mtd microwave techniques and devices module i&ii part1

8/7/2019 MTD Microwave Techniques and Devices MODULE I&II PART1

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Principles and Applications of

Microwave Devices and Circuits


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Microwave techniques have been increasingly adopted in such

diverse applications as

Radio Astronomy

Long Distance communications

Space Navigation

RadarSystems

Medical Equipment

Missile Electronic Systems, etc.


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The range of frequencies that lie in between 109 Hz (ie.,

1 GHz) and 1012 Hz (ie., 1 THz) are called the

microwave frequencies.

These include the UHF, SHF and EHF ranges of

frequencies.

Why do we go for Microwave frequencies?

What are their specialties?

What are their applications?


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Advantages of Microwaves Microwave frequencies are extremely high. Hence they

offer extremely high BW

for communication channels.For example, a typical microwave communication

channel can carry 4000 audio or 4 video channels. Only,

optical communication channels have more BW than

this.

They propagate along line-of-sight paths throughtroposphere, where losses that we face in the ground-

wave and sky-wave propagations are absent or

minimum. Therefore, for transmission of signals, the

transmitter power used will be much smaller than thatrequired in the cases of the ground-wave and the sky-

wave propagations. (With microwave frequencies, even

a 100W transmitter is considered as a high power

transmitter.)


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Antennas required at microwave frequencies are much

smaller in size than those used at lower frequencies.

Because of this, and because of the low losses,microwave frequencies are used in modern mobile

communications system.

Usually, noise interference from disturbances such as

automobile ignition switches will not affect microwaves

as they occur at much lower frequencies.

Microwave communication is a point-to-point

communication scheme through cables or antennas.Hence, tapping is difficult at these frequencies as they

require highly specialized and costly equipments for

carrying out those tasks.


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Microwave tend to flow through the surface of

conductors due to what is known as the skin effect.T

hisin turn has been effectively used in a heating process

called the induction heating. Induction heaters are being

widely used in the manufacturing of blades, knives

semiconductor chips etc.

Since microwaves are also AC, they can flow through

capacitors. However, since frequencies are very high, as

they flow through capacitors, microwave frequencies

heat up the dielectric material in between the capacitorplates. This type of heating is called the dielectric

heating. This is the principle of operation of modern

microwave ovens.


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Disadvantages

A major disadvantage of microwaves is the nature ofpropagation itself. As has been seen, microwaves

propagate in the line-of-sight mode. In this mode,

microwave frequencies travel in straight-line paths,

which are limited by the horizon. The maximum range of

transmission, hence, is limited to a radius of 80 kmaround a transmitter. For long distance transmission, we

must use repeating stations at approximately 80 km

interval.

Microwave transmission towers are complex in

construction, and are highly expensive.


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Wave Guides (Single Lines)


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Comparison ofWave Guides with 2 WireTransmission Lines


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Types ofWave Guides


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Propagation ofWaves in RectangularWave Guides


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Dominant Mode and Degenerarate Mode


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THANK YOU


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CAVITY RESONATORSCAVITY RESONATORS


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A cavity resonator is a metallic enclosure that confinesA cavity resonator is a metallic enclosure that confinesthe electromagnetic energy.the electromagnetic energy.

The stored Electric and Magnetic energies inside theThe stored Electric and Magnetic energies inside thecavity determines its equivalent Inductance andcavity determines its equivalent Inductance andCapacitance.Capacitance.

The energy dissipated by the finite conductivity of theThe energy dissipated by the finite conductivity of thecavity walls determines its equivalent Resistance.cavity walls determines its equivalent Resistance.

In practice the RectangularIn practice the Rectangular--cavity resonator andcavity resonator andCircular cavity resonator are commonly used in manyCircular cavity resonator are commonly used in manymicrowave applications.microwave applications.


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Standing wave pattern in a Rectangular CavityStanding wave pattern in a Rectangular Cavity

ResonatorResonator


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Theoretically a given resonator has an infiniteTheoretically a given resonator has an infinite

number of resonant modes, and each modenumber of resonant modes, and each modecorresponds to a definite resonant frequency.corresponds to a definite resonant frequency.

When the frequency of an impressed signal isWhen the frequency of an impressed signal isequal to a resonant frequency, a maximumequal to a resonant frequency, a maximum

amplitude of the standing wave occurs, and theamplitude of the standing wave occurs, and the

peak energies stored in the Electric andpeak energies stored in the Electric and

Magnetic fields are equal.Magnetic fields are equal.


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Rectangular Cavity Resonator.Rectangular Cavity Resonator.

The electromagnetic field inside the cavityThe electromagnetic field inside the cavity

should satisfyshould satisfyMaxwells equationsMaxwells equations, subject to, subject tothe boundary conditions that thethe boundary conditions that the electric fieldelectric field

tangential totangential to andand magnetic field normal tomagnetic field normal to thethemetal walls must vanish.metal walls must vanish.


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b

a

d

x

y

z

Coordinates of a rectangular cavity.


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For the rectangular cavity, TEFor the rectangular cavity, TE101101 is the primary mode.is the primary mode.

For TEFor TEmnpmnp modes the field components Emodes the field components Exx,, EEyy, H, Hxx andandHHyycan becan beobtained from the Hobtained from the Hzz expression given below.expression given below.

and the same can be used to sketch the field patterns of theand the same can be used to sketch the field patterns of thevarious cavity modes.various cavity modes.

The subscripts m, n, and p actually represent the half waveThe subscripts m, n, and p actually represent the half waveperiodicity of the field in the x, y and z directionsperiodicity of the field in the x, y and z directionsrespectively.respectively.

cos cos sin

j t z

z

m x n y p z H C e

a b d

[ KT T T !

Q lit F t r (Q) f C itQ lit F t r (Q) f C it


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Quality Factor (Q) of CavityQuality Factor (Q) of Cavity

ResonatorResonator

The quality factor (Q) of any resonant or antiThe quality factor (Q) of any resonant or anti--

resonant circuit is a measure of frequencyresonant circuit is a measure of frequencyselectivity and is defined by the equation.selectivity and is defined by the equation.

(1)(1)

Where W= Maximum energy storedWhere W= Maximum energy stored

P = Average power lossP = Average power loss

00 = Resonant frequency= Resonant frequency

0 WQP

[

!


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ie.,ie.,

At resonant frequency, the electric and magneticAt resonant frequency, the electric and magneticenergies are equal and in time quadrature.energies are equal and in time quadrature.

When the electric energy is maximum, theWhen the electric energy is maximum, the

magnetic energy is zero and vice versa.magnetic energy is zero and vice versa.

The total energy stored in the resonator isThe total energy stored in the resonator is

obtained by integrating the energy density overobtained by integrating the energy density overthe volume of the resonator: ie.,the volume of the resonator: ie.,

2Ma xi u Energy Stored percycle

Q

Energy Dissipated percycle

T!


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.(2).(2)

Where are the peak values of the field Where are the peak values of the fieldintensities.intensities.

The average power loss in the resonator can beThe average power loss in the resonator can beevaluated by integrating the power density asevaluated by integrating the power density asgiven in eqn.given in eqn. (where R(where Rss is known asis known asthe skin effect and the magnitude of thethe skin effect and the magnitude of theconductor resistance.) over the inner surface ofconductor resistance.) over the inner surface ofthe resonator.the resonator.

2 2

2 2e m

v v

W E dv W H dv WI Q

! ! ! !

E and H

21

2s

p H R!

2R


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HenceHence (3)(3)

where Hwhere Htt is the peak value of the tangentialis the peak value of the tangential

magnetic intensity and Rmagnetic intensity and Rss is the surfaceis the surfaceresistance of the resonator.resistance of the resonator.

Substitution of eqn. (2) and (3) in eqn (1)Substitution of eqn. (2) and (3) in eqn (1)

yieldsyields ..(4)..(4)

2

2

st

s

R P H da!

2

2

v

s t

s

H dvQ

R H da

[Q

Si h k l f h i i i iSi h k l f h i i i i


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Since the peak value of the magnetic intensity isSince the peak value of the magnetic intensity is

related to its tangential and normal componentsrelated to its tangential and normal components

bybywhere Hwhere Hnn is the peak value of the normalis the peak value of the normal

magnetic intensity, the value ofmagnetic intensity, the value of at theat the

resonator walls is approximately twice the valueresonator walls is approximately twice the valueof averaged over the volume.of averaged over the volume.

So the Q of a cavity resonator as shown in eqn.So the Q of a cavity resonator as shown in eqn.(4)(4) can be expressed approximately by can be expressed approximately by

(5)(5)

2 2 2

t n!

2

t

2

( )

2 ( )

volumeQ

urf ce re

[Q!


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An unloaded resonator can be represented by either aAn unloaded resonator can be represented by either aseries or a parallel resonant circuit.series or a parallel resonant circuit.

The resonant frequency and the unloaded QThe resonant frequency and the unloaded Q00 of a cavityof a cavityresonator are .resonator are .

....(6)(6)

..(7)..(7)

If the cavity is coupled by means of an ideal N:1If the cavity is coupled by means of an ideal N:1transformer and a series inductance Ltransformer and a series inductance Lss to a generatorto a generatorhaving internal impedance Zhaving internal impedance Zgg, then the coupling circuit, then the coupling circuitand its equivalent are as shown in fig. Below.and its equivalent are as shown in fig. Below.

0

1f

LCT!

0

0

LQ

R

[

!


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The loaded QThe loaded Qll of the system is given by ......of the system is given by ......

forfor .(8).(8)

The coupling coefficient of the system is definedThe coupling coefficient of the system is definedas .as . (9)(9)

And the loaded QAnd the loaded Qllwould become would become == .(10).(10)

0

l

g

LQ

R N Z

[

!

2 2

S gN L R N Z

2

gN ZK

R!

0

(1 )l

LQ

R K

[

0

1

Q

K


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Rearrangement of the above eqn. yields.Rearrangement of the above eqn. yields.

..(11)..(11)

where Qwhere Qextext == QQ00 / K/ K== 00 LL /(KR)/(KR) is theis the

external Q.external Q. There are three types of coupling coefficients:There are three types of coupling coefficients:

1. Critical coupling: If the resonator is matched to1. Critical coupling: If the resonator is matched to

the generator, thenthe generator, then K=1.K=1.The loaded QThe loaded Qll is given byis given by

(12)(12)

0

1 1 1

l ext Q Q Q

!

0

1 1

2 2l ext Q Q Q! !


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2. Overcoupling: If K > 1, the cavity terminals are2. Overcoupling: If K > 1, the cavity terminals areat a voltage maximum in the input line atat a voltage maximum in the input line atresonance. The normalized impedance at theresonance. The normalized impedance at the

voltage maximum is the standing wave ratiovoltage maximum is the standing wave ratio ..

ie.,ie., K =K =

The loaded QThe loaded Qll is given byis given by3. Undercoupling: If K < 1, the cavity terminals3. Undercoupling: If K < 1, the cavity terminals

are at a voltage minimum and the input terminalare at a voltage minimum and the input terminalimpedance is equal to the reciprocal of theimpedance is equal to the reciprocal of thestanding wave ratio. That is,.standing wave ratio. That is,.

0

1l

QQ

V

1K

V


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The loaded QThe loaded Qll is given by is given by

The relationship of the coupling coefficient KThe relationship of the coupling coefficient K

and the standing wave ratio is shown in fig.and the standing wave ratio is shown in fig.below.below.

0

1lQ Q

V

V!


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Thank youThank you


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Microwave Junctions


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mtd microwave techniques and devices module i&ii part1

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