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9 aS

Perfect Ground Aluminum Car Body

Real Ground Aluminum Car Body

9 aS

Perfect Ground Aluminum Car Body

3 deg "evation

3 deg "evation

V—^13 deg Elevation

V—^13 deg Elevation

Real Ground Steel Car Body

All Three Plots Superimposed

Real Ground Steel Car Body

All Three Plots Superimposed

3 deg ation

3 deg ation

All .Azimuth Plots at 13 Degree Elevation Angles

Fig 5—The effect of ground quality and car body material on radiation patterns.

Fig 4 shows both the elevation pattern with the antenna at the center of the isolated roof, 56 inches above the ground, and the pattern with the antenna on a roof-and-hood combination. Both of these show horizontal gains of only a few dBi, largely because the roof is an inefficient ground plane of only about A/2 radius. The horizontal azi-muthal plots are roughly circular, but they are not shown here. Instead, the azimuthal plots shown have elevation angles of 30° and pass through the maxima of the elevation lobes. The conical elevated azimuthal pattern with the antenna at the center of the roof-hood combination is decidedly not circular.

As expected from the previous results for the complete car, the azimuthal patterns (at elevations of 20° and 30°, respectively) with the antenna at the corner of the roof are not circular, regardless of the presence or absence of the hood.

For brevity, I won't describe details of further model versions, which added the trunk lid, sides, front and rear panels of the car body. I found that radiated energy from the body side panels significantly affects the radiation pattern.

How Does Ground Quality and Car-Body Material Affect the Patterns?

Fig 5 shows the effect a typical real ground (conductivity of0.005 Seimens and a relative dielectric constant of 13) an aluminum car body (conductivity of 4E-8 il-meter and relative permeability of 1.00), and a mild-steel car body (1.2E-7 -meter and relative permeability of 200). All plots in Fig 5 are for the antenna positioned at the left-front corner of the roof. The three elevation/azimuth pairs of patterns are, starting from the left: "Perfect" ground with an aluminum car body (one of the cases shown in Fig 2), "Real" ground with aluminum car body and "Real" ground with mild-steel car body. The final pair of patterns shown on the right are simply a superposition of the three pairs to allow ready comparison.

As expected, using "real" instead of "perfect" ground significantly affects the horizontal gain, but has very little effect on whether patterns are circular. This demonstration is the objective of this paper.

Since the combination of a "real" ground and the 56 inch height of the car roof elevates the main lobes to 13°, the azimuthal pattern for the "perfect" ground case was replotted for 13°. The resulting azimuthal plot shows that for 13° and higher elevation angles, the patterns are essentially the same regardless of the ground or the car body material.

Limitations

I know that the patterns shown by this model are simplistic. Both experience and mathematical analyses have shown that gaps around the doors, hood, trunk lid, and other nonconduc-tive joints in the car body can significantly affect the radiation patterns. However, these complex effects are unlikely to make the patterns circular. Accurate modeling of a three-dimensional object sufficiently large to sustain standing waves in all directions requires very detailed models. Fortunately, they were not necessary for the present purposes.

Conclusion

The height, size and shape of all conductive portions of a car body and the location of the antenna each contribute to the three-dimensional radiation pattern of a vertical antenna.

From one point of view, the location of the actual antenna on the car merely serves to determine the patterns and relative current densities of a complicated set of standing waves. To a large degree the electromagnetic radiation emanates from the entire structure— not just from the piece of metal we call the antenna.

Few mobile installations have a circular radiation pattern. Therefore, turning a car permits the driver to place the maximum lobe in a desired direction. This can significantly increase the field strength in a given direction, as the car body acts as a contoured ground plane. CO

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