Multi-mode Composite Antenna
The multi-mode composite antenna developed by researchers at Stellenbosch University achieves near hemispherical coverage with reduced power gain variation and improved polarisation discrimination capabilities when compared to conventional dual-polarised dipole antennas. This is currently the only antenna in the world which radiates over such a wide angle. This technology is ideally suited for wireless Multiple-Input Multiple-Output (MIMO) access point applications where it is desirable to receive and transmit signals from a wide variety of possible angles, yet standard antenna elements rarely offer this type of performance and are never completely omnidirectional.
The multi-mode composite antenna radiates over a full hemisphere. This is currently the only antenna in the world which radiates over such a wide angle. This antenna technology has been implemented and tested successfully in MIMO wireless access points for WiFi and 5G communication. The antenna technology can also be used in low- and mid-frequency RF scanning antenna applications, including radio astronomy devices.
A number of problems exist in currently available wireless access points, which this antenna technology addresses:
• Standard Wi-Fi antennas only radiate in certain directions, leaving certain areas around the antenna completely without coverage.
• These antennas radiate a signal with one polarisation only, requiring a perfectly aligned receive antenna with the same polarisation for optimum signal transmission.
• Most current access points cannot be mounted at arbitrary angles due to the limited antenna patterns.
• Current systems mostly employ multiple single elements, e.g. multiple monopoles, set at arbitrary angles with respect to each other, to create different possible signal paths (formally called spatial and polarisation diversity) to improve signal integrity and data throughput. This creates problems for installations where systems must be spaced at arbitrary points in space, and where multiple obstacles exist. One example of this is a multi-storey office block where some walls may be reinforced concrete (high shielding), metal furniture is scattered throughout the environment, and where vertical signal paths are required in addition to horizontal paths.
For the 5G systems being planned, the signal blockage will be a much bigger problem than for present systems. Here, signal path diversity will be a prime design parameter, yet current solutions have limited options to achieve this. An example of this would be street-lamp mounted access points which will have to connect to points in moving vehicles – a very dynamic environment where the signal blockage will also be dynamic. The multi-mode antenna technology provides for dynamic reconfigurability for use in dynamic environments.
Furthermore, current antennas have a significant influence on the final size and form factors of access points. Available planar (flat) technology has undesirable radiation properties, unlike the proposed technology.
The main result of all these problems is a reduction in signal path integrity, requiring data packets to be re-sent multiple times, causing sometimes dramatic reductions in data rates. Furthermore, in a world where aesthetics of electronic equipment are becoming more and more important, form factors are severely constrained by current technology.
The multi-mode composite antenna element can function as radiating element for antenna arrays in applications where a physically fixed system must electronically scan the whole of space above the horizon for signals. Alternatively, such an antenna can be mounted with arbitrary physical orientation, as determined by available surfaces, and have the gain maximised in any required direction. In a MIMO configuration the ability of the antenna to receive signals from antennas scattered geographically extend beyond that of standard antenna elements.
The antenna element addresses the problems identified with currently available MIMO access points in the following ways:
• The wide full hemispherical coverage achieved by this antenna technology makes it ideally suited for fixed MIMO configurations. MIMO access points do not require a direct line-of-sight channel to client devices. Instead, channels are formed along multiple propagation paths resulting from scattering and reflection of signals within the environment.
• Due to the multiplicity of elements at the same point in space, dynamic reconfigurability can be achieved, with dynamic electronic control of the antenna radiation direction and selective partial nulling of directions from which disturbance signals originate. This will be very important in dynamic environments.
• Alternatively, the system can be configured for optimal signal diversity (i.e. the highest number of signal paths). This is possible due to the inherent spatial and polarisation diversity of each multi-mode element.
• The system is suited to planar construction (unlike present wire antennas) which allows for high-volume manufacturing and good form factors.
In summary, the antenna technology, when used in a MIMO access point demonstrates improved coverage, signal diversity, dynamic flexibility, and provides good form factors.
The most important feature is the full hemispherical radiation pattern, for an element mounted over a ground plane (such as the top of an access point enclosure). No undesirable nulls, therefore, exist in the pattern, as is the case for e.g. standard monopoles which have a perfect null in the vertical direction. This is currently the only element in the world which can achieve such wide field-of-view coverage.
The composite antenna also provides accurate collocation, improved polarisation capabilities, dynamic reconfigurability, optimal signal diversity and excitation can be achieved by a single multi-modal transmission line.
The invention combines co-located monopole and crossed dipole antennas to enable transmission in any direction within a full hemisphere.
By utilising weighted combinations of four different modes of excitation, a radiation pattern is obtained which contains no nulls, and only a small variation of gain within a hemisphere extending down to the horizon.
This technology was developed initially to address the needs presented by the specification of the Square Kilometre Array (SKA) telescope, where it was desired that the antenna elements that form part of the phased arrays of the telescope have a wider field-of-view with a minimum scan angle of 60° from zenith compared to the scan angle of 45° from the zenith of purely differential antenna elements implemented in the majority of phased array radio telescopes.
Prof P. Meyer, Professor of Electric and Electronic Engineering at Stellenbosch University.
Dr. D.S.V. Prinsloo, Antenna Design Engineer at Astron.
Prof R. Maaskant, Assistant Professor at Chalmers University of Technology (Gothenburg, Sweden).
Prof M.V. Ivashina, Assistant Professor at Chalmers University of Technology (Gothenburg, Sweden).