Transmission Line Lab (4/2008)
In this lab you will be working with an open-line transmission line. Please refer to your
text to learn about the advantages and disadvantages of this type of line. The purpose is
two-fold: 1) determine the characteristic impedance of an open-line from physical
measurement and experimentally, and 2) to observe standing wave voltages in order to
calculate the standing-wave ratio (SWR) and predict the conditions for minimum SWR.
VHF Oscillator (HP 3200B)
Function Generator (HP 3312A)
Digital Oscilloscope (Tektronix TDS 210)
Open feed line w/150 MHz detector
Resistors – 220, 390, 680 Ohm ½ watt
Soldering station w/solder
3 – RG58 coax, ~3ft, w/BNC on both ends
Micrometer and metric rulers
1. Place the open feedline on the lab bench. This requires 2 persons to carry. Gather
the rest of your equipment and place the instruments on the lab bench shelf.
2. VHF Oscillator. Ensure the power is OFF. Set frequency to 150 MHz. The RF out
is at the rear. Connect a piece of coax with 2 BNCs from the RF out to the RF
transformer at one end of the feedline.
3. Function Generator. Ensure power is turned off. Set to 50 kHz, sine wave, NO
modulation, maximum output. Use another RG58 coax to connect the Vpp output
to the AM/B+ input of the VHF Oscillator.
4. Digital Oscilloscope. Use the remaining RG58 coax to connect Channel 1 to the
150 MHz Detector on the open feedline. We will be using only Ch1. May the
following changes/settings. Ch1 – DC coupling, 200mv. Horizontal – 1ms.
Display – dots. Acquire – Average, 16 averages. Return to viewing Ch1.
5. Check physical condition of feedline
and wooden container. Do not over
tighten the lines as it will bend the ends.
Place 2 meter stick beneath the parallel
wire so you can measure distances. Us
the micrometer to measure the diameter
of the feedline wires and the distance
between the centers of the wires. Refer
to this diagram.
d (measured) __________
Z0 (predicted) __________
6. Turn on all equipment.
7. Move the detector without pulling on the wires or having other objects close to
the wires. You should see the voltage on the oscilloscope gently increase/decrease
as you move the detector along the length of the wire.
8. At 150 MHz, what is one wavelength (in meters)? _________________
9. As you move the detector note the distance along the feedline between maximum
voltages. Record your results in this table. Voltage vs. distance (cm)
10cm 50cm 90cm 1.3m 1.7m
20cm 60cm 1m 1.4m 1.8m
30cm 70cm 1.1m 1.5m 1.9m
40cm 80cm 1.2m 1.6m 2m
10. Is there some correlation between the wavelength and the distance between
voltage peaks (what is it)?
11. Using the equation for SWR = Vmax/Vmin,
What is the SWR? ______
What is the ideal SWR?__________
Why is the measured SWR not the ideal?
12. Now solder a resistor at the far end of the feedline, across the two wires. Then
measure the SWR again for each resistor. Which has the best SWR? _____
You may try additional resistors to see if you can get close to SWR = 1.
13. How close is the resistor value that produces the best SWR to the characteristic
impedance you predicted in part 5(above)? Why would this be so?