Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds

Samson, Timothy W.

Graduate Thesis Or Dissertation

Characterization of Alternative Mounting of Vehicular Antenna Systems Near Real Grounds Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/bg257f402

Abstract

Vehicular antenna systems for jamming and communications are frequently mounted on either the roof or rear bumper of the vehicle. While this facilitates easier mounting and maintenance of the antenna systems, it reduces the maneuverability and increases the visual profile of the vehicle. This thesis researches alternative mounting locations such as underneath the vehicle or on the sides of the vehicle. For either mounting location, the presence of the ground electrically close to the antenna system may drastically modify its performance and characteristics. Therefore, it is vital to understand how the presence of the ground affects the antenna's patterns, impedance, efficiency, and axial ratio when the antennas are designed to be circularly polarized. Once the effects of the grounds on the antenna systems are understood, steps can be taken to mitigate potential adverse effects and capitalize on any benefits. To better understand propagation close to the ground, both with and without the vehicle model, ideal sources were initially studied. More conductive soils offer higher efficiencies, but cause a greater amount of depolarization for circularly polarized sources. For sources under the vehicle, the source is not just having a stronger coupling with the ground, but the vehicle body redirects energy back towards the ground. To mitigate this, the parallel plate source is proposed for HF and VHF frequencies. Small modifications to the geometry can achieve significantly higher efficiencies for sources under the vehicle. Other antennas are considered for operation in the UHF region. For antenna systems mounted on the side of the vehicle, two mounting approaches (for spiral antennas) are considered. First, a cavity-backed aperture is mounted flush with the vehicle's body. The second configuration has the spiral antenna aperture offset from the vehicle thus requiring minimum changes to the vehicle's body. While the first approach is more commonly used, the latter provides greater gain and efficiency at the lower end and comparable gain and efficiency at higher frequencies while maintaining good axial ratio. For side mounted sources, the four arm spiral provides a lower and more stable axial ratio when compared to the two arm spiral. Both antennas offer similar efficiency, but the four arm spiral maintains higher quality performance with the introduction of a cavity backing. The performance of side mounted sources can be further improved with the use of the spiral-helix that improves both the low and high frequency performance.

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