Radiation Resistance Enhancement Techniques for Ultracompact Half-Wavelength Helical Omnidirectional Circularly Polarized Antennas

This paper demonstrates the techniques that effectively enhance the radiation resistance of a conventional λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> /2 omnidirectional circularly polarized (OCP) normal mode helical antenna. The original single-arm OCP antenna, although has a very compact size, only exhibits an extremely low radiation resistance that makes it impossible for practical applications. To sufficiently enhance the radiation resistance while maintaining a compact size, innovative techniques as folding the radiator, adding near-field coupled parasitic elements and their hybrid combination have been successfully developed. An OCP antenna based on above techniques has been designed. Results show that the radiation resistance has a substantial enhancement from 3.5 to 80 ohms. Meanwhile, the overall size is still ultracompact, e. g., only 0.0006 λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> 3 , with the ka value less than 0.42. These techniques are generally effective approaches that also enable a fundamental understanding of the methodology to enhance radiation resistance of small antennas.