1. Radar Sources Researched by Islam Ayman
Ziad Maged
Mohamed Mostafa
Karim Ahmed
Adam Ahmed
Yasmine Azzazi    Supervised by  Prof. Abdelmegid Allam Head of Communications Department
Vice Dean of IET German University in Cairo

2. Outline Conventional vacuum tubes Klystron Reflex Klystron
Travelling Wave Tubes (TWT)
Magnetron
Semiconductors

3. Chapter 1 Conventional Vacuum Tubes Triodes (Lighthouse Tubes), Tetrodes and Pentodes

4. Microwave tubes working as generators in the microwave region can be divided into two main groups:
Devices with linear movement of charged particles:
Devices with electrostatic controlled introduction (triodes, tetrodes and pentodes)
Devices with introduction in a small gap of resonators (two and multi-cavity klystrons and reflex klystron)
Devices working with a long introduction of electron beam with velocity modulation (O- type of travelling wave tubes)
Devices with curvilinear movement of charged particles:
Devices with crossed electric and magnetic field (electrons move perpendicular to the magnetic lines ) like magnetron,
M- type of travelling wave tubes.
b) Electron beam of parametric amplifiers (low noise)

5. The basic principle of all these devices consists in a transformation from kinetic energy and potential energy of charged particles to electromagnetic field energy.
Conventional vacuum triodes, tetrodes and pentodes are less useful signal sources at frequencies above 1 GHz because of transit angle effect and lead inductances and inter-electrode capacitance effect

6. Difficulties of using tubes in the microwave band:
The transit time between the electrodes is of the same order as the signal period.
Parasitic capacitance and lead inductances of electrodes limit the upper frequency used. Fig (1.1) shows these parasitic elements.

7. 1. Transit time of electrons (inertia of electrons):
Consider two plates A&B are subjected to a voltage difference
UAB ,fig (1.2). Calculating the transit time of an electron from
the plate A to plate B.
This transit time causes time delay between Ug (grid voltage) and igk, which means phase shift Ф , (1.7).
For maximum higher frequency (fh):

8. The first electron is able to reach the anode.
Space Time Diagram
Fig (1.3) shows the motion of electron moving from cathode to grid at different times during the period control voltage Ug
The first electron is able to reach the anode.
The second electron is able to reach the anode during the second half of the period.
The third electron can oscillate long time between cathode & grid.
The fourth electron returns back to cathode (cause heating).

9. 2. Parasitic capacitance and lead inductance:
Consider an approximate equivalent circuit diagram of a triode. Neglect the lead inductance of the cathode, since Llk has a small influence on the circuit, fig (1.4).

11. We can solve circuit of fig (1.8) to get the oscillating
frequency graphically or analytically

13. Graphical solution
Plotting X1 & X2 & X3 of the three resonant circuit against frequency :
fig (1.10) if ω1 < ω2 < ω3 and fig (1.11) ω1> ω2 > ω3 , we can get oscillating frequency.

15. To have X1& X2 have the same sign.
Analytic Solution
To have X1& X2 have the same sign.

16. The lighthouse tube  
These tubes we can work up to 6 GHz (as amplifier) and up to 10 GHz (as oscillator).
These tubes have planar electrodes in the form of discs with decreasing diameters from the cathode to the anode. This construction is suitable to get very small inter-electrode distance.

17. Fig (1. 13a) shows the construction of lighthouse tube
Fig (1.13a) shows the construction of lighthouse tube. It is practically used in some stations.
Fig (1.13b) shows practical connection of lighthouse tube in some stations with cylindrical cavities short circuited with plungers to represent capacitances and inductances needed.

18. Linear beam tubes (O-type)
Cavity
Resonant
Klystron
Reflex Klystron
Slow-wave
Forward-wave
Helix TWT
Coupled-cavity T.W.T
Backward-wave
BWA, BWO

19. Linear Beam O-tubes
The Paramount O-type tube is the two cavity klystron followed by the reflex klystron.
Slow wave structures are also O-type but have non-resonant periodic structures for electron interactions.
Twystron is a hybrid amplifier that uses combination of klystron and TWT components.

20. Two cavity Klystron

21. Two cavity Klystron
The klystron consists of two main parts: an input cavity (Buncher) and an output cavity (Catcher).
There is a space between the buncher cavity and the catcher cavity called the “drift space”.
When used as an amplifier the input and output cavities are not coupled to each other.

22. Basic Scheme of the Klystron

23. Principle of operation
The Cathode and the anode act as an electron gun to produce a high velocity stream of electrons.
The beam of electrons first passes through the buncher cavity, through grids attached to each side.
The electrons are either accelerated or decelerated by the MW field in the MW interaction region.
Finally, the electrons are collected by the collector having the same potential.

24. Principle of operation
The electrons enter the input gap ( space between G1 and G2 ) with velocity 𝑉 0 which is given by: 𝑉 0 = 2𝑒 𝑚 𝑈 0 Where, 𝑉 0 is the velocity of electrons when entering the input gap. 𝑒 is the charge of electrons. 𝑚 is the mass of electrons. 𝑈 0 is the initial energy provided by the DC beam voltage.

25. Principle of operation
The electrons leave the input gap ( space between G1 and G2 ) with velocity 𝑉 1 which is given by: 𝑉 1 = 𝑉 𝑈 𝑈 0 cos 𝑤 𝑡 0 Where, 𝑉 1 is the velocity of electrons when leaving the input gap. 𝑈 cos 𝑤 𝑡 0 is due to the alternating field in the input gap. 𝑡 0 is the time at which the electrons pass the center of the input gap (Buncher). This represents velocity modulation.

26. Principle of operation
The electrons enter the drift space with velocity 𝑉 1 , the time at which the electrons reach a distance 𝑍 from the input cavity is t which is given by: 𝑡= 𝑡 0 + 𝑍 𝑉 0 1− 1 2 𝑈 𝑈 0 cos 𝑤 𝑡 0 Where, 𝑡 is the time at which the electrons reaches the output cavity. 𝑍 is the distance between the buncher and catcher cavities.

27. Velocity Diagram

28. How to achieve maximum power output ??
Now we shall try to get the optimum distance between buncher and catcher cavities to achieve maximum output power. First, we define the instantaneous current which is given by: 𝐼= 𝐼 0 1+𝑥 sin 𝑤 𝑡 0 Where, 𝐼 0 is the DC current. 𝑥 is the bunching parameter which is given by: 𝑥= 1 2 w 𝑧 𝑣 0 𝑈 𝑈 0 Where w represents the angular velocity.

29. How to achieve maximum power output ??
After some derivations, we reached that formula: Since the catcher cavity is tuned to the fundamental, we need only the fundamental term with angular velocity w. For maximum amplitude of first harmonic:

30. Characteristics of Two cavity Klystron
Efficiency of about 40%.
Peak power output up to 30 MW.
Average output power is up to 700 KW at frequency of 10 GHz.
Power Gain of about 30 dB.
Bandwidth range from 1% to 8%.

31. Three (Multi) cavity Klystron
The signal in the first gap results in a small bunching of electrons.
The voltage in the second gap is much greater than in the first one.
The bunched beam enter the third gap and produce a large amount of power.

32. Three (Multi) cavity Klystron
The three cavity klystron can be used as a self excited oscillator.
The power is fed back from the second cavity to the first one.
The first two cavities determine the frequency and the third cavity is responsible for power amplification.

33. Characteristics of Reflex Klystron
Single cavity Klystron.
Efficiency of about 20-30%.
Low Power output from mW.
Frequency range of 1-25 GHz.

34. Reflex Klystron

35. Principle of operation
There is only one cavity and grid system is used with a –ve Reflector and the cavity fulfils the function of buncher and catcher .
The electron beam passes through a single resonant cavity.
The electrons are fired into one end of the tube by an electron gun.
After passing through the resonant cavity they are reflected by a negatively charged reflector electrode through the cavity, where they are collected.
The formation of electron bunches takes place in drift space between reflector and cavity.

36. Principle of operation
The voltage on reflector must be adjusted so that the bunching is at max as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is transferred from electron beam to RF oscillations in the cavity.
The reflector voltage may be varied slightly from the optimum value, which results in some loss of output Power, but also in a variation in frequency.
Finally, Modern semiconductor Technology has effectively replaced the reflex klystron in most applications.

37. Principle of operation
The electrons leave the grids and enters the retarding field with intensity which is given by: 𝐸 𝑟 = 𝑈 0 + 𝑈 𝑅 𝑆 Where, 𝐸 𝑟 is the retarding field intensity. 𝑈 0 is the initial energy provided by the DC beam voltage. 𝑈 𝑅 is the reflector voltage. 𝑆 is the distance between the reflector and the grids.

38. Principle of operation
The average transient angle of electrons from grids to grids is given by: 𝞠 = 2𝑚𝑤 𝑣 0 𝑒 𝐸 𝑟 Where, 𝞠 is the transient angle. 𝑚 is the mass of electrons . 𝑤 is the angular velocity. 𝑉 0 is the velocity of electrons when entering the input gap. 𝑒 is the charge of electrons.

39. Electronic power and efficiency
The input power is given by: 𝑃 0 = 𝐼 0 𝑈 0 where 𝐼 0 is the DC current.
The electronic power is given by:
𝑃 𝑒𝑙𝑒𝑐 = 𝐼 0 𝑈 0 cos 𝜃− 3𝜋 𝑥. 𝐽 1 (𝑥) 𝜃
The Electronic efficiency is given by:
𝜂= 𝑃 𝑒𝑙𝑒𝑐 𝑃 0 = cos 𝜃− 3𝜋 𝑥. 𝐽 1 (𝑥) 𝜃

40. For Starting Current: I 0,st = 2 𝑈 0 𝑅 𝑒𝑞 𝜃 where 𝜃 = 3𝜋 2 +2𝜋𝑘 For 𝑘 = 0 represents the first mode. For 𝑘 = 1 represents the second mode. For 𝑘 = 2 represents the third mode. This is called the minimum cathode current at which oscillating can start.

41. Modes of oscillation

42. Figures show that the output power and frequency against reflector voltage.
At maximum negative reflector voltage we have maximum output power, which is the 1st mode.
The point at which the maximum power of the mode, there is output frequency.
Moving left or right power decreases till zero and frequency increases if U R negatively increases and decreases if U R negatively decreases.

44. The figure shows the phase of returning current in reflex Klystron to have oscillations for the different modes against sinusoidal oscillations in the cavity.
Returning current must return the grids of the cavity in such phase to deliver the energy to maintain oscillation.

45. Travelling Wave Tube

46. Introduction:
The traveling-wave tube (TWT) was invented in 1944 by Kompfner.
The Traveling-Wave Tube (TWT) is an amplifier of microwave energy.
It accomplishes this through the interaction of an electron beam and an RF circuit known as a slow wave structure.
TWT are commonly used as amplifiers in satellite transponders, where the input signal is very weak and the output needs to be high power.
TWT transmitters are used extensively in radar systems, particularly in airborne fire-control radar systems, and in electronic warfare and self- protection systems.

47. Difference between TWT & Klystron:
In the case of the TWT, the microwave circuit is non-resonant.
The interaction of electron beam and RF field in the TWT is continuous over the entire length of the circuit, but the interaction in the klystron occurs only at the gaps of a few resonant cavities.
The wave in the TWT is a propagating wave; the wave in the klystron is not.
In the coupled-cavity TWT there is a coupling effect between the cavities, whereas each cavity in the klystron operates independently.

48. Types of TWT:
Helix Travelling wave Tube:

49. How it works:
A helix traveling-wave tube consists of an electron beam and a slow-wave structure. The electron beam is focused by a constant magnetic field along the electron beam and the slow-wave structure.
The commonly used slow-wave structure is a helical coil with a concentric con­ducting cylinder

50. The electron beam can be accelerated only to velocities that are about a fraction of the velocity of light.
A slow-wave structure must be incorporated in the microwave devices so that the phase velocity of the microwave signal can keep pace with that of the electron beam for effective interactions.
It can be shown that the ratio of the phase velocity vp along the pitch to the light velocity along the coil is given by:
Where c = 3 x 108 m/s is the velocity of light in free space
p = helix pitch
d = diameter of the helix
ψ = pitch angle

51. 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.

52. The applied signal propagates around the turns of the helix and produces an electric field at the center of the helix, directed along the helix axis.
When the electrons enter the helix tube, an interac­tion takes place between the moving axial electric field and the moving electrons.
This interac­tion causes the signal wave on the helix to be amplified.

54. The characteristics of the Traveling-wave tube are:
Frequency range: 3 GHz and higher
Bandwidth: about 0.8 GHz
Efficiency: 20 to 40%
Power output: up to 10 kW
Average Power gain: up to 60 dB

55. Types of TWT:
Coupled Cavity Travelling wave Tube:

56. 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 is velocity modulated by an RF input signal at the first resonant cavity.
This RF energy 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.

57. Microwave Crossed Field Tubes
1-Classification of M-type tubes is shown in the figure below.
2- we shall study in detail magnetron as commonly used M- type tube.

58. Magnetron Types of Magnetron
The magnetron is the earliest source of Microwave power.
 The magnetron serves as an oscillator, generating a microwave signal from direct current electricity supplied to the vacuum tube.
It is still the most efficient Microwave tube(The device is capable of converting dc to Microwave power with efficiencies of between 50 to 80 percent).
The latter tubes have supplanted the magnetron as the transmitting tube in high power radars.
Types of Magnetron
1) Cyclotron frequency magnetron
2) The negative, resistance magnetron
3) The multi-cavity traveling wave magnetron

59. Multi-Cavity Magnetic ( Cylindrical Magnetic )
There is a continuous magnetic field with intensity " H " between anode and cathode
At H=h0 the electrons starts to tocuh the anode
H should be greater than Hc in to make the electrons don’t reach the anode

60. Multi-Cavity Magnetic ( Cylindrical Magnetic )
Operating d. c. voltage must be less than UC for a given Ho or by other mean for a given U. d .c magnetic field ' H" must be greater than " Hc"
Equation of Parabola of Critical State:

61. Multi-Cavity Magnetic ( Cylindrical Magnetic )
The cavity magnetron  generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities.
Electrons pass by the openings to these cavities and cause radio waves to oscillate within.
The frequency of the microwaves produced, the resonant frequency, is determined by the “cavities” physical dimensions.
Magnetic field
Electric field

62. Multi-Cavity Magnetic ( Cylindrical Magnetic )
Equation of the synchronizing voltage line:
It is the minimum voltage needed to make electron pass the distance (L) between two neighbouring cavities in half a period ( pi-mode).
Anode Radius L=
Number of cavities

63. Multi-Cavity Magnetic ( Cylindrical Magnetic )
After some cylindrical motion electrons must reach the anode(to supply dc energy to the oscillation)
If Uo < UOH no electron will reach the anode and therefore there will be no oscillation.
Finally, oscillation zone of magnetron in the ∏ mode is determined by the three curves: parabola of critical state, synchronizing voltage line and Hartree line.

64. Equivalent circuit and modes of oscillations :
The phase shift from cavity to the next one is
Where N is the number of cavities.

65. How to ensure magnetron operation in the π – mode?
It is normal to keep operation of in the π -mode for good frequency stability also magnetron operating in the π mode has greater power. There are two main methods to keep magnetron operation in the π – mode :
1-Strapping (for Wavelength>3 cm)
2-Rising sun method (For Wavelength<3 cm)

66. Strapping Method
We connect odd sections together by one strapping ring and even sections together by another strapping ring to prevent inefficient modes to oscillate.

67. Tuning of magnetron 2-Mechanical tuning
Tuning of conventional magnetron can be by:
1-Changing supply voltage Uo but it is a very weak dependence
2-Mechanical tuning
a) Capacitive tuning is made by changing of the slot capacitor by inserting a movable dielectric in it.
b) Inductive tuning is made by changing the magnetic volume of the cavity by inserting a metallic rod in the cavity.

68. Applications of Magnetron
1-Radar
Magnetron may be used in radar transmitters as either pulsed or cw oscillators at frequencies ranging from approximately 600
to 30,000 megahertz(one of the disadvantages is the magnetron's instability in its transmitter frequency which may lead to frequency shifts from one pulse to the next) 
The magnetron's waveguide is connected to
an antenna
2-Heating(Microwave Ovens)

69. Microwave Semiconductors
Electronic devices that are used in high frequency applications

70.   Microwave Semiconductors are surveyed into two overlapping classifications:
1.By type of device behavior
a.Varistors (variable resistance)
b.Varactor
c.Controllable impedance diodes [P-I-N ]
d. Negative resistance
2.By Device Structure:
P-N junction like varactor, P-I-N, Impatt and tunnel diodes.
Schottky Barrier, i.e., intimate contact of a metal and a semiconductor.
Voltage breakdown like avalanche diode and tunneling diode.
Transit time effect like Impatt or Gunn diodes
MW Bipolar transistor
Field Effect Transistor (F.E.T.)

71. Some Types of Microwave Semiconductors
(Diodes)

72. P-I-N Diode
The PIN diode receives its name from the fact that is has three main layers. Rather than just having a P type and an N type layer, the PIN diode has three layers:
P-type layer
Intrinsic layer
N-type layer
The instrinic layer of the PIN diode is the one that provides the change in properties when compared to a normal PN junction diode. The intrinsic region comprises of the undoped, or virtually undoped semiconductor, and in most PIN diodes it is very thin - of the order of between 10 and 200 microns.
There are a two main structures that can be used, but the one which is referred to as a planar structure is shown in the diagram. In the diagram, the intrinsic layer is shown much larger than if it were drawn to scale. This has been done to better show the overall structure of the PIN diode.

73. PIN diode characteristics
The intrinsic layer between the P-type and N-type regions of the PIN diode enable it to provide properties such as a high reverse breakdown voltage, and a low level of capacitance, and there are also other properties such as carrier storage when it is forward biased that enable it to be used for certain microwave applications.
It is found that at low levels of reverse bias the depletion layer become fully depleted. Once fully depleted the PIN diode capacitance is independent of the level of bias because there is little net charge in the intrinsic layer. However the level of capacitance is typically lower than other forms of diode and this means that any leakage of RF signals across the diode is lower.
When the PIN diode is forward biased both types of current carrier are injected into the intrinsic layer where they combine. It is this process that enables the current to flow across the layer.
The particularly useful aspect of the PIN diode occurs when it is used with high frequency signals, the diode appears as a resistor rather than a non linear device, and it produces no rectification or distortion. Its resistance is governed by the DC bias applied. In this way it is possible to use the device as an effective RF switch or variable resistor producing far less distortion than ordinary PN junction diodes.

74. PIN diode uses and advantages
The PIN diode is used in a number of areas as a result of its structure proving some properties which are of particular use.
High voltage rectifier:   The PIN diode can be used as a high voltage rectifier. The intrinsic region provides a greater separation between the PN and N regions, allowing higher reverse voltages to be tolerated.
RF switch:   The PIN diode makes an ideal RF switch. The intrinsic layer between the P and N regions increases the distance between them. This also decreases the capacitance between them, thereby increasing he level of isolation when the diode is reverse biased.
Photodetector:   As the conversion of light into current takes place within the depletion region of a photdiode, increasing the depletion region by adding the intrinsic layer improves the performance by increasing he volume in which light conversion occurs.

75. GUNN Diode Gunn diode basics
The Gunn diode is a unique component - even though it is called a diode, it does not contain a PN diode junction. The Gunn diode or transferred electron device can be termed a diode because it has two electrodes.
The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance – this being dependent upon the fact that when a voltage is placed across the device, most of the voltage appears across the inner active region. This inner region is particularly thin and this means that the voltage gradient that exists in this region is exceedingly high.
The device exhibits a negative resistance region on its V/I curve as seen below. This negative resistance area enables the Gunn diode to amplify signals, enabling it to be used in amplifiers and oscillators. However it is the Gunn diode oscillators are the most commonly used.

76. This negative resistance region means that the current flow in diode increases in the negative resistance region when the voltage falls - the inverse of the normal effect in any other positive resistance element. This phase reversal enables the Gunn diode to act as an amplifier and as an oscillator.

77. How a Gunn diode acts as an oscillator
Whilst the Gunn diode has a negative resistance region, it is interesting to see a little more about how this happens and how it acts as an oscillator.
At microwave frequencies, it is found that the dynamic action of the diode incorporates elements resulting from the thickness of the active region.
When the voltage across the active region reaches a certain point a current is initiated that travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created.
It can be seen that the time taken for the current pulse to traverse the active region largely determines the rate at which current pulses are generated. It is this that determines the frequency of operation.
To see how this occurs, it is necessary to look at the electron concentration across the active region. Under normal conditions the concentration of free electrons would be the same regardless of the distance across the active diode region. However a small perturbation may occur resulting from noise from the current flow, or even external noise - this form of noise will always be present and acts as the seed for the oscillation. This grows as it passes across the active region of the Gunn diode.
The increase in free electrons in one area cause the free electrons in another area to decrease forming a form of wave.
The peak will traverse across the diode under the action of the potential across the diode, and growing as it traverses the diode as a result of the negative resistance.
A clue to the reason for this unusual action can be seen if the voltage and current curves are plotted for a normal diode and a Gunn diode. For a normal diode the current increases with voltage, although the relationship is not linear. On the other hand the current for a Gunn diode starts to increase, and once a certain voltage has been reached, it starts to fall before rising again. The region where it falls is known as a negative resistance region, and this is the reason why it oscillates.

78. Like any form of component, the Gunn diode has a number of advantages and disadvantages that need to be considered when looking at suitable components for a particular circuit design.

79. Tunnel Diode
The tunnel diode is a type of microwave semiconductor diode that can be used in oscillators and also amplifiers.
Rather than using the standard physics of the ordinary PN junction, the tunnel diode uses a quantum mechanical effect called tunneling – from which it gains its name.
The tunneling effect gives the tunnel diode a negative resistance region and this enables it to be used as an oscillator and also in pre-amplifier applications at frequencies well into the microwave region.
Although tunnel diodes are not as widely used today, they can still be used in a good number of RF applications. They were used in television receiver front end oscillators and oscilloscope trigger circuits, etc. They have been shown to have a very long life and can offer a very high level of performance when used as an RF pre-amplifier.
However today, tunnel diode applications are less widespread because three terminal devices can often offer better levels of performance in many areas.

80. The tunnel diode is not as widely used these days as it was oat one time. With the improvement in performance of other forms of semiconductor technology, they have often become the preferred option. Nevertheless it is still worth looking at a tunnel diode, considering its advantages and disadvantages to discover whether it is a viable option.
Advantages
Very high speed:   The high speed of operation means that the tunnel diode can be used for microwave RF applications.
Longevity:   Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other semiconductor devices may have degraded.
Disadvantages
Reproducibility:   It has not been possible to make the tunnel diode with as reproducible performance to the levels often needed.
Low peak to valley current ratio:   The negative resistance region and the peak to valley current is not as high as is often be required to produce the levels of performance that can be attained with other devices.

81. Applications
Although the tunnel diode appeared promising some years ago, it was soon replaced by other semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as an amplifier. Nevertheless the tunnel diode is a useful device for certain applications.
One area where the tunnel diode can be usefully used is within military and other equipment that may be subject to magnetic fields, high temperature and radioactivity. The tunnel diode is more resilient to the effects of these environments and as such may still be usefully used.
Another advantages of the tunnel diode which is beginning to be discovered is its longevity and reliability. Once manufactured its performance remains stable over long periods of time despite its use where other devices may degrade or fail.