Download ELECTROMAGNETICS EXERCISE

Applied Electromagnetics
Name:
WLS 501
Winter 2011
Student Number:
INTRODUCTION
When a voltage reading is taken in an RF circuit, that reading refers to the voltage at
that particular point on the line. What many people do not know is that the reading is
actually the sum of two separate voltages: the forward-traveling voltage and the
reverse-traveling voltage. Since RF signals are really electromagnetic waves, the
forward and reflected (reverse) waves interact with one another and the effects are
shown in the reading.
Since voltage and power are closely related, the same is true for power: A standard RF
power meter will give you a combined (forward + reverse) reading. The Directional
Wattmeter, however, allows power in only one direction to be measured. We will be
taking some of these measurements in order to gain insight into what is happening to
signals on RF transmission lines.
EQUIPMENT
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2-way radio
12V power supply
coaxial cables
RF adapters
Isolator
Two 50-Ohm load terminations
Directional Wattmeter
PROCEDURE
1. Connect the 2-way radio to the power supply. Make sure that it has been set to
the correct voltage!
2. Connect the isolator to the RF output (antenna) port of the radio. This device is
to protect the radio against reflected power.
3. Connect the directional Wattmeter to the isolator.
4. What we have is an open circuit at the end of the transmission line. When an RF
signal reaches an open circuit, 100% of the power is reflected back toward the
generator. This is also the case for a short circuit.
Applied Electromagnetics
Name:
WLS 501
Winter 2011
Student Number:
5. Take measurements of both forward power and reverse power for all of the loads
indicated in the table below, and enter them into the table.
LOAD
Pforward
Preverse
VSWR
25
50
SHORT
OPEN
6. Which of the loads is reflecting the least power?
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7. Which of the loads is reflecting the most power?
8. The voltage standing wave ratio on a transmission line is a measure of
reflections on the line. Reflected waves (in the reverse direction) interact with
forward-traveling waves, producing standing waves. A VSWR of 1:1 means that
there are no reflections, while a VSWR of 
ALL of the power is
. Use the formula below to determine the VSWR associated with
each of the loads under test, and enter the values into the table on the previous
page.
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Preflected
1 
Pforward
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VSWR  
1  Preflected
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Pforward
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