Tracking Antenna Mounts

Tracking Science

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A moving tracking antenna commonly uses one of three different mounts.

An Azimuth-Elevation, Az-El, antenna mimics a human standing looking north. To see an object in the sky the ‘feet’– the azimuth drive – move around horizontally clockwise to the appropriate azimuth angle, between 0° and 360°, and the ‘head’– the elevation drive – tilts up to the appropriate elevation angle, between 0° horizontally and 90° vertically. Az-El antennas (and humans) find that directly overhead (the zenith) is the most difficult area in the sky to access.

An X-Y antenna avoids the overhead problem, as you could do, by lying down with your ‘head’ to the north and your ‘feet’ to the south; directly upwards being 0° X and 0° Y. The X-axis drives the antenna from east (+90°) to west (-90°); like turning your head from left to right. The Y-axis drives the antenna from north (+90°) to south (-90°); like tilting your head from right 'up' to right 'down'. Lie on the ground and try it! Tracking overhead now becomes easy; the accuracy of measurement of anglular velocity is maximised for satellites passing across the zenith area. But there are new inaccessible areas depending on which axis drive is on top of the other; two small ‘keyholes’ where no tracking is possible, usually planned to be ‘north and south’ for earth-orbit tracking and ‘east and west’ for deep-space tracking.

A Polar mount is commonly used for an antenna tracking objects in deep space particularly in astronomy. Here the Y-axis is tilted by the amount of the antenna’s latitude (24.900° towards the north for Carnarvon in the southern hemisphere); akin to a human lying on a tilted bed. The X-axis now becomes an hour-angle axis and the Y-axis becomes a declination axis. This makes star tracking - star-gazing - easier because once the declination angle is set for a particular star only the hour-angle axis moves - from east to west at a rate very close to 15° per hour.

Carnarvon had several of each type of mount: X-Y mounts for USB and the two R&RR antennas; Polar mounts for the SPAN telescopes and the Jupiter Monitor antenna; and Az-El mounts for all other movable antennas on site.

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