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SMW OA 1600 dish
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<blockquote data-quote="archive10" data-source="post: 987920"><p>If I understand you right, your dish have vertical paraboloid profile, and horizontal circular profile. You should therefore be well off as you will end up having roughly same configuration as the OA-1600.</p><p></p><p>But I do think it will be essential to build in a mechanism that allows for skewing the dish.</p><p></p><p>The reasoning goes something like this:</p><p></p><p>1) A Prime Focus dish is parabolic in all planes, including what will be horizontal and vertical once mounted.</p><p></p><p>2) An offset dish (technically an offset Prime Focus dish) will (should) still be parabolic in all planes, both in vertical and horizontal plane, except that the reflector is now tilted a bit (most often in the vertical plane). But it is still parabolic as seen from the LNB.</p><p></p><p>3) A true toroidal dish is *not* parabolic in any direction, instead is it circular in both horizontal and vertical directions. Luckily, circular is an approximation of parabolic when looking a low angular sections of the parabolic curve. The circular dish does not have a single focus point, but still a spatial region of increased amplification (in which the LNB can be placed). This allows a toroidal dish to work, as it will perform only somewhat worse than a a true parabolic dish.</p><p></p><p>4) I believe the OA-1600 is a hybrid, with a parabolic vertical profile, and a circular horizontal one. This means that the dish has a horizontal focus line (rather than multiple focus points). This is a direct consequence of the dish having a circular horizontal profile. (This also means the performance is not as good as a dedicated single-focus-point dish.)</p><p></p><p>5) Ideal placement of LNB is:</p><p> a) vertically in the parabolic focal point, and</p><p> b) horizontally so that the LNB is pointing at 90 degrees to the surface of the dish. Note that since we have a focal line, rather than a focal point, you can still get reception when pointing at, sat, 85 degrees at the dish, but you will get max performance at 90 degrees.</p><p></p><p>The above points have an impact on your set-up without skew of the dish. Let's assume you have the dish horizontal (as in your pictures). This meanst that the LNB at the center of the dish, you will have best possible reception at whatever sat your aim at.</p><p></p><p>If you place an LNB at either end of the dish, the parabolic section is no longer pointing in the direction of the SAT (due to the arc of the sats). Seen from the direction of the satellite you are trying to receive, the reflector is not a true paraboloid viewed spot on, so your focal point is now also a focal cloud in the vertical direction.</p><p></p><p>With strong sats you may still be able to receive something, but it's far from optimal. And the problem is bigger the further from the "true" parabolic center of the aligned dish.</p><p></p><p>If you struggle with getting a feel for the focal behaviour of the dish, you can visualise it quite vividly by getting hold of some reflective tape, or even carefully applied kitchen foil (that you have already used in he build).</p><p>Carefully attach this to your dish in the vertical and horizontal directions (but only cover a small section of the dish!), point the dish *at the sun*(!), and hold a coloured piece of paper where the LNBs would go:</p><p></p><p>[ATTACH]98477[/ATTACH] [ATTACH]98478[/ATTACH] </p><p></p><p>( Taken from slide 20 of <a href="http://www.ntms.org/files/Nov2015/Fun_with_Off-set_dish.pdf" target="_blank">http://www.ntms.org/files/Nov2015/Fun_with_Off-set_dish.pdf</a> )</p><p></p><p></p><p>Recommendation</p><p></p><p>SO: to get the most from your dish, I believe you need a mechanism for skewing the dish. If it is not heavy, it can literally be two plates with a central bolt that allows rotating the dish around the geometric center, and mounting one plate on the dish and the other on the mount.</p><p></p><p>Principle can be seen here in the mounting bracket for my WaveFrontier T55:</p><p></p><p>[ATTACH]98480[/ATTACH] [ATTACH]98479[/ATTACH]</p><p></p><p>At the same time, I would attach the feed-arms mechanically to that back plate, in the same the OA-1600 has them. This will remove most of the load on the reflector that might distort the surface of the dish when putting a number of LNBs on the feed-arms. Distorting the reflector even a little bit can be detrimental to the performance (as eg [USER=175005]@John[/USER] has found out with a Gibertini 125 elsewhere on this site):</p><p></p><p><a href="https://www.satellites.co.uk/forums/threads/1-2-precision-octagon-lnbf-mod.169117/page-2#post-980256" target="_blank">Just Sharing This - 1.2 Precision / Octagon lnbf mod</a></p><p></p><p>And while you are in the metal workshop, you can be inspired by John's image in the post above and consider putting in a support rod from the plate attached to the reflector, to near the top or bottom of each of the ends. This will address the lack of rigidity you mention earlier, and limit unwarranted twisting in the reflector in high winds.</p><p></p><p>Best of luck with the project!</p></blockquote><p></p>
[QUOTE="archive10, post: 987920"] If I understand you right, your dish have vertical paraboloid profile, and horizontal circular profile. You should therefore be well off as you will end up having roughly same configuration as the OA-1600. But I do think it will be essential to build in a mechanism that allows for skewing the dish. The reasoning goes something like this: 1) A Prime Focus dish is parabolic in all planes, including what will be horizontal and vertical once mounted. 2) An offset dish (technically an offset Prime Focus dish) will (should) still be parabolic in all planes, both in vertical and horizontal plane, except that the reflector is now tilted a bit (most often in the vertical plane). But it is still parabolic as seen from the LNB. 3) A true toroidal dish is *not* parabolic in any direction, instead is it circular in both horizontal and vertical directions. Luckily, circular is an approximation of parabolic when looking a low angular sections of the parabolic curve. The circular dish does not have a single focus point, but still a spatial region of increased amplification (in which the LNB can be placed). This allows a toroidal dish to work, as it will perform only somewhat worse than a a true parabolic dish. 4) I believe the OA-1600 is a hybrid, with a parabolic vertical profile, and a circular horizontal one. This means that the dish has a horizontal focus line (rather than multiple focus points). This is a direct consequence of the dish having a circular horizontal profile. (This also means the performance is not as good as a dedicated single-focus-point dish.) 5) Ideal placement of LNB is: a) vertically in the parabolic focal point, and b) horizontally so that the LNB is pointing at 90 degrees to the surface of the dish. Note that since we have a focal line, rather than a focal point, you can still get reception when pointing at, sat, 85 degrees at the dish, but you will get max performance at 90 degrees. The above points have an impact on your set-up without skew of the dish. Let's assume you have the dish horizontal (as in your pictures). This meanst that the LNB at the center of the dish, you will have best possible reception at whatever sat your aim at. If you place an LNB at either end of the dish, the parabolic section is no longer pointing in the direction of the SAT (due to the arc of the sats). Seen from the direction of the satellite you are trying to receive, the reflector is not a true paraboloid viewed spot on, so your focal point is now also a focal cloud in the vertical direction. With strong sats you may still be able to receive something, but it's far from optimal. And the problem is bigger the further from the "true" parabolic center of the aligned dish. If you struggle with getting a feel for the focal behaviour of the dish, you can visualise it quite vividly by getting hold of some reflective tape, or even carefully applied kitchen foil (that you have already used in he build). Carefully attach this to your dish in the vertical and horizontal directions (but only cover a small section of the dish!), point the dish *at the sun*(!), and hold a coloured piece of paper where the LNBs would go: [ATTACH]98477[/ATTACH] [ATTACH]98478[/ATTACH] ( Taken from slide 20 of [URL]http://www.ntms.org/files/Nov2015/Fun_with_Off-set_dish.pdf[/URL] ) Recommendation SO: to get the most from your dish, I believe you need a mechanism for skewing the dish. If it is not heavy, it can literally be two plates with a central bolt that allows rotating the dish around the geometric center, and mounting one plate on the dish and the other on the mount. Principle can be seen here in the mounting bracket for my WaveFrontier T55: [ATTACH]98480[/ATTACH] [ATTACH]98479[/ATTACH] At the same time, I would attach the feed-arms mechanically to that back plate, in the same the OA-1600 has them. This will remove most of the load on the reflector that might distort the surface of the dish when putting a number of LNBs on the feed-arms. Distorting the reflector even a little bit can be detrimental to the performance (as eg [USER=175005]@John[/USER] has found out with a Gibertini 125 elsewhere on this site): [URL="https://www.satellites.co.uk/forums/threads/1-2-precision-octagon-lnbf-mod.169117/page-2#post-980256"]Just Sharing This - 1.2 Precision / Octagon lnbf mod[/URL] And while you are in the metal workshop, you can be inspired by John's image in the post above and consider putting in a support rod from the plate attached to the reflector, to near the top or bottom of each of the ends. This will address the lack of rigidity you mention earlier, and limit unwarranted twisting in the reflector in high winds. Best of luck with the project! [/QUOTE]
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SMW OA 1600 dish
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