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DISH SETUP: Single sat, Multi-Sat & Motorised
Dish Setup Guides, Information threads and FAQs
Multifeed installation and exact calculation of LNB position
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<blockquote data-quote="a33" data-source="post: 957569" data-attributes="member: 332642"><p>Well, I've done some googling myself in the meantime, and this is what I found out and what I now assume to be true:</p><p></p><p>Q1</p><p>For subjects as 'mirror' and 'focal line' or 'focal plane', google gives quite a few hits; most of which concern optical mirrors though; both spherical and parabolical.</p><p><strong>For both is stated, that the focal line/plane is a straight line(plane), perpendicular to the main axis of the mirror. </strong></p><p>As for instance in the attached drawing (source: <a href="http://www.astronomyasylum.com/telescopeopticstutorial.html" target="_blank">Telescope Optics Tutorial</a> ):</p><p></p><p>[ATTACH]89602[/ATTACH]</p><p></p><p>So, assuming that what is valid for optics is also valid for satellite beams, <strong>a multifeed bracket for a satellite dish should best be straight, in the horizontal plane</strong>.</p><p></p><p>As the focal line is straight for both spherical mirrors and parabolical mirrors, this would then be <strong>the same for normal satellite dishes and for (spherical) multifeed-dishes</strong>.</p><p>(But not, of course, for purely toroid dishes. See this topic: <a href="https://www.satellites.co.uk/forums/threads/smw-oa-1600-dish.164740/page-4" target="_blank">SMW OA 1600 dish</a> .)</p><p></p><p></p><p>Q2</p><p>I think my assumption about a 'horizontal G-spot' has much chance to be correct.</p><p>The message is: Spill-over losses should be avoided as much as possible!</p><p><strong>So aiming at half the aperture angle, at the 'horizontal G-spot', would be best.</strong></p><p></p><p></p><p>Q3</p><p>Looking at the drawing of the focal plane and the fixed main axis, another way of calculating LNB-distances comes to mind, using exactly those given lines. I would think a proper way to calculate LNB-distances along those lines is to <strong>always calculate LNB-distances relative to the main axis (or 'central LNB', so to say)</strong>, using the tangens-formula:</p><p> <u><strong>distance between LNB and main axis = fG x tan (dA)</strong></u></p><p>[ fG = focal distance, dA = Azimuth difference ]</p><p></p><p>The distance between two LNBs somewhere on the multifeed bracket could then be calculated, as the <strong>difference between the two calculated distances</strong> to the central LNB (in the case that they are at the <u>same</u> offset-side of the main axis; otherwise [at opposite offset-sides] it would be the <u>sum</u> of course instead of the difference).</p><p></p><p>If this way of calculating the distances is in fact correct, then the other ways of calculating I gave in my starting post are still pretty good approximations for little azimuth differences to the main axis.</p><p>The further away from the central LNB, though, these ways of calculating give <strong>too low</strong> an outcome.</p><p></p><p>For wide-offset calculations, <strong>I guess the above formula would be more appropriate and more precise</strong>.</p><p>I would love to see practical confirmation, though, that this way of calculating is actually right!</p><p></p><p>Personally, for simple calculations, I prefer to use the 'radial-calculation' (method b), because it is easy to calculate with, and can be written as <strong>distance between LNBs = 0,01745 x fG x dA</strong></p><p></p><p></p><p>Q4, I notice, is allready answered under Q1, above.</p><p></p><p></p><p>So far for assumptions and theory now.</p><p>Would love to discuss if this proves to be right in practice!</p><p></p><p></p><p>Greetz,</p><p>A33</p></blockquote><p></p>
[QUOTE="a33, post: 957569, member: 332642"] Well, I've done some googling myself in the meantime, and this is what I found out and what I now assume to be true: Q1 For subjects as 'mirror' and 'focal line' or 'focal plane', google gives quite a few hits; most of which concern optical mirrors though; both spherical and parabolical. [B]For both is stated, that the focal line/plane is a straight line(plane), perpendicular to the main axis of the mirror. [/B] As for instance in the attached drawing (source: [URL='http://www.astronomyasylum.com/telescopeopticstutorial.html']Telescope Optics Tutorial[/URL] ): [ATTACH]89602[/ATTACH] So, assuming that what is valid for optics is also valid for satellite beams, [B]a multifeed bracket for a satellite dish should best be straight, in the horizontal plane[/B]. As the focal line is straight for both spherical mirrors and parabolical mirrors, this would then be [B]the same for normal satellite dishes and for (spherical) multifeed-dishes[/B]. (But not, of course, for purely toroid dishes. See this topic: [URL='https://www.satellites.co.uk/forums/threads/smw-oa-1600-dish.164740/page-4']SMW OA 1600 dish[/URL] .) Q2 I think my assumption about a 'horizontal G-spot' has much chance to be correct. The message is: Spill-over losses should be avoided as much as possible! [B]So aiming at half the aperture angle, at the 'horizontal G-spot', would be best.[/B] Q3 Looking at the drawing of the focal plane and the fixed main axis, another way of calculating LNB-distances comes to mind, using exactly those given lines. I would think a proper way to calculate LNB-distances along those lines is to [B]always calculate LNB-distances relative to the main axis (or 'central LNB', so to say)[/B], using the tangens-formula: [U][B]distance between LNB and main axis = fG x tan (dA)[/B][/U] [ fG = focal distance, dA = Azimuth difference ] The distance between two LNBs somewhere on the multifeed bracket could then be calculated, as the [B]difference between the two calculated distances[/B] to the central LNB (in the case that they are at the [U]same[/U] offset-side of the main axis; otherwise [at opposite offset-sides] it would be the [U]sum[/U] of course instead of the difference). If this way of calculating the distances is in fact correct, then the other ways of calculating I gave in my starting post are still pretty good approximations for little azimuth differences to the main axis. The further away from the central LNB, though, these ways of calculating give [B]too low[/B] an outcome. For wide-offset calculations, [B]I guess the above formula would be more appropriate and more precise[/B]. I would love to see practical confirmation, though, that this way of calculating is actually right! Personally, for simple calculations, I prefer to use the 'radial-calculation' (method b), because it is easy to calculate with, and can be written as [B]distance between LNBs = 0,01745 x fG x dA[/B] Q4, I notice, is allready answered under Q1, above. So far for assumptions and theory now. Would love to discuss if this proves to be right in practice! Greetz, A33 [/QUOTE]
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DISH SETUP: Single sat, Multi-Sat & Motorised
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Multifeed installation and exact calculation of LNB position
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