Monday, 4 April 2011

How do KAIRA and LOFAR work? — Part 8 : Steering a phased array

Last time, we saw that a couple of detectors can be used to collect signals and that because they will add in phase from some directions and not others, there is a certain directionality to the system.

In principle, you can ‘steer’ this system to look in different directions by tipping it, but that’s not particularly efficient. The real advantage is that without moving the antennas you can change the directionality of the overall system by changing the lengths of the cables.

Let’s consider the off-zenith case from the last part.

If we now add a bit of extra cable, these off-zenith signals now add in-phase again.

In fact, the zenith signals are the ones now out of phase when they are combined electrically. So, by adding some cable length, you can control the direction in which the array is sensitive. That is, you can steer its ‘looking direction’ around the sky without actually moving the antennas themselves.

Some phased arrays indeed use cables to adjust their pointing direction. The VHF radar in Tromsø is one such system. It is mechanically steered in the vertical direction and horizontally pointed with a phased array. By manually changing the cables, the horizontal pointing direction of the array can be altered by 15 degrees.

A view inside the feeder bridge of the VHF system
at Tromsø. (Photo courtesy Mike Rietveld)

Because this needs to be done by hand, it is not patch the cables that quickly, so these sorts of directional changes are not done too often.

Although still useful (rotating a 120×40m antenna in azimuth is tricky!) there is another technique which has recently become affordable and which makes arrays like KAIRA, LOFAR and the SKA practical.

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