Microwave engineering

EC402 Microwave Engineering

2National Institute of Technology, Warangal 18/04/2023Microwave Engineering

Electromagnetic Spectrum

Microwave frequency range 1-30GHz wave length 30cm-1cm

Microwave Frequency Range

1. Small size wavelength f=1GHzλ=c/f=3x1010/1x109=30cm f=30GHzλ=c/f=3x1010/30x109=1cm

Wave lengths are same as dimensions of components, so distributed circuit elements or transmission theory is applied.

National Institute of Technology, Warangal 18/04/2023Microwave Engineering

Characteristics of Microwaves

Characteristics-Large Bandwidth

Large BandwidthHigh transmission rates used for communication World’s data, TV and telephone communications are

transmitted long distances by microwaves between ground stations and communications satellite

Characteristics-Line of sight propagation

Characteristics-Transmission Through Ionosphere

Characteristics- Reflection From Metallic Surfaces

Characteristics

Characteristics- Heating

Characteristics- Microwave Resonance

Microwave Resonance: Molecular, atomic and nuclear systems exhibit resonance when Present electromagneticFields Several resonance absorption lines are in microwave range

2318/04/2023Microwave Engineering

Application- Communications

National Institute of Technology, Warangal

Point to point communications

GSM 1.8 and 1.9 GHzDVB-SH, 1.452, 1.492 GHz

Wireless LAN networks 2.4GHz ISM band

Wimax(Worldwide Interoperability for Microwave Access) 2 to 11 GHzPMP-Point to multipoint links

Wimax, WiFi

Satellite Communications

L band (1-2 GHz )Global Positioning System (GPS) carriers and also satellite mobile phones, such as Iridium; Inmarsat providing communications at sea, land and air; WorldSpace satellite radio.

Satellite Communications

Radar is an object-detection system that uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain.AviationMarineMeteorologists

Heating

Domestic Application: Heating, Microwave oven

Industrial Application: Food, Rubber, leather, chemical and textile , pharmaceutical industries

Remote Sensing

Remote sensing: Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on site observation.

Remote Sensing

Radio Astronomy

Radio Astronomy: Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies.

Radio Astronomy

Arecibo 305 m ( about 20 acres) radio telescope, located in a natural valley in Puerto Rico.

Radio Interferometery

The Very Large Array, an interferometric array formed from many smaller telescopes

Medical Application

Microwave Imaging

Microwave imaging is a science which has been evolved from older detecting/locating techniques (e.g., radar) in order to evaluate hidden or embedded objects in a structure (or media)using electromagnetic (EM) waves in microwave regime (i.e., ~300 MHz-300 GHz)

Microwave Imaging

•concealed weapon detection at security check points, structural health monitoring•through-the-wall imaging. •Disbond detection in strengthened concrete bridge• Corrosion and precursor pitting detection in painted aluminum and steel substrates •Flaw detection in spray-on foam insulation

Industry Applications

Microwave oven Drying machines – textile, food and paper industry for drying

clothes, potato chips, printed matters etc. Food process industry – Precooling / cooking, pasteurization /

sterility, hat frozen / refrigerated precooled meats, roasting of food grains / beans.

Rubber industry / plastics / chemical / forest product industries Mining / public works, breaking rocks, tunnel boring, drying /

breaking up concrete, breaking up coal seams, curing of cement.

Drying inks / drying textiles, drying / sterilizing grains, drying / sterilizing pharmaceuticals, leather, tobacco, power transmission.

Biomedical Applications ( diagnostic / therapeutic ) – diathermy for localized superficial heating, deep electromagnetic heating for treatment of cancer, hyperthermia ( local, regional or whole body for cancer therapy).

Advantages

Large Bandwidth: It is very good advantage, because of this, Microwaves are used for Point to Point Communications.

Advantages

Better Directivity

Advantages

Better Directivity: At Microwave Frequencies, there are better directive properties. This is due to the relation that as Frequency Increases, Wavelength decreases and as Wavelength decreases Directivity Increases and Beam width decreases. So it is easier to design and fabricate high gain antenna in Microwaves.

Advantages

Small Size Antenna

Advantages

Low Power Consumption

Advantages

Low Power Consumption:The power required to transmit a high frequency signal is lesser than the power required in transmission of low frequency signals. As Microwaves have high frequency thus requires very less power.

Advantages

Effect Of Fading

Space wave Sky wave

Advantages

Effect Of Fading: The effect of fading is minimized by using Line Of Sight propagation technique at Microwave Frequencies. While at low frequency signals, the layers around the earth causes fading of the signal.

Space wave Sky wave

Fresnel Zone

there should be no reflective objects in the 1st Fresnel zoneeven Fresnel zone are out of phase with the direct-path wave and reduce the power of the received signalodd Fresnel zone are in phase with the direct-path wave and can enhance the power

Limitations of Tubes at High Frequencies

Vacuum tubes- Triode

Triode Amplifier Circuit

Limitations at Higher Frequencies

Inter electrode Capacitance

At frequencies greater than 1 GHz

Leads:Leads are used for physical support, to transfer power and sometimes as a Heatsink.

Limitations at Higher Frequencies Limitations at Higher Frequencies

Leads:Leads are used for physical support, to transfer power and sometimes as a Heatsink.

In fact, any wires or component leads that have current flowing through them create magnetic fields. When these magnetic fields are created, they can produce an inductive effect. Thus, wires or components leads can act as inductors if they are long enough

Parasitic Inductance and capacitance becomes very large At Microwave frequencies

Limitations at Higher Frequencies Limitations at Higher Frequencies

Reduce length of and area of leads, in turn reduces Power handled.

Input conductance loads the circuitry, efficiency reduces.

Lead Inductance

Input Impedance

Input Voltage

Input Impedance

Input Current

Input Impedance

Input Admittance

Input Impedance

Input conductance loads the circuitry, efficiency reduces.

Gain Bandwidth

Gain bandwidth product is independent of frequency, hence is constant. Hence resonant circuits are reentrant or slow wave structures

Transit Time

•In the positive half-cycle, grid potential attracts the electron beam and supplies energy to it

Transit Time

•In the negative half-cycle, it repels the electron beam and extracts energy from it.

Transit Time

As a result, the electron beam oscillates back and forth in the region between the cathode and the grid, and may even return to the cathode.

The overall result is a reduction of the operating frequency of the vacuum tube.

Transit Time

Reduce Transit Time•Increasing the anode voltage •Decreasing the inter-electrode spacing

However, the increase in anode voltage will increase the power dissipation,

whereas the decrease in inter-electrode spacing will increase the inter-electrode capacitance.

Transit Time

The increase in inter-electrode capacitance can be reduced by reducing the area of the electrodes, but this will reduce anode dissipation and hence the output power.

RF Loss- Skin Effect Loss

RF Loss- Skin Effect Loss Skin effect loss At a high frequency, current has a tendency to concentrate around the surface rather than being distributed throughout the cross section. This is known as skin effect. It reduces the effective surface area, which in turn increases the resistance and hence the loss of the device. Resistance loss is also proportional to the square of the frequency. Losses due to skin effect can be reduced by increasing the current-carrying area, which, in turn, increases the inter-electrode capacitance and thus limits high frequency operations.

RF Loss- Dielectric Loss

Dielectric loss Dielectric loss in a material is proportional to frequency, and hence plays an important role in the operations of high-frequency tubes. This loss can be avoided by eliminating the tube base and reducing the surface area of the dielectric materials, and can be reduced by placing insulating materials at the point of minimum electric field.

Radiation Loss

Radiation loss At higher frequencies, the length of the leads approaches the operating wavelength, and as a result these start radiating. Radiation loss increases with the increase in frequency and hence is very severe at microwave frequencies. Proper shielding is required to avoid this loss. Radiation loss can be minimized by enclosing the tubes or using a concentric line construction

Resonator

A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others.

Resonant Circuit

An electrical circuit composed of discrete components can act as a resonator when both an inductor and capacitor are included. Such resonant circuits are also called RLC circuits after the circuit symbols for the components.

Cavity Resonator

A cavity resonator, usually used in reference to electromagnetic resonators, is one in which waves exist in a hollow space inside the device

Cavity Resonator Due to the low resistance of their conductive walls, cavity resonators have very high Q factors; that is their bandwidth, the range of frequencies around the resonant frequency at which they will resonate, is very narrow.

Thus they can act as narrow bandpass filters. Cavity resonators are widely used as the frequency determining element in microwave oscillators.

Their resonant frequency can be tuned by moving one of the walls of the cavity in or out, changing its size.

Rectangular Cavity Resonator

For a > b < d, the dominant mode is the TE101 mode.

The electric field lines start from top and bottom, positive and negative charges are induced, hence forms capacitor

The current flows via side walls and hence serve as inductor, hence the enclosed volume behaves as tank circuit.

Circular Cavity Resonator

TE111 mode is the dominant mode.

Quality Factor

The Q factor (quality factor) of a resonator is a measure of the strength of the damping of its oscillations, or for the relative linewidth.

the Q factor is 2π times the ratio of the stored energy to the energy dissipated per oscillation cycle

the Q factor is the ratio of the resonance frequency ν0 and the full width at half-maximum (FWHM)bandwidth δν of the resonance:

Quality Factor

Reentrant Cavity Resonator

Excitation Wave Modes

Loop coupling Probe coupling

Probe coupling

Loop coupling

Aperture Coupling

Aperture coupling

Coupling Between Waveguides

Directional Coupler

Linear Beam Tubes-Otype Tubes

Electric Field is applied to the accelerate or decelerate the Electron beam

Magnetic Field is applied along the axis toFocus the electron beam.

Klystron

an electron tube that generates or amplifies microwaves by velocity modulation.

Klystron

an electron tube that generates or amplifies microwaves by velocity modulation.

Klystron- Velocity Modulation

Velocity of electrons accelerated by high DC Voltage

Gap Voltage applied at Buncher grids

Average transit time through buncher gap

Average Voltage across the buncher gap

Equation for Velocity Modulation

Klystron- Bunching Process

Distance travelled by the electrons in drift space.

Klystron- Current Modulation

Beam Current varies with the applied RF voltage –current modulation.

Fundamental component of current

Current becomes maximum at

Klystron- Current Modulation

Optimum distance for bunching

Klystron

Applegate Diagram

Output Power

Efficiency

Theoretical efficiency is 58%Where as practical efficiency is 15% to 30%

Voltage Gain

Typical Values

Applications

As power output tubes1. in UHF TV transmitters2. in troposphere scatter

transmitters3. satellite communication

ground station4. radar transmitters

Klystron

Multi cavity Klystron

Reflex Klystron

Velocity Modulation

Velocity of the electrons in entering the cavity gap

Velocity Modulation

Exit Velocity of the electrons in leaving the cavity gap

Velocity Modulation

Retarding Electric Field

Velocity Modulation

Force equation of one electron assuming V1<<(Vr+Vo)

Reflex Klystron

Integrating

Reflex Klystron

Integrating

Reflex Klystron

Integrating

Reflex Klystron

Integrating

Reflex Klystron

Transit Time

Round trip transit time in the repeller region

Transit Time

Applegate Diagram

Efficiency

Efficiency of Reflex Klystron

Characteristics of Reflex Klystron

Electronic Admittance

Bunched electrons return to the cavity gap a little before the transit time, current leads the behind the field-capacitance appears in the circuit

Bunched electrons return to the cavity gap a little after toThe ac current lags the field –inductance reactance appears in the circuit

Condition for oscillation Ge is negative and total conductance in the circuit is negative –Ge>Gc+Gl

Applications

Low power oscillator- 10mw to 500mwFrequency 1-25GHz

Local Oscillator in commercial , Military, Air borne Doppler radar and missiles.

Tuning Klystron

Electronic Tuning

Tuning Klystron

Mechanical Tuning: By changing capacitance or inductance

Klystron

Output is via a co-axial pin, and the device can be mechanically tuned with the screw on the left, which applies vertical compression to the metal envelope.

Amplitude Modulation -Klystron

Frequency Modulation Klystron

Slow Wave Structures

Non Resonant periodic circuitsProduce large gain over wide bandwidth

Slow Wave Structures

Slow Wave Structure

Phase Velocity

Group Velocity

Travelling wave tube

Amplifiers in satellite transponders, where the input signal is very weak and the output needs to be high power.

TWTA transmitters are used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems

Linear Beam tubes –O type

Klystron – Resonant , standing waveReflex Klystron- Resonant, standing waveTravelling wave tube- Non resonant, travelling wave

Amplifies a wide range of frequencies, a wide bandwidth and low noise.

Bandwidth two octaves, while the cavity versions have bandwidths of 10–20%.

Operating frequencies range from 300 MHz to 50 GHz.

The power gain of the tube is on the order of 40 to 70 decibels

Output power ranges from a few watts to megawatts

Octave

A frequency is said to be an octave in width when the upper band frequency is twice the lower band frequency

Crossed Field tubes –M type

Crossed-field tubes derive their name from the fact that the dc electric field and the dC magnetic field are perpendicular to each other.

They are also called M –type tubes

Cylindrical Magnetron

Travelling wave Magnetron

Depend upon the interaction of electrons with a rotating electromagnetic field of same angular velocity.

Provide oscillations of very high peak power and hence are useful in radar applications

Cavity Magnetron

Fig (i) Major elements in the Magnetron oscillator

Anode Assembly

Construction

Each cavity in the anode acts as an inductor having only one turn and the slot connecting the cavity and the interaction space acts as a capacitor.

These two form a parallel resonant circuit and its resonant frequency depends on the value of L of the cavity and the C of the slot.

The frequency of the microwaves generated by the magnetron oscillator depends on the frequency of the RF oscillations existing in the resonant cavities.

Reentrant Cavity

Description

Magnetron is a cross field device as the electric field between the anode and the cathode is radial whereas the magnetic field produced by a permanent magnet is axial.

A high DC potential can be applied between the cathode and anode which produces the radial electric field.

Depending on the relative strengths of the electric and magnetic fields, the electrons emitted from the cathode and moving towards the anode will traverse through the interaction space as shown in Fig. (iii).

In the absence of magnetic field (B = 0), the electron travel straight from the cathode to the anode due to the radial electric field force acting on it, Fig (iii) a.

Cavity Magnetron

Description

If the magnetic field strength is increased slightly, the lateral force bending the path of the electron as given by the path ‘b’ in Fig. (iii).

The radius of the path is given by, If the strength of the magnetic field is made sufficiently high then the electrons can be prevented from reaching the anode as indicated path ‘c’ in Fig. (iii)),

The magnetic field required to return electrons back to the cathode just grazing the surface of the anode is called the critical magnetic field (Bc) or the cut off magnetic field.

If the magnetic field is larger than the critical field (B > Bc), the electron experiences a greater rotational force and may return back to the cathode quite faster.

Fig (iii) Electron trajectories in the presence of crossed electric

and magnetic fields (a) no magnetic field

(b) small magnetic field(c) Magnetic field = Bc

(d) Excessive magnetic field

Effect of electric field Effect of magnetic field

Effect of Crossed-Fields

Hull Cut off Condition

Force due to magnetic field on charge Q moving with velocity v

Force on electron moving with velocity v

Force due to electric field on electron

Magnetic Field Bz az

Equations of electrons in motion

Acceleration due to electric field

Equations of Electrons in motion

Rearranging the equation (2)

Angular Velocity

Kinetic Energy of Electrons

Velocity of electrons

Hull Cutoff Magnetic Equation

Hull Cutoff Voltage Equation

Cyclotron Angular Frequency

Time Period

Phase shift between adjacent cavities

Phase constant

π Mode

RF Field

PH0101 Unit 2 Lecture 5 235

WorkingFig (iv) Possible trajectory of electrons from cathode to anode in an eight cavity

magnetron operating in mode

Working The RF Oscillations of transient nature produced when

the HT is switched on, are sufficient to produce the oscillations in the cavities, these oscillations are maintained in the cavities reentrant feedback which results in the production of microwaves.

Reentrant feedback takes place as a result of interaction of the electrons with the electric field of the RF oscillations existing in the cavities.

The cavity oscillations produce electric fields which fringe out into the interaction space from the slots in the anode structure, as shown in Fig (iv).

Energy is transferred from the radial dc field to the RF field by the interaction of the electrons with the fringing RF field.

Working Due to the oscillations in the cavities, the either sides of

the slots (which acts as a capacitor) becomes alternatively positive and negative and hence the directions of the electric field across the slot also reverse its sign alternatively.

At any instant the anode close to the spiraling electron goes positive, the electrons gets retarded and this is because; the electron has to move in the RF field, existing close to the slot, from positive side to the negative side of the slot.

In this process, the electron loses energy and transfer an equal amount of energy to the RF field which retard the spiraling electron.

On return to the previous orbit the electron may reach the adjacent section or a section farther away and transfer energy to the RF field if that part of the anode goes positive at that instant.

Working This electron travels in a longest path from cathode to the

anode as indicated by ‘a’ in Fig (iv), transferring the energy to the RF field are called as favoured electrons and are responsible for bunching effect and give up most of its energy before it finally terminates on the anode surface.

An electron ‘b’ is accelerated by the RF field and instead of imparting energy to the oscillations, takes energy from oscillations resulting in increased velocity, such electrons are called unfavoured electrons which do not participate in the bunching process and cause back heating.

Every time an electron approaches the anode “in phase” with the RF signal, it completes a cycle. This corresponds to a phase shift 2.

For a dominant mode, the adjacent poles have a phase difference of radians, this called the - mode.

Fig (v) Bunching of electrons in multicavity magnetron

Working

At any particular instant, one set of alternate poles goes positive and the remaining set of alternate poles goes negative due to the RF oscillations in the cavities.

AS the electron approaches the anode, one set of alternate poles accelerates the electrons and turns back the electrons quickly to the cathode and the other set alternate poles retard the electrons, thereby transferring the energy from electrons to the RF signal.

This process results in the bunching of electrons, the mechanism by which electron bunches are formed and by which electrons are kept in synchronism with the RF field is called phase focussing effect. electrons with the fringing RF field.

Working

The number of bunches depends on the number of cavities in the magnetron and the mode of oscillations, in an eight cavity magnetron oscillating with - mode, the electrons are bunched in four groups as shown in Fig (v).

Two identical resonant cavities will resonate at two frequencies when they are coupled together; this is due to the effect of mutual coupling.

Commonly separating the pi mode from adjacent modes is by a method called strapping. The straps consist of either circular or rectangular cross section connected to alternate segments of the anode block.

Performance Characteristics

1. Power output: In excess of 250 kW ( Pulsed Mode), 10 mW (UHF band), 2 mW (X band), 8 kW (at 95 GHz)

2. Frequency: 500 MHz – 12 GHz

3. Duty cycle: 0.1 %

4. Efficiency: 40 % - 70 %

Applications of Magnetron

1. Pulsed radar is the single most important application with large pulse powers.

2. Voltage tunable magnetrons are used in sweep oscillators in telemetry and in missile applications.

3. Fixed frequency, CW magnetrons are used for industrial heating and microwave ovens.

Mode Jumping

Strapping Rising sun structure

Disadvantages

•They are costly and hence limited in use.

•Although cavity magnetron are used because they generate a wide range of frequencies , the frequency is not precisely controllable.

•The use in radar itself has reduced to some extent, as more accurate signals have generally been needed and developers have moved to klystron and systems for accurate frequencies.

Cross Field Amplifier

Cross Field AmplifierThe Crossed-Field Amplifier (CFA), is a broadband microwave amplifier that can also be used as an oscillator (Stabilotron).

It is a so called Velocity-modulated Tube . The CFA is similar in operation to themagnetron and is capable of providing relatively large amounts of power with high efficiency.

In contrast to the magnetron, the CFA have an odd number of resonant cavities coupled with each other. These resonant cavities work to as a slow-wave structure: an oscillating resonant cavity excites the next cavity.

The actual oscillation will be lead from the input waveguide to the output waveguide.

The electric and magnetic fields in a CFA are perpendicular to each other (“crossed fields”). Without an input signal and the influence of both the electric field (anode voltage) and the magnetic field (a strong permanent magnet) all electrons will move uniformly from the cathode to the anode on a cycloidal path as shown in figure

Cross Field AmplifierIf the input-waveguide introduces an oscillation into the first resonator, the vanes of the resonator gets a voltage difference synchronously to the oscillation.

Under the influence of this additionally field flying past electrons get acceleration (at the positively charged vane) or they are decelerated (at the negatively charged vane). This causes a difference in speed of the electrons. The faster electrons catch the slower electrons and the forms electron bunches in the interaction space between the cathode and the anode.

These bunches of electrons rotates as like as the “Space-Charge Wheel” known from the magnetron operation. But they cannot rotate in full circle, the “Space-Charge Wheel” will be interrupted because the odd number of cavities causes an opposite phase in the last odd cavity (this bottom one between the waveguides).

To avoid a negative feedback, into this resonant cavity may exist a bloc containing graphite to decouple input and output.

Cross Field Amplifier

The bandwidth of the CFA, at any given instant, 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, with efficiency ratings in excess of 70 percent, possible with crossed-field amplifiers.

To avoid ineffective modes of operation the construction of CFA contains strapping wires like to as used in magnetrons.

Because of the desirable characteristics of wide bandwidth, high efficiency, and the ability to handle large amounts of power, the CFA is used in many applications in microwave electronic systems.

When used as the intermediate or final stage in high-power radar systems, all of the advantages of the CFA are used.

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