Quiet cellular antenna technology for SKA

Two South Africans have given their home country a boost with its Square Kilometre Array (SKA) bid by inventing cellular antenna technology which reduces ‘noisy’ emissions from cellular base stations in the area.

South Africa has been short-listed along with Australia to host the SKA – a partnership between 67 organisations in 20 countries which will ultimately produce the world’s largest radio telescope.

The SKA is projected to be 50 to 100 times more sensitive than any other radio telescope ever built, and an area without much radio emissions is essential for the success of the project.

To protect and improve the low levels of RF interference, a large part of the Northern Cape Province is protected as a radio-astronomy reserve by the South African Astronomy Geographic Advantage Act.

Notwithstanding the general low levels of RF interference at the proposed SKA core site, a number of strong RF signals can still be measured in this area. Some of these are due to GSM cellular base stations.

The GSM signals are strong enough to potentially block the receivers of the radio telescopes, and the SKA Project Office approached Vodacom in 2006 to reducing these signal levels.

Dr. Gordon Mayhew-Ridgers and Paul van Jaarsveld were tasked to come up with a solution to the problem.

Mayhew-Ridgers and van Jaarsveld looked at a range of options, but settled on developing their own antenna technology to solve the problem.

The solution was an antenna based on phased-array principles, providing omnidirectional coverage but also blocking the RF transmissions along a single direction (that would correspond with the bearing of the SKA core site).

“The antenna has since been tested in the Karoo and performs extremely well. Trialling measurements have shown that the RF signal levels at the proposed SKA core site can be reduced significantly, while at the same time, much of the original GSM coverage can be retained,” Vodacom said.

The first production unit is currently being manufactured and should soon be installed on a permanent basis.

Full details of the project are provided below

In 2003, the Department of Science and Technology and the National Research Foundation decided to enter into a race to host the world’s largest radio telescope. The Square Kilometre Array (SKA), as it is known, began as an international project in 1991 and is currently a partnership between 67 organisations in 20 countries.

The SKA is projected to be 50 to 100 times more sensitive than any other radio telescope ever built. It is expected to help answer fundamental questions in astronomy, physics and cosmology, including the nature of dark energy and dark matter.

It should also allow scientists to go back in time to explore the origins of the first galaxies, stars and planets. It is anticipated that the SKA will consist of about 3,000 antennas with a total collecting area of one square kilometre, the equivalent of more than 1,000,000 DStv satellite dishes.

While the SKA footprint should cover an entire continent, the current design calls for about half of the antennas to be located at a core site with about 10 km diameter. The rest of the antennas should be located in clusters that spiral out of the core site. The furthest of these could be up to 3000 km away.

Current projections indicate that the construction cost of the SKA should be around 1.5 billion Euros, while running costs should amount to about 100 to 150 million Euros per year.

After South Africa submitted its bid in 2005 to host the SKA, it made the final short-list along with Australia. An announcement should be made early in 2012 regarding the final location of the SKA.

In order to illustrate South Africa’s commitment to the SKA project, it was decided to construct a South African SKA science and technology pathfinder at the proposed SKA core site in the Karoo. The Karoo Array Telescope, or MeerKAT as it is now known, will consist of 64 dishes and will be the most sensitive radio telescope in the southern hemisphere until the SKA is finally completed.

Construction work on the first 7 dishes that constitute the engineering and science prototype, also known as KAT-7, has already been finished. In order to protect and improve the low levels of RF interference, a large part of the Northern Cape Province is protected as a radio-astronomy reserve by the South African Astronomy Geographic Advantage Act.

This allows South Africa’s Minister of Science and Technology to regulate all RF transmissions and any other activities that could interfere with radio astronomy.

Notwithstanding the general low levels of RF interference at the proposed SKA core site, a number of strong RF signals can still be measured in this area. Some of these are due to GSM cellular base stations that provide much sought-after communications to many of the small Karoo towns and farming communities.

The power levels associated with these GSM signals are high enough to potentially block the receivers of the radio telescopes and therefore the SKA Project Office approached Vodacom in 2006 to enquire about the possibilities of reducing these signal levels.

As Dr Gordon Mayhew-Ridgers and Paul van Jaarsveld’s role in the company includes the mitigation of RF interference and the development of special RF coverage solutions, they were tasked to come up with a solution to the problem.

Given that many people in the Karoo rely on cellular communications, the challenge was to reduce emissions from the various base stations towards the SKA core site while still retaining most of the existing coverage around these base stations.

Initially, quite a number of ideas were evaluated. These varied from simply using panel antennas that could be pointed away from the SKA core site to low-power base stations and highly directional antennas.

However, none of these ideas proved to be an optimal solution. These could either reduce the RF interference at the expense of losing significant coverage or the other way around.

It was soon realised that a base-station antenna was needed that could provide more or less omnidirectional coverage, but at the same time block the RF transmissions along a single direction that would correspond with the bearing of the SKA core site.

The only problem was that such an antenna does not appear in the catalogue of any vendor and would have to be developed specifically for this purpose.

As both Gordon and Paul have some experience in the design and development of passive RF devices, and also have access to very good modelling software and antenna testing facilities, they decided to develop the antenna themselves.

The antenna design, which is based on phased-array principles, went through a number of iterations and physical prototypes, eventually resulting in a very compact assembly, just slightly larger than a commercial base-station antenna that is normally used on omnidirectional sites.

Some aspects of the antenna design appeared to be original and worthy of protection through a patent. As such, the Legal Affairs Division within Vodacom was approached and, together with some guidance from Vodafone Group Legal, a provisional patent for the antenna was obtained.

The antenna has since been tested in the Karoo and performs extremely well. Trialling measurements have shown that the RF signal levels at the proposed SKA core site can be reduced significantly, while at the same time, much of the original GSM coverage can be retained.

The International SKA Site Engineer has indicated that he is very impressed with the results, while Naledi Pandor, the South African Minister of Science and Technology, has explicitly referred to these developments in her address at the SKA Forum meeting in Canada recently.

The first production unit is currently being manufactured and should soon be installed on a permanent basis.