MICROWAVES Introduction to Laboratory Classes

MEEC, 1st semester 2011/2012

Custódio Peixeiro

DEEC

Laboratory Classes

• 4 Sessions

• T1 – Contact with a microwave bench

• T2 – Matching on an impedance load

• T4 – Measurement of the S-parameters of a waveguide “T”-junction

• T5 – Measurement of dielectric properties of materials

• Classes concentrated in 4 weeks (separated 4 weeks)

• T1 – 19-23 September

• T2 – 17-21 October

• T4 – 14-18 November

• T5 – 12-16 December

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• Maximum duration: 3 hours

• 2 students per group (few exceptions with 3 students)

• 5 groups per class (5 benches)

• On-line registrations in Fénix (from 15h30 September 13 until 23h59 September 15)

• Support to students on the days of the laboratory classes, 15h30-17h00, room 11.24 North Tower (ext. 2167)

• Classes start next week (19 September)

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Evaluation Marks

• Final Mark = 70% Written Exam Mark + 30% Laboratory Mark

Laboratory Mark = 50% Oral Evaluation Mark +

50% Written Report Mark

Witten Exam Mark – individual (0 to 20)

Oral Evaluation Mark – individual (0 to 10)

Written Report Mark – group (0 to 10)

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Microwave Bench

• Students use a microwave didactic bench (X band: 8-12 GHz) with rectangular metallic waveguide

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6

Reports Editor

• Use of the desktop computer available in each bench to write the session report

• Fixed template for each session report

• The templates follow the laboratory guide (Portuguese version available in “Secção de Folhas”, AEIST)

• Report printing at the end of the class

• Report marks given two weeks after the last class of each

session

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Important Notes

• It is absolutely necessary to prepare the laboratory sessions

by reading the corresponding guide. If you don’t do that

usually you will not be able to finish the session.

• Always follow thoroughly the safety recommendations given

in the first session

• Although low power is used (10 mW) microwaves radiation

hazards must be avoided

• Always follow the handling recommendation given in the first

session to handle the components and equipment with care

• Very expensive and fragile equipment

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• Take a USB flash drive to each class to store the report

• Register in paper all the results obtained

• Download the evaluation record sheet file from the Fénix web

page, print it and take it (with name and photo of each

student) for the first session

• Take a calculator for every session

• The classes start at the indicated time (08h00 or 17h00) and

FINISH at the indicate time (11h00 or 20h00)

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Transmission Line Basics

• As already indicated the microwave bench uses metallic

rectangular waveguide components

• TE (transversal electric) and TM (transversal magnetic) modes

are defined for the metallic rectangular waveguide

• The fundamental mode of the metallic rectangular waveguide

(ab) is the TE10 (a > b). The corresponding cutoff frequency

is

10

a2

cf

10TEc

2z

a2

λ1

λλ

10TE

11

• Each waveguide mode can be represented by equivalent

voltage and current (fictitious) distributions

• Transmission line theory can be used to study the field

distribution along the microwave bench

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Zg

Vg Z0

I1 I2

Zc V1 V2

z -l 0

zkj2

zkj2 eVreVi(z)Vr(z)Vi(z)V

)eVreVi(Z

1(z)Ir(z)Ii(z)I zkj

2zkj

2o

θj

2

2 e|Γ|Vi

VrΓ Load (voltage) reflection coefficient

)eΓe(Vi(z)V zkjzkj2

)eΓe(Z

Vi(z)I zkjzkj

o

2

)Γ1(Z

ViZ)Γ1(ViIZV

o

2c22c2

1y

1yΓΓ

y1

y1

1z

1zΓ

Γ1

Γ1

Z

Zz

c

cI

c

c

c

c

o

cc

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14

l)(ktanZjZ

l)(ktanZjZZ

eΓe

eΓeZ

l)z(I

l)z(VZ

co

ocolkjlkj

lkjklj

o1

)(tan1

)(tan11

lkzj

lkjz

Z

Zz

c

c

o

zk2j

zkj2

zkj2 eΓeVi

eVr

Vi

Vr(z)Γ

Reflection coefficient along the line

Voltage standing wave ratio (VSWR)

[,[1VSWR10,|Γ|1VSWR

1VSWR|Γ|

|Γ|1

|Γ|1

V

VVSWR

min

max

In space and time

θ)]zktω(cos|Γ|)zktω(cos[Vit)(z,V 2

Download VSWR.EXE from Fénix web page

Smith Chart

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xjrz1z

1ze|Γ|Γ c

c

cθj

bjgy

1y

1ye|Γ|Γ c

c

cθjII

I

πθθ|Γ||Γ| II

Re ( or I)

Im ( or I)

1

(r + j x)

I (g + jb)

or I

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Example Zc = 25 + j 50 Zo = 50 zc = 0.5 + j 1 yc= 0.4 – j 0.8

π0.46je0.62Γ

dB4.15)(0.62log20|Γ| dB

VSWR = 4.27

Impedance Matching with Stub

Goal is y- = 1 + j 0 (perfect matching)

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Zc Z0

Z0

ls

d

z 0

)(z'ylimy

(z)ylimy

(z)ylimy

slz's

left)(fromdz

right)(fromdz

s

)b(bj)gg(

)bjg()bjg(

yybjgy

ss

ss

s

0bb

1gg

s

s

bb

1g

s b)l(kcotanb ss

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Example Zc = 25 + j 50 Zo = 50 zc = 0.5 + j 1 yc= 0.4 – j 0.8

Two solutions (a, b) da / = (0.115+0.178)=0.293 lsa / = (0.340-0.250) = 0.090 db / = (0.115+0.322)=0.437 lsb / = (0.250+0.160)=0.410

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Solutions a and b

• are equivalent (|| = 0 = - dB, VSWR = 1) at central frequency

• (but) have different bandwidths (BW)

For VSWR 2 (|| -9.54 dB)

Solution a BW = 16.2 %

Solution b BW = 8.1 %

-40

-35

-30

-25

-20

-15

-10

-5

0

0,7 0,8 0,9 1 1,1 1,2 1,3

Inp

ut

Re

fle

ctio

n C

oe

ffic

ien

t [d

B]

Normalized Frequency

Solution a Solution b

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1

1,5

2

2,5

3

3,5

4

4,5

5

0,7 0,8 0,9 1 1,1 1,2 1,3

VSW

R

Normalized Frequency

Solution a Solution b

Several other specifications may be used

VSWR 1.5 || 0.2 -14 dB

VSWR 3 || 0.5 -6 dB

RF Virtual (Software Tool)

• Down load the software tool from the Fénix web page

• Basic setup (T1)

• Choose frequency and type of load

• Include (or not) slide screw tuner

• Visualization of the VSWR meter measurement

• Can be very useful to prepare T1 and T2

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VSWR Meter

• It is a dedicated equipment to measure directly the VSWR in

the slotted waveguide

• Tuned (1 kHz) amplifier and voltmeter

• Calibration needed each time the load conditions are changed

• Pointer deflection (stronger voltage) clockwise

• Very important to learn you to use it

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VSWR Measurement Procedure

1. Move the probe carriage to the right limit of the slotted waveguide (closest to the load)

2. Move the probe carriage slowly to the left until you find a maximum

3. Adjust the gain (use the buttons for 10 dB steps and the knob for continuous adjustment) until the meter indicates 1.0 on the upper scale

4. Move the probe carriage to a minimum

• Move to the right if in 2 you have passed a minimum and to the left otherwise

• Do not change the gain

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5. Read the VSWR directly in the upper scale (1 - )

6. If VSWR > 4 then increase the gain 10 dB and read the VSWR in the second scale (3.2 – 10)

7. If VSWR > 10 then increase the gain 10 dB again, read the VSWR in the first scale and multiply by 10

8. If VSWR > 40 then increase the gain 10 dB again, read the VSWR in the second scale and multiply by 10

9. You can continue this increase of gain and change of scale procedure if needed (VSWR > 100)

10. The third (red) scale is used to obtain more accurate measurements when VSWR 1.3. In that case you need to press the (red) “EXPAND” button.