Frequency coverage

How low can you go?

Classical radio telescopes were designed to operate at a few very specific frequencies, with a little bit of leeway either side. Over the last decade, there is a trend for astronomers to want access to as much of the radio frequency space as possible. This trend will culminate in the SKA, where astronomers want to be able to observe from 50MHz up to several GHz, without any gaps.

Split personality

There is no technology which can receive such a large frequency range, so by design the SKA will consist of at least three different instruments. The highest frequencies (>1.5GHz) are most efficiently covered with dishes. The lowest frequencies (<500MHz) will consist of aperture arrays. For the middle range, a decision has to be made between classical dishes, dishes with a phased array, and aperture arrays.

Even with this split, the low-frequency aperture arrays will have to be able to observe from at least 70MHz up to 450MHz. That is a factor of at least 6.5, which is huge for a radio telescope at these frequencies! The problem lies not with the design of an antenna which can receive such a large range. For a Vivaldi antenna, the width of the base defines the highest accessible frequency, the length of the element sets the lowest possible frequency. So by making very narrow and tall elements, you can cover enormous frequency ranges.

Pinhole vision

The problem occurs when you want to use the element in an array to do science. A very narrow and tall element is only sensitive in a single direction. As it is fixed to the ground, it could only see the sky which is directly above it, as if it were looking through a pinhole. Imagine it looks like a capital letter V, and the opening between the legs of the V defines the area where you can see a signal. For a very narrow and tall V, this opening is very small. This clashes with another crucial request from astronomers: they want to see as much sky as possible.

The problem persists for alternative antenna designs. As a rule of thumb: the broader the frequency range that the element is sensitive to, the smaller the area of the sky it can see. So a decision has to be made, balancing two critical elements of the science.

Well balanced

As the sky rotates, it is still possible to look at a significant part of the entire sky, even if the antenna elements can only 'see' in a circle with diameter of 45 degrees. This value seems to be possible for the aperture arrays both at low and middle frequencies, as long as the frequency range doesn't change. The element designs currently considered for the SKA middle frequency range will still outperform dish technology. They can see a much larger part of the sky compared to dishes, and are sensitive to a much larger range of frequencies.