HF Mobile Antenna Efficiency
The power radiated by any antenna is equal to the current flowing through its radiation resistance squared, times its radiation resistance. Therefore, increasing the current at a given radiation resistance or the radiation resistance at a given current increases radiated power. All antennas with a size in the longest dimension that is a small fraction of a wavelength have low radiation resistance. Practical considerations generally limit automobile antenna sizes to small fractions of a wavelength throughout the HF spectrum, but especially in the lower frequency portion, so HF mobile antennas generally have low radiation resistance. To maximize mobile antenna and radiation efficiency: 1) Maximize the radiation resistance, which generally means, make the antenna as large as possible. 2) Minimize all series loss resistances to maximize current, which generally means use a large antenna conductor size, a high-Q loading coil, and the lowest-resistance ground connection to the automobile possible. 3) Minimize shunt currents which bypass the radiation resistance by minimizing shunt capacitances and leakage currents through mechanical support insulators.