjacent segments, are shown at an instant of maximum magnitude of one alternation of the RF oscillations occurring in the cavities. Forming of a ‘’Space-Charge Wheel’’ The high-frequency electrical field Rotating space-charge wheel in an twelve-cavity magnetron

28. Radar Transmitters Modes of Oscillation : • The operation frequency depends on the sizes of the cavities and the interaction space between anode and cathode. • But the single cavities are coupled over the interaction space with each other. • Therefore several resonant frequencies exist for the complete system. • Two of the four possible waveforms of a magnetron with 8 cavities are in the figure 8 represented. • Several other modes of oscillation are possible (3/4π, 1/2π, 1/4π), but a magnetron operating in the π mode has greater power and output and is the most commonly used strapping cutaway view of a magnetron (vane-type), showing the strapping rings and the slots Waveforms of the magnetron (Anode segments are represented „unwound”)

29. Radar Transmitters Magnetron Coupling Methods : • RF energy can be removed from a magnetron by means of a coupling loop. • At frequencies lower than 10GHz, the coupling loop is made by bending the inner conductor of a coaxial line into a loop. • The loop is then soldered to the end of the outer conductor so that it projects into the cavity. • Locating the loop at the end of the cavity, as shown figure, causes the magnetron to obtain sufficient pickup at higher frequencies. • The segment-fed loop method. • The loop intercepts the magnetic lines passing between cavities. • The strap-fed loop method, intercepts the energy between the strap and the segment. • Aperture, or slot, coupling is illustrated in view. Magnetron Coupling Magnetron Coupling Views

30. Radar Transmitters Magnetron Tuning : • A tunable magnetron permits the system to be operated at a precise frequency anywhere within a band of frequencies, as determined by magnetron characteristics. • The resonant frequency of a magnetron may be changed by varying the inductance or capacitance of the resonant cavities Tuner frame Additional inductive tuning elements Anode block Resonant Cavities of an Hole-and-slot- Type Magnetron With Inductive Tuning Elements Inductive magnetron tuning

31. Radar Transmitters An example of Magnetron: • An example of a tunable magnetron is the M5114B used by the ATC- Radar ASR-910. • To reduce mutual interferences, the ASR-910 can work on different assigned frequencies. • This magnetron is provided with a mechanism to adjust the TX- frequency of the ASR-910 exactly. Magnetron M5114B of the ATC-radar ASR-910 Magnetron VMX1090 of the ATC-radar PAR-80. This magnetron is even equipped with the permanent magnets necessary for the work.

32. Radar Transmitters Fully Coherent Radar: The output device would typically be a Klystron, TWT or Solid State Block diagram of a fully coherent radar

33. Radar Transmitters Other Names: Crossed Field Amplifier (CFA): • Amplitron • Platinotron • Stabilotron • Crossed-field amplifier (CFA), is a broadband microwave amplifier that can also be used as an oscillator (Stabilotron). • The CFA is similar a magnetron and is capable of providing relatively large amounts of power with high efficiency. • The bandwidth of the CFA is approximately plus or minus 5 percent of the rated center frequency. • Any incoming signals within this bandwidth are amplified. • Peak power levels of many megawatts and average power levels of tens of kilowatts average are possible with CFA. • Efficiency ratings in excess of 70 percent. • Typically, the first stage and the second stage are traveling-wave tubes (TWT) and the final stage is a CFA. • Recent technological advances in the field of solid-state microwave amplifiers have produced solid-state amplifiers with enough output power to be used as the first stage in some systems. • Transmitters with more than three stages usually use CFA, in the third and any additional stages. • Both TWT and CFA have a very flat amplification response over a relatively wide frequency range.

34. Radar Transmitters CFA have another advantage: • Design of CFA allows RF energy to pass through the tube unaffected when the tube is not pulsed. • When no pulse is present, the tube acts as a section of waveguide. • If less than maximum output power is desired, the final and preceding CFA stages can be shut off as needed. • This feature also allows a transmitter to operate at reduced power, even when the final CFA is defective. Cathode Anode with resonant-cavities Space-charge wheel Delaying strapping rings Schematically view of a crossed-field amplifier Water-cooled CFA, L-4756A in its transport case

35. Radar Transmitters Extended Interaction Klystron (EIK) : • EIK technology preserves the ruggedness and high power capability of the conventional KLYSTRON. • The EIK can be considered as a refinement of both two-cavity klystron and COUPLED-CAVITY TWT. • The EIK is a velocity modulated tube as linear beam device which combines the advantages of both tubes, the ruggedness and high power capability of a klystron and the larger bandwidth of a TWT. • It achieves enhanced power, bandwidth and efficiency at millimeter frequencies through the introduction of cavities with multiple coupled gaps. • A Ladder-type RF circuit supports high efficiency and thermal stability at millimeter and sub-millimeter frequencies, while operating with moderate electron beam voltages. • The EIKs currently operate at frequencies from 18 to 280 GHz.

36. Radar Transmitters Klystron Amplifier: • Klystron amplifiers are high power microwave vacuum tubes. • Klystrons are velocity-modulated tubes that are used in some radar equipments as amplifiers. • Klystrons make use of the transit-time effect by varying the velocity of an electron beam. • A klystron uses one or more special cavities, which modulate the electric field around the axis the tube. Mode of operation of a klystron A klystron uses special cavities which modulate the electric field around the axis the tube. In the middle of these cavities, there is a grid allowing the electrons to pass. The first cavity together with the first coupling device is called a “buncher”, while the second cavity with its coupling device is called a “catcher”.

37. Radar Transmitters Klystron Amplifier: • Multi Cavity Power Klystrons • Reflex Klystron Repeller Klystron Repeller Klystron circuit diagram with a repeller klystron Cavity of a repeller klystron Repeller klystron K-806

38. Radar Transmitters Traveling Wave Tube (TWT) : 4 1/3 ft. High Power TWT VTR 57 15 1/8 in. Russian Low Power T WT UV-1B (Cyrillic: УВ-1Б)

39. Radar Transmitters • Traveling Wave Tube (TWT): • TWT is a high-gain, low-noise, wide-bandwidth microwave amplifier. • It is capable of gains greater than 40 dB with bandwidths exceeding an octave. (A bandwidth of 1 octave is one in which the upper frequency is twice the lower frequency.) • TWT have been designed for frequencies as low as 300 megahertz and as high as 50 gigahertz. • Wide-bandwidth and low noise characteristics make the TWT ideal for use as an RF amplifier in microwave equipment. • On reason of the special low-noise characteristic often they are in use as an LNA in receivers additional. • There are two different groups of TWT: • Low-power TWT for receiversoccurs as a highly sensitive, low-noise and wideband amplifier in radar equipments. • High-power TWT for transmittersthese are in use as a pre-amplifier for high-power transmitters.

40. Radar Transmitters Physical Construction and Functional Describing: • The TWT contains an electron gun which produces and then accelerates an electron beam along the axis of the tube. • The surrounding magnet provides A magnetic field along the axis of the tube to focus the electrons into A tight beam. • The helix, at the center of the tube, is A coiled wire that provides A low-impedance transmission line for the RF energy within the tube. • The RF input and output are coupled onto and removed from the helix by waveguide directional couplers that have no physical connection to the helix. • The attenuator prevents any reflected waves from traveling back down the helix. Physical construction of a TWT Amplified Helix signal

41. Radar Transmitters Electric Fields That Are Parallel To The Electron Beam Inside The Helical Conductor. • Power-amplification are essentially dependent on the following factors: • Constructive details (e.g. Length of the helix) • Electron beam diameter (adjustable by the density of the focusing magnetic field) • Power input • Voltage UA2 on the helix electron- beam bouncing and a detail photo of a helix (Measure detail for 20 windings)

42. Radar Transmitters Characteristics of a TWT : • Gain of the TWT has got a linear characteristic of about 26 db at small input power. • If you increase the input power, the output power doesn't increase for the same gain. • The relatively low efficiency of the TWT partially offsets the advantages of high gain and wide bandwidth. • Given that the gain of an TWT effect by the electrons of the beam that interact with the electric fields on the delay structure, the frequency behavior of the helix is responsible for the gain. Characteristic of a traveling wave tube • The bandwidth of commonly used TWT can achieve values of many gigahertz's. • The noise figure of recently used TWT is 3 ... 10 dB. • The helix may be replaced by some other slow wave structure such as a ring-bar, ring loop, or coupled cavity structure. • The structure is chosen to give the characteristic appropriate to the desired gain/bandwidth and power characteristics

43. Radar Transmitters Ring-Loop TWT : • A Ring Loop TWT uses loops as slow wave structure to tie the rings together. • These devices are capable of higher power levels than conventional helix TWTs, but have significantly less bandwidth of 5…15 percent and lower cut-off frequency of 18 GHz. • The feature of the ring-loop slow wave structure is high coupling impedance and low harmonic wave components. • Therefore ring-loop traveling wave tube has advantages of high gain (40…60 Decibels), small dimension, higher operating voltage and less danger of the backward wave oscillation Ring-Loop slow wave structure Ring-Bar TWT : • The Ring-Bar TWT has got characteristics likely the Ring-Loop TWT. • The slow wave structure can be made easier by cut-out the structure of a copper tube. Ring-Bar slow wave structure

44. Radar Transmitters Coupled Cavity TWT : Coupled-cavity slow wave structure • The Coupled-cavity TWT uses a slow wave structure of a series of cavities coupled to one another. • The resonant cavities are coupled together with a transmission line. • The electron beam velocity modulated by an RF input signal at the first resonant cavity. • This RF energy (displayed as blue arrow) travels along the cavities and induces RF voltages in each subsequent cavity. • If the spacing of the cavities is correctly adjusted, the voltages at each cavity induced by the modulated beam are in phase and travel along the transmission line to the output, with an additive effect, so that the output power is much greater than the power input.

45. Radar Transmitters Solid State Amplifier : • The PSR transmitter of the ASR-E operates in the S-Band and is solid state. • It comprises four clusters, each of which contains eight power modules. • All power modules are identical. • BITE and status information are displayed on the transmitter front panel, as well as at the operator workstation. • The modules can be replaced during transmitter operation (hot replacement) without the disconnection of any cables. Solid state amplifier of the ASR-E(Manufacturer: EADS)

46. Radar Transmitters Solid State Amplifier Solid state amplifier • All high power transistors are protected against consequential damage. • The availability of the transmitter is nearly 100% because of the graceful degradation capability. • The un-serviceability of one or more power modules will not cause the complete loss of the transmitter and consequently of the ASR-E system. • A temporary slight reduction in performance has, however, to be accepted. • Driver and power supply modules are also redundant.

47. Radar Transmitters Solid State Amplifier Design of Transmitter Modules • Solid State transmitters are employed in radar sets nowadays however too. • At constant phases several MESFET- power amplifiers operates parallel by means of simple power splitters and adders • The high performance is assembled by many low-power amplifiers (or amplifier modules). • The modules are feed in phase by power splitters. • Its respective output powers then are in phase summed up to the complete transmit power. • To achieve adequate range with relatively low pulse power, the pulses are intra-pulse modulated often.

48. Radar Transmitters Active Antenna : Functional view of feeding the antenna-arrays • Active phased array antennae are antennae at which the transmit power is produced by many RX/TX- Modules of low performance on the antenna directly. • An active phased array uses a special type of solid state transmitter module. • e.g. the TORNADO-NOSE-RADAR and the air defense radar RRP-117. • The special transmitter modules come up on this page.

49. Radar Transmitters Transmitter Modules of Active Antenna • All components are assembled in one single T/R module. • This module integrates the electronic phase shifter, the digital controlled attenuator, the solid-state power amplifier, the low noise amplifier (LNA) and two circulators and a Duplexer. • It is also usual for the module to have self test and status features so that the overall performance of the system can be assessed (BITE). • A limiter is sometimes added between the LNA and antenna. • This reduces very strong incoming signals from jammers or large targets (with very high Radar Cross Section) close to the radar. • The limiter also protects the receiver against duplexer failure. A simple limiter implementation consists of two pin diodes connected in parallel but with inverted polarities. backside of an active antenna

50. Radar Transmitters RRP-117 (Remote Radar Post) • In many recently developed radar sets like the RRP-117 (Remote Radar Post) are used power-amplifier transmitters. • RRP-117 (Remote Radar Post) is a completely D-Band 3D radar system . • This transmitter works multiplexed with two frequencies. • These two frequencies are linear FM- modulated for the pulse compression. • By this pulse compression method the radar don't need so high pulse power, therefore a good maximum range is obtained despite the lower pulse power limited by semiconductor amplifiers. • Every row of the phased array antenna has got an own solid-state power-amplifier module and an own receiver. • The combination of the received signals by the monopulse antennae concept is carried out after the mixer stages on the IF-frequency. • The power moduls providing with needed currents is carried out via a bus system.