Practical_Antenna_Handbook_0071639586

636 p a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s
Folded Dipole Construction
As discussed in Chap. 6, the folded dipole (Fig. 28.9) consists of two close-Âspaced conductors
shorted together at both ends, but only one of the conductors is split in the
center for the feedpoint. The free-Âspace feedpoint impedance at resonance is around 290
W, so it makes a good match to 300-ÂW, television antenna twin-Âlead. (Often the antenna
itself is cut from the same roll of twin-Âlead!) Alternatively (as shown in Fig. 28.9), a 4:1
balun transformer at the feedpoint provides a match to 75-ÂW, coaxial cable. Despite the
fact that 300-ÂW, TV twin-Âlead is disappearing from the market, the folded dipole maintains
a following among SWLs and hams. A preassembled folded dipole and feedline
rolls up into a small storage space, weighs very little, and can be rapidly deployed in an
emergency or portable scenario.
Neither the conductors nor the dielectric of ordinary TV twin-Âlead are particularly
strong, so some ingenuity is required to keep the terribly weak 300-ÂW, twin-Âlead from
breaking at the slightest provocation. Figure 28.10 shows a solution developed by one
fellow (admittedly a fine worker in plastics and other materials); he fashioned a center
insulator and two end insulators from a piece of strong Lucite material.
The 300-ÂW TV twin-Âlead is prepared as shown in Fig. 28.10A. One conductor of the
twin-Âlead is cut at the planned feedpoint, and the insulated wire freed from the center
dielectric material a distance of about 0.5 to 0.75 cm in both directions, using a match
and/or small diagonal (side) cutters. Use the same tools to strip the insulation from
the two wire ends just created. A hand punch—either a large-Âsize leather punch or a
paper punch—places two or three holes in the center insulation on either side of the
break.
The center insulator is shown in Fig. 28.10B. It is made from a piece of strong plastic,
Lucite, or other insulating material. Two identical sections, front and back, are
needed. A number of 5-Âmm holes are drilled into both pieces at the points shown to
clamp the twin-Âlead. A pair of solder lugs provides a mechanical tie-Âpoint for connections
between the antenna element and the transmission line. The nuts and small bolts
that fasten the two halves of the insulator are nylon to eliminate any possibility of
short-Âcircuiting the two terminals or the two sides of the twin-Âlead that form the antenna.
A side view is shown in Fig. 28.10C. Note that the twin-Âlead causes a gap that can
catch water and makes it possible to break one or both insulators by overtightening the
A
B
B
4:1
BALUN
75-OHM
COAX
Figure 28.9 Folded dipole.