Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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24.07.2018 Views

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 191 Figure 6.7 Continued. Sloping Dipole (“Sloper”) The sloping dipole (Fig. 6.8) is popular with those operators who need a low angle of radiation and have at least one tall support. This antenna, informally referred to as a sloper, is a half-wavelength dipole that is erected with one end at the top of a support and the other end close to the ground so as to make a 45-degree (more or less) angle with the support. Just like a horizontal dipole, it is usually fed in the center by coaxial cable or two-wire balanced transmission line. When fed with a coaxial (unbalanced) line, common practice is to connect the outer conductor or shield of the line to the lower half of the antenna, but there is little provable basis for doing so. In fact, the antenna is so unbalanced with respect to nearby ground that how the coaxial cable is connected is essentially immaterial. Some of the comments made regarding the inverted-vee antenna also apply to the sloping dipole, so please see that section also. Like the inverted-vee, a sloping dipole sacrifices the high-current portions of the antenna to lower heights, but unlike the Âinverted-vee, the vertically polarized contributions to the radiated field from the two halves of the antenna are in phase because the two halves of the antenna are not folded back toward each other. The vertical components of the E-field are, however, offset spatially, with the result that the antenna exhibits a small amount of gain in the direction of the lower end. That “forward” gain is accompanied by a modest reduction at the rear (i.e., in the direction of the higher end), so the antenna exhibits a small but noticeable front-to-back ratio, as shown in Fig. 6.9.

192 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s R I I Coax to transmitter I Insulator S Support stake R Rope I R S Figure 6.8 Sloping dipole (“sloper”). Figure 6.9 Sloping dipole radiation patterns.

192 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s<br />

R<br />

I<br />

I<br />

Coax to<br />

transmitter<br />

I Insulator<br />

S Support stake<br />

R Rope<br />

I<br />

R<br />

S<br />

Figure 6.8 Sloping dipole (“sloper”).<br />

Figure 6.9 Sloping dipole radiation patterns.

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