Miniature VHF/UHF conformal spirals with inductive and ferrite loading

There is significant interest in developing small ultra wide band (UWB) antennas that operate from 20 MHz up to several GHz. Often, numerous antennas are used to cover such a large frequency range leading to issues such as space limitations, payload and electromagnetic interference (EMI). Moreover, antennas used at lower frequencies tend to be large and protruding making them unsuited for many applications. Size reduction can be often achieved at the expense of gain. However, as size is reduced, it is important to ensure that optimal gain is achieved for a given size. Toward this goal, theoretical Chu antenna size limits were developed subject to a pre-specified gain. For example, a min. aperture size of Dcong24" is needed to operate down to 30 MHz with -15 dBi gain (f_MHz=18/D_meters). Also, a min. size of D=35" is needed to operate down to 30 MHz with - 10 dBi gain (f_MHz=24/D_meters). These are optimal miniaturization limits and cannot be exceeded based on the prevailing Chu limit theory. Nevertheless, it is challenging to achieve them, and over the years, we have made consistent progress in antenna miniaturization of wideband antennas. Such miniaturization are even more challenging when conformal (as part of the platform\´s skin) design are required. This paper reviews spiral miniaturization using a variety of loading (inductive, ferrite and combinations).