ELCT564
Spring 2012
Chapter 7: Power Dividers and Directional
Couplers
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ELCT564
1
Power Dividers, Couplers and Hybrids
• Single components which perform power generation/amplification are
interconnected to combine outputs to achieve more power
• Requirements: components with low loss and high isolation between ports:
component with 3 or 4 ports
• Most commonly known: power dividers, couplers and directional couplers
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T Junctions
T-junction
Lossless and matched at
all ports.
Can not be lossless, reciprocal and
matched at all ports.
Counterclockwise circulation
Clockwise circulation
• It is impossible to construct a 3-port network that is lossless, reciprocal and
matched at all ports
• If only two ports are matched, then it can be lossless and reciprocal
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Directional Couplers
Can be lossless, reciprocal and
matched at all ports.
The Symmetrical (90o) Coupler
Ideal coupler: input power is split
between direct and coupled ports, no
power is reflected back or delivered to
isolated power.
The Antisymmetrical (180o) Coupler
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Hybrids
90o or 180o couplers which split the power equally between direct and coupled ports
are called Hybrids
Quadrature Hybrid (90o phase shift)
Quadrature Hybrid (180o phase shift)
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T-Junction Power Divider
E plane waveguide T, H plane waveguide T and Microstrip T-junction
Transmission line model of a lossless Tjunction
At low frequencies, parasitic junction capacitance is negligible. At high frequency,
performance is affected.
T Junction can only have two of the following: lossless, matched, reciprocal
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T-Junction Power Divider Example
A lossless T-junction power divider has a source impedance of 50 Ω. Find the
output characteristic impedances so that the input power is divided in a 2:1 ratio.
Compute the reflection coefficients seen looking into the output ports.
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Resistive Divider
To match all the ports
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The Wilkinson Divider
Matched at all ports, with isolation between output ports and lossless
When divider is driven at port 1 and matched output ports, no power is dissipated in the resistor
Only reflected power from ports 2 and 3 is dissipated in the resistor
S23=S32=0, ports 2 and 3 are isolated
Can also be designed for unequal power splits: k2=P3/P2
′
𝑍03 =
1 + 𝑘2
𝑘3
Design an equal-split Wilkinson power divider for a 50 Ω system impedance at
Frequency f0,and plot the returnloss(S11),insertion loss(:S21=S31),and isolation
(S23=S32)v ersus Frequency from 0.5f0 to 1.5f0.
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Coupled Line Couplers
• When 2 lines are close together, power can be coupled from one to the other (EM
filed interaction)
• C12 is capacitance between the 2 strip conductors in the absence of ground
conductor, C11 & C22 are capacitance between strip and ground in the absence of
the other strip
• Use more section to increase bandwidth
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The Quadrature (90o) Hybrid
•
Excellent isolation and directivity, equal split with 90o
difference between outputs
Bandwidth is 10-20%: more bandwidth with multiple
sections
Equal power division (unequal also possible)
•
180o Hybrid
•
•
Outputs have 180o difference
•
Different ways for fabrication, most popular is the “rat race”
coupler
Input port 1, even split in ports 2 & 3, port 4 will be isolated
Input port 4, even split in ports 2 & 3, port 1 will be isolated
•
•
Combiner
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The Lange Coupler
•
Coupling in coupled line coupler is to loose to achieve of 3 to dB
•
In order to increase coupling between edge-coupled lines : use several lines parallel to
each other so that the fringing fields at both edges of a line contribute to the coupling.
•
•
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Four coupled lines are used with interconnection to get
tight coupling
Can achieve 3 dB coupling with an octave or more
bandwidth
•
90o phase difference between outputs (ports 2 and 3)
•
Drawback: very narrow lines close together, difficult to
fabricate bonding wires
•
Interdigitated geometry
ELCT564