LOWEST PROFILE OMNI DIRECTIONAL ANTENNA

In mobile communications it is important, not only should to have adequate signal strength, but the direction of the electric field, also be parallel with the antenna i.e. there be minimum polarization mismatch. However because of the scattering environment, a part of vertical electric field generated at the transmitter can turn into horizontal & vice versa. An antenna capable of receiving or generating equal amplitudes of electric fields both in horizontal and vertical directions will improve reception by providing diversity. The simplest form of such an antenna is a crossed dipole which consists of two dipoles orientated at 90^o to each other. The crossed dipole provides equal vertical and horizontal electric field over narrow regions and its height would be approximately half wavelength. But many applications such as in mobile handsets and aerodynamic surfaces at jet speeds the antenna has to be low profile either to appeal the aesthetics or to reduce air resistance. In addition equal amplitude of electric field both in vertical and horizontal directions is required in all directions for reliable communication.

To meet such and many other potential applications, a faculty and research member of Communication Systems Engineering Department at IST, Dr. Muhammad Amin participated in the design and development of a very low profile antenna that can generate equal vertical and horizontal electric field components. This antenna has a helical shape and has a feed at the centre of the helical section of one side.

This antenna, known as side-fed bifilar helix antenna, does not require a ground plane [“Single Feed Low Profile Omnidirectional Antenna with Slant 45^o Linear Polarization” by Amin, M.; Cahill, R.; Fusco, V. in IEEE Transactions on Antennas and Propagation, Volume 55, Issue 11, Nov. 2007 Page(s):3087 - 3090]. Doing away with a ground plane is an advantage for installation of antenna at places where many new non-metallic composite materials are being used such as in high speed aerodynamic structures and in places where enough plane metallic surface area is not available such as in compact communications devices. The independence of the electric field from ground plane also means less proximity effects due to nearby objects having material characteristics.

This side-fed bifilar helix antenna is a lot more compact than a crossed dipole and in addition since the latter type of structure generates two toroidal patterns which are orientated at 90^o, the coverage offered by the side fed bifilar helix antenna is significantly larger.  It is hoped that the antenna will find many applications that require omni-directional coverage.