Practical_Antenna_Handbook_0071639586

C h a p t e r 2 6 : T h e S m i t h C h a r t 585
input impedance of the line. The length of the line is 0.60l, so we must step back (0.60
– 0.50) l or 0.1l as marked off on the “WAVELENGTHS TOWARD GENERATOR”
outer circle. A radius is drawn from this point to the 1.0, 0.0 center point of the circular
chart. The new radius intersects the smaller VSWR circle at 0.76 + j0.4, which is the
normalized input impedance (Z′ IN ) corresponding to an actual input impedance of 38 +
j20 W.
This example exaggerates the loss per unit length to clearly show the inward spiral
of a lossy transmission line’s VSWR curve. In fact, if a lossy line is long enough, the
VSWR will appear to be 1.0:1 regardless of the nature of the load attached to it!
Frequency Response Plots
A complex passive network (such as a filter or an antenna) may contain resistors, inductors,
and capacitors—either lumped or distributed. Both the resistive and the reactive
components of the input impedance of such a network are generally functions of frequency;
we say the network response is frequency-Âsensitive. For antennas, in particular,
the Smith chart becomes a valuable tool for plotting (and recording for future reference)
Z IN versus frequency.
Figure 26.11 shows Z IN for four different antennas currently in use by the author.
The curves were obtained with a VNA. The exact values on these curves are unimportant,
but the general shapes of the curves, which span varying percentage bandwidths,
are typical of what various practical monoband antennas exhibit for Z IN . The same
curves can be created for your antennas with even simpler equipment. All that is necessary
is an antenna analyzer that provides a readout of both the resistive and the reactive
parts of Z IN at each frequency, as well as the sign of the reactance.