Figure 4-1B.Typical electromagnetic probe.
NEON-LAMP AND MILLIAMMETER METHODS
In this method of measurement, a neon bulb or milliammeter is moved along the two-wire parallel
transmission line. Points of maximum voltage (standing-wave voltage peaks) with the lamp or points of
maximum current (standing-wave current peaks) with the indicator will have maximum brilliance or
At what points along a transmission line will a neon lamp glow the brightest?
The bridge method permits measurement of the standing-wave ratio without actually measuring the
standing waves. The bridge method is applicable because the input impedance of a line terminated in its
characteristic impedance is a pure resistance equal to the characteristic impedance. A line terminated in
this way can be used as the unknown resistance in a bridge circuit and a null can be obtained in the
indicating device when the other resistance arms of the bridge are properly adjusted.
Many types of bridges can be used. For example, an ac bridge that is independent of the applied
frequency can be used. The bridge will become unbalanced when the line is no longer properly
terminated. Improper termination will produce a reactive component as well as a resistive component in
the input impedance of the line and result in a standing wave. The reading of the indicating device
depends on the degree of imbalance, which becomes more severe as the mismatch caused by the
termination becomes worse. The indicating device can be calibrated directly to indicate the standing-wave
ratio. The most common indicator consists of a crystal rectifier, a filtering circuit, and a sensitive dc meter
movement in series with a high resistance.
A resistance-capacitance bridge circuit is shown in view A of figure 4-2. The bridge is theoretically
independent of the applied frequency.