The spiral-pole antenna: An electrically small, resonant hybrid dipole with structural modification for inherent reactance cancellation

Khair, Ishrak

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The spiral-pole antenna: An electrically small, resonant hybrid dipole with structural modification for inherent reactance cancellation

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A small “spiralpole” antenna – the hybrid structure where one dipole wing is kept, but another wing is replaced by a coaxial single-arm spiral, is studied both theoretically and experimentally. Such a structure implies the implementation of an impedance-matching network (an inductor in series with a small dipole) directly as a part of the antenna body. The antenna impedance behavior thus resembles the impedance behavior of a small dipole in series with an extra inductance, which is that of the spiral. However, there are two improvements compared to the case when an equivalent small dipole is matched with an extra lumped inductor. First, the spiralpole antenna has a significantly larger radiation resistance – the radiation resistance increases by a factor of two or more. This is because the volume of the enclosing sphere is used more efficiently. Second, a potentially lower loss is expected since we only need a few turns of a greater radius. The radiation pattern of a small spiralpole antenna is that of a small dipole, so is the first (series) resonance. The Q-factor of the antenna has been verified against the standard curves. The antenna is convenient in construction and is appealing when used in conjunction with passive RFID tags such as SAW temperature sensors.

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