microwave lab report

8/11/2019 Microwave Lab Report

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A Report on

Microwave Laboratory Components

Submitted as a part of course requirement of

Microwave Laboratory

Submitted by

Pushkar Lal Vijayvergiya

M.Tech, 1st

year, RF & Microwave Engineering

Department of Electronics and Communications Engineering

Indian Institute of Technology, Roorkee

Submitted to:

Dr. N P Pathak

Associate Professor

Department of Electronics and Communications Engineering

Indian Institute of Technology, Roorkee


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ContentsMicrowave Overview .............................................................................................................................. 4

Microwave Frequency Band and Applications ........................................................................................ 5

Microwave Laboratory Components ...................................................................................................... 6

Waveguides ............................................................................................................................................. 6

Standard sizes of rectangular waveguide ........................................................................................... 7

Waveguide Bends ................................................................................................................................... 8

Waveguide Tees ...................................................................................................................................... 9

E-plane tee .......................................................................................................................................... 9

H-plane tee.......................................................................................................................................... 9

Magic Tees ........................................................................................................................................ 10

Directional Couplers .............................................................................................................................. 10

Circulator ............................................................................................................................................... 11

Isolators ................................................................................................................................................. 12

Attenuators ........................................................................................................................................... 12

Matched Load\Terminations ................................................................................................................ 13

Waveguide short/Variable shorts ......................................................................................................... 14

Frequency Meter .................................................................................................................................. 15

Horn Antennas ...................................................................................................................................... 15

Waveguide Detector Mount (Tunable) ................................................................................................. 17

Slide Screw Tuner .................................................................................................................................. 17

Slotted Line and tunable probe ............................................................................................................ 17

Klystron and Klystron Power Supply ..................................................................................................... 19

VSWR meter .......................................................................................................................................... 20


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Figure 1 Waveguide ................................................................................................................................ 6

Figure 2 Gradual Bends ........................................................................................................................... 8

Figure 3 Sharp Bend ................................................................................................................................ 8

Figure 4 Waveguide twist....................................................................................................................... 9

Figure 5 Waveguide Tees ........................................................................................................................ 9

Figure 6 Magic Tee ................................................................................................................................ 10

Figure 7 Directional Couplers ................................................................................................................ 11

Figure 8 Circulator ................................................................................................................................. 11

Figure 9 Isolators ................................................................................................................................... 12

Figure 10 Attenuators (Fixed and Variable) .......................................................................................... 13

Figure 11 Matched Loads ...................................................................................................................... 14

Figure 12 Waveguide short and variable short ..................................................................................... 14

Figure 13 Frequency meter ................................................................................................................... 15

Figure 14 Horn Antennas ...................................................................................................................... 16

Figure 15 Detector Mount .................................................................................................................... 17

Figure 16 Slide Screw Tuner .................................................................................................................. 17Figure 17 Slotted Line and Tunable Probe ............................................................................................ 18

Figure 18 Reflex Klystron ...................................................................................................................... 19

Figure 19 Reflex Klystron and power supply ......................................................................................... 20

Figure 20 VSWR Meter .......................................................................................................................... 21
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Microwave Overview

Microwavesare a form of electromagnetic radiation with wavelengths ranging from

as long as one meter to as short as one millimeter, or equivalently,

with frequencies between 300 MHz (0.3 GHz) and 300 GHz. The prefix "micro-" in

"microwave" is not meant to suggest a wavelength in the micrometer range. It

indicates that microwaves are "small" compared to waves used in typical radiobroadcasting, in that they have shorter wavelengths.

The term microwavealso has a more technical meaning

in electromagnetic and circuit theory. Apparatus and techniques may be described

qualitatively as "microwave" when the frequencies used are high enough that

wavelengths of signals are roughly the same as the dimensions of the equipment, so

that lumped-element circuit theory is inaccurate. As a consequence, practical

microwave technique tends to move away from the discrete resistors, capacitors,

and inductors used with lower-frequency radio waves. Instead, distributed circuit

elements and transmission-line theory are more useful methods for design andanalysis. Open-wire and coaxial transmission lines used at lower frequencies are

replaced by waveguides and stripline, and lumped-element tuned circuits are

replaced by cavity resonators or resonant lines. In turn, at even higher frequencies,

where the wavelength of the electromagnetic waves becomes small in comparison to

the size of the structures used to process them, microwave techniques become

inadequate, and the methods of optics are used.

High-power microwave sources use specialized vacuum tubes to generate

microwaves. These devices operate on different principles from low-frequency

vacuum tubes, using the ballistic motion of electrons in a vacuum under the influenceof controlling electric or magnetic fields, and include the magnetron (used

in microwave ovens), klystron, traveling-wave tube (TWT), and gyrotron. These

devices work in the density modulated mode, rather than the current modulated

mode. This means that they work on the basis of clumps of electrons flying

ballistically through them, rather than using a continuous stream of electrons.

Low-power microwave sources use solid-state devices such as the field-effect

transistor (at least at lower frequencies), tunnel diodes,Gunn diodes, and IMPATT

diodes. Low-power sources are available as bench top instruments, rack mount

instruments, embeddable modules and in card-level formats. A maser is a solid state

device which amplifies microwaves using similar principles to the laser, which

amplifies higher frequency light waves.


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Microwave Frequency Band and Applications

Letter

Designation

Frequency

rangeTypical uses

L band 1 to 2 GHzmilitary telemetry, GPS, mobile phones (GSM), amateur

radio

S band 2 to 4 GHz

weather radar, surface ship radar, and some

communications satellites (microwave ovens, microwave

devices/communications, radio astronomy, mobile phones,

wireless LAN, Bluetooth, GPS, amateur radio)

C band 4 to 8 GHz long-distance radio telecommunications

X band 8 to 12 GHzsatellite communications, radar, terrestrial broadband,

space communications, amateur radio

Kuband 12 to 18 GHz satellite communications

K band 18 to 26.5 GHzradar, satellite communications, astronomical observations,

automotive radar

Kaband 26.5 to 40 GHz satellite communications

Q band 33 to 50 GHzsatellite communications, terrestrial microwave

communications, radio astronomy, automotive radar

V band 50 to 75 GHzmillimeter wave radar research and other kinds of scientific

research

W band 75 to 110 GHz

satellite communications, millimeter-wave radar research,

military radar targeting and tracking applications, and some

non-military applications, automotive radar

F band 90 to 140 GHz

SHF transmissions: Radio astronomy, microwave

devices/communications, wireless LAN, most modern

radars, communications satellites, satellite television

broadcasting, DBS, amateur radio

D band 110 to 170 GHz

EHF transmissions: Radio astronomy, high-frequency

microwave radio relay, microwave remote sensing, amateur

radio, directed-energy weapon, millimeter wave scanner


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Microwave Laboratory Components

In Microwave laboratory, various components are used to characterize and

measurement of various microwave component and parameters. The details

descriptions of components used are given subsequently.

WaveguidesWaveguides are used in a variety of applications to carry radio frequency energy

from one pint to another. In their broadest terms they can be described as a system

of material that is designed to confine electromagnetic waves in a direction defined

by its physical boundaries.Typically a waveguide is thought if as a transmission line

comprising a hollow conducting tube, which may be rectangular or circular within

which electromagnetic waves are propagated. Unlike coaxial cable, there is no

centre conductor within the waveguide. Signals propagate within the confines of the

metallic walls that act as boundaries. The signal is confined by total internal

reflection from the walls of the waveguide

Waveguides will only carry or propagate signals above a certain frequency, known

as the cut-off frequency. Below this the waveguide is not able to carry the signals.

Since waveguides are really only hollow metal pipes, the installation and the physical

handling of waveguides have many similarities to ordinary plumbing. In light of this

fact, the bending, twisting, joining, and installation of waveguides is commonly called

waveguide plumbing

In order to determine the EM field configuration within the waveguide, Maxwells

equations should be solved subject to appropriate boundary conditions at the walls

of the guide. Such solutions give rise to a number of field configurations. Each

configuration is known as a mode. The following are the different modes possible in

a waveguide system

TE modes (Transverse Electric) have no electric field in the direction of propagation.

TM modes (Transverse Magnetic) have no magnetic field in the direction of

propagation

Figure 1 Waveguide


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The mode with the lowest cutoff frequency is termed the dominant mode of the

guide. It is usual to choose the size of the guide such that only this one mode can

exist in the frequency band of operation. In rectangular and circular (hollow pipe)

waveguides, the dominant modes are designated the TE1,0mode and TE1,1modes

respectively.

Standard sizes of rectangular waveguideWaveguide

name

Recommended frequency band of

operation (GHz)

Cutoff frequency of lowest order

mode (GHz)

Inner dimensions of waveguide

opening (inch)

WR2300 0.320.45 0.257 23.000 11.500

WR2100 0.350.50 0.281 21.000 10.500

WR1800 0.450.63 0.328 18.000 9.000

WR1500 0.500.75 0.393 15.000 7.500

WR1150 0.630.97 0.513 11.500 5.750

WR975 0.751.15 0.605 9.750 4.875

WR770 0.971.45 0.766 7.700 3.850

WR650 1.151.72 0.908 6.500 3.250

WR510 1.452.20 1.157 5.100 2.550

WR430 1.722.60 1.372 4.300 2.150

WR340 2.203.30 1.736 3.400 1.700

WR284 2.603.95 2.078 2.840 1.340

WR229 3.304.90 2.577 2.290 1.145

WR187 3.955.85 3.153 1.872 0.872

WR159 4.907.05 3.712 1.590 0.795

WR137 5.858.20 4.301 1.372 0.622

WR112 7.0510.00 5.260 1.122 0.497

WR90 8.2012.40 6.557 0.900 0.400

WR75 10.0015.00 7.869 0.750 0.375

WR62 12.4018.00 9.488 0.622 0.311

WR51 15.0022.00 11.572 0.510 0.255

WR42 18.0026.50 14.051 0.420 0.170

WR34 22.0033.00 17.357 0.340 0.170

WR28 26.5040.00 21.077 0.280 0.140

WR22 33.0050.00 26.346 0.224 0.112

WR19 40.0060.00 31.391 0.188 0.094

WR15 50.00

75.00 39.875 0.148 0.074WR12 60.0090.00 48.373 0.122 0.061

WR10 75.00110.00 59.015 0.100 0.050

WR8 90.00140.00 73.768 0.080 0.040

WR6, WR7 110.00170.00 90.791 0.0650 0.0325

WR5 140.00220.00 115.714 0.0510 0.0255

WR4 172.00260.00 137.243 0.0430 0.0215

WR3 220.00330.00 173.571 0.0340 0.0170


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Waveguide Bends

The size, shape, and dielectric material of a waveguide must be constant throughout

its length for energy to move from one end to the other without reflections. Any

abrupt change in its size or shape can cause reflections and a loss in overall

efficiency. When such a change is necessary, the bends, twists, and joints of the

waveguides must meet certain conditions to prevent reflections

Waveguides may be bent in several ways that do not cause reflections. One way is

the gradual bend shown in figure. This gradual bend is known as an E bend because

it distorts the E fields. The E bend must have a radius greater than two wavelengths

to prevent reflections

Another common bend is the gradual H bend. It is called an H bend because the H

fields are distorted when a waveguide is bent in this manner. Again, the radius of the

bend must be greater than two wavelengths to prevent reflections. Neither the E

bend in the "a" dimension nor the H bend in the "b" dimension changes the normal

mode of operation

Figure 2 Gradual Bends

A sharp bend in either dimension may be used if it meets certain requirements.

Notice the two 45-degree bends in figure; the bends are 1/4l apart. The reflections

that occur at the 45-degree bends cancel each other, leaving the fields as though no

reflections have occurred

Figure 3 Sharp Bend

Sometimes the electromagnetic fields must be rotated so that they are in the proper

phase to match the phase of the load. This may be accomplished by twisting the

waveguide as shown in figure. The twist must be gradual and greater than 2l


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Figure 4 Waveguide twist

Waveguide Tees

. In microwave circuits a waveguide or coaxial line junction with three independent

ports is commonly referred to as a tee junction. From the S-parameter theory of a

microwave junction it is evident that a tee junction should be characterized by amatrix of third order containing nine elementsBelow are some pictures of somewaveguide splitters found in the lab. Note that basic network theory says that you

can't make a three-port splitter that is lossless and matched at all three ports

E-plane tee (series tee) An E-plane tee is a waveguide in which the axis of the

side arm is parallel to the E-field of the main guide. The signal entering the first port

ofthis T - junction will be equally dividing at second and third ports of the same

magnitude but in opposite phase

H-plane tee (shunt tee)

An H-plane tee is a waveguide tee in which the axis of theside arm is shunting the E-field or parallel to the H field of the main guide. It can be

seen that if the two input waves are fed in port 1 and port 2 of the collinear arm, the

output wave at port 3 will be in phase and additive. On the other hand, if the input is

fed into port 3, the wave will split equally into port 1 and port 2 in phase and in same

magnitude

Figure 5 Waveguide Tees


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Magic Tees (Hybrid tees)-A magic tee is a combination of the E-plane tee and H-

plane tee. These Tees are employed in balanced mixers, AFC circuits and

impedance measurement circuits etc. The magic tee has several characteristics

1. If the two ports of equal magnitude and the same phase are fed into port 1 and

port 2, the output will be zero at port 3 and additive at port 4.

2. If a wave is fed into port 4 (H arm), it will be divided equally between port 1 and

port 2 of the collinear arms and will not appear in port 3.

3. If a wave is fed into port 3 (E arm), it will produce an output of equal magnitude

and opposite phase at port 1 and port 2. The output at port 4 is zero.

4. If a wave is fed into one of the collinear arms at port 1 or port 2, it will not appear

in the other collinear arm at port 2 or port 1 because the E-arm causes a phase

delay while the H-arm causes a phase advance.

Figure 6 Magic Tee

Directional Couplers

Directional Couplers couple a defined amount of the electromagnetic power in

a transmission line to a port enabling the signal to be used in another circuit. An

essential feature of directional couplers is that they only couple power flowing in one

direction. Power entering the output port is coupled to the isolated port but not to the

coupled port.

The symbols most often used for directional couplers are shown in figure 1. The

symbol may have marked on it a number in dB: this refers to the coupling factor of

the coupler. Directional couplers have four ports. Port 1 is the input port where power

is applied. Port 3 is the coupled port where a portion of the power applied to port 1

appears. Port 2 is the transmitted port where the power from port 1 is outputted, less

the portion that went to port 3. Directional couplers are frequently symmetrical so

there also exists port 4, the isolated port. A portion of the power applied to port 2 will

be coupled to port 4. However, the device is not normally used in this mode and port

4 is usually terminated with a matched load (typically 50 ohms). This termination canbe internal to the device and port 4 is not accessible to the user. Effectively, this


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results in a 3-port device.Directional couplers are useful for sampling a part of

Microwave energy for monitoring purposes and for measuring reflections and

impedance.

Figure 7 Directional Couplers

Circulator

A circulator is a ferrite device (ferrite is a class of materials with strange magnetic

properties) with usually three ports. The beautiful thing about circulators is that they

are non-reciprocal. That is, energy into port 1 predominantly exits port 2, energy into

port 2 exits port 3, and energy into port 3 exits port 1. In a reciprocal device the same

fraction of energy that flows from port 1 to port 2 would occur to energy flowing the

opposite direction, from port 2 to port 1The selection of ports is arbitrary, and

circulators can be made to "circulate" either clockwise (CW) or counterclockwise

(CCW). A circulator is sometimes called a "duplexer", meaning that is duplexes two

signals into one channel (e.g. transmit and receive into an antenna).Circulators have

low electrical losses and can be made to handle huge powers, well into kilowatts.They usually operate over no more than an octave bandwidth, and are purely an RF

component (they don't work at DC).

The make a great antenna interface for a transmit/receive system. Energy can be

made to flow from the transmitter (port 1) to the antenna (port 2) during transmit, and

from the antenna (port 2) to the receiver (port 3) during receive.

Figure 8 Circulator


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Isolators

By terminating one port, a circulator becomes an isolator, which has the property that

energy flows on one direction only. This is an extremely useful device for "isolating"

components in a chain, so that bad VSWRs don't contribute to gain ripple.

An isolator is a nonreciprocal transmission device that is used to isolate one

component from reflections of other components in the transmission line. An ideal

isolator completely absorbs the power for propagation in one direction and provides

lossless transmission in the opposite direction. Thus the isolator is usually called

uniline. Isolators are generally used to improve the frequency stability of microwave

generators such as klystrons and magnetrons in which the reflection from the load

affects the generating frequency. In such cases the isolator is placed between the

generator and load to prevent the reflected power from the unmatched load from

returning to the generator. As a result the isolator maintains the frequency stability of

the generator

Figure 9 Isolators

AttenuatorsAttenuators are meant for inserting a known attenuation in a wave guide system.

This consists of a lossy vane inserted in a section of wave guide, flanged on both

ends. These are useful for isolation of wave guide circuits, padding and extending

the range of measuring equipments. Attenuators are passive resistive elements that

do the opposite of amplifiers, they kill gain.

Fixed attenuators: - All of the standard fixed attenuators are manufactured from

selected waveguide tube. The attenuating element is manufactured from a metalized

glass fiber reinforced PTFE, resistive card vane or an absorptive composite material.

The vane version is supported in the waveguide using two metal rods and is

accurately positioned to give a desired value between 0 and 40dB as required. The

composite absorber is positioned and glued into the tube (the attenuation is based

on the length of the absorber).

Variable Attenuators:-Based upon the same construction as the Fixed Attenuators,

the metalized glass fiber reinforced PTFE resistive card vane is positioned in the

Waveguide using a backlash free, spring controlled piston, precisely fitted in a

machined housing to give a high degree of mechanical stability. The attenuation is

varied by means of a knurled finger-control knob, and a locking screw is provided forrepetitive measurements, or, in the case of the variable precision devices, the


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attenuation is varied by means of a standard micrometer drive. For some type of

variable attenuator, a guillotine principle is used for the vane insertion into the broad

wall of the waveguide. Movement of the vane is achieved by the means of an

eccentric cam attached to the control knob.

Figure 10 Attenuators (Fixed and Variable)

Matched Load\TerminationsWaveguide terminations absorb energy and prevent RF signals from reflecting back

from open-ended or unused waveguide ports. They are passive devices which

dissipate radio frequency (RF) energy by producing heat energy.A waveguide may

also be terminated in a resistive load that is matched to the characteristic impedance

of the waveguide. One method is to fill the end of the waveguide with a

graphite and sand mixture .When the fields enter the mixture, they induce a

current flow in the mixture that dissipates the energy as heat. Another method

is to use a high-resistance rod placed at the center of the E field. The E field

causes current to flow in the rod, and the high resistance of the rod dissipatesthe energy as a power loss, again in the form of heat. Still another method for

terminating a waveguide is the use of a wedge of highly resistive material. The

plane of the wedge is placed perpendicular to the magnetic lines of force.

When the H lines cut through the wedge, current flows in the wedge and

causes a power loss .As with the other methods, this loss is in the form of

heat. Since very little energy reaches the end of the waveguide,

reflections are minimum

Sliding Loads:-The standard sliding loads are assembled in the same way as the

standard fixed loads using selected waveguide tube and precise positioning of theflange interface. Operation of the sliding element is by a push/pull mechanism and a

locking screw is provided for repetitive measurements


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Figure 11 Matched Loads

Waveguide short/Variable shorts

Waveguide short plates are designed to terminate round or rectangular waveguide connectors atthe mating plane. These are simple metallic plates of high conductivity to reflect incident field

wave. They are used to establish a reference plane in systems and in making loss, Wavelength

measurement etc.

Variable Waveguide short circuit terminations provide standard reflection at desired, precisely

measurable positions. The basic idea behind it is to provide short-circuit by changing reactance

of the termination. The simplest form of an adjustable waveguide short consists of a sliding block

of a good conductor which makes as snug fit in the waveguide. However the electric short

position deviates from the physical short circuit position in a random manner owing to the erratic

contact between the sliding block and the wall. Furthermore some power leakage past the block

may occur thereby making the reflection coefficient less than unity. This too is highly undesirable. These problems are overcome in chock-type shorting plungers.

Figure 12 Waveguide short and variable short


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flared sides, but wide in the plane of the narrow sides. These types are often used as

feed horns for wide search radar antennas.

E-plane hornA sectoral horn flared in the direction of the electric or E-field in the

waveguide.

H-plane hornA sectoral horn flared in the direction of the magnetic or H-field inthe waveguide.

Conical hornA horn in the shape of a cone, with a circular cross section. They are

used with cylindrical waveguides.

Exponential horn(e)A horn with curved sides, in which the separation of the

sides increases as an exponential function of length. Also called a scalar horn, they

can have pyramidal or conical cross sections. Exponential horns have minimum

internal reflections, and almost constant impedance and other characteristics over a

wide frequency range. They are used in applications requiring high performance,such as feed horns for communication satellite antennas and radio telescopes.

Corrugated hornA horn with parallel slots or grooves, small compared with a

wavelength, covering the inside surface of the horn, transverse to the axis.

Corrugated horns have wider bandwidth and smaller side lobes and cross-

polarization, and are widely used as feed horns for satellite dishes and radio

telescopes.

Ridged hornA pyramidal horn with ridges or fins attached to the inside of the

horn, extending down the center of the sides. The fins lower the cutoff frequency,

increasing the antenna's bandwidth.

Septum hornA horn which is divided into several subhorns by metal partitions

(septums) inside, attached to opposite walls.

Aperture-limited horna long narrow horn, long enough so the phase error is a

negligible fraction of a wavelength, so it essentially radiates a plane wave. It has an

aperture efficiency of 1.0 so it gives the maximum gain and minimum beam width for

a given aperture size. The gain is not affected by the length but only limited by

diffraction at the aperture. Used as feed horns in radio telescopes and other high-

resolution antennas.

Figure 14 Horn Antennas


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Waveguide Detector Mount (Tunable)

Tunable Detector Mount is simple and easy to use instrument for detecting

microwave power through a suitable detector. It consists of a detector crystal

mounted in a section of a Wave guide and shorting plunger for matching purpose.

The output from the crystal may be fed to an indicating instrument

Figure 15 Detector Mount

Slide Screw Tuner

Tuners are based on precision slide screw technology that utilizes broadband slab

line transmission structure and passive probes to create impedances for devices.

The probes are designed to be very close to one-quarter wavelength in the linear

dimension at the mid-band of each range. Slide screw tuners are used for matching

purposes by changing the penetration and position of a screw in the slot provided in

the centre of the wave guide. These consist of a section of wave guide flanged on

both ends and a thin slot is provided in the broad wall of the Wave guide. A carriage

carrying the screw is provided over the slot. A VSWR upto 20 can be tuned to a

value less than 1.02 at certain frequency

Figure 16 Slide Screw Tuner

Slotted Line and tunable probe

Slotted lines are used for microwave measurements and consist of a movable probe

inserted into a slot in a transmission line It consists of a waveguide, with a movable

insulated probe inserted into a longitudinal slot cut into the line. In a rectangular

waveguide, the slot is usually cut along the centre of the broad wall of the

waveguide. Circular waveguide slotted lines are also possible.


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The slotted line works by sampling the electric field inside the transmission line with

the probe. For accuracy, it is important that the probe disturbs the field as little as

possible. For this reason the probe diameter and slot width are kept small (usually

around 1 mm) and the probe is inserted in no further than necessary. It is also

necessary in waveguide slotted lines to place the slot at a position where the current

in the waveguide walls is parallel to the slot. The current will then not be disturbed by

the presence of the slot as long as it is not too wide. For the dominant mode this is

on the centre-line of the broad face of the waveguide, but for some other modes it

may need to be off-centre. This is not an issue for the co-axial line because this

operates in the TEM (transverse electromagnetic) mode and hence the current is

everywhere parallel to the slot. The slot may be tapered at its ends to avoid

discontinuities causing reflections.

The disturbance to the field inside the line caused by the insertion of the probe is

minimized as far as possible. There are two parts to this disturbance. The first part is

due to the power the probe has extracted from the line and manifests as a lumpedequivalent circuit of a resistor. This is minimized by limiting the distance the probe is

inserted into the line so that only enough power is extracted for the detector to

operate effectively. The second part of the disturbance is due to energy stored in the

field around the probe and manifests as a lumped equivalent of a capacitor.

This capacitance can be cancelled out with an inductance of equal and

opposite impedance. Lumped inductors are not practical at microwave frequencies;

instead, an adjustable stub with an inductive equivalent circuit is used to "tune out"

the probe capacitance. The result is an equivalent circuit of a high impedance in

shunt across the line which has little effect on the transmitted power in the line. The

probe is more sensitive as a result of this tuning and the distance it is inserted can

be further limited as a result.The probe is connected to a detector and a VSWR

meter. The detector can be a crystal detector or a Schottky barrier diode. The

detector is mounted on the probe assembly, usually a distance /4from the probe

tip. This is because the detector looks almost like a short circuit to the transmission

line, and this distance will convert it to an open circuit through the quarter-wave

impedance transformer effect. Thus, the detector has minimal effect on loading the

line. The probe tuning stub branching from the line linking the probe to the detector

Figure 17 Slotted Line and Tunable Probe


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Klystron and Klystron Power Supply

A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-

beam tube. The pseudo-Greek word klystron comes from the stem form -(klys) of a

Greek verb referring to the action of waves breaking against a shore, and the end of

the word electron

Klystrons are used as an oscillator or amplifier at microwave and radio frequencies

to produce both low power reference signals for super heterodyne radar receivers

and to produce high-power carrier waves for communications and the driving force

for linear accelerators. It has the advantage (over the magnetron) of coherently

amplifying a reference signal and so its output may be precisely controlled in

amplitude, frequency and phase. Many klystrons have a waveguide for coupling

microwave energy into and out of the device, although it is also quite common for

lower power and lower frequency klystrons to use coaxial couplings instead. In some

cases a coupling probe is used to couple the microwave energy from a klystron into

a separate external waveguide.

Figure 18 Reflex Klystron

In Reflex klystron, electron beam passes through 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 for

another pass through the cavity, where they are then collected. The electron beam is

velocity modulated when it first passes through the cavity. The formation of electron

bunches takes place in the drift space between the reflector and the cavity. The

voltage on the reflector must be adjusted so that the bunching is at a maximum as

the electron beam reenters the resonant cavity, thus ensuring a maximum of energyis transferred from the electron beam to the RF oscillations in the cavity. The


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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. This effect is used to

good advantage for automatic frequency control in receivers, and in frequency

modulation for transmitters. The level of modulation applied for transmission is small

enough that the power output essentially remains constant. At regions far from the

optimum voltage, no oscillations are obtained at all. There are often several regions

of reflector voltage where the reflex klystron will oscillate; these are referred to as

modes.

Three power sources are required for reflex klystron operation: (1) filament power,

(2) positive resonator voltage (often referred to as beam voltage) used to accelerate

the electrons through the grid gap of the resonant cavity, and (3) negative repeller

voltage used to turn the electron beam around. The electrons are focused into a

beam by the electrostatic fields set up by the resonator potential in the body of the

tube. In addition to these voltage klystron power supply has the mode control switch

of selecting CW or AM or FM. There also knobs provided for changing amplitude andfrequency of the AM and FM.

Figure 19 Reflex Klystron and power supply

VSWR meter

The VSWR meter measures the standing wave ratio in a transmission line. A VSWR

meter basically consists of high gain, high Q, low noise voltage amplifier normally

tuned at fixed frequency (1 kHz) at which the microwave signal modulated. The

VSWR meter used the detected signal out of microwave detector as its input,

amplifies the same and provides the output on a calibrated voltmeter. The meter

itself can be calibrated in terms of VSWR.

The VSWR meter has a gain control to adjust the reading to a desired value, by fine

or coarse adjusting knobs. Normally, the overall gain is about 125dB that can be

adjusted in steps of 10. Also there are three scales on the VSWR meter- normal

SWR, Expanded SWR and dB scales. Normal SWR scale can be used when the

VSWR is between 1 and 4, when value up to 10 bottom of normal scale is used. The


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expanded scale is graduated from 1 to 1.3. The dB scale is at the bottom along with

an expanded dB for measuring VSWR directly in dB.

Figure 20 VSWR Meter

microwave lab report

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