However, I have been searching for a method that will work for mini-dishes as well.
I now have a formula which relies solely on the height of the dish measured rim to rim, and the distance of the top and bottom rims to the face of the LNB, but the trouble is, for a dish of unknown offset I have here, it disagrees with the other one by about 2.5 degrees! So which, if indeed either, is right?
Really I need a few dishes of known offset given in the manufacturer's specs to measure up, so I can check to see if indeed either formula is correct. But I don't have any.
For as many dishes of known offset as possible, I'd like to know (units are degrees and mm): Offset Height measured from rim next to LNB arm to opposite rim Distance of rim next to LNB arm to front-face of LNB Ditto for opposite rim.
Sorry, it would also be useful to know the width measured rim to rim at right-angles to the height measurement previously explained.
On Mon, 02 Nov 2009 13:49:52 +0000, Java Jive <j...@evij.com.invalid> wrote:
> For as many dishes of known offset as possible, I'd like to know > (units are degrees and mm): > Offset > Height measured from rim next to LNB arm to opposite rim > Distance of rim next to LNB arm to front-face of LNB > Ditto for opposite rim.
> If anybody can help out, I'd be most grateful. In hope, I'll dare to > say TIA.
> Sorry, it would also be useful to know the width measured rim to rim > at right-angles to the height measurement previously explained.
> On Mon, 02 Nov 2009 13:49:52 +0000, Java Jive <j...@evij.com.invalid> > wrote:
>> For as many dishes of known offset as possible, I'd like to know >> (units are degrees and mm): >> Offset >> Height measured from rim next to LNB arm to opposite rim >> Distance of rim next to LNB arm to front-face of LNB >> Ditto for opposite rim.
> Not exactly useful advice to someone in the middle of nowhere, with a > minidish of unknown offset ...
> On Mon, 2 Nov 2009 21:31:13 -0000, "-<GB>-Carpy" <ca...@nothanks.com> > wrote:
>> Wouldn't it be easier to just buy a £10 sat bleeper? Line the dish up & >> get >> down the pub for a few jars. > -- > ========================================================= > Please always reply to ng as the email in this post's > header does not exist. Or use a contact addresses at: > http://www.macfh.co.uk/JavaJive/JavaJive.html > http://www.macfh.co.uk/Macfarlane/Macfarlane.html
OK. How about a compass, and use the Sky box, wife and a cordless phone as your alignment meter?
At 13:57:10 Mon, 2 Nov 2009, Java Jive <j...@evij.com.invalid> wrote in article <61pte55gvhbbcgsv3h72rr6jsesii7u...@4ax.com>:
>Sorry, it would also be useful to know the width measured rim to rim >at right-angles to the height measurement previously explained.
I suggest you analyse the curvature of the dish. Place a straight edge across the dish from top to bottom and measure the distance to the surface at, say, three suitably spaced points. Together with the end points which touch the rim of the dish, this will give you five points which must lie a parabolic curve, the axis of which will be at some (as yet unknown) angle to the straight edge, and will have its origin at some unknown point along the curve of the dish.
Now write a small program which will allow you to repeatedly plug in the unknown factors until you find the best possible match to the equation of a parabola of the form y = a.x^2 where 'a' is a constant equal to the reciprocal of four times the (as yet unknown) focal length.
With my dish, I found that that the vertex of the parabola was located at the lower rim of the dish, and that the axis formed an angle to the straight edge of 70 degrees. This (I think) implies an offset angle of 20 degrees, though I have no idea what the offset of this dish is supposed to be. It also turned out that the LNB was mounted at the correct distance away from the dish, but was a couple of centimetres lower than it should have been.
> I suggest you analyse the curvature of the dish. Place a straight edge > across the dish from top to bottom and measure the distance to the > surface at, say, three suitably spaced points. Together with the end > points which touch the rim of the dish, this will give you five points > which must lie a parabolic curve, the axis of which will be at some > (as yet unknown) angle to the straight edge, and will have its origin at > some unknown point along the curve of the dish.
> Now write a small program which will allow you to repeatedly plug in the > unknown factors until you find the best possible match to the equation > of a parabola of the form y = a.x^2 where 'a' is a constant equal to > the reciprocal of four times the (as yet unknown) focal length.
Although there are already around in the public domain such programmes I could copy, the method you suggest depends on a series of fairly accurate measurements by the user of my site. I'd rather keep it simple, though, of course that may not prove possible. I'm taking the attitude that I'll only cross that bridge when I find I have come to it.
> With my dish, I found that that the vertex of the parabola was located > at the lower rim of the dish, and that the axis formed an angle to the > straight edge of 70 degrees. This (I think) implies an offset angle > of 20 degrees, though I have no idea what the offset of this dish is > supposed to be. It also turned out that the LNB was mounted at the > correct distance away from the dish, but was a couple of centimetres > lower than it should have been.
This is a common, perhaps even a universal (I don't know for sure) arrangement for commercial dishes for broadcast TV, which my new method of calculation assumes to be the case. The dish is a section of a parabaloid of revolution about an x-axis through the bottom edge and the focal point within the LNB. The angle between a straight edge placed vertically across the dish and the y-axis is the offset, and therefore that between the same straight edge and the x-axis is 90-Offset. The other measurements, the distances to the LNB (that between the lower rim and the LNB being the focal length), now form a triangle which can be solved using the cosine rule. Hence you have cos(90-Offset), or sin(Offset).
The big question is, is this reasoning correct? That why I need measurements of dishes of known offset to confirm or deny. -- ========================================================= Please always reply to ng as the email in this post's header does not exist. Or use a contact addresses at: http://www.macfh.co.uk/JavaJive/JavaJive.html http://www.macfh.co.uk/Macfarlane/Macfarlane.html
> How is a compass supposed to help with the problem of not knowing the > dish offset?
You'd never make it into the A-Team with a defeatist attitude like that ! They made flame throwing tanks from a few bits of cord and a handful of old antique furniture.
I (perhaps wrongly) assumed that you were trying to setup a minidish? What exactly is the outcome you are searching for? Is this just a purely academic excercise?
> This is a common, perhaps even a universal (I don't know for sure) > arrangement for commercial dishes for broadcast TV, which my new > method of calculation assumes to be the case. The dish is a section > of a parabaloid of revolution about an x-axis through the bottom edge > and the focal point within the LNB. The angle between a straight edge > placed vertically across the dish and the y-axis is the offset, and > therefore that between the same straight edge and the x-axis is > 90-Offset. The other measurements, the distances to the LNB (that > between the lower rim and the LNB being the focal length), now form a > triangle which can be solved using the cosine rule. Hence you have > cos(90-Offset), or sin(Offset).
This reminds me of when I went on a satellite TV course in the very early days. Several of us were earning money installing steerables, and we'd learnt by experience how to find the arc, set the declination offset, set the polarization offset, etc. The lecturer was all theory, blathering maths at us all day about azimuths and elevations and so forth. Pages of maths just to point a dish -- how we laughed!
As an aside, the college decided to install a dish on the flat roof, so they called in the Works Department to make a base. They must have envisaged something like Jodrell Bank because we were provided with a platform made from 10" square baulks of timber 15ft long.
On Tue, 3 Nov 2009 19:29:13 -0000, "Wrights" <em...@f2s.com> wrote:
> This reminds me of when I went on a satellite TV course in the very early > days. Several of us were earning money installing steerables, and we'd > learnt by experience how to find the arc, set the declination offset, set > the polarization offset, etc. The lecturer was all theory, blathering maths > at us all day about azimuths and elevations and so forth. Pages of maths > just to point a dish -- how we laughed!
That's all very well, Bill, but I dare say you had a lot of related experience and any required supporting technical equipment contemporarily consistent with the profession.
IIRC also, there was a thread here in the last year or so where a regular contributor to this ng, who might therefore be expected to have enough background knowledge, attempted to install his own dish and ended up on the wrong sat. So if people with such a background can get into a difficulty, then others with less knowledge will surely do so.
And my site is aimed not so much at pros like yourself, but amateurs with a bit of DIY knowhow but no particular knowledge of satellite TV who wish to do the job themselves. For such people, any unexpected difficulty such as having a dish with no scale and of unknown offset poses quite a difficult problem, as I know very well - neither of my dishes have scales, and of neither do I know the manufacturers' quoted offset!
"A dish is just a dish isn't it?" I guess it doesn't seem sensible to the manufacturers' to incur any significant cost in designing and stamping a scale that may only be used once for the entire dish's life, when most installers have meters that will do a better job anyway. This may be the reason for the seemingly increasing prevalence of dishes with badly or incorrectly stamped scales, or even none at all, and no supporting documentation.
Yes, meters are cheap these days, but even so, for the amateur it's a purchase of something that will likely be used once and then lie in a draw until the owner moves house, or some such. And, to use a meter, you still have to get the initial alignment, preferably to avoid confusion on the *correct* sat, sufficiently accurately. Hence, some systematic method would be useful for aligning dishes with no scales, and this is simply what I'm trying to achieve.
In the analogy of driving a car, as long as I can turn on the ignition and leave the rest to nature, so to speak, the fount of related mechanical knowledge that I have is not useful. However, when it breaks down ...
So as, or so I would judge, you are more likely than most to be in a position to provide the information required, any chance? To recap, it's: Manufacturer's Quoted Offset (degrees) Height (mm) Width (mm) Distance of both top and bottom rims to face of LNB (mm).
> As an aside, the college decided to install a dish on the flat roof, so they > called in the Works Department to make a base. They must have envisaged > something like Jodrell Bank because we were provided with a platform made > from 10" square baulks of timber 15ft long.
Are you quite certain it's a satellite TV and not a radio-astronomy dish they're after? :-)
> On Tue, 3 Nov 2009 19:29:13 -0000, "Wrights" <em...@f2s.com> wrote:
>> This reminds me of when I went on a satellite TV course in the very early >> days. Several of us were earning money installing steerables, and we'd >> learnt by experience how to find the arc, set the declination offset, set >> the polarization offset, etc. The lecturer was all theory, blathering >> maths >> at us all day about azimuths and elevations and so forth. Pages of maths >> just to point a dish -- how we laughed!
> That's all very well, Bill, but I dare say you had a lot of related > experience and any required supporting technical equipment > contemporarily consistent with the profession.
> IIRC also, there was a thread here in the last year or so where a > regular contributor to this ng, who might therefore be expected to > have enough background knowledge, attempted to install his own dish > and ended up on the wrong sat. So if people with such a background > can get into a difficulty, then others with less knowledge will surely > do so.
> And my site is aimed not so much at pros like yourself, but amateurs > with a bit of DIY knowhow but no particular knowledge of satellite TV > who wish to do the job themselves. For such people, any unexpected > difficulty such as having a dish with no scale and of unknown offset > poses quite a difficult problem, as I know very well - neither of my > dishes have scales, and of neither do I know the manufacturers' quoted > offset!
> "A dish is just a dish isn't it?" I guess it doesn't seem sensible to > the manufacturers' to incur any significant cost in designing and > stamping a scale that may only be used once for the entire dish's > life, when most installers have meters that will do a better job > anyway. This may be the reason for the seemingly increasing > prevalence of dishes with badly or incorrectly stamped scales, or even > none at all, and no supporting documentation.
> Yes, meters are cheap these days, but even so, for the amateur it's a > purchase of something that will likely be used once and then lie in a > draw until the owner moves house, or some such. And, to use a meter, > you still have to get the initial alignment, preferably to avoid > confusion on the *correct* sat, sufficiently accurately. Hence, some > systematic method would be useful for aligning dishes with no scales, > and this is simply what I'm trying to achieve.
> In the analogy of driving a car, as long as I can turn on the ignition > and leave the rest to nature, so to speak, the fount of related > mechanical knowledge that I have is not useful. However, when it > breaks down ...
> So as, or so I would judge, you are more likely than most to be in a > position to provide the information required, any chance? To recap, > it's: > Manufacturer's Quoted Offset (degrees) > Height (mm) > Width (mm) > Distance of both top and bottom rims to face of LNB (mm).
>> As an aside, the college decided to install a dish on the flat roof, so >> they >> called in the Works Department to make a base. They must have envisaged >> something like Jodrell Bank because we were provided with a platform made >> from 10" square baulks of timber 15ft long.
> Are you quite certain it's a satellite TV and not a radio-astronomy > dish they're after? :-)
At 13:21:12 Tue, 3 Nov 2009, Java Jive <j...@evij.com.invalid> wrote:
>Thanks for the first sensible piece of advice John ... >> Now write a small program which will allow you to repeatedly plug in the >> unknown factors until you find the best possible match to the equation >> of a parabola of the form y = a.x^2 where 'a' is a constant equal to >> the reciprocal of four times the (as yet unknown) focal length.
>Although there are already around in the public domain such programmes >I could copy, the method you suggest depends on a series of fairly >accurate measurements by the user of my site. I'd rather keep it >simple, though, of course that may not prove possible. I'm taking the >attitude that I'll only cross that bridge when I find I have come to >it.
The problem can be simplified if the vertex of the parabola is assumed to be located on the lower rim of the dish. Only one measurement is then needed from the straight-edge to the surface of the dish, say at the exact centre. This leads to an equation which I think should be solvable without using some iterative procedure, though my maths doesn't seem to be quite up to it at the moment...
While the experts here may say that knowing the offset angle of a dish doesn't really matter, the instructions that came with my dish didn't even mention that there was such a thing, saying simply that the dish should be pointed at the sky at the angle of elevation - and helpfully giving a long list of the elevations for Astra 28E for places all over the UK. So I started off by tilting the dish at an angle of 25 degrees. Luckily, I had also read somewhere that it was best to begin by pointing the dish towards the horizon. Of course, anyone who's installed more than one dish will have a pretty good idea of what the angle should be in their area.
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