Q22. What is the radiation pattern of a quarter-wave antenna?
Q23. Describe the physical arrangement of a ground screen.
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 views A and B of
Figure 4-20.Folded-dipole antennas.
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.
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 view B.
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 more slowly as the frequency is varied from resonance.
Because of this the folded dipole can be used over a much wider frequency range than is possible with a