Microwave couplers are passive devices that divide and distribute power between transmission lines. There are different types of couplers including directional couplers, hybrid couplers, and Lange couplers. Key specifications for couplers include coupling factor, isolation factor, directivity, and losses. Recent developments aim to reduce size and increase bandwidth, such as using novel phase inverter designs in hybrid couplers.

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Microwave Couplers
Presentation by:
MD ERFAN (33108908)
Masters in Electrical Communication Engineering

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Outline
• Introduction
• Directional coupler
• Differences between couplers
• Key specification
• Losses
• Types of coupler
• Recent development
• Summary
• References
Microwave Couplers

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COUPLERS
•Passive device that divides 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.
Couplers is a device having 4 port , which couples the power from
input port, to two out ports, while the fourth port is isolated.
Microwave Couplers
[13]

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Directional Couplers
• Symmetric Device
• Directional couplers
• Ports 1&2 are Primary ports
• Ports 3&4 are Secondary ports
•Power is defined to a specific direction,
while no power is reflected back at any port.
•Power is divided between transmitted port
and coupled port
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Differences
[14]
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Forward Coupling
• Energy that propagates down transmission line starts a parallel wave down to
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.
• Asymmetric multi-section coupled structures provide forward-wave, in-phase
response. [15]
Microwave Couplers

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Backward Coupling
• Reverse coupler
• Energy that propagates down transmission line starts a reverse wave down
to transmission line 2.
• Single-section coupled transmission lines are always backward-wave
couplers (and outputs are in quadrature)
• Example: Lange coupler
• Symmetric multi-section couplers provide backward-wave, quadrature
response.
[15]
Microwave Couplers

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Key Specifications
The key specifications for a directional coupler relay on its purpose.
Each of them should be carefully evaluated to ensure that the meet
assemble its intended use.
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Coupling Factor
• Indicates the fraction of input power that is coupled to
the output port.
• Coupling is not constant, but varies with frequency.
• Coupling values of 10 and 20 dB are most common.
• Larger the coupling value smaller the coupled power.
Microwave Couplers

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Coupling Factor
• Directional couplers are specified in terms of the coupling
accuracy at the frequency band centre.
• 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.
Microwave Couplers

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Isolation Factor
• 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.
• Also 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.
Microwave Couplers

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Isolation Factor
• The isolation among the input and the isolated ports may be
dissimilar 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.
• Larger the bandwidth, higher the frequency, difficult to provide
Isolation.
• Higher the isolation, better the performance (Ideally Infinite)
Microwave Couplers

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Directivity
• Indicates the coupler’s ability to isolate the forward and
backward waves.
• How well the forward travelling wave in the primary
couples only to a specific port in secondary.
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 (Ideally
Infinite).
Microwave Couplers

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Directivity
• The directivity is very high at the designed frequency
• More sensitive function of frequency because it depends
on the cancellation of two wave components.
• Waveguide directional couplers will have the best
directivity.
Ref: [13]
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 SWR:
Standing wave ratio for power dividers and couplers, 50
Ohm match over all frequencies.
• Best performance
• Minimize reflections
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losses
Ref: [16]
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Main line loss
• Indicates the power loss as the signal travels
from Input port to Through Port.
• Ratio of input power through Input port (port1)
to power out of the Through Port(port 2).
Microwave Couplers

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Coupling loss:
• Indicates the portion of Mainline loss that is due
to coupling some of the input power into the
coupling port.
• This loss is unavoidable
• Very small value
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Insertion loss
 Additional loss
Causes:
• reflections,
• dielectric absorption,
• radiation effects,
• and conductor losses
 Broadband designs tend to have higher
insertion losses.
Causes:
• accumulate more dielectric,
• Reflection loss
• radiation, and
• conductor losses 1) skin effect 2) surface roughness
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TYPES OF COUPLERS
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Bethe-hole coupler
 One of the most common, and simplest, waveguide directional
couplers is the Bethe-hole directional coupler
Ref: [17]
• Two parallel staked WG
•Power pass through hole
•Backward coupler
•λ/4 a parted hole
•Bandwidth extended
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Two couplers in series, in opposing directions, with the
isolated ports internally terminated.
This component is the basis for the reflectometer.
Using internal, well-matched loads helps remove errors
with poor terminations that might be present in real
systems.
Dual-directional coupler
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A hybrid coupler is a special case, where a 3 dB split is desired
between the through path and the coupled path.
Hybrid couplers
There are two types of hybrid
couplers
90 degree couplers
and 180 degree hybrids (such as
rat-races and magic tees).
Microwave Couplers

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• 3 dB coupler divides the power
equally (within a certain tolerance)
between the output and coupled
output ports
Quadrature hybrid couplers
90° Hybrids or hybrid couplers are basically 3 dB directional couplers in which
the phase of the coupled output signal and the output signal are 90° apart.
Application:
1) Variable attenuators
2) Microwave mixers
3) Modulators
90° Hybrid Schematic
Microwave Couplers

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180° Hybrid Coupler [7].
 Depending on the junction
three types:
 Ring hybrid or rat-race
 Tapered coupled line and
 Magic-T hybrid.
 Application: combiner and
splitter.
180180oo
Hybrid CouplerHybrid Coupler
Four-port network with 180°
phase shift output
 The output are in phase
Microwave Couplers

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• Four-port component
• Coherent power by means of
simple Tee junctions
• E-plane and H-plane
• split power equally
• Tends to compensate phase
velocities
Lange Coupler [9].
 Coupling is too loose to achieve 3 or 6 dB
 90°phase difference between port 2 and 3
Advantages: Small Size and large Bandwidth
Disadvantage: lines are very narrow and close
together
Application: power combiners and splitters
The Lange Coupler and Magic T
Wave guide-magic T [8]
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• Power Measurements
• Frequency measurements
• Signal levelling
• Reflection coefficient measurements
• Signal sampling
• Signal injection
• Measure incident and reflected power
to determine VSWR
Applications
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Recent Development
Rat-race coupler
3D Phase Inverter Structure [11].
Top View Of Phase Inverter [11].
• Great features a 40% size reduction and
over 60% bandwidth improvement
• high port-to-port isolation in the D-band
frequency range.
• 90° and 180° output phase differences
• Phase inverters are based on CMOS process
• Performance dependent on phase inverter.
Advantages:
• Wide operation bandwidth
•Acceptable return loss,
•Good port-to-port isolation,
•Excellent amplitude and phase
imbalance in the whole D band
frequency range.
Based on novel phase inverter and developed in a standard CMOS process
Microwave Couplers

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Summary
• Couplers are often called directional couplers.
• Directional coupler is ideally matched at all ports.
• In waveguide technology, bethe hole couplers and multihole couplers are
widely used.
• Microstrip couplers are hybrid couplers, coupled line directional couplers
and lange couplers.
• Phase inverter can simultaneously increase bandwidth and can reduce
size for the hybrid coupler application.
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References
1. http://publications.lib.chalmers.se/records/fulltext/143058.pdf
2. http://www.aps.anl.gov/asd/people/nassiri/USPAS2003/Lecture10.pdf
3. http://www.nikoha.co.jp/eg/highvacu_com_eg.html
4. Pozar, David M., Microwave Engineering, 4th
ed.p.cm.
5. http://www.cernex.com/PRODUCTS_NEW/Coupler-PDF/Directional%20Couplers.pdf
6. http://news.thomasnet.com/fullstory/Quadrature-Hybrid-Coupler-has-compact-90-degree-
design-605196
7. http://www.militaryaerospace.com/articles/2013/08/krytar-hybrid-coupler.html
8. http://www.aps.anl.gov/asd/people/nassiri/USPAS2003/Lecture10.pdf
9. http://www.amitecltd.com/photo-gallery2.html
10. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6552367
11. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6259758
12. ML Sismondi, GS Raghuvanshi, Basic Microwave Techniques and Laboratory Manual, 1st
ed.1987
13. http://www.rtftechnologies.org/physics/fusor-mark3-ecrf-driver.htm
14. http://www.markimicrowave.com/Assets/appnotes/microwave_power_dividers_and_couplers_p
rimer.pdf
15. http://www.microwaves101.com/encyclopedias/directional-couplers
16. http://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/couplers.pdf
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