This document discusses microwave devices, specifically directional couplers and isolators. It begins by defining microwaves and their applications such as telecommunications and radar. It then describes how directional couplers are passive devices that divide power through four ports and discusses their key figures of merit like coupling factor, isolation, and directivity. Isolators are also covered as two-port non-reciprocal devices that allow high power transmission in one direction while providing high attenuation in the opposite direction using Faraday rotation in a ferrite rod.

1. MICROWAVE DEVICES
DIRECTIONAL COUPLER AND
ISOLATOR
E. CINTHURIYA -
ME - COMMUNICATION ENGINEERING
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2. MICROWAVES
 Microwaves are electromagnetic waves whose
frequencies range from about 300 MHz – 300 GHz
(1 MHz = 106 Hz and 1 GHz =109 Hz) or
wavelengths in air ranging from 100 cm – 1 mm.
 Microwave is an electromagnetic radiation of short
wavelength.
 They can reflect by conducting surfaces just like optical
waves since they travel in straight line.
 Microwave currents flow through a thin outer layer of an
ordinary cable.
 Microwaves are easily attenuated within short distances.
 They are not reflected by ionosphere
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3. APPLICATIONS
 Microwaves have a wide range of applications in
modern technology, which are listed below
1. Telecommunication: Intercontinental Telephone and
TV, space communication (Earth – to – space and space
– to – Earth), telemetry communication link for railways
etc.
2. Radars: detect aircraft, track / guide supersonic
missiles, observe and track weather patterns, air traffic
control (ATC), burglar alarms, garage door openers,
police speed detectors etc.
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4. DIRECTIONAL COUPLERS
 Passive device that divided and distributes power.
 Couples part of the transmission power by a
known amount out through another port
 Uses two transmission lines set close enough
together such that energy passing through one
is coupled to the other.
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5. DIRECTIONAL COUPLER
 The device has four ports:
Input, Port
Through Port
Coupled, Port
Isolated Port
 The term "main line" refers to the section between ports 1
and 2.
 Ports 1&2 are Primary ports
 Ports 3&4 are Secondary ports 5
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6. FORWARD COUPLING
 Energy that propagates down transmission line starts a parallel wave
down transmission line 2 .
 Most common forward coupler is the multi-hole coupler realized in
waveguide.
 In this case the holes are spaced a quarter wave apart so that the reverse
wave cancels out.
 Asymmetric multi-section coupled structures provide forward-wave, in-
phase response.
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7. BACKWARD COUPLING:
 Reverse coupler
 Energy that propagates down transmission line starts a reverse wave down
transmission line 2.
 Single-section coupled transmission lines are always backward-wave
couplers (and outputs are in quadrature),
 Eg: Lange coupler
 Symmetric multi-section couplers provide backward-wave, quadrature
response.
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8. FIGURES OF MERIT:
COUPLING
FACTOR
DIRECTIVITY
DIRECTIONAL
COUPLER
ISOLATION
FACTOR 8
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9. COUPLING FACTOR
• The coupling factor is defined as:
where P1 is the input power at port 1 and P3 is the
output power from the coupled port.
• The coupling factor represents the primary property of a
directional coupler.
• Coupling is not constant, but varies with frequency.
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10. • Directional couplers are specified in terms of the coupling
accuracy at the frequency band center. For example, a 10 dB
coupling ± 0.5 dB means that the directional coupler can have
9.5 dB to 10.5 dB coupling at the frequency band center.
• The accuracy is due to dimensional tolerances that can be
held for the spacing of the two coupled lines.
• Another coupling specification is frequency sensitivity. A larger
frequency sensitivity will allow a larger frequency band of
operation.
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11. ISOLATION
• Isolation of a directional coupler can be defined as the
difference in signal levels in dB between the input port and the
isolated port when the two other ports are terminated by
matched loads.
• Isolation:
• It can also be defined between the two output ports. In this
case, one of the output ports is used as the input; the other is
considered the output port while the other two ports (input and
isolated) are terminated by matched loads.
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12. • Consequently:
• The isolation between the input and the isolated ports may be
different from the isolation between the two output ports.
• For example, the isolation between ports 1 and 4 can be 30 dB
while the isolation between ports 2 and 3 can be a different
value such as 25 dB. Isolation can be estimated from the
coupling plus return loss.
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13. DIRECTIVITY
• Directivity is directly related to isolation. It is
defined as:
where: P3 is the output power from the coupled
port and P4 is the power output from the
isolated port.
• The directivity should be as high as possible.
The directivity is very high at the design
frequency and is a more sensitive function of
frequency because it depends on the
cancellation of two wave components.
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14.  Waveguide directional couplers will have the best
directivity.
 Directivity is not directly measurable, and is calculated
from the difference of the isolation and coupling
measurements as:
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15. APPLICATIONS
 Frequency measurements
 Signal levelling
 Reflection coefficient measurements
 Signal sampling
 Signal injection
 Measure incident and reflected power to determine VSWR
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16. ISOLATORS
• An isolator is a two-port non-reciprocal device which produces
a minimum attenuation to wave propagation in one direction
and very high attenuation in the opposite direction.
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17. ISOLATOR - CONSTRUCTION
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18. ISOLATOR WORKING
• Thus when inserted between a signal source and load almost
all the signal power can be transmitted to the load and any
reflected power from the load is not fed back to the generator
output port.
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19. Faraday rotation Isolator, the input resistive card is in y-z
plane, the output resistive card is displaced 45o with respect
to the input card.
The magnetic field which is applied longitudinally to the
ferrite rod rotates the wave plane by 45o.
This is normal to the output resistive card
As the result of rotation the wave arrives at the out put end
without attenuation at all.
On the other end a reflected wave from the output end is
similarly rotated clockwise 45o by the ferrite rod, since the
reflected wave is parallel to the input resistive card the wave
is absorbed by the input card.
ISOLATOR WORKING
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20. FARADAY ROTATION ISOLATOR
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21. FARADAY ROTATION
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22. ADVANTAGES
• This eliminates variations of source power output and
frequency pulling due to changing loads.
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