Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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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 189 220 L (feet) = F(MHz) (6.8) After the length L is calculated, the actual length is found from the same cut-and-try method used to tune the dipole in the previous section. Bending the elements downward also changes the feedpoint impedance of the antenna and narrows its bandwidth. Thus, some adjustment in these departments is in order. You might want to use an impedance-matching scheme at the feedpoint or an antenna tuner at the transmitter. As we shall note numerous times in this book, there is no “free lunch”. The inverted-vee is definitely a compromise antenna. In effect, we gain some mechanical advantages (previously described) while incurring the following electromagnetic disadvantages: • Part of the radiation from each leg of the inverted-vee is horizontally polarized and part is vertically polarized. For a = 90 degrees, these two components are roughly equal; simple trigonometry then tells us each is about 70 percent of the magnitude of the original horizontally polarized E-field from a horizontal dipole. • Remembering that the radiation field far from the antenna is the sum of the radiation from all the many very small segments of wire making up the antenna, we note that the average height of the high-current portions of the antenna is less than that of a comparable dipole whose entire length is at the same height as the inverted-vee’s center. Thus, the horizontally polarized radiation from the inverted-vee tends to favor higher takeoff angles, compared to a horizontal dipole at the same height as the center of the inverted-vee. “Aha!” you say. “Since some of the radiation is now vertically polarized, doesn’t that mean that there will be an increased amount of low takeoff angle radiation for working distant stations?” The answer is,“Not where you’re expecting it!”, as explained here: • Just as the currents on opposite sides of a balanced two-wire transmission line feeding a dipole are 180 degrees out of phase with each other (thus minimizing radiation from the feedline at distant points by cancellation of the fields from the two sides), the vertical component of the inverted-vee’s radiation on one side of the center insulator is out of phase with the vertical component on the other side. Since the average spacing of the high-current portions of the two sides of the inverted-vee is much wider than that of a parallel-wire transmission line—perhaps l/8—there is incomplete cancellation and some vertically oriented energy is, in fact, radiated. However, the direction of maximum radiation is not broadside to the wire, as it is for the horizontal component. Rather, the vertically polarized pattern is typically maximum along the wire axis, at right angles to the optimum direction of horizontal radiation, and over average ground about 4 dB lower in amplitude at a 20 degree elevation angle than a simple l/4 vertical would be.

190 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 Figure 6.7 shows the modeled patterns for the horizontally and vertically polarized E-field components of an 80-m inverted-vee with a 90-degree included angle and a center support height of 50 ft over average ground. Note the 90-degree difference in the compass headings of the horizontal and vertical E-field lobes. Figure 6.7 Inverted-vee radiation patterns.

190 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 />

Figure 6.7 shows the modeled patterns for the horizontally and vertically polarized<br />

E-field components of an 80-m inverted-vee with a 90-degree included angle and a<br />

center support height of 50 ft over average ground. Note the 90-degree difference in the<br />

compass headings of the horizontal and vertical E-field lobes.<br />

Figure 6.7 Inverted-vee radiation patterns.

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