Optimum inductive loading of short whip antennas

Abstract

Short whips (monopoles) are often used on vehicles when a quarter wavelength is too long. It is common knowledge that a loading coil located in series with the whip improves narrow band performance. This series inductive loading of monopole (dipole) antennas is analyzed by a piecewise sinusoidal moment method. When loaded to resonance, the current rises from the feed value to a peak slightly beyond the load point with decay to zero at the end. Radiation resistance improvement factors as large as 5 have been calculated. The resonant loading reactance is shown to vary inversely with the antenna length for most lengths. Efficiency varies slowly with load point and peaks at roughly 0.4 (of the whip length) from the feed in contrast to earlier approximate theory that predicted higher efficiency for loads farther out toward the end. The curves allow a tradeoff between slightly reduced efficiency and larger input resistance (to allow easier matching) to be made. For this case, a 2/3 loading point may be a good compromise. The length for which efficiency is 50 percent is a useful design guide. This length varies slowly with Q and h/a and is in the range 0.06 to 0.09λ. Thus shorter monopoles will have low efficiency. Extensive graphical data are presented.