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Precision RF integration

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Case Study – Development of single axis tracking feed.

In 2014 iSat began to develop a range of Ka-terminals.Through experimenting with the exact focal point of the reflector we learnt how critical beamwidth, focus, and positioning is at Ka-band. This realization lead to the development of our manual adjusted feed positioning tool, that allows installers to achieve the best cross-polar performance on the feed, without having to move the Az and El screws- which have backlash and are dangerous to move while transmitting. The focus tool is adjusted by micrometer, and can be moved slowly while transmitting with an offset reflector. So after fixing the best place for receive frequency, the amplifier can be turned on and with Az and El locked off, the feed can be moved to the best place for transmit cross-polarisation.

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Ka-band satellites keep their orbit almost exactly in sync with the ground, so the look angle changes very little; within 0.05 degrees, or sometimes 0.1 degrees. It was observed that even this small movement at Ka-band can affect the cross pol performance of reflectors at 1.8 and more at 2.4mtr. The manual adjustment tool allowed the feed to be positioned exactly at the middle of the satellite station, thereby ensuring that the miss-point error would be maximum 0.025 or 0.05 degrees. At these sizes of antenna, the feed needs to move only 1 or 2 mm (1 wavelength at 30GHz is 10mm), but these movements are almost impossible with Az and El bolts that have to withstand 100mph wind. The differences are marginal compared to rain fade, but every user wants to maximize their investment in expensive amplifiers, and the last 0.5dB seemsalwaysthe most critical for performance and competitive tendering.

 

Having studied Ka-band carefully, iSat started to explore compensating the movement of inclined satellites with feed positioning. Inclined satellites move 1, 2, up to about 4 or 5 whole degrees. The feed tracking system would offer lower costs than Az El tracking systems, retain the wind loading of the antenna, and provide satellite operators with a way to extend their satellite life. It would offer customers of satellite operators lower cost equipment to use competitive bandwidth on inclined satellites.

Distortion of the beam occurs as the feed moves off the focal point. The higher the frequency, the narrower the beamwidth, the greater the distortion for a given mm movement. In principal therefore, C-band is the best suited to this type of compensation, because the beam-width is the largest commercially available (after L-band) and so effects of moving off the focal point are least. In feasibility studies conducted on the far field range, it was clear that even at Ku-band common angles of inclined satellites could be received while not exceeding the side-lobe mask.  iSat have subsequently not only developed the motorized hardware and motor controller but also a sophisticated computer controller to provide GUI interface, alarms and unattended automatic updates of tracking information.

 

In dedicating time and resources to the development, iSat won a support contract for the development from the European Space Agency.

 

In creating the GUI interface, iSat connected the back-end computer functions of alarm monitoring and setup, with a simple user interface.

 

The project underwent final milestone acceptance from ESA and provides a unique competitive advantage to the inclined satellite market.