Practical_Antenna_Handbook_0071639586

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Figure 10.11 Bobtail curtain.
example of a 1:2:1 binomial array, as discussed in Chap. 5.) Since the currents feeding
the two end elements are traveling in opposite directions along the top, and since the
current to either end element goes through a reversal at the middle of the horizontal
half-section, the horizontal radiation is partially canceled out.
Because all three elements are in phase, the principal lobe is broadside to the plane of
the three radiating elements. For maximum signal north and south, for instance, the
array should be suspended between supports on an east-west line.
The radiating elements are l/4 monopoles and normally would require a low-Â
resistance ground path for their return currents. However, if each vertical element is
visualized as one side of a l/2 dipole and half the horizontal distance along the phasing
line to the adjacent element is seen as the other side of the dipole (or as an elevated
groundplane), the vertical elements themselves have no particular need for a good
ground system. However, unless there is an excellent RF ground at the base of the middle
element for the matching network to connect to, there is no low-loss return path for
RF currents flowing to/from the ground end of the matching network, and excessive
transmitter power will be wasted in some vague but very lossy return path. Thus, the
matching network at the base of the middle element may require a good system of radials.
Since radials lying on the surface of the earth are not resonant, their exact length is
not particularly critical. It might be helpful, depending on the quality of your ground,
to put radial fields under the end elements, as well.
The bobtail curtain has an extremely low angle of radiation. At one time the author
had an 80-m bobtail curtain aimed north and south at his home in the northeastern
United States; it was a “killer” antenna for very long haul contacts over the north
pole into the far reaches of Asiatic Russia and the western Pacific region—especially
ÂIndonesia—but it was totally useless for anything much closer.
Half-Square
If one end element of the bobtail curtain of Fig. 10.11 and its associated phasing line are
removed, the resulting antenna is known as a half-square. Now we have two vertical
radiators joined by a l/2 horizontal phasing line. As with the original bobtail curtain,
the antenna is fed at the bottom of one element—a high impedance point. (Keep in
mind that both the fed end and the far end are high-voltage points.) We can consider
that element and half the horizontal wire as constituting an L-shaped l/2 dipole feeding
a second l/2 dipole. But since the current in consecutive l/2 sections reverses, the