Practical_Antenna_Handbook_0071639586

368 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
wave grounded vertical monopole antenna on that band. The tuner will then accommodate
frequencies above and below 20 m. Remember: A monopole will not work well at all
without an appropriate ground or system of radials beneath it. Radials should be relatively
easy to hide, although you may have to wait until midnight to lay them in the
thatch of the lawn surrounding your flagpole! (If caught in the act, you can always claim
that the radials are part of the lightning discharge system associated with your flagpole.)
Another alternative is the version shown in Fig. 15.3B. In this case the wire radiator
is replaced with a section of aluminum tubing. A wooden or plastic insert is fashioned
with a drill and file to support the aluminum tubing inside the PVC tubing. One way to
make the support is to use a core bit in an electric drill to cut out a disk that fits snugly
inside the PVC tubing. Rat out the center hole left by the core bit pilot to the outside
diameter of the aluminum tubing. The support can be held in place with screws from
the outside or simply glued in place. One advantage to using tubing instead of a wire is
a modest increase in the bandwidth of the vertical.
Coming up with the right concept for operating with a hidden antenna can pose a
serious challenge. But with some of these guidelines and a little creativity, you should
be able to get on the air and enjoy your hobby.
Limited-Space Antennas
Many people live in situations where it is permissible to install an outdoor antenna, but
it is not practical to install a full-size antenna. Here we examine some of the options
open to those with limited space for amateur radio, citizens band (CB), or shortwave
listener (SWL) antennas.
Once again we return to the simple dipole as the basis for our discussion. Figure
15.4 sketches several alternatives for installing an outdoor antenna in a limited space. In
Fig. 15.4A, the slanted dipole antenna uses the standard dipole configuration, but one
end is connected to the high point of the building, while the other is anchored near the
ground. The coaxial cable is connected to the midpoint of the antenna in the usual manner
for regular dipoles. If the end of the dipole is within reach of people on the ground,
someone may get a nasty RF burn if the antenna is touched while you are operating.
Take precautions to keep people and pets away from any part of your antenna.
Another method is the vee dipole shown in Fig. 15.4B. In a regular dipole installation,
the ends of the antenna are along the same axis. (In other words, they form an
angle of 180 degrees.) In the example of Fig. 15.4B, however, the angle between the elements
is less than 180 degrees but greater than 90 degrees. In some cases we might need
to bend the elements partway along their length, rather than installing them in a vee
shape. Figure 15.4C shows an angled or bent dipole with four segments. In all three examples,
Figs. 15.4A through 15.4C, you can expect to find that the length needed for
resonance will vary somewhat from the standard 468/F(MHz) value, and the feedpoint
impedance is very likely to be something other than 73 Ω. Also, the pattern will be
somewhat distorted by any house wiring, aluminum siding, or other nearby conducting
materials with at least one dimension roughly comparable to l/4 or more. Nonetheless,
although these antennas may not work quite as well or as predictably as a high
dipole clear of nearby objects, the performance can be surprisingly good.
Another limited-space wire antenna is the (so-called) half-sloper shown in Fig.
15.4D. Although single-band versions are often seen, the example shown is a multiband
version. Resonant traps result in relatively low SWR across multiple frequency ranges,