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DISH SETUP: Single sat, Multi-Sat & Motorised
Fixed mount for Channel Master
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<blockquote data-quote="archive10" data-source="post: 995137"><p>I'm not entirely sure I agree with you.</p><p>AFAIU, the paraboloid reflection causes phase distortions (polarisations changes) in the reflected wavefront, meaning that the result is not optimally reflected as it would be from a flat mirror.</p><p>It is for this reason that the Gregorian dishes has a seond (or sub) reflector in the signal path before the LNB.</p><p>Apart from getting the LNB and possible transmit electronics closer to the centre of gravity, I believe that it actually corrects the phase abberations introduced by the main reflector.</p><p></p><p>What is not clear to me is if the abberations are different in an off-set configuration, where the mirror is not physcially symmetrical - only optically when seen from the offset angle. One could speculate that the larger physical surface might diffract the signal more in the top half of the dish.</p><p>Or, the wavefront is slightly phase-shifted due to the difference in physical distance between top and bottom of reflector, meaning that some signal is lost as the reflecte waves interfere.</p><p></p><p>This might just have an influence when you have H and V polarised signals, where the abberations may be kept to a minimum when the H axis is aligned with the H axis of the feed attached to the LNB.</p><p>"Well", you could say, "doesn't that mean that the distortion is greatest in the H aligned direction?"</p><p>Possibly, yes, but I am speculating this may have be a reason for the apparent mis-alignment of the LNB that RimaNTSS has documented so diligently in the past.</p><p></p><p>But then again, I am only speculating. And perhaps the effect is so small, that it is overshadowed by other factors on smaller dishes (say, <1.5m)</p></blockquote><p></p>
[QUOTE="archive10, post: 995137"] I'm not entirely sure I agree with you. AFAIU, the paraboloid reflection causes phase distortions (polarisations changes) in the reflected wavefront, meaning that the result is not optimally reflected as it would be from a flat mirror. It is for this reason that the Gregorian dishes has a seond (or sub) reflector in the signal path before the LNB. Apart from getting the LNB and possible transmit electronics closer to the centre of gravity, I believe that it actually corrects the phase abberations introduced by the main reflector. What is not clear to me is if the abberations are different in an off-set configuration, where the mirror is not physcially symmetrical - only optically when seen from the offset angle. One could speculate that the larger physical surface might diffract the signal more in the top half of the dish. Or, the wavefront is slightly phase-shifted due to the difference in physical distance between top and bottom of reflector, meaning that some signal is lost as the reflecte waves interfere. This might just have an influence when you have H and V polarised signals, where the abberations may be kept to a minimum when the H axis is aligned with the H axis of the feed attached to the LNB. "Well", you could say, "doesn't that mean that the distortion is greatest in the H aligned direction?" Possibly, yes, but I am speculating this may have be a reason for the apparent mis-alignment of the LNB that RimaNTSS has documented so diligently in the past. But then again, I am only speculating. And perhaps the effect is so small, that it is overshadowed by other factors on smaller dishes (say, <1.5m) [/QUOTE]
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Satellite TV receivers & systems support forums
DISH SETUP: Single sat, Multi-Sat & Motorised
Fixed mount for Channel Master
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