ECE 6th SEMESTER Subject MICROWAVE & RADAR ENGG. By: Rajesh Kumar HOD ECE GPCG Jalandhar Punjab EDUSAT society
TOPIC Introduction to Microwaves Punjab EDUSAT society
OUTLINE OF LECTURE Introduction Microwave Frequency Range Application Performance of vacuum tube at microwave Microwave amplifier and oscillator Punjab EDUSAT society
Microwaves • Microwaves are the electromagnetic waves with wavelengths ranging from as long as a few centimeter to as short as one millimeter and with frequencies ranging from 1GHz to 1000 GHz. • Microwaves include the entire SHF band (3 to 30 GHz).
Advantage of Microwaves • Larger Bandwidth : The band width of microwaves is larger than the low frequency signals. So due to this larger bandwidth, more information can be transmitted using microwaves.
Advantage of Microwaves • Better Directive Properties : At microwave frequencies, it is easier to design and fabricate a high gain antenna as compared to low frequency signals. Beam width Φ=λ/D=c/fD Φ is beam width λ is wavelength D is Directivity c is velocity of light
This is because of the fact that as the frequency increases ,directivity increases and beam width decreases.
Advantage of Microwaves • Lower Power Requirement : The power required by the microwaves is very less as compared to low frequency signals. • Fading effect : At the lower frequencies , various layers around the earth sometimes cause fading of the received signals , however at microwave frequencies by using line of sight propagation technique ,the amount of fading is minimized.
Frequency-Wavelength • A definite relationship exists between the frequency (f) and the corresponding wavelength (λ) of electromagnetic waves .The product of these two i.e. (f) and (λ) gives the velocity of propagation of electro-magnetic waves and it is equal to the velocity of light . • This is expressed as c = f * λ c= velocity of light. (approx. 3* 108 m/sec ).
Applications Of Microwaves 1.Communication : • Microwaves is used in broadcasting and Telecom. transmisssion, due to their short wavelength, highly directional antennas are smaller . Mobile phone networks, like GSM, use the low microwave/UHF frequencies around 1.8 and 1.9GHz .
Microwaves are used in television signal to transmit a signal from a remote location to a television station from a specially equipped van. • Microwave are used for comm. from one point to another via satellite. • Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct –broadcast satellite.
2. Remote Sensing : • The most important application of remote sensing is RADAR, that uses a transmitter to illuminate an object and a receiver to detect its position and velocity. • Another class of remote sensing is radio astronomy .It is a sub-class of astronomy that studies celestial objects at radio frequencies.
3.Heating Application • Baking : The heating property of microwaves are used for baking, cooking using microwave oven. In microwave oven ,the food is heated directly by microwave radiations without heating the container. The cooking time very small as compare to conventional heating
Concentrating : Permits concentration of heat sensitive solution and slurries at relatively low temperatures. Also applicable to highly corrosive or viscous solutions • Drying : microwaves are used for drying the solids. Drying is uniform throughout the product moisture present in the product is evaporated out .Drying is at relatively low temp.
Enzyme Inactivation : The enzyme inactivation can be achieved by rapid and uniform heating which can control and terminate enzyme reactions. • Precooking : Microwaves are ideal for precooking the food items because there is no overcooking of the surface and cooking losses are negligible i.e is nutrients in the food are not lost.
Parameters Of Microwaves System • Frequency Characteristics : Microwaves are very short frequency radio waves that have many of the characteristics of light waves they travel in line of sight paths and can be reflected and focused . By focusing these ultra high radio waves into a narrow beam, their energies are concentrated and relatively low transmitting power is required for reliable transmission over long distance .
System Capacity: • Microwaves communication systems are used to carry telephony, television and data signals. • Majority of the system carry multi- channel telephone signals (base band ). Individual telephone channels , 4KHz wide are multiplexed together in a multiplexer equipment to get the base band. At microwave due to high bandwidth capacity is more.
Microwave Frequency Bands As already mentioned ,microwave is an electromagnetic wave ranging from approximately 1GHz in frequency, but older usage includes lower frequencies . Most common applications are within the range 1 to 40GHz.
Classification of Microwaves on the basis of Frequency bands : L-band: L-band (20-cm radar long band) is a portion of the microwave band of the electromagnetic spectrum ranging roughly from 0.39 to 1.55 GHz. It is used by some communication satellite and by terrestrial. 2. S-band: S-band or 10 cm. radar short band, is the part of microwave band of the electromagnetic spectrum ranging roughly from 1.55 to 5.2 GHz. It is used by weather radar and some communication satellites
3. C-band: C-band (“ Compromise” band) is a portion of electromagnetic spectrum in the microwave range of frequencies ranging from 4 to 6 GHz. 4. X-band: The X-band (3 cm radar spot band) of the microwave band of the electromagnetic spectrum roughly ranges from 5.2 to 10.9 GHz. It is used by some communication satellite and X-band radar. 5. Ku-band: The Ku-band (Kurz-under band) is a portion of electromagnetic spectrum in the microwave range of frequency range 11 to 18 GHz. It’s primarily used for satellite communication.
6. K-band: It is a portion of the EM wave spectrum in the microwave range of frequency range between 12 to 40 GHz. The K comes from Kurz. K-band between 18 to 26.5GHz is absorbed easily by water vapour. 7. Ka-band: The Ka-band ( Kurz-above band is a portion of the K-band) of the microwave band of the electromagnetic spectrum. Ka-band roughly ranges from 18 to 40 GHz.
Introduction to microwave devices • A wide range of semiconductor devices have been developed for detection, mixing, amplification, attenuation etc. • Microwave tubes are preferred over vacuum tubes. At microwave frequency range, the conventional tubes become less effective when used as an amplifier and oscillator. • Microwave tubes usually operate on the theory of velocity modulation, a concept that avoids the problems encountered in conventional tubes.
Electron Emission • The liberation of electrons from the surface of a substance is called electron emission. • In metals, the electrons in the outermost orbit are very loosely held by the nucleus. • Those loosely attached electrons called free electrons, can be easily detached by some external energy.
Methods of Electron Emission • The liberation of electron is possible only when external energy supplied to a metal is equal to or more than the work function. • This external energy may be supplied from variety of sources such as heat energy, kinetic energy supplied by the moving electron or energy stored in the electrical field.
Accordingly there are four main methods of obtaining electron emission from the surface of metal :- • Thermionic emission • Secondary emission • Photo-electric emission • High Field emission
Thermionic Emission • A thermionic emission include heating of the metal. • When a metal is heated, some of heat energy is converted into kinetic energy which accelerates the motion of free electrons. • When the temperature is raised sufficiently, these electrons acquire sufficient energy equal to work function of the metal. • Consequently they overcome the opposing forces and leave the metal surface.
Thermionic emitter: • Oxide coated cathode: low work function(1ev),high efficiency, made up of cobalt, iron,nickel,titanium • Thoriated tungsten: work function 2.6ev,and used for high power • Tungsten: work function 4.52 ev , high melting point , high mechanical strength and long life Punjab EDUSAT society
Vacuum Tubes • An electronic device in which electrons flow through vacuum is called vacuum tubes. • A vacuum tube consists of a evacuated glass envelope which contains a cathode, an anode and one or more electrodes called grids. • Diode, triode, tetrode, pentode
Vacuum Tubes Punjab EDUSAT society
Features of Vacuum Tubes • Vacuum tubes are voltage controlled device. • These can operate at very high voltages. • High power can be easily developed by vacuum tubes. According to the number of electrodes, vacuum tubes can be classified as • Vacuum diode • Vacuum triode
(iii) Vacuum Tetrode (iv) Vacuum Pentode • Conventional tubes such as triodes, tetrodes and pentodes are useful only at low microwave frequencies. • These tubes cannot operate at high frequencies due to their limitations at those frequencies.
High Frequency Limitations of Conventional Tubes • The conventional tubes become less effective at microwave frequency range when these are used as an amplifier and oscillator. The limitations of conventional tubes at high frequencies is due to : (a) Inter-electrode capacitance effect (b) Lead Inductance effect (c) Transit Time effect
Inter-electrode Capacitance Effect • The capacitance exists when two pieces of metal are separated by a dielectric. Vacuum has a dielectric constant of 1. • The elements of the triode are made up of metal and are separated by dielectric material . • So there must exist capacitance between them. This capacitance is called interelectrode capacitance.
Inter-electrode Capacitance Effect Punjab EDUSAT society
Lead Inductance Effect • The common lead inductance is the inductance associated with the common connection of vacuum triode. • This effect is more when the frequency of the signal is high. • As the frequency increases, the inductive reactance increases and due to high inductive reactance there is an input matching problem.
The lead inductance affects the performance of vacuum triode with most of input voltage lost across inductance and only small fraction of input reach to terminal for amplification. • These inductances form unwanted tuned circuit with the capacitance and parasitic oscillations are produced. As frequency increases, the reactance increases.
Effect of Transit Time • The time taken by an electron to travel from cathode to anode is called transit time. • At low frequencies, the transit time is very small i.e. the electrons reach instantaneously the anode plate from cathode. • At high frequencies, the transit time becomes large because the source driving the grid becomes loaded and the gain of the vacuum tube becomes less than unity.
Effect of Transit Time Punjab EDUSAT society
This loading is due to the dissipation of the power at the grid. The effect of loading is such that the noise in the circuit increases. • To minimize this effect, the distance between the electrodes is to be reduced and high voltage must be applied. • This will increase the interelectrode capacitance.
Klystron Tubes • Klystron tube is a vacuum tube that can be operated either as an oscillator or as an amplifier at microwave frequencies. Two basic configurations of klystron tubes are : 1. Multicavity klystron which is used as a low power microwave amplifier. 2. Reflex klystron which is used as a low power microwave oscillator.
Multicavity Klystron Punjab EDUSAT society
Application • As power output tubes • in UHF TV transmitters • in troposphere scatter transmitters • satellite communication ground station • radar transmitters • As power oscillator (5 – 50 GHz), if used as a klystron oscillator Punjab EDUSAT society
Reflex Klystron oscillator Punjab EDUSAT society
Construction • A reflex klystron consists of an electron gun, a cavity with a pair of grids and a repeller plate as shown in the above diagram. • In this klystron, a single pair of grids does the functions of both the buncher and the catcher grids. • The main difference between two cavity reflex klystron amplifier and reflex klystron is that the output cavity is omitted in reflex klystron and the repeller or reflector electrode, placed a very short distance from the single cavity, replaces the collector electrode. Punjab EDUSAT society
Working of reflex klystron • The cathode emits electrons which are accelerated forward by an accelerating grid with a positive voltage on it and focused into a narrow beam. • The electrons pass through the cavity and undergo velocity modulation, which produces electron bunching and the beam is repelled back by a repeller plate kept at a negative potential with respect to the cathode. • On return, the electron beam once again enters the same grids which act as a buncher, therby same grids acts simultaneously as a buncher for the forward moving electron and as a catcher for the returning beam Punjab EDUSAT society
The feedback necessary for electrical oscillations is developed by reflecting the electron beam, the velocity modulated electron beam does not actually reach the repeller plate, but is repelled back by the negative voltage. • Thus the repeller voltage is so adjusted that complete bunching of the electrons takes place at the catcher grids, the distance between the repeller and the cavity is chosen such that the repeller electron bunches will reach the cavity at proper time to be in synchronization. • Due to this, they deliver energy to the cavity, the result is the oscillation at the cavity producing RF frequency. Punjab EDUSAT society
Application • The reflex klystrons are used in • Radar receivers • Local oscillator in microwave receivers • Signal source in microwave generator of variable frequency • Portable microwave links • Pump oscillator in parametric amplifier Punjab EDUSAT society