EECS 420 – Electromagnetics II Lab
Lab 5 – Standing Wave Ratio and Impedance Measurements
1. Objectives: At the completion of the lab, you should have each done the following:
a. Used a network analyzer and recorded the VSWR on a slotted line for several
different loads.
b. Determine the value of a load based on its VSWR.
2. Background: We will be using a slotted line to measure the voltage on a
transmission line at different z-coordinates. A slotted line is a section of waveguide
or transmission line with a slot cut in the side to allow insertion of a detecting probe.
This probe detects the magnitude of the electric field that occurs as the wave
propagates through the guide. Because the probe detects the electric field, it is
referred to as a voltage probe.
In this lab, we set the network analyzer to continuous frequency mode at 3 [GHz]. In
this mode, the network analyzer outputs a sinusoid (single frequency) just as a
function generator would.
This output is fed to the slotted line device.
The
impedance of an unknown load can be determined by measuring the voltage minima
when the line is terminated with the load and with a short circuit. The parameters
required to make this calculation are, the SWR of the unknown load, the position of a
voltage minimum with the unknown load termination, and the position of a voltage
minimum with a short termination. The distance between two successive minima is a
half-wavelength. An important point to remember in this calculation is whether the
minima of an unknown load occur on the load or generator side of the minima of a
short termination. When moving toward the generator we are decreasing the angle of
the reflection coefficient, when moving toward the load, we are increasing the angle
of the reflection coefficient.
The solution of using the Smith chart can be verified by an analytical technique. The
mathematical expression is:
ZL 
1  j SWR  tan  X 
Z
SWR   j tan  X  0
or
ZL  Z 0
1  L
1  L
where
ZL = the impedance of the unknown load
Z0 = the characteristic impedance (assume 50 [)
ΓL = the load reflection coefficient (complex)
SWR = the Standing Wave Ratio of the unknown load
d
2

d = the shift in minima position when the short is applied. It is positive when
the minima shift toward the load and negative when the minima shift toward the
generator. You can use either the positive or negative distance to find your
answer.
X 
The magnitude of the reflection coefficient in terms of the VSWR is:
L 
VSWR  1
VSWR  1
The angle of the reflection coefficient in terms of the minima positions is:
 d 1 

  4 
 L  4 
3. Equipment: You will need the following pieces of equipment:
a. Agilent network analyzer
b. Two 2 [ft] transmission line cables
c. Short, 50 [] matched load, two different loads that are unmatched.
4. Procedure:
Network
Analyzer
PORT 1
PORT 2
Cable
Slotted Line
Cable
Load
Figure 6-1: Test setup for this lab
1) Turn on the network analyzer.
2) Preset the network analyzer.
3) Press [SWEEP SETUP]. Change [SWEEP TYPE] to [POWER SWEEP].
4) Press [SWEEP SETUP] [POWER]. Set [CW FREQ] to 3 GHz.
5) Perform a full two-port calibration.
6) Attach a 2 [ft] transmission line from port one of the network analyzer to the input
of slotted line. Use adapters as necessary.
7) Attach another 2 [ft] transmission line from port two of the network analyzer to
the slotted line’s probe output (the vertical output that is on the top of the device).
Use adapters as necessary.
8) Place a short on the load side of the slotted line (right side). Use adapters as
necessary.
9) Press [MEAS] [S21].
10) Move the carriage along the line and record the position and values of the voltage
maxima and voltage minima. You should see a flat line moving up and down as
you move the carriage along the line. Use markers to record S21 values. Save the
measurement plot.
11) Repeat steps 8-10 for a 50 [] load and the two unknown loads.
12) Determine the VWSR for the short, 50 [] load, and the two unknown loads.
13) Determine the reflection coefficient (ΓL) and load impedance (ZL) for the 50 []
matched load, and for the given loads from the VSWR and voltage minima
locations.
 Measure the impedance values of the given loads at 3GHz by using the NA.
15) Calculate what the VSWR and reflection coefficient should have been. How well
did the measured VSWR, reflection coefficient, and impedance compare with
what you expected from the nominal value of each load?
16) Was the VSWR of the matched load what you expected? Explain what
might cause the 50 [to vary so much.