# HF Mobile Antenna Efficiency

July 14th Fact-of-the-DayThe 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. *©2005
Martek International All rights reserved.*

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