Practical_Antenna_Handbook_0071639586

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 197
cles during the 1950s as the so-called Wonder Bar antenna for 10 m. It still finds use, but
its popularity has faded in the intervening period.
Cage Dipole
The cage dipole (Fig. 6.13) is similar in concept, if not construction, to both the bow-tie and
the three-wire dipole. (“If three wires are good, five or six must be better, right?”) Again,
the idea is to connect several parallel dipoles to the same transmission line in an effort to
increase the apparent cross-sectional area. Insulated spreaders to keep the wires separated
are typically made from plexiglass, lucite, or ceramic. They can also be made from
materials such as wood, if the wood is properly treated with varnish or polyurethane, or
from any other lightweight insulating material that has been rendered impervious to
moisture. The spreader disks are held in place with wire jumpers (see inset to Fig. 6.13)
that are soldered to—or tightly wrapped around—the main element wires.
Fan Dipole
A tactic used by some 80-m and 10-m amateurs is to parallel two or more dipoles cut for
different parts of the same band, fanning the individual wires on each side of the common
center insulator out with distance from the center. Unlike the broadbanding
schemes previously described, the wires at each end are not connected to each other.
This “stagger tuning” method sends most of the RF to the shorter dipole at the upper
end of the band and to the longer dipole at the lower end of the band. The overall result
is to somewhat flatten variations in VSWR across the entire band. Modeling this configuration
beforehand, to get a handle on the desired difference in lengths for optimum
coverage of the desired frequency, is highly recommended. If three or four separate
half-wavelength elements are employed, it should be possible to overlap even narrower
sections of the band in order to create an even flatter VSWR characteristic. Remember:
The ends of the wires in fan dipoles should not be tied together!
R
S
I
S
I
I
R
I Insulator
R Rope (etc.)
S Spreader
Jumper
Solder
6 to 12
inches
Spreader
detail
Figure 6.13 Cage dipole.