The folded dipole perspective of electrical small spherical helix antennas

Abstract

The dimensions of resonant antennas are typically in the same scale as the wavelength of the operating frequency. As the operating frequency decreases, the dimension of the antenna would increase along with the wavelength in general. On the other hand, low frequency operations are preferred in some scenarios of wireless communications because the intercepted power by the receiving antenna is proportional to the square the wavelength, resulting a lower propagation path loss. Electrically small antennas with dimensions in a fraction of the wavelength are thus needed. In addition to the well known fundamental limit of Q in those antennas [1], another significant disadvantage associated with electrically small antennas is their small radiation resistance, which results in poor radiation efficiency and difficulty in designing impedance matching circuits. Recently, spherical helix antennas have been studied [2&3] and it is found that such antennas can provide reasonable bandwidth performance and dipole-like radiation patterns. Regardless of the good performance of the spherical helix antenna, how to design and analyze the spherical helix antennas remains as a challenging task due to its complex matching and radiation mechanisms. Here, it is proposed that the spherical helix antenna operates in a similar way to folded dipole antennas [4]. It is well known that folded dipole antennas can quadruple the radiation impedance of a common dipole of impedance 73+42.5j ohms to a “twin-lead” 300 ohm transmission line. It will be presented the spherical helix antenna can be analyzed in a way similar to [4] as the multiple parasitic arms of the helix antenna plays a role of impedance “boosting up” similar to that of the folded arm in the folded dipole antenna. The small radiation impedance associated with a single arm miniaturized dipole antenna is compensated through this impedance transformation so that an radiation impedance of 50 ohms can be realized. The analysis provides a direction on how to modify and extend the spherical helix antennas for different applications.