Practical_Antenna_Handbook_0071639586

404 p a r t V I : a n t e n n a s f o r O t h e r F r e q u e n c i e s
within the ATU itself may be less than the reduction in transmission line losses from
high SWR, and it may also represent the only way to get full performance and/or legal
power from the transceiver or amplifier feeding the line.
Shortened Verticals
The antennas in this section are all variations on the basic l/4 vertical of the preceding
paragraph. In general, each is a shortened monopole augmented by distributed and/or
lumped-constant circuit elements to keep the radiation efficiency as high as possible
and to minimize the VSWR on the transmission line back to the transmitter.
Again, these antennas will not work quite as well as a properly installed full-size
antenna, but they will serve to get you on the air with a very passable signal from
cramped quarters that would otherwise preclude operation on these frequencies.
Several different popular configurations are shown in Fig. 18.1. The basis for all
these antennas is a vertical that is physically short as compared to the “standard”: a
quarter-wavelength (l/4) (i.e., 90-degree electrical length) monopole with an ideal
ground plane beneath it.
Recall that a vertical monopole too short for its operating frequency (i.e., less than
l/4) will exhibit capacitive reactance. In order to resonate such an antenna, it is necessary
to cancel the capacitive reactance with an equal amount of inductive reactance,
such that |X L | = |X C |. By placing an inductance in series with the radiating element,
therefore, we can effectively “lengthen” it electrically, as measured at the feedpoint of
the combined antenna element and series inductor.
An antenna that is reactance-compensated is said to be loaded; in the preceding example,
the antenna is inductively loaded. Three generic forms of loading are popular:
discrete loading, continuous loading, and linear loading.
Discrete loading means that there is a discrete, or lumped, component (inductance or
capacitance) connected to the antenna radiator. The simplest form of inductive loading
inserts a loading coil near the center (Fig. 18.1A) or at the base (Fig. 18.1B) of the radiator
element. Because there is very little current near the open circuit end of a vertical,
inductive loading at the top does little; instead, a capacitive hat (Fig. 18.1C)—often in
conjunction with a center loading coil—is much more effective at increasing the area
under the current-versus-length curve.
You may recognize these configurations as being the same as those found on mobile
antennas. Indeed, low-band mobile antennas can be used in both mobile and fixed installations.
Note, however, that although it is convenient to use mobile antennas for
fixed locations (because they are easily available in “store-bought” form), they are far
less efficient than other, longer versions of the same concept. The reason is that the mobile
antenna for the lower HF bands has historically been based on the standard 96- to
102-in whip antenna used by amateur operators on 10 m or by citizens band operators
on 11 m, with the overall height of the antenna limited by safety and mounting considerations.
In fixed locations, on the other hand, longer radiator elements (which are more
efficient) are usually possible. For example, a 16- to 30-ft-high aluminum radiator element
can easily be constructed of readily obtainable materials.
Loaded antennas tend to be rather high Q devices, and their bandwidth is quite
narrow. An antenna tuned for the center of a band may present a high VSWR at the ends
of the band—especially on 160 m, where the U.S. amateur band extends ±5 percent
from the center frequency. One way around this problem is to make the inductor variable,
so that slightly different inductance values can be selected for different band seg-