Unmanned aerial vehicles (UAVs) have become increasingly popular for scientific research, remote sensing, transportation of goods, search and rescue as well as military applications. UAVs have several key advantages over piloted aircrafts including low cost and the ability to penetrate unattainable areas that would be classified as unsafe. Technological advances and miniaturization allow communication devices to be placed on small UAVs. To improve aerodynamics it is thus necessary to design antennas conformal to host structure of UAVs. However, at VHF/UHF the UAVs often become electrically small, making it challenging to design wideband communication antennas.
In this research, the theory of characteristic modes (CMs) is used for the analysis and design of complex conformal antennas. Traditionally, CM theory is used as an analysis tool. However, research efforts in this dissertation are focused on expanding characteristic mode theory for the design of antennas. First, two systematic simplification procedures are developed which reduce the number of characteristic modes considered for complex antenna structures. This lays the foundation for simplifying the analysis of complex antenna structures allowing the designer to focus on a small subset of critical modes. Later, thorough analysis of the input admittance of CMs is conducted. It is shown that if a mode contributes to the conductance, it ultimately contributes to the radiated pattern. For higher order modes this is typically undesirable. Thus, it will be shown how to suppress the effects of higher order modes by proper feed placement. By suppressing a higher order mode’s admittance, the bandwidth of an antenna can be increased.
Using the simplification procedures, an investigation of electrically small square and rectangular ground planes is conducted. The analysis identified four important modes and allowed thorough analysis of the eigen properties of each mode. Results showed that antenna elements with electrically small ground planes should be fed like a dipole. Dipole excitation resulted in extended pattern bandwidth, relative to monopole excitation. Furthermore, the Qmin feed location was identified, by suppressing the first resonant higher order mode. Qmin feed location results in largest pattern bandwidth, for antennas considered. Rectangular ground plane effects were also investigated when the antenna element is offset on the ground plane. Design tradeoffs and practical limitations are discussed.
Finally, a 5-turn bifilar helix GPS antenna which fits inside the tail of the Dakota UAV covering L1-L5 GPS bands was designed and verified experimentally. The antenna uses a linearly varying pitch to enhance the bandwidth and was fabricated using a novel mesoplasma direct write technology. Furthermore, based on cumulative knowledge obtained from CM analysis of simplified electrically small structures, a wideband VHF/UHF antenna conformal to the vertical tail and fuselage of the Dakota UAV was designed. Utilization of the fuselage increases the occupied volume, lowering the Q. CM analysis shows that to extend the pattern bandwidth the antenna must be fed like a dipole.