Practical_Antenna_Handbook_0071639586
432 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s 38 in 19 in 19 in 38 in B 19.5 in B A 19.5 in C C 1 in. VHF balun 4:1 1 in. Figure 19.5 Vertical collinear antenna. A Coax to XMTR in an omnidirectional pattern. Each array consists of a quarter-wavelength section A and a half-wavelength section C separated by a phase reversing stub B. (The total length of the stub— out on one side and back on the other—is a half-wavelength, but if made from open-wire line, only a l/4 line is required. The phase reversal stub adds a 180-degree phase shift between the inner l/4 section and the outer l/2 section. Because either outboard half-wavelength section is l/2 or 180 degrees above or below the center half-wave section, the in-phase currents in the sections cancel in the vertical direction. In any horizontal direction, however, the current in each outer section is in phase with that of its respective inner section, and the antenna exhibits about 3 dB gain at low elevation angles over a conventional l/2 vertical dipole or l/4 ground-plane vertical. Note that the phase reversal stub, even if made out of open-wire line, is not being operated as a transmission line and the usual length correction for velocity factor is not applied. As is the case with an ordinary dipole, the feedpoint is at the midpoint of the array (i.e., between the A sections). Unlike an ordinary dipole, the freespace feedpoint impedance is around 280 Ω resistive at 146 MHz—a reasonably good match to a 4:1 balun transformer (see Fig. 19.6), especially if fed with 72-Ω coaxial cable. Alternatively, 300-Ω twin-lead can be connected directly to the two sides of the dipole center. If the transmitter lacks the balanced output needed to feed twin-lead, use an appropriate balun at the input end of the twin-lead (i.e., right at the transmitter). For proper performance, make sure the feedline is kept at right angles to the dipole axis near the dipole.
C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 433 A 4Z 0 L 5904 V Inches F MHz Z o B Figure 19.6 Coaxial 4:1 balun transformer. Grounds for Ground-Plane Verticals With the exception of the sleeve antenna and the collinear dipole, all the preceding verticals are monopoles—in effect, one side of a dipole. All will be poor performers if a good return path simulating the other half of the dipole is not provided. When the base of these antennas is elevated more than a small fraction of a wavelength above ground, an artificial ground plane is an absolute necessity. The simplest way to provide this is with
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C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 433<br />
A<br />
4Z 0<br />
L 5904 V Inches<br />
F MHz<br />
Z o<br />
B<br />
Figure 19.6 Coaxial 4:1 balun transformer.<br />
Grounds for Ground-Plane Verticals<br />
With the exception of the sleeve antenna and the collinear dipole, all the preceding verticals<br />
are monopoles—in effect, one side of a dipole. All will be poor performers if a good<br />
return path simulating the other half of the dipole is not provided. When the base of<br />
these antennas is elevated more than a small fraction of a wavelength above ground, an<br />
artificial ground plane is an absolute necessity. The simplest way to provide this is with
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