venting currents from flowing away from the base of the whip.

How Does Height Affect the Patterns?

The roof of a car is almost 3U % above ground. Fig3 shows how height affects radiation patterns by modeling a simple vertical whip antenna. First the whip location is directly on a "perfect" ground. The models for various heights above ground include a set of radial wires to provide a ground plane.

The lowest plot in Fig 3 shows the elevation pattern of a 19.1 inch long vertical antenna mounted directly on the ground. [Note: All the azimuth patterns are circular because the antenna and ground planes are symmetrical.]

The vertical antenna, augmented by the addition often radials and lengthened to 19.6 inches (to adjust the reactive impedance), is raised in eight-inch

( Ay 10) steps up to a height of 56 inches (0.7 X). That is, to the height of the roof of the car modeled in this paper. The use of X/4 radials over a "perfect" ground keeps the main lobe horizontal at all heights, while various higherangle lobes grow and change angle as the height changes.

Table 2 shows how the gain and input impedance depend on height

How Does the Shape of the Car Affect Patterns?

I made two additional copies of the car model. One eliminates all elements except those representing the vertical antenna and the flat roof. The second was similar except that it retained the antenna, roof, hood and the pair of windshield window posts that connect them. The first model shows how poorly the roof alone acts as a ground plane. The second shows that the hood has a significant effect.

2 Meter Vertical Ground-Plane Antenna Performance at Various Heights

Height (lnches)(k)

Gain Resistance Reactance

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