Fig. 6.23 Waveguide irises.

An inductive iris (Fig. 6.23a) allows a current to flow where noneflowed before. The iris is placed in a position where the magnetic fieldis strong ( or where electric field is relatively weak). Since the plane ofpolarisation of electric field is parallel to the plane of iris, the currentflow due to iris causes a magnetic field to be set up. Energy storage ofmagnetic field takes place and there is an increase in inductance at thatpoint of the waveguide.

In capacitive iris (Fig. 6.23b), it is seen that the potential whichexisted between the top and bottom walls of the waveguide now existsbetween surfaces which are closer and therefore the capacitance hasincreased at that point. The capacitive iris is placed in a position wherethe electric field is strong.

The inductive and capacitive irises if combined suitably (correctlyshaped and positioned) the inductive and capacitive reactancesintroduced will be equal and the iris becomes a parallel resonant circuit(Fig. 6.23c). For the dominant mode, the iris presents a high impedanceancl..the-shunting effect for this mode will be negligible. Other modesare completely attenuated and the resonant iris acts as a band pass filterto suppress unwanted modes.

Figure 6.23d, shows a series resonant iris which is supported by anon-metallic material and is transparent to the flow of microwaveenergy .6.8 POSTS AND TUNING SCREWS

When a metallic cylindrical post is introduced into the broader side ofwaveguide, it produces the same effect as an iris in providing lumped

! reactance at that point. If the post extends only a short distance.

~

< A.g/4) into the waveguide, it behaves capacitively (Fig. 6.24a), and thiscapacitive susceptance increases with depth of penetration. When the

epth is equal to 'A.g/4, the post acts as a series resonant circuit (Fig..24b), if it is > A.g/4, the post behaves inductively (Fig. 6.24c) and this

i ductive susceptance decreases when the post is moved further away

'risD Ir::::::]l I tf I

Wa'. .@guide -- -- £: --

Q0

tL0

Eau;,al.nt Iimpedance Lcircuit TC0 0

(0) (b) (c) (d)

from the centre ofthe waveguide. When the post is extended completelyacross the waveguide, the post becomes inductive (Fig. 6.24d). Thesusceptance vs penetration (h) characteristics is.shown in Fig. 6.25.

Post

I<~-mn

4 LJ_m

o---Lo

(0)

Post

I

>ii!!] - I(c)

Post

,I

Iir!] -(b)

Post

I

[IJ-IF~g. 6.24 Waveguide posts.

(d)

<I>

II(f)

IIg/4--h

Fig. 6.25

The amount of susceptance decreases as the diameter of the post isreduced. If the post is made thicker, effective Q will be lowered and canact as a 'band \)ass filter similar to an iris. .

The big advantage of the post .over an iris is that it is readilyadjustable. An adjustable post is known as a screw or slug. Theadjustable or tuning screws are shown in Fig. 6.26. As in case of posts

rv'\

~cm(0)

<~4

1 J!i](d)

(b) Fig. 6.26 (c)

I depending upon the depth of penetration, the tuning screw mayintroduce inductive or capacitive susceptance.

A combination of two screws A.g/4 apart can be used to match awaveguide to its load similar to use of two fixed stubs in a transmissionline. A very effective waveguide matcher can be realised when twotuning screws are placed in close proximity separated by 3 A.g/8 asshown in Fig. 6.27. This is almost similar to the double stub matchingin transmission lines.

h,

1-3"--1I 8II

C L

Fig. 6.27

6.9 COUPLING PROBES AND COUPLING LOOPS

When a short antenna in the form of a probe or a loop in inserted into awaveguide, it will radiate and if it is placed correctly, the wanted modewill be set up. Figure 6.28 shows the correct positioning of the couplingprobes for launching dominant TElOmode. The probe is placed at adistance of A.g/4 from the shorted end of the waveguide I¥1dthe centreof broader dimension of the waveguide because at that point electricfield is maximum. This probe will now act as an antenna which ispolarized in the plane parallel to that of electric field.

Co-axialcable

Fig. 6.28 Coupling probe and loop.

The coupling loop placed at the centre of shorted end pl,ate of thewaveguide can also be used to launch TElOmode Le. coupling is achievedby means of a loop antenna located in a plane perpendicular to the planeof the probe. It is thus seen that probes couple primarily to the electricfield and loops to a magnetic field but in each case both are set, upbecause electric and magnetic fields are inseparable.

6.10 WAVEGUIDE TERMINATIONS

In a waveguide system it is not possible to attach a fixed resistive loadas a termination. Graphited sand at the end of the waveguide as shownin Fig. 6.29a can dissipate energy to achieve SWR's of less than 1.01.Alternatively a resistive rod placed at a point in the waveguide whereelectric field strength is maximum can also do a similar job (Fig. 6.29b).Fig. 6.2~c shows a wedge of resistive material (powdered iron or carbonmixed with a binder deposited on a dielectric strip) in the form of a taperthat can act as a termination. All these are matched terminations andalmost no reflection occurs.

A permanent metal plate welded at the end ofthe waveguide (a shortcircuit) may be employed for complete reflection. This could be mademovable (similar to a choke joint discussed earlier) by having anadjustable plunger as shown by Fig. 6.29d and e.

Energ~- ~ II/

Waveguide Graphited sand

(a)

(c)

~

J

Energy - 11;

/

Waveguide~

Resistive rod

(b)

~dP(d)

~§;J(e)

Fig.6.29

6.11 FERRITE DEVICES

Ferrites are non-metallic materials with resistivities (p) nearly 1014times greater than metals and with dielectric constants (Er) around10-15 and relative permeabilities of the order of 1000. They havemagnetic properties similar to those of ferrous metals. They are oxidebased compounds having general composition of the form MeO.Fe20ai.e." a mixture of a metallic oxide and ferric oxide where MeO representsany divalent metallic oxide such as MnO, ZnO, CdO, NiO or a mixtureof these. They are obtained by firing powdered oxides of materials at1l00.C or more and pressing them into different shapes. Thisprocessing gives them the added characteristics of ceramic insulatorsso that they can be used at microwave frequencies.

Ferrites have atoms with large number of spinning electrons. resulting in strong magnetic properties. These magnetic properties aredue to the magnetic dipole moment associated with the electron spin.Because of the above properties, ferrites find application in a number ofmicrowave devices to reduce reflected power, for modulation purposesand in switching circuits. Because of high resistivity they can be usedupto 100 GHz.

Ferrites have one more peculiar property which is useful atmicrowave frequencies i.e." the non-reciprocal property. When twocircularly polarised waves one rotating clockwise and otheranticlockwise are made to propagate through ferrite, the material reactsdifferently to the two rotating fields, thereby presenting differenteffective permeabilities to both the waves. i.e." Ert, Jlrt, PI for leftcircularly polarised wave and Er2,Jlr2,P2for the right circularly polarisedwave.

6.11.1 FaradayRotation in Ferrites

Consider an infinite lossless medium. A static field Bo is applied along. the z-direction. A plane TEM wave that is linearly polarised along thex-axis at t =0 is made to propagate through the ferrite in the z-direction.The plane of polarisation of this wave will rotate with distance, aphenomenon known as Faraday Rotation.

Any linearly polarised wave can be regarded as the vector sum oftwo counter rotating circularly polarised waves (Eo/2 vectors shown inFig. 6.30). The ferrite material offers different characteristics to thesewaves, with the result that the phase change for one wave is larger thanthe other wave resulting in rotation '9' of the linearly polarized wave, atz =l.

It is observed that a rotation of 100 degrees or more per cm offerritelength is typical for ferrites at a frequency of 10 GHz. If the directionof propagation is reversed, the plane of polaris at ion continues to rotatein the same direction i.e." from z = l to z = 0, the wave will arrive back

x

y

Fig. 6.30 Faraday rotation.

at z =0 polarised at an angle 2 e relative to x-axis.

In fact, the angle ofrotation 'e' is given by1

e = 2 (~+- ~-) ...(6.75)

where, 1= length of the ferrite rod

~+= Phase shift for the right circularly polarised (component inclockwise direction) wave with respect to some reference.

~- =phase shift for the left circularly polarised (component inanticlockwise direction) wave with respe<:t to the samereference.

In a practical ferrite medium, there will be finite losses. Thepropagation constant for circularly polarised wave will have unequalattenuation constants and unequal phase constant. Due to this, thedirection of Faraday rotation will be different in the two regions aboveand below the resonant frequency (O)o).A two port ferrite device is shownin Fig. 6.31 when a wave is transmitted from port (i)port~, it undergoesrotation in the anticlockwise direction as shown. Even if the same waveis allowed to propagate from port ~ port (i), it will undergo rotation inthe same direction (anticlockwise). Hence the direction of rotation oflinearly polarised wave is independent of the direction of propagationofthe wave.

Port 1 Port 2

l

Fig. 6.31

6.11.2 Microwave Devices which make use of Faraday rotation

We discuss three important devices which make use of faraday rotation

(a) Gyrator

(b) Isolator

(c) Circulator

(a) Gyrator: It is a two portdevice that has a relativephase difference of 180. fortransmission from port CDport@and 'no' phase shift (0. phaseshift) 'for transmission fromport @ to port CDshown in Fig.6.32.

The construction of a gyrator is as shown in Fig. 6.32. It consists ofa piece of circular waveguide carrying the dominant TEll mode withtransitions to a standard rectangular waveguide with dominant mode(TE10) at both ends. A thin circular ferrite rod tapered at both ends islocated inside the circular waveguide supported by polyfoam and thewaveguide is surrounded by a permanent magnet which generates dc .

magnetic field for proper operation of ferrite. To the input end a 90.twisted rectangular waveguide is connected as shown. The ferrite rodis tapered at both ends to reduce the attenuation and also for smoothrotation of the polarized wave.

Operation: When a wave enters port (j) its plane of polarizationrotates by 90. because of the twist in the waveguide. It again undergoesFaraday rotation through 90' because offerrite rod and the wave whichcomes out of port @will have a phase shift of 180' compared to the waveentering port CD.

But when the same wave (TEIO mode signal) enters port @, itundergoes faraday rotation through 90' in the same anticlock wisedirection. Because of the twist, this wave gets rotated back by 90. comesout of port (j)with O' phase shift as shown in Fig. 6.30. Hence a wave

at port (i)undergoes a phaseshift off radians (or 180.) but a wave fedfrom port @ does not change its phase'in a gyrator.

(b) Isolator: An isolator is a 2 port device which provides very smallamount of attenuation for transmission from port (i) to port @ butprovides maximum attenuation for transmission from port @to port (j).This requirement is very much desirable when we want to match asource with a variable load.

In most microwave generators, the output amplitude and frequencytend to fluctuate very significantly with changes in load impedance.This is due to mismatch of generator output to the load resulting inreflected wave from load. But these reflected waves should not be

Fig. 6.32

- .'-' 1\

Port 1 Port 2- Radians -'-'

lar0 to

lar lar0 toO

",TElO1

III

/

Transition

ler0

WG

TElO

lar0

(1)1I1IIII1

fi

Ferrite

, ,

, ,, ,

Fig. 6.33

allowed to reach the microwave generator, which will cause amplitudeand frequency instability of the microwave generator.

When isolator is inserted between generator and load, the generatoris coupled to the load with zero attenuation and reflections if any fromthe load side are completely absorbed by the isolator without affectingthe generator output. Hence the generator appears to be matched forall loads in the presence of isolator so that there is no change infrequency and output power due to variation in load. Thisis shown inFig. 6.34.

I TElO

I TE11V

Permanent

t ',

magnet

":K

"

-/J-W LoadSource

\ ',..

/J-W IsolatorSource Load

=0

Fig. 6.34

~- ------.-

Construction: The construction of isolator (Fig. 6.35) is similar togyrator except that an isolator makes use of 45' twisted rectangularwaveguide (instead of 90' twist) and 45' faraday rotation ferrite rod(instead of 90' in gyrator), a resistive card is placed along the largerdimension of the rectangular waveguide, so as to absorb any wave whoseplane of polarisation, is parallel to the plane of resistive card. Theresistive card does not absorb any wave whose plane of polarization isperpendicular to its own plane.

lor0

WG

TE10 Po (2)fi

TElO

'.CD~p=o I',

TE11

45' ~',

45..J/X " D'~WGII~' ~

71,"45' TE1~Resistive

card

TElO Pi (2)~p.I ,

CDfi ' ,,

Fig. 6.35 Constructional details of isolator.

Operation: A TEIOwave passing from port (l)through the resistivecard and is not attenuated. After coming out of the card, the wave getsshifted by 45' because of the twist in anticlockwise direction a& thenby another 45' in clockwise direction because of the ferrite rod and hencecomes out of port @with the same polarization as at port (l)without anyattenuation.

But a TEIOwave fed from port ~ gets a pass from the resistive cardplaced near port @ since the plane of polarization of the wave isperpendicular to the plane of the resistive card. Then the wave getsrotated by 45' due to Faraday rotation in clockwise direction and furthergets rotated by 45' in clockwise direction due to the twist in the

waveguide. Now the plane of polarization of the wave will be parallelwith that of the resistive card and hence the-wave will be completlyabsorbed by the resistive card and the output at port CDwill be zero. Thispower is dissipated in the card as heat. In practice 20 to 30 dB isolationis obtained for transmission from port (2)to port @.

(c) Circulator: A circulator is afour port microwave device whichhas a peculiar property that eachterminal is connected only to thenext clockwise terminal. i.e., portCDis connected to port CDonly andnot to port G>and (i) and port (2)isconnected only to port @ etc. This @is shown in Fig. 6.36. Althoughthere is no restriction on thenumber of ports, four ports aremost commonly used. They areuseful in parametric amplifiers,tunnel diode, amplifiers andduplexer in radars. Fig. 6.36

Construction: A four port Faraday rotation circulator is shown inFig. 6.37. The power entering port CDis TEIOmode and is converted toTEll mode because of gradual rectangular to circular transition. Thispower passes port (j)unaffected since the electric field is not significantlycut and is rotated through 45° due to the ferrite, passes port (i)unaffected(for the same reason as it passes port @) and finally emerges out of port@. Power from port @ will have plane of polarization already tilted by45° with respect to port (i). This power passes port (i)unaffected becauseagain the electric field is not significantly cut. This wave gets rotatedby another 45° due to ferrite rod in the clockwise direction. This powerwhose plane of polarization is tilted through 90° finds port G>suitablyaligned and emerges out of it. Similarly port @in coupled only to port(i) and port (i)to port (i).

Port Q)lor alor

T0 to' X

J

TElO TE"8=45'

~)

CD

~

Q)

Fig. 6.37 Four port circulator.