Practical_Antenna_Handbook_0071639586

C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 127
The phase of the reflected voltage can be deduced from the relationship between
load impedance and transmission line characteristic impedance.
For resistive loads (Z L = R L + j0):
• If the ratio Z L /Z 0 is 1.0, there is no reflection.
• If Z L /Z 0 is less than 1.0, the reflected signal is 180 degrees out of phase with the
incident voltage.
• If the ratio Z L /Z 0 is greater than 1.0, the reflected signal is in phase with the
incident voltage.
The step-function (or pulse) response of the transmission line leads to a powerful
means of analyzing both the line and the load on an oscilloscope. Figure 4.7A shows (in
schematic form) the test setup for time domain reflectometry (TDR) measurements. (As
discussed in Chap. 27, the vector network analyzer, or VNA, is capable of providing TDRlike
measurements at far lower cost to the hobbyist.) An oscilloscope and a pulse (or
square-wave) generator are connected in parallel across the input end of the transmission
line. Figure 4.7B shows a pulse test jig built by the author for testing lines at HF.
The small shielded box contains a square-wave oscillator circuit built with a few inexpensive
integrated circuits. Although a crystal oscillator can be used, an RC-timed circuit
running at 1 MHz or less is sufficient. In Fig. 4.7B, the test pulse generator box is
connected in parallel with the cable under test and the input of the oscilloscope. A
Oscilloscope
Z o
Z s =Z o
Z L
Z s
Pulse
generator
Figure 4.7A Time domain reflectometry setup.