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Microwave Bench Measurements Guide

This document describes how to measure an unknown load impedance using a slotted line microwave bench. There are two key measurements needed: (1) measuring the voltage standing wave ratio (VSWR) on the line, and (2) measuring the distance from the load to the first voltage minimum (lmin). These two measurements are sufficient to determine the unknown impedance ZL. The document provides an example of making such measurements on a microwave bench and calculating ZL using both analytical calculations and the Smith chart.

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348 views32 pages

Microwave Bench Measurements Guide

This document describes how to measure an unknown load impedance using a slotted line microwave bench. There are two key measurements needed: (1) measuring the voltage standing wave ratio (VSWR) on the line, and (2) measuring the distance from the load to the first voltage minimum (lmin). These two measurements are sufficient to determine the unknown impedance ZL. The document provides an example of making such measurements on a microwave bench and calculating ZL using both analytical calculations and the Smith chart.

Uploaded by

ghadei_debasish
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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MICROWAVE BENCH

MEASUREMNT
SU

PROF. A. BHATTACHARYA
DEPT. OF E & ECE
INDIAN INSTITUTE OF TECHNOLOGY
KHARAGPUR – 721 302
Slotted Line with Carriage
g
Components used in
Microwave Laboratory
Some more components
p
Microwave Bench
A Gunn based Bench
THE MICROWAVE BENCH
Waveforms in Bench
Detector
Rectangular
g Waveguide
g
Measurement of unknown
Load Impedance
(1)Measurement of VSWR on line
(2)Measurement of lmin (distance from
load to 1st voltage minima)

These two information sufficient for


determination of ZL
Magnitude
M it d
Voltage
g minima occurs when
where θ is phase of Reflection
Coefficient
So, θ= π + 2βlmin
Voltage minima repeats every λ/2 along line
S above
So, b relation
l ti holds f any lmin + n λ/2
h ld for
l=0
Bench Measurement
In a measurement lab,
a)) A short
h placed
l d at load
l d plane
l
b) From arbitrarily positioned scale on slotted line,
recorded voltage minima →
Z = 0.2 cm,, 2.2 cm,, 4.2 cm.
c) Short removed and unknown load put
d) SWR measured 1.5 15
e) Voltage minima at Z = 0.72 cm, 2.72 cm, 4.72 cm
f) Why not voltage maxima →
sharply defined
Solution
• Voltage minima repeats every λ/2
S λ = 2(2.2
So, 2(2 2 – 0.2)
0 2) = 4 cm.
• Reflection coefficient and input
p impedance
p also
repeats every λ/2.
• We can consider load terminals at minima of
step (b)
• Let load at 4.2 cm.
• Nearest minima under load at 2.72
2 72 cm.
cm
Lmin = 4.2 – 2.72 = 1.48 cm = 1.48/4 = 0.37 λ
How do you do the calculation by 
smith Chart?
• Step I: 
Locate the point where r=1.5 circle cuts real axis of 
p
Smith Chart (point A)

Why???
h ???

Left or Right point???
Voltage Maxima and minima in Smith Chart
g

1 + ρ (z)
• Variation of Line Impedance
Variation of Line Impedance Z ′( z ) =
1 − ρ (z)
If l li () h
•If we move along line, ρ(z) changes 
but ρ0 (load reflection coefficient) 
does not change
does not change 

If we move along Transmission Line, 
•If we move along Transmission Line
we cross real axis at two points.
Voltage Maxima and minima in Smith Chart 
(contd.)

•Line Impedance is real at these two 
Line Impedance is real at these two
points
•Reflection Coefficient is also real at
•Reflection Coefficient is also real at 
these two points
One value is +|ρ
•One value is +|ρ0|, other 
| other ‐|ρ
|ρ0|
1+ | ρ 0 |
•One corresponds to Impedance  ′
Rmax =
Maxima (Voltage Maxima, Current 
( g , 1− | ρ 0 |
Minima)
•Another to Impedance Minima  1− | ρ 0 |
′ =
Rmin
(Voltage Minima, Current Maxima) 1+ | ρ 0 |
•Note VSWR
How do you do the calculation by 
smith Chart?

Step 2:
p
Draw constant VSWR circle with radius OA
Step 3:
Step 3:
Where is the short at Smith Chart? Voltage minima 
exists there. Locate that as C
i h h
• Step 4
λ
– Load is 0.37      from the voltage minima
– Which way to go?
– Note: 
Note: j (θ − 2 βl )
e
– To move to generator, we go clockwise (why?) 
– So, to reach load, we should go anticlockwise
Step 5
Reach D on the Smith chart periphery
• Step 6: 
– Join DO, it intersects VSWR circle at E
,
Step 7:
Read impedances of E This is the unknown
Read impedances of E. This is the unknown 
impedance
z = 0.95 + j 0.4
'

z = 47.5 + j 20Ω
L

Close agreement with analytical calculations


Close agreement with analytical calculations

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