MW2 Lab Sheet (EMG2016) - FOE

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MULTIMEDIA UNIVERSITY
FACULTY OF ENGINEERING
LAB SHEET
ELECTROMAGNETIC THEORY
EMG2016
MW2 – IMPEDANCE MEASUREMENT AND MATCHING
EM Theory
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EXPERIMENT MW2:
IMPEDANCE MEASUREMENT AND MATCHING
OBJECTIVES:
a) To measure the normalized impedance of an unknown load.
b) To perform impedance matching using a slide-screw tuner and Smith Chart.
APPARATUS:
Klystron Source
Klystron Power Supply
Isolator
Slotted-line probe detector
SWR measurement amplifier/meter
Slide-Screw Tuner
Attenuator
Short-circuit Plate x 2
BNC coax cable x 1
Spanner
INTRODUCTION:
The ratio between the electric and the magnetic fields at a particular point along a
waveguide is defined as the impedance at that point. It may be denoted as Z = E / H = R +
jX. If there is no reflected wave, this ratio is the same at all points along the line. The
standing wave ratio (SWR) will be unity. In this case, the load absorbs all the incident
energy and is said to be matched to the characteristic impedance Zo of the waveguide. If a
reflected wave is present, causing a standing wave, then the impedance will vary
periodically with distance along the transmission path. It is therefore necessary to specify
the plane to which the impedance is defined when measuring an unknown impedance. It
can be shown that the relationship between SWR, reflection coefficient , and the
impedance Z at any point on the line be given by
Z - Zo
 = 
Z + Zo
1 + 
SWR = 
1  
= Z / Zo
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Smith Chart (see Appendix A) is usually used to analyze the values of R and X at
any distance along the transmission line. On Smith Chart, impedance values are
normalized with respect to the characteristic impedance Zo. Complete circles in the chart
are curves of constant resistance R/Zo. Incomplete circles which are at right angles to the
complete circles are curves of constant reactance jX/Zo. Thus, every point in the chart
specifies a normalized impedance Z/Zo=(R+jX)/Zo. The impedances at various points
along the waveguide lie on a circle with its centre at the centre of the Smith Chart. This
circle is often called the circle of constant mismatch (or the SWR circle). When the load
is equal to Zo, there is no mismatch and the radius of the SWR circle will be zero. The
impedances at various distances from a point (where the impedance is known) can be
established by moving around the SWR circle through the appropriate angles. The point
where the SWR circle crosses the horizontal axis on the right-side of Smith Chart (i.e.
R/Zo + j0) corresponds to a point of maximum electric field in the standing wave pattern.
It also gives the VSWR, since at this point the normalized resistance value is equal to the
VSWR (i.e. R/Zo = VSWR). The point at the opposite end of the horizontal diameter (on
the left-side of Smith Chart) corresponds to a point of minimum electric field and has
impedance Z = (1/VSWR) + j0.
To measure an unknown impedance, the SWR (=So) of the device connected to a
slotted-line waveguide and the position of one minimal point xo is determined (see Fig-1).
A circle corresponds to R+j0 where R=So is drawn on the Smith Chart. After that, the
unknown device is substituted by a short circuit plate. Two successive minimum points,
x1 and x2, are noted. Twice the distance between them is the guide wavelength g. One of
the minimum points is used as reference. Let d = (xo - x1) / g. The impedance at the input
terminal of the unknown device can be found on the constant mismatch circle at a
distance d from the point of minimum electric field towards load if d  0; otherwise
towards generator. It should be noted that the impedance along the line is equal to the
load impedance at any integral number of half-wavelengths from the load.
Plane of
load
Emax2
minimum with
load, Emin2
g / 2
xo d
x2
x1
Fig 1 : Illustration to Impedance Measurement.
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When the load doesn’t match with the transmission line impedance, the reflected
energy is usually lost as heat and, in some cases, may destroy the source amplifier. It may
also cause a wide variation in performance when the condition is disturbed by, for
example, temperature change or signal frequency drift. Therefore, it is usually desirable
that the load accepts all the RF energy or additional impedance matching network can be
incorporated to the line in order to minimize the amount of energy reflected to the
generator. In the case of waveguide, a slide-screw tuner which consists of a slotted
waveguide section and a movable metal rod penetrating through the slot can be used for
this purpose. A variable capacitive susceptance is introduced in parallel to the
transmission line by varying the tuner’s protrusion depth.
A convenient feature of Smith Chart is that, if the normalized impedance is
represented by one point Z, then the normalized admittance (reciprocal of impedance) is
found by simply moving to the opposite end of the diameter through Z on the constant
mismatch circle. The admittance Y = G + jB changes along the waveguide according to
the values represented by the circle. A point Y1 can be found where the conductance G is
equal to the characteristic admittance Yo. If the susceptance at this point has a negative
value (ie. -jB), a slide-screw tuner can be used to add a susceptance +jB in parallel with Y
so that the combined susceptance will be zero and the resulting admittance Y’ will simply
become Yo. There may be a standing wave in the waveguide section between the tuner
and the load but, as far as the source generator is concerned, no part of the incident wave
is reflected.
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PREPARATION





Conduct background study about the experiment, e.g. read the lab sheet.
Bring log book for recording observations and results.
Bring scientific calculator.
Bring Smith Charts.
Bring a pair of compasses.
PROCEDURE:
CAUTION 1: The RF power levels in the following experiments are not
harmful, but a human eye may be damaged by low level of radiation. DO NOT
LOOK INTO THE WAVEGUIDE at any time when the equipment is on.
CAUTION 2: Klystron tube gets extremely HOT when it is operated and must
not be handled by hand.
A. Initial Setup (30 minutes)
The experimental setup (see Fig. 2) is similar to the source-transmission line-load setup
used in MW1. The difference being a slide-screw tuner is inserted in between the
transmission line and the load. The slide-screw tuner allows a metal rod to protrude into
the waveguide introducing a shunt capacitance at the point of protrusion. The shunt
capacitance introduced is proportional to the depth of protrusion. It is used to perform
impedance matching in this experiment. As in MW1, the SWR meter used in the setup is
a power meter with SWR scales on it and it has a built-in variable gain amplifier. Power
increases from the left to the right side of the scales.
A primary objective of this initial setup is to tune the components for maximum signal
strength at the SWR meter. This includes tuning the Klystron source’s repeller voltage for
maximum output power, the tuning gear and the source’s modulation frequency and
amplitude for maximum reception at the SWR meter.
1. Set up the equipment as shown in Fig 2. (already done by the lab technician but do
verify). Caution: Make sure the SWR meter input setting is ‘XTAL LOW’.
Klystron
Source
Klystron Power
Supply
Isolator
Slotted-line
Detector
Slide-screw
Tuner
SWR
Meter
Fig 2: Experiment Setup
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Attenuator
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2. Set the slotted-line probe’s depth to 0.5 mm. (preset by the lab technician)
3. Set the attenuator to >2.8mm (> 20 dB return loss).
4. Make sure that the SWR meter is in the normal deflection mode (set by the smaller
dial at and on top of the range-gain knob).
5. Make sure that the tuning screw of the slide-screw tuner is totally out of the
waveguide (micrometer setting < 0).
6. Switch on the Klystron power supply (with internal modulation active) and the SWR
meter.
7. Adjust the SWR meter’s gain until a significant deflection appears on the meter.
8. Adjust the repeller voltage knob for maximum reception at the SWR meter*.
9. Adjust the tuning gear of the slotted-line probe for maximum reception at the SWR
meter*.
10. Adjust the modulation frequency and amplitude for maximum reception at the SWR
meter*.
11. At this stage, the SWR meter should give good deflection at gain settings of 30 to 40
dB. If necessary, increase the slotted-line probe’s depth to achieve the stated
condition. Make sure that a full deflection can be obtained by adjusting the vernier
gain knob.
* As one is tuning towards the maximum, the SWR meter will indicate that power is
increasing. As one’s tuning reaches the maximum and then moves away from the
maximum, the SWR meter will indicate that power reaches a maximum and then
decreases. Adjust the SWR meter’s gain if necessary to have a decent deflection on the
meter and to avoid overdriving the meter.
Note:
 Do not disturb the repeller voltage, slotted-line probe’s depth, and tuning gear
beyond this point. Else, you might have to repeat the above.
 For slotted-line measurements below, make your measurements at the central part of
the waveguide; avoid the two ends of the waveguide.
 All measurement readings have to be in the correct precision. Please make it a habit.
 Make your own tables of observations. Make it a habit too.
Evaluation A: (cognitive – knowledge, level 1) [10 marks]
i. Please make sure you know all components of the setup and their functions. You may
consult the lab instructor if necessary.
ii. Request for evaluation when you are ready.
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B. Determine the Normalised Load Impedance (50 minutes)
12. Replace the load (attenuator and short-circuit) by a short circuit plate. You need
spanners; borrow them from the lab technician. Do not turn off the Klystron Power
Supply. Do not look into the waveguide. Do not lose the screws.
13. Move the slotted-line probe along the waveguide to locate two successive points of
minimum power. Record the positions of these two points, x1 and x2. Determine the
waveguide wavelength g .
14. Restore the load. Do not turn off the Klystron Power Supply. Do not look into the
waveguide. Make sure the joints are tight, e.g. cannot be displaced by hand.
15. Set the attenuator to 1.1 mm (about 6 dB return loss).
16. Move the detector probe along the waveguide to locate the point of maximum power.
Henceforth, for convenience, it will be referred to as the maximum point (or
minimum point for minimum power).
17. Adjust the gain of the SWR meter so that its reading is 0 dB on the bottom scale (i.e.
full scale deflection). Do not overdrive the SWR meter.
18. Move the probe to locate a minimum point (any minimum point). Record its position,
x0 . Read and record the SWR from the SWR meter**.
** There are 5 scales on the SWR meter. From the top, the first two are meant for the
expand mode (not used). Third and fourth are linear scales. The third scale is used
when the maximum and minimum are observed using the same range-gain. The fourth
scale is used when the minimum is observed using a range-gain 10dB higher than that
of the maximum. The fifth scale is a log scale (dB). Difference in range-gain between
the maximum and minimum should be added to the reading on the log scale to obtain
the SWR in dB.
19. – (it means skip this line)
20. Calculate d = (x0 – x1) / g .
21. On the Smith Chart, locate the resistance grid having a value equal to the recorded
SWR. Locate the intersection between this resistance grid and the horizontal line in
the middle of the Smith Chart. See Appendix A.
22. Using a pair of compasses, draw a circle that goes around the center of the Smith
Chart and passes through the intersection point of step 21. It is the circle of constant
mismatch, also known as the SWR circle. See Appendix A.
23. On the Smith Chart, locate the short circuit point zsc = 0 + j0. Starting from the short
circuit point, move around the ‘toward load’ scale to locate the distance d if d > 0
(hereafter known as point E); use the ‘toward generator’ scale for d < 0. See
Appendix A.
24. Draw a straight line that passes through point E and the centre of the Smith Chart, and
crosses all the circles. There are two intersections between this line and the SWR
circle. The normalized load impedance zL is given by the intersection nearest to point
E. The farther intersection gives the normalized load admittance yL. With the aid of
the resistance/conductance grids and reactance/susceptance grids, read zL and yL. See
Appendix A and B.
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Evaluation B: (psychomotor – complex overt response, level 5) [30 marks]
i. Tabulate your measurements and draw on the Smith chart.
ii. Be ready to demonstrate how the measurements were done.
iii. Request for evaluation when you are ready.
C. Impedance Matching Using Smith Chart (20 minutes)
25. The line drawn in step 24 also makes two intersections with the ‘toward generator’
scale. Record the distance d1 on the ‘toward generator’ scale (irrespective of the sign
of d) for the farther intersection. See Appendix B.
26. Determine the point y1 where the SWR circle intersects the unity constant resistance
circle. There are two intersections; use the one with negative susceptance. Draw a line
from the center of the Smith Chart, through y1, to the outer most circle. The line will
intersect the ‘toward generator’ scale. Record the distance d2 on the ‘toward
generator’ scale at the intersection point. See Appendix A and B.
27. Calculate d = (d2 - d1)g
d is the distance from the load terminal towards generator where the input
conductance G will be equal to the characteristic admittance Yo.
Evaluation C: (cognitive – application, level 3) [20 marks]
i. Draw on the Smith chart.
ii. Request for evaluation when you are ready.
D. Verify and Fine Tune Impedance Matching Conditions (60 minutes)
28. The slide-screw tuner (hereafter referred to as tuner) acts as a variable shunt capacitor.
For impedance matching, it should be placed d from the load terminal. Note that d
is measured from the load towards generator. d computed from step 27 may not give
a practical position to insert a tuning screw. In that case, add an integral number of
half-wavelengths (ng/2) to d in order to determine a practical position to insert a
tuning screw. Record the distance Do = d + ng/2 from the load, where the tuning
screw is to be inserted.
29. Position the tuner at distance Do from the load terminal. The distance between the
load terminal and the beginning of the vernier scale on the tuner is 50 mm.
30. Measure the SWR before impedance matching (see step 16. to 18. ). Henceforth, it
will be referred to as the initial SWR.
31. Increase the tuner (not slotted-line probe!) protrusion depth slowly until there is a
noticeable change in the meter reading. Re-measure the SWR at this point.
32. If the new SWR is less than the initial SWR, proceed to step 33. Otherwise, there
were errors in impedance matching calculations and Do is not correct. Move the tuner
to the left or right (choose a direction with ample movement space) by 1 mm. Remeasure the SWR. Repeat the 1 mm movement and re-measure the SWR of course,
until the SWR is noticeably less than the initial SWR.
33. At this point, you should have a new SWR less than the initial SWR.
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34. Increase the tuner protrusion depth slowly until there is a noticeable change in the
meter reading. Re-measure the SWR at this point.
35. Repeat step 34. until the new SWR is more than the previous one. You have just
missed the optimum protrusion depth giving minimum SWR.
36. Fine tune the protrusion depth to locate the point of minimum SWR.
37. If the new SWR is less than 1.1, proceed to step 39. Otherwise, further fine tuning is
required. With the protrusion depth intact, move the tuner to the left or right by 1 mm
Use a smaller movement value (this requires heuristic judgments) if you are close to
SWR 1.1. Re-measure the SWR. If the new SWR is more than the previous one, very
likely the movement direction is wrong, try moving in the opposite direction and remeasure the SWR. Repeat the process until you have the optimum tuner position
giving minimum SWR.
38. Repeat steps 36. and 37. until you have the SWR less than 1.1.***
39. Record the final SWR = S1 achieved, the position D1 of the tuner, and the protrusion
depth p.
*** Note that, for simplicity, the above tuning procedure assumes that independent tuning
of tuner position and protrusion depth will converge to the optimum point. From previous
experience, it is generally true, but not always true. In the case that it is not true, one has
to find the optimum depth for each tuner position in order to locate the optimum tuner
position.
Evaluation D: (psychomotor – adaptation, level 6) [40 marks]
i. Tabulate your results.
ii. Be ready to demonstrate the impedance matched state.
iii. Request for evaluation when you are ready.
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APPENDIX A
toward generator scale (23)
reactance/susceptance grids
(24)
SWR circle (22)
unity constant
resistance/conductance circle
centre
0 + j0
(23)
SWR (21)
resistance/conductance grids (21,24)
Intersections with
the unity circle
toward load scale (23)
http://www.sss-mag.com/pdf/smithchart.pdf
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APPENDIX B
SWR circle (22)
d1 (25)
yL (24)
d
zL (24)
E (23)
y1 (26)
d2 – d1
(27)
d2 (26)
http://www.sss-mag.com/pdf/smithchart.pdf
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http://www.sss-mag.com/pdf/smithchart.pdf
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http://www.sss-mag.com/pdf/smithchart.pdf
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