__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.