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UNIT II MICROWAVE COMPONENTS
Waveguide Attenuators- Resistive card, Rotary Vane types. Waveguide Phase Shifters : Dielectric, Rotary Vane types. Waveguide Multi port Junctions- E plane and H plane Tees, Magic Tee, Hybrid Ring. Directional Couplers- 2 hole, Bethe hole types. Ferrites-Composition and characteristics, Faraday Rotation. Ferrite components: Gyrator, Isolator, Circulator. S-matrix calculations for 2 port junction, E & H plane Tees, Magic Tee, Directional Coupler, Circulator and Isolator
WAVEGUIDE ATTENUATORS :
In order to control power levels in a microwave system by partially absorbing the transmitted microwave signal, attenuators are employed. Resistive films (dielectric glass slab coated with aquadag) are used in the design of both fixed and variable attenuators. A co-axial fixed attenuator uses the dielectric lossy material inside the centre conductor of the co-axial line to absorb some of the centre conductor microwave power propagating through it dielectric rod decides the amount of attenuation introduced. The microwave power absorbed by the lossy material is dissipated as heat.
In waveguides, the dielectric slab coated with aquadag is placed at the centre of the waveguide parallel to the maximum E-field for dominant TEIO mode. Induced current on the lossy material due to incoming microwave signal, results in power dissipation, leading to attenuation of the signal. The dielectric slab is tapered at both ends upto a length of more than half wavelength to reduce reflections as shown in figure 5.7. The dielectric slab may be made movable along the breadth of the waveguide by supporting it with two dielectric rods separated by an odd multiple of quarter guide wavelength and perpendicular to electric field. When the slab is at the centre, then the attenuation is maximum (since the electric field is concentrated at the centre for TEIO mode) and when it is moved towards one sidewall, the attenuation goes on decreasing thereby controlling the microwave power corning out of the other port.
If the resistive card in the centre section is kept at an angle 8 relative to the E-field direction of the TE11 mode, the component E cos8 parallel to the card get absorbed while the component E sin 8 is transmitted without attenuation. This component finally comes out as E sin2θ as shown in figure 5.10.
WAVEGUIDE PHASE SHIFTERS :
A microwave phase shifter is a two port device which produces a variable shift in phase of the incoming microwave signal. A lossless dielectric slab when placed inside the rectangular waveguide produces a phase shift.
PRECISION PHASE SHIFTER
The rotary type of precision phase shifter is shown in figure 5.12 which consists of a circular waveguide containing a lossless dielectric plate of length 2l called "half- wave section", a section of rectangular-to-circular transition containing a lossless dielectric plate of length l, called "quarter-wave section", oriented at an angle of 45° to the broader wall of the rectangular waveguide and a circular-to-rectangular transition again containing a lossless dielectric plate of same length 1 (quarter wave section) oriented at an angle 45°.
After emerging out of the half-wave section, the field components E3 and E4 as given by equations (5.19) and (5.20), may again be resolved into two TE modes, polarized parallel and perpendicular to the output quarterwave plate. At the output end of this quarterwave plate, the field components parallel and perpendicular to the quarter wave plate, by referring to figure 5.12 (d), can be expressed as
Comparison of equation (5.21) and (5.22) yields that the components Es and E are identical in both magnitude and phase and the resultant electric field strength at the output is given by
All the above listed parameters can be represented as the ratio of either voltage to current or current or voltage under certain conditions of input or output ports.
At microwave frequencies it is impossible to measure: 1 total voltage and current as the required equipment is not available. 2 Over a broad band region, it is difficult to achieve perfect open and short circuit conditions. 3 The active devices used inside the two port network such as microwave power transistors will tend to become unstable under open and short circuit conditions.
WAVE GUIDE MULTI PORT JUNCTIONS:
A waveguide Tee is formed when three waveguides are interconnected in the form of English alphabet T and thus waveguide tee is 3-port junction. The waveguide tees are used to connects a branch or section of waveguide in series or parallel with the main waveguide transmission line either for splitting or combining power in a waveguide system There are basically 2 types of tees namely
. H- Plane Tee junction . E-plane Tee junction A combination of these two tee junctions is called a hybrid tee or “Magic Tee”.
E-plane Tee(series tee):
An E-plane tee is a waveguide tee in which the axis of its side arm is parallel to the E field of the main guide. if the collinear arms are symmetric about the side arm.If the E-plane tee is perfectly matched with the aid of screw tuners at the junction, the diagonal components of the scattering matrix are zero because there will be no reflection. When the waves are fed into side arm, the waves appearing at port 1 and port 2 of the collinear arm will be in opposite phase and in
same magnitude.
- If two waves of equal magnitude and same phase are fed into port 1 and port the output will be zero at port 3 and additive at port 4.
- If a wave is fed into port 4 it will be divided equally between port 1 and port 2 of the collinear arms and will not appear at port 3.
- If a wave is fed into port 3 , it will produce an output of equal magnitude and opposite phase at port 1 and port 2. the output at port 4 is zero.
- If a wave is fed into one of the collinear arms at port 1 and port 2, it will not appear in the other collinear arm at port 2 or 1 because the E-arm causes a phase delay while H arm causes a phase advance.
Hybrid Rings( Rat Race circuits): A hybrid ring consists of an annular line of proper electrical length to sustain standing waves, to which four arms are connected at proper intervals by means of series or parallel junctions.
The hybrid ring has characteristics similar to those of the hybrid tee. When a I wave is fed into port 1, it will not appear at port 3 because the difference of phase shifts for the waves traveling in the clockwise and counterclockwise direction is 180°. Thus the waves are canceled at port 3. For the same reason, the w aves fed into port 2 will not emerge at port 4 and so on. The S matrix for an ideal hybrid ring can be expressed as
It should be noted that the phase cancellation occurs only at a designated frequency for an ideal hybrid ring. In actual hybrid rings there are small leakage coupling s and therefore the zero elements in the matrix are not equal to zero.
WAVE GUIDE CORNERS , BENDS AND TWISTS: The waveguide corner, bend, and twist are shown in figure below, these waveguide components are normally used to change the direction of the guide through an arbitrary angle.
In order to minimize reflections from the discontinuities, it is desirable to have the mean length L between continuities equal to an odd number of quarter wave lengths. That is,
where n = 0, 1, 2, 3, ... , and Ag is the wavelength in the waveguide. If the mean length L is an odd number of quarter wavelengths, the reflected waves from both ends of the waveguide section are completely canceled. For the waveguide bend, the minimum radius of curvature for a small reflection is given by Southworth as
where P1 = power input to port 1 P3 = power output from port 3 P4 = power output from port 4
It should be noted that port 2, port 3, and port 4 are terminated in their characteristic impedances. The coupling factor is a measure of the ratio of power levels in the primary and secondary lines. Hence if the coupling factor is known, a fraction of power measured at port 4 may be used to determine the power input at port 1. This significance is desirable for microwave power measurements because no disturbance, which may be caused by the power measurements, occurs in the primary line. The directivity is a measure of how well the forward traveling wave in the primary waveguide couples only to a specific port of the secondary waveguide ideal directional coupler should have infinite directivity. In other words, the power at port 3 must be zero because port 2 and port A are perfectly matched.
Actually well-designed directional couplers have a directivity of only 30 to 35.
Several types of directional couplers exist, such as a two-hole direct coupler, four-hole directional coupler, reverse-coupling directional coupler , and Bethe- hole directional coupler the very commonly used two-hole directional coupler is
described here.
TWO HOLE DIRECTIONAL COUPLERS:
A two hole directional coupler with traveling wave propagating in it is illustrated. the spacing between the centers of two holes is
A fraction of the wave energy entered into port 1 passes through the holes and is radiated into the secondary guide as he holes act as slot antennas. The forward waves in the secondary guide are in same phase , regardless of the h ole space and are added at port 4. the backward waves in the secondary guide are out of phase and are cancelled in port 3.
