A novel structure for improved directivity

The authors report on their experience with a novel structure which may be used with existing vertical elements, not only to increase the low-angle ground wave coverage, but also to reduce significantly the high-angle sky wave radiated by traditional towers. Such high-angle radiation results in two undesirable effects: skywave interference with adjacent and cochannel stations, and the creation of a fading zone surrounding the transmitter. This structure, a toroidal helix, can be used in a variety of applications to tailor the radiated pattern of wire antennas of any particular polarization state. The structure is formed by taking a contrawound, normal-mode helix and forming it into a self-resonant, closed, multiply connected region (torus, for example) of arbitrary cross section. The resulting low profile structure can be viewed as a leaky resonator. It not only combines the tuning and matching elements with the radiating element itself, for increased efficiency, but also provides the designer with the flexibility of polarization state and pattern shape control.<>