Practical_Antenna_Handbook_0071639586

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.