Efficiency and Matching Tradeoffs for Inductively Loaded Short Antennas

Abstract

Efficiency and input resistance of short monopoles (or dipoles) may be significantly improved through use of a loading coil in series with the monopole wire. The piecewise sinusoidal moment method is used to analyze the loaded monopole. 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 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 0.09 λ. Thus shorter monopoles will have low efficiency. Bandwidth is improved due to loading coil losses at the expense of efficiency; lossless loading bandwidth is unchanged over the unloaded monopole as the radiation resistance and reactance slope increase commensurately. Extensive graphical data are presented.