Daily Image
05-02-2014
What does the presence of grating lobes do to your effective area - part II
| Submitter: | Michel Arts & Stefan Wijnholds |
| Description: | On January 29th we showed a picture of the array pattern of a 20x20 uniform rectangular array with isotropic elements (see http://www.astron.nl/dailyimage/main.php?date=20140129 ). We showed that that the gain (and therefore the effective area) reduces when a grating lobe appears. Today we investigate this effect further. Instead of using the gain of the array we consider the effective area. Two pictures are shown. The first picture shows the effective area of a 20x20 uniform rectangular array with an element spacing of 1.25 meter and an isotropic element pattern as a function of frequency and scan angle w.r.t. zenith. It is assumed that the array is scanned along one of its primary axes. The black line shows the scan angle where the grating lobe appears as a function of frequency (the grating lobe is present for scan angles larger than that angle). One can see that the effective area drops dramatically as soon as the grating lobe appears. The second picture shows the effective area of the same array but with a cos^2 element pattern instead of an isotropic pattern. It is clearly visible that the decrease of the effective area due to the grating lobe is less than in the case of an isotropic pattern. The reduced effect of the appearance of the grating lobe is caused by the suppression of the grating lobe by the cos^2 element pattern. From these pictures, we can conclude that the effect of the appearance of grating lobes on the effective area strongly depends on the element pattern. For broad element patterns, grating lobes cause a significant reduction of the effective area. For narrow element patterns, the effect of grating lobes on the effective area is less. Next time we will show a comparison of the measured and simulated sensitivity (Aeff/Tsys) of a LOFAR high band tile to illustrate the effect of grating lobes on effective area (and therefore on sensitivity) of the LOFAR high band antenna system. |
| Copyright: | ASTRON |
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