Building blocks

Low frequency receivers

When you build a big thing like the Square Kilometer Array, you want to make sure that the parts you use to build it from are nearly flawless and meet al the required specifications. For that purpose, each component goes through rigorous design and testing phases. This also goes for the receiver elements. Meet the low frequency antenna concepts for the SKA.

Frequency range

The requirements for the low frequency system for SKA are harsh. The receivers have to be able respond to frequencies from at least 70MHz to 450MHz, a range of 6.5 to 1, which is an enormous range for a radio telescope. The current discussion even moves to a lower frequency of 50MHz, which seems a small step down, but will stretch the range to 9 to 1, which is a huge increase.

Luckily, there are receivers which by design are sensitive to such a large range. A single Vivaldi element is a broad-band receiver, capable of reaching 9 to 1 (some manufacturers even claim a 10 to 1 range is possible). The second option is a conical spiral antenna, which has a major difference in the sense that it is sensitive to circularly polarized radiation, while the other options are linearly polarized. Last but not least, the log-periodic antenna is considered. In the last two cases, extending towards a lower frequency is a matter of making the element larger.

Polarization

For a radio telescope it is essential that the full polarization information is received, as it is used by astronomers to reveal structures which are not visible directly, such as magnetic fields. The conical spiral poses problems when trying to manufacture an array which could receive both left-handed and right-handed polarized radiation. So in spite of the many benefits, it is no longer a very serious option until this problem is solved. This doesn't mean that the design was a waste of time, there are non-astronomical applications for which a single polarization antenna over a large bandwidth is useful, so the team continues the testing and characterization of the element.

Further tests

The two remaining candidates are the log-periodic and the Vivaldi. Both are currently being tested in a small array to study their behaviour in a group. As they both meet the requirement for the large frequency range, the comparison now focuses on additional requirements. One aspect that is being tested is the effect of different types of soil on which the antenna is placed. For LOFAR the receivers have a so-called ground plane: a wire mesh is placed on the ground below the antenna to reflect the radiation back to the antenna, and boosting the sensitivity. For SKA this may not be an option, due to the large number of receivers required, a ground plane for each of them is a costly exercise.

Decision

Once all possible tests are done, the difficult step arises on having to decide which design is most suitable for the SKA. This involves a lot of parameters, and for each we have to understand the impact it will have on the final instrument and the science we want to do with it. However, the decision also has a large impact on the teams which have worked for many years on the project. Everything has to be well balanced, which is a challenge.