A knowledge of Smith chart operations is assumed, along with the companion understanding of transmission lines. Circular polarization came into vogue for FM broadcast during the seventies, and is now the standard. Particularly when end fed, modulation of the beam width and elevation occurs with FM, but is not normally a factor with antennas of six elements or less, due to the very wide vertical pattern. Corporate feeding eliminates this, but is much more complex due to the large number of interconnecting lines. Sometimes, due to conversions, repairs, or radome installation the impedance match deteriorates and the antenna needs to be re-tuned.
Individual elements are factory tuned close to resonance by size.
Center conductor diameters are customized to transform the impedance of the single element so that the parallel in-phase equivalent of the total number of elements is near 50 ohms. This is ERI's implementation of the slug tuner, functioning essentially the same as the common single stub tuner commonly explored in transmission lines 101. Steatite 410 is a ceramic commonly used for electrical and electronic insulating applications with favorable properties for the task. A capacitor is in the making. These steatite slugs are available from ERI and are a major profit center for them in terms of markup, but they do accept returns on unused slugs.