6.3 Coil Loading

Top-loads are not very aerodynamic, and they are generally not self-supporting. Above 800 MHz it s easy to integrate them into the packaging, but below 30 MHz the smallest mobile platform that can use them is a ship. One solution that s been around for over 50 years is to put a coil partway up the monopole. The coil is adjusted to series resonate the portion of the antenna above it, which makes the current in the lower section nearly uniform. In effect, the coil and the upper antenna section are a top load for the lower antenna section. See Figure 6.15 for an illustration. This raises the radiation resistance and improves system efficiency over that of a simple monopole with a series coil at the base. The improvement is limited by the fact that the section of antenna above the coil has less capacitance than the unloaded antenna, so the coil inductance has to be larger than one for base tuning, and so the coil loss resistance is larger. The earliest analysis I know of was published by J. S. Belrose in 1953 [7] and was based on an assumed linear current distribution on each part of the antenna. A more elaborate analysis was published by C. W. Harrison [8] in 1963 based on general sinusoidal current models. As noted in Chapters 2 and 3, the plain monopole is not itself inefficient; its low radiation resistance combined with the fact that a tuning or matching coil will have...