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<blockquote data-quote="2old4this" data-source="post: 1209" data-attributes="member: 174998"><p>More points for consideration:</p><p></p><p>- mesh is said to have only 80% of the gain of a same-sized solid. However, this rule-of thumb originates from the time that the most common form of mesh dish was the huge "DIY" types used for C-band. That they would have a lower gain is more likely due to their imperfect form (typically being segmented, rather than smoothly curving) than to the intrinsic quality of mesh vs. solid. Logically, if the mesh surface was perfectly formed and the gaps smaller than the microwave wavelength ("micromesh"), there should be no difference.</p><p>Any professionals out there that can clarify...?</p><p>In the meantime, read this: http://www.drdish.com/features/mesh.html</p><p></p><p>- a solid picks up more wind and so is more susceptible to being blown out of alignment in storms. </p><p></p><p>- so long as the dish is correctly formed and the surface is smooth then the underlying metal (steel, aluminium, whatever) is not of significance (but note: there must be a metal layer somewhere; it is the metal that reflects the microwaves). However, dents or a rough paint job will cause incoming signals to be reflected away from the ideal focal point, so reducing gain.</p><p></p><p>- The form of both a prime-focus (as used, eg, for C-band) and offset dish (LNB on an arm at the bottom) must be a parabola. However, the degree of curvature is not the same. This link explains it: http://www.mlesat.com/antennas.html</p><p>If you were to misuse an offset dish as a prime-focus dish by rigging up your own central LNB mount you would not get a (very good) signal!</p><p></p><p>- lighter dishes are less likely to damage the mountings/wall/roof, and will cause less strain on any motor used. This may be an important consideration if the size of the dish is approaching the maximum supported by a particular motor (especially for DiSEqC motors) </p><p></p><p>- some dishes are matched to particular LNBs. The Sky elliptical dishes don't just have any old LNB on them. The internal construction of the LNB is matched to the geometry of the dish. If you were to put any old LNB on such a dish you could expect less gain. See here: http://www.satcure.co.uk/tech/lnb.htm</p><p></p><p>- A Cassegrain dish looks much like a Gregorian dish inasmuch as they both employ subreflectors and an outward-looking LNB to pick up the refelection back from the subreflector. However, a Gregorian dish has an elliptical subreflector whereas a Cassegrain has a hyperbolic sub-reflector. The elliptical form can be made smaller, so there is less obscuration of the incoming signals. And an offset Gregorian offers the least possible obscuration - hence is the most efficient of all dishes.</p><p></p><p>- the dish is usually supplied with an arm for the LNB/LNBF, and they can vary in construction. Beware of cheap constructions that may wear/shake loose and cause the LNB to move out of the focal point of the dish - that would more than negate any gain from using a solid rather than mesh dish (for example).</p><p></p><p>- when all said and done, ask yourself if you NEED the extra gain. After all, a clear digital picture received via a 60cm dish does not get any clearer on a 1m dish (or even a 5m dish...). That is the nature of digital - it is "all or nothing". You would only ever notice the difference when specifically seeking out weak signals or else in severe weather conditions.</p><p></p><p>More info needed?</p><p>Check this out too: http://www.geocities.com/TheTropics/1975/glosarium.html</p><p></p><p>2old</p></blockquote><p></p>
[QUOTE="2old4this, post: 1209, member: 174998"] More points for consideration: - mesh is said to have only 80% of the gain of a same-sized solid. However, this rule-of thumb originates from the time that the most common form of mesh dish was the huge "DIY" types used for C-band. That they would have a lower gain is more likely due to their imperfect form (typically being segmented, rather than smoothly curving) than to the intrinsic quality of mesh vs. solid. Logically, if the mesh surface was perfectly formed and the gaps smaller than the microwave wavelength ("micromesh"), there should be no difference. Any professionals out there that can clarify...? In the meantime, read this: http://www.drdish.com/features/mesh.html - a solid picks up more wind and so is more susceptible to being blown out of alignment in storms. - so long as the dish is correctly formed and the surface is smooth then the underlying metal (steel, aluminium, whatever) is not of significance (but note: there must be a metal layer somewhere; it is the metal that reflects the microwaves). However, dents or a rough paint job will cause incoming signals to be reflected away from the ideal focal point, so reducing gain. - The form of both a prime-focus (as used, eg, for C-band) and offset dish (LNB on an arm at the bottom) must be a parabola. However, the degree of curvature is not the same. This link explains it: http://www.mlesat.com/antennas.html If you were to misuse an offset dish as a prime-focus dish by rigging up your own central LNB mount you would not get a (very good) signal! - lighter dishes are less likely to damage the mountings/wall/roof, and will cause less strain on any motor used. This may be an important consideration if the size of the dish is approaching the maximum supported by a particular motor (especially for DiSEqC motors) - some dishes are matched to particular LNBs. The Sky elliptical dishes don't just have any old LNB on them. The internal construction of the LNB is matched to the geometry of the dish. If you were to put any old LNB on such a dish you could expect less gain. See here: http://www.satcure.co.uk/tech/lnb.htm - A Cassegrain dish looks much like a Gregorian dish inasmuch as they both employ subreflectors and an outward-looking LNB to pick up the refelection back from the subreflector. However, a Gregorian dish has an elliptical subreflector whereas a Cassegrain has a hyperbolic sub-reflector. The elliptical form can be made smaller, so there is less obscuration of the incoming signals. And an offset Gregorian offers the least possible obscuration - hence is the most efficient of all dishes. - the dish is usually supplied with an arm for the LNB/LNBF, and they can vary in construction. Beware of cheap constructions that may wear/shake loose and cause the LNB to move out of the focal point of the dish - that would more than negate any gain from using a solid rather than mesh dish (for example). - when all said and done, ask yourself if you NEED the extra gain. After all, a clear digital picture received via a 60cm dish does not get any clearer on a 1m dish (or even a 5m dish...). That is the nature of digital - it is "all or nothing". You would only ever notice the difference when specifically seeking out weak signals or else in severe weather conditions. More info needed? Check this out too: http://www.geocities.com/TheTropics/1975/glosarium.html 2old [/QUOTE]
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