Lec.11 Properties of Dividers and Couplers

RF & Microwave EngineeringBETE-Spring 2009

Department of Electrical EngineeringAir University

Properties of n-port Networks

Introduction to Passive Microwave Devices

Lecture No. 11

RF & Microwave EngineeringBETE-Fall 2009

Basit Ali ZebDepartment of Electrical Engineering, AU

Introduction

• Passive devices are widely used in microwave circuits, antenna transmission and reception, feeding networks, radars, test and measurement etc.

• Power Dividers (combiners) and directional couplers are the most common passive microwave devices.

• They can be characterized as n-port networks (3 and 4-ports)while it is common in practice to find even more than that.– A T-junction and a 2-way power splitter/combiner– A directional coupler is a 4-port device

• Implementation is possible/feasible using microstrip and waveguide technology.



A simple Divider/Combiner



Basic Properties of 3-port Networks

• Matched at all ports

• Reciprocal

• Lossless

As we will show here, it is not possible to construct a

3-port network that fulfills all three properties!



3-Port Network S-parameters

1. Matched




2. Reciprocal




3. Lossless

At least two of the three S-parameters (S12,S13,S23) must be zero to hold equations 4 - 6 true.

Eq.1

Eq.6

Eq.5

Eq.4

Eq.2

Eq.3

If this is the case, then none of the equations 1 - 3 can be satisfied.



Lossless, Reciprocal, Matched

• What does the result of previous equations tell us about the three properties of a 3-port microwave network?

• It is not possible to construct a three-port network fulfilling all the above properties.

• These basic properties limits our design expectations for power dividers and combiners which have three ports.



Conclusion

So our conclusion is that a 3-port network cannot be

reciprocal, lossless and matched at all the ports. We

have to design the component around this limitation!

However, one can realize and design such a 3-port

network if one of the three constraints are loosened.



Three Possibilities!!

1.



Three Possibilities!

2.

3.



Basic Properties of 4-port Networks

A four port network can be lossless, reciprocal and matched

at all ports unlike 3-port network. The S matrix of such a network is shown here:



Symmetric Coupler

Phases of the terms having amplitude β are chosen equal.



Asymmetric Coupler

Phases of the terms having amplitude β are chosen to be 180o apart.



Directional Coupler

Arrows indicate the assumed direction of time average

power flow.



Operation of Directional Coupler

We can easily deduce the operation of a symmetrical or

asymmetrical directional coupler from its S matrix.

Port 1---Input port

Port 2,3---Output port

Port 4--- Isolated port

Observe the pattern in S-matrix

of the couplers



Directional Coupler

The performance of directional coupler is characterized by three terms: Coupling, Directivity and Isolation.



Passive RF/Microwave Components

• Resistors, Capacitors, Inductors

• Connectors, Transformers, Transitions

• Matching networks, Baluns, Tuners

• Directional Couplers, Power dividers and Combiners

• Resonators, Filters, Isolators

• Circulators, Delay lines, Antennas

• Multiplexers, Hybrids

Examples

Lec.11 Properties of Dividers and Couplers

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