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Notes 19

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ECE 5317-6351

Microwave Engineering

Fall 2011

Prof. David R. Jackson

Dept. of ECE

Notes 19

Power Dividers and Couplers

Part 1

Three-port networks

Power Dividers and Directional Couplers

General 3-port:

Power Dividers and Directional Couplers (cont.)

Not physically possible

(There are three distinct values.)

Lossless [S] is unitary

These cannot all be satisfied.

(If only one is nonzero, we cannot satisfy all three.)

At least 2 of S13, S12, S23must be zero.

(If only one is zero (or none is zero), we cannot satisfy all three.)

Power Dividers and Directional Couplers (cont.)

“Circulator”

These equations will be satisfied if:

(There are six distinct values.)

1

2

Lossless

or

Note that Sij Sji.

Circulators

Clockwise (LH) circulator

1

2

Note: We have assumed here that the phases of all the S parameters are zero.

Circulators can be made using biased ferrite materials.

Counter clockwise (RH) circulator

T-Junction: lossless divider

If we match at port 1, we cannot match at the other ports!

Assuming port 1 matched:

We can design the splitter to control the powers into the two output lines.

For each port we have:

Also, we have

If port 1 is matched:

The output ports are not isolated.

Powers:

Hence

Summary

- The input port is matched, but not the output ports.
- The output ports are not isolated.

Example: Microstrip T-junction power divider

Same for Zin1 and Zin2

All ports are matched.

By reciprocity and symmetry

Hence we have

All ports are matched, but 1/2 Pinis dissipated by resistors, and the output ports are not isolated.

(This is needed for analyzing the Wilkenson.)

Obviously,

Example: We want to solve for V.

using even/odd mode analysis

“even” problem

“odd” problem

Even-Odd Mode Analysis (cont.)

Even-Odd Mode Analysis (cont.)

short circuit (SC) plane of symmetry

Even-Odd Mode Analysis (cont.)

“odd” problem

“even” problem

By superposition:

Wilkenson Power Divider

Equal-split (3 dB) power divider

- All ports matched (S11 = S22 = S33 = 0)
- Output ports are isolated (S23 = S32 = 0)

Note: No power is lost in going from port 1 to ports 2 and 3.

Obviously not unitary

Wilkenson Power Divider (cont.)

Example: Microstrip Wilkenson power divider

Wilkenson Power Divider (cont.)

- Even and odd analysis is required to analyze structure when port 2 is excited.

To determine

- Only even analysis is needed to analyze structure when port 1 is excited.

To determine

The other components can be found by using symmetry and reciprocity.

Wilkenson Power Divider (cont.)

A microstrip realization is shown.

Split structure along plane of symmetry (POS)

Even voltage even about POS place OC along POS

Odd voltage odd about POS place SC along POS

Wilkenson Power Divider (cont.)

How do you split a transmission line? (This is needed for the even case.)

top view

Voltage is the same for each half of line (V)

Current is halved for each half of line (I/2)

(magnetic wall)

Z0microstrip line

For each half

Wilkenson Power Divider (cont.)

Note: The 2Z0 resistor has been split into two Z0 resistors in series.

Ports 2 and 3 are excited in phase.

Wilkenson Power Divider (cont.)

Note: The 2Z0 resistor has been split into two Z0 resistors in series.

Ports 2 and 3 are excited 180o out of phase.

Wilkenson Power Divider (cont.)

Port 2 excitation

Port 2

Wilkenson Power Divider (cont.)

Port 2 excitation

Port 2

Wilkenson Power Divider (cont.)

We add the results from the even and odd cases together:

Note: Since all ports have the same Z0, we ignore the normalizing factor Z0in the S parameter definition.

In summary,

Wilkenson Power Divider (cont.)

Port 1 excitation

Port 1

When port 1 is excited, the response, by symmetry, is even. (Hence, the total fields are the same as the even fields.)

Even Problem

Wilkenson Power Divider (cont.)

Port 1 excitation

Hence

Wilkenson Power Divider (cont.)

Port 1 excitation

Along g/4 wave transformer:

(reciprocal)

Wilkenson Power Divider (cont.)

For the other components:

By symmetry:

By reciprocity:

Wilkenson Power Divider (cont.)

All three ports are matched, and the output ports are isolated.

Wilkenson Power Divider (cont.)

- When a wave is incident from port 1, half of the total incident power gets transmitted to each output port(no loss of power).
- When a wave is incident from port 2 or port 3, half of the power gets transmitted to port 1 and half gets absorbed by the resistor, but nothing gets through to the other output port.

Wilkenson Power Divider (cont.)

Figure 7.15 of PozarPhotograph of a four-way corporate power divider network using three microstrip Wilkinson power dividers. Note the isolation chip resistors.Courtesy of M.D. Abouzahra, MIT Lincoln Laboratory.