MM0474
LESSON 1.
INTRODUCTION TO POWER MEASUREMENT
AIPD SUBCOURSE NUMBER MM0474................. Power Measurements
LESSON OBJECTIVE ............................. After studying this lesson you
will be able to explain the
purpose, principles of operation
and types of units used for power
measurements.
CREDIT HOURS ................................. One
TEXT
1.
GENERAL.
a. When electronic power flows in a transmission line at low frequencies,
there is definite current flowing in each line conductor, and a definite
voltage across any two terminals of the line.
Because of this, power is
usually measured in terms of voltage, current and power factor.
Impedance
magnitude is defined as the ratio of voltage to current. Also, meters exist
which measure power and impedance directly. These instruments read the product
of voltage, current and power factor, or the ratio of voltage to current.
b. At frequencies of 400 MHz or higher, it is difficult to describe
circuit performance with the conventional concepts of current and voltage. For
example, the voltage supplied to a rectangular waveguide by a UHF oscillator,
or the current drawn by a cavity, varies depending on the terminal plane at
which these quantities are measured. Furthermore, there are no electrodynamic
instruments available for measurements at microwave frequencies, so that
crystal and thermal detectors are generally employed in specially calibrated
circuits to permit the measurement of voltage, current and impedance. However,
absolute power may readily be measured at microwave frequencies by means of
various types of thermally sensitive elements. These elements and methods for
using them are the subject of this subcourse.
2.
THE NATURE OF MICROWAVE POWER AND POWER MEASUREMENTS.
a. Micro Power.
(1) Power measurements at microwave frequencies are subdivided into
several categories usually depending on power level, waveform, and frequency.
These subdivisions are arbitrary and based to a large extent upon the type of
measuring equipment available. Power is divided into three levels, low, medium
and high, covering ranges from 0 to 10 mW, 10 mW to 1 Watt and above 1 Watt.
Microwave power meters usually handle one power range. Low-power measurements
are made with devices
1
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