TC 9-64 _________________________________________________________________________
usually elevated above the earth. No specific dimensions are necessary in the
construction of a counterpoise nor is the number of wires particularly critical.
A practical counterpoise may be assembled from a large screen of chicken
wire or some similar material. This screen may be placed on the ground, but
better results are obtained if it is placed a few feet above the ground.
4-89. The use of parasitic elements and various stacking arrangements
causes a reduction in the radiation resistance of a center-fed, half-wave
antenna. Under these conditions, obtaining a proper impedance match
between the radiator and the transmission line is often difficult. A convenient
method of overcoming these difficulties is to use a folded dipole in place of the
center-fed radiator. (See figure 4-21, views A and B).
Figure 4-21. Folded-Dipole Antennas
4-90. A folded dipole is an ordinary half-wave antenna that has one or more
additional conductors connected across its ends. Additional conductors are
mounted parallel to the dipole elements at a distance equal to a very small
fraction of a wavelength. Spacing of several inches is common.
4-91. The feed-point impedance can be further increased by using three or
four properly spaced parallel conductors. Standard feed-line spreaders are
used to maintain this spacing when required. In any folded dipole, the
increase of impedance is the square of the number of conductors used in the
radiator. Thus, a three-wire dipole has nine times (32) the feed-point
impedance of a simple center-fed dipole. A second method of stepping up the
impedance of a folded dipole is to use two conductors with different radii, as
shown in figure 4-21, view B.
4-92. The directional characteristics of a folded dipole are the same as those
of a simple dipole. However, the reactance of a folded dipole varies much
4-28
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