Just wondering what the differences are between a regular Ku lnb typically used on an offset dish compared to a Ku lnb designed to work on a prime feed dish? Just wondering why an LNB would perform differently if it was positioned at the focal point and how an lnb could be "designed" for one or the other. I'm not in anyway disputing there are differences rather I am just trying to understand why there is a difference. I'm not talking about combo (C/Ku) lnbs, just Ku lnb.
Ku LNBs prime vs. offset, whats the difference?
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Here is a test I did a while ago for someone in Hawaii. Used a prime focus KU LNB on 97W and then swapped to a regular LNB (offset)
There are some differences....where the prime focus shines is the weak signals.
http://www.satelliteguys.us/free-air-fta-discussion/181927-bsc321sp-f-d-ratio.html
There are some differences....where the prime focus shines is the weak signals.
http://www.satelliteguys.us/free-air-fta-discussion/181927-bsc321sp-f-d-ratio.html
It's all because of the feedhorn. The shape and position of the scalar plate on the feed determines how much and what part of the surface area of the dish that the feed can see. If it's not enough then the feed is losing signal by not seeing all the dish, and if it's too much then the feed is receiving earth noise from the area just to the outside of the dish. The pattern of how a feedhorn sees the surface of an offset dish is much different than the symmetrical nature of a prime focus.
Actually both dishes appear symmetrical to their feeds (circularly symmetric). The main difference is a prime-focus feed normally has a wider 'acceptance angle' than an offset feed. This is because prime-focus dishes tend to have a nominal f/D of 0.35 versus 0.6 for an offset. In this respect a prime-focus feed is more like a wide-angle lens and an offset feed is more like a telephoto lens.
As ACRadio indicated, you need to match the feed to the dish. If one puts an offset feed on a prime-focus dish, the offset feed will only 'see' the center part of the dish and miss the energy reflected from the outer part. If one puts a prime-focus feed on an offset dish, the feed will 'see' the whole dish, but also 'see' beyond the dish edge. This will increase the background noise and reduce the effective gain.
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25 words or less...
Think of BUDs (prime feed dishes) as having feedhorn/LNBs set for wide angle.
Think of offset dishes using feedhorn/LNBs set for telephoto.
Think of BUDs (prime feed dishes) as having feedhorn/LNBs set for wide angle.
Think of offset dishes using feedhorn/LNBs set for telephoto.
First of all, my first impression of this whole thread is that it's pretty much calling LNBFs LNBS. An LNB doesn't care what kind of dish it's getting the signal from. If anything, as Radar said, it's the feed that might care, not the LNB. I know it's easier sometimes to just call an LNBF an LNB, but it really isn't accurate in a discussion like this.
If you're talking LNB though, one difference is that consumers like us don't really have much access to any high quality LNBs that are part of an LNBF, whereas the separate LNBs available to BUDs users with feedhorns provides a much wider spectrum of quality.
Despite the discussion of whether the view of the dish is circular or not (for a dish that's taller than wide, yes, it will look circular to an offset feed, but dishes that are wider than tall obviously won't), the main goal, as Radar said, is to get the feed to look at the whole surface of the dish and no more, ie to get the maximum benefit out of the dish without picking up any extraneous noise.
Now, though, the part of this that has always interested me, and I have never really come to grips with it, is the analogy of a satellite dish to a camera or a telescope. Being or having been both a photography and astronomy hobbiest, I was very quick to trying to compare sat dishes with telescopes and cameras, with respect to the telephoto and resolution aspects of long focal lengths, and light gathering benefits of low F/D ratios. However, despite some 24 years of thinking about this, I really cannot find any scientific description of a TVRO dish that supports the analogy to the optical devices.
All the equations I have run across describe the resolution of a sat dish as being related only to the gain, and describe the gain as only being related to the diameter of the dish, not in any way to the focal length or F/D ratio. The only way I can rationalize the issue is to assume that it has to do with the fact that the telephoto/resolution/light gathering aspects of cameras and telescopes all really relate to viewing a field of view with light coming from the entire field, while a sat dish is only looking at a point source, but even that doesn't explain why the performance equations don't include FL or F/D at all, so I'm really having a hard time with this aspect.
I really believe that the main difference in performance between a high F/D dish and a low F/D dish is the rays coming from the extreme edges of a low F/D dish hit the dish at extreme angles, compared to the rays coming from the edges of a high F/D dish come in at a much lower angle. So a high F/D feedhorn is generally more efficient at receiving signal from the entire dish, not just the middle, particularly on Ku. One interesting observation I once made, was once, I didn't realize the several pine trees had grown 10 or 15' since I planted my dish, and were blocking the bottom half of my BUD. I was getting very poor reception on Ku, and couldn't figure out why. I went up on a ladder, and was playing with the aim of the feedhorn, and found that I could double or triple my signal by tilting my feedhorn upward by about 20 or more degrees, basically aiming the feed midway between center and the top of the dish. Since that was where most of my signal was coming from, I improved signal by improving the efficiency of the feed on off angle approach to the feed. Basically, since the bottom half of my dish was blocked, my prime focus dish was really behaving as an offset dish, like the bottom half wasn't even there.
Basically, for Ku, except for the fact that most offset dishes are high F/D and most prime focus dishes are low F/D, there really isn't any difference between an offset dish and a prime focus dish relative to performance, other than the obvious fact that the feedhorn blocks part of the signal on a prime focus dish. But I would really be interested in some scientific explanation as to why the equations for gain and resolution do NOT contain any reference to FL or F/D, when the telescope/camera analogy would seem to suggest that they should. BTW, I've asked this same question a dozen times before on various forums, and have yet to get what I thought was an answer that directly addressed the question.
If you're talking LNB though, one difference is that consumers like us don't really have much access to any high quality LNBs that are part of an LNBF, whereas the separate LNBs available to BUDs users with feedhorns provides a much wider spectrum of quality.
Despite the discussion of whether the view of the dish is circular or not (for a dish that's taller than wide, yes, it will look circular to an offset feed, but dishes that are wider than tall obviously won't), the main goal, as Radar said, is to get the feed to look at the whole surface of the dish and no more, ie to get the maximum benefit out of the dish without picking up any extraneous noise.
Now, though, the part of this that has always interested me, and I have never really come to grips with it, is the analogy of a satellite dish to a camera or a telescope. Being or having been both a photography and astronomy hobbiest, I was very quick to trying to compare sat dishes with telescopes and cameras, with respect to the telephoto and resolution aspects of long focal lengths, and light gathering benefits of low F/D ratios. However, despite some 24 years of thinking about this, I really cannot find any scientific description of a TVRO dish that supports the analogy to the optical devices.
All the equations I have run across describe the resolution of a sat dish as being related only to the gain, and describe the gain as only being related to the diameter of the dish, not in any way to the focal length or F/D ratio. The only way I can rationalize the issue is to assume that it has to do with the fact that the telephoto/resolution/light gathering aspects of cameras and telescopes all really relate to viewing a field of view with light coming from the entire field, while a sat dish is only looking at a point source, but even that doesn't explain why the performance equations don't include FL or F/D at all, so I'm really having a hard time with this aspect.
I really believe that the main difference in performance between a high F/D dish and a low F/D dish is the rays coming from the extreme edges of a low F/D dish hit the dish at extreme angles, compared to the rays coming from the edges of a high F/D dish come in at a much lower angle. So a high F/D feedhorn is generally more efficient at receiving signal from the entire dish, not just the middle, particularly on Ku. One interesting observation I once made, was once, I didn't realize the several pine trees had grown 10 or 15' since I planted my dish, and were blocking the bottom half of my BUD. I was getting very poor reception on Ku, and couldn't figure out why. I went up on a ladder, and was playing with the aim of the feedhorn, and found that I could double or triple my signal by tilting my feedhorn upward by about 20 or more degrees, basically aiming the feed midway between center and the top of the dish. Since that was where most of my signal was coming from, I improved signal by improving the efficiency of the feed on off angle approach to the feed. Basically, since the bottom half of my dish was blocked, my prime focus dish was really behaving as an offset dish, like the bottom half wasn't even there.
Basically, for Ku, except for the fact that most offset dishes are high F/D and most prime focus dishes are low F/D, there really isn't any difference between an offset dish and a prime focus dish relative to performance, other than the obvious fact that the feedhorn blocks part of the signal on a prime focus dish. But I would really be interested in some scientific explanation as to why the equations for gain and resolution do NOT contain any reference to FL or F/D, when the telescope/camera analogy would seem to suggest that they should. BTW, I've asked this same question a dozen times before on various forums, and have yet to get what I thought was an answer that directly addressed the question.
We can either be complicated or simple here, and I don't think there's much in between.
If we neglect the practicalities of the feed and consider only paraboloid reflectors, then in the ideal case any such reflector will concentrate all of its reflected energy at its focal point regardless of the focal length. Thus the dish gain will be only dependent on the diameter of the paraboloid and not on f or f/D. This seems intuitively obvious to me, so it's hard for me to get my head around this from a different perspective.
If however we were to additionally consider the physics of the feed, we would immediately dive far deeper into theory than would be appropriate for this forum. It's more than dangerous to generalize because there are many variables involved in feed design, but there is a tendency for low f/D feeds (and therefore associated dishes) to have lower gains, but higher delivered CNRs than high f/D feeds and associated dishes. So there can be a gain dependence on f/D, but it's more complicated than at first appearance: if you're transmitting, you want gain and for receiving you want CNR.
I don't particularly see an issue with the wide angle/telephoto analogy. In this case I was only applying it to the view seen by the feed, which is of course the dish. Regardless of the focal length, a feed positioned at the focal point will only see the parallel rays impinging on the dish from the direction it is pointed. But everything said also applies to parabolic mirror optical telescopes, f-stops, etc. The 'gain' or light-gathering potential of such a telescope is determined by the diameter of its primary mirror, not its focal length. The field of view in an optical telescope is more of interest than with a dish reflector, but we still position feeds off-axis in FTA. The reflector f/D does make a difference for both cases.
In terms of trees shadowing Ku reception on your BUD, it would appear the f/D of your Ku feed was larger than that of your dish. This might have allowed you to point the feed to the top of the dish where it was not shadowed, without overscanning the dish edge.
There is one other minor twist I can think of with respect to offset and prime-focus dishes. Even though the gain pattern of a feed is tapered to reject radiation beyond the edge of the dish, nothing is perfect and some thermal noise will still be accepted. A prime-focus dish at normal latitudes will tend to point the feed towards the earth when at 'true south' and less so as the dish moves toward the horizons. Thus there will theoretically be more of such thermal noise induced at 'true south' than at the horizons for a prime-focus dish, because the earth has a higher temperature than the sky. The opposite tends to be true for an offset dish. I doubt this is of much consequence.
If there's something I'm not directly addressing, please let me know so we can clear up the mysteries once and for all.
If we neglect the practicalities of the feed and consider only paraboloid reflectors, then in the ideal case any such reflector will concentrate all of its reflected energy at its focal point regardless of the focal length. Thus the dish gain will be only dependent on the diameter of the paraboloid and not on f or f/D. This seems intuitively obvious to me, so it's hard for me to get my head around this from a different perspective.
If however we were to additionally consider the physics of the feed, we would immediately dive far deeper into theory than would be appropriate for this forum. It's more than dangerous to generalize because there are many variables involved in feed design, but there is a tendency for low f/D feeds (and therefore associated dishes) to have lower gains, but higher delivered CNRs than high f/D feeds and associated dishes. So there can be a gain dependence on f/D, but it's more complicated than at first appearance: if you're transmitting, you want gain and for receiving you want CNR.
I don't particularly see an issue with the wide angle/telephoto analogy. In this case I was only applying it to the view seen by the feed, which is of course the dish. Regardless of the focal length, a feed positioned at the focal point will only see the parallel rays impinging on the dish from the direction it is pointed. But everything said also applies to parabolic mirror optical telescopes, f-stops, etc. The 'gain' or light-gathering potential of such a telescope is determined by the diameter of its primary mirror, not its focal length. The field of view in an optical telescope is more of interest than with a dish reflector, but we still position feeds off-axis in FTA. The reflector f/D does make a difference for both cases.
In terms of trees shadowing Ku reception on your BUD, it would appear the f/D of your Ku feed was larger than that of your dish. This might have allowed you to point the feed to the top of the dish where it was not shadowed, without overscanning the dish edge.
There is one other minor twist I can think of with respect to offset and prime-focus dishes. Even though the gain pattern of a feed is tapered to reject radiation beyond the edge of the dish, nothing is perfect and some thermal noise will still be accepted. A prime-focus dish at normal latitudes will tend to point the feed towards the earth when at 'true south' and less so as the dish moves toward the horizons. Thus there will theoretically be more of such thermal noise induced at 'true south' than at the horizons for a prime-focus dish, because the earth has a higher temperature than the sky. The opposite tends to be true for an offset dish. I doubt this is of much consequence.
If there's something I'm not directly addressing, please let me know so we can clear up the mysteries once and for all.
To continue the photographic analogy, LNBs are cameras with the ability to accept interchangeable lenses (feedhorns). LNBFs are cameras with a built-in lens (either wide angle for prime focus or telephoto for offset), with no ability to swap lenses.
As Mr. Spock might say, not a precise analogy, but a fairly rough comparison...
As Mr. Spock might say, not a precise analogy, but a fairly rough comparison...
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On reading the above couple of comments, and on thinking about this a bit more, I realized that I have been thinking about these issues all wrong.
The issue I was mainly troubled by was resolution, and I was looking at the equations for resolution for sat dishes, but only thinking intuitively when it came to cameras and telescopes. Never bothered to even look at the equations there.
When considering telescopes and cameras, I was intuitively, confusing resolution with the magnification/field of view. Pretty much, I was thinking that the resolution of a long focal length lens or mirror is greater than the resolution of a short lens, pretty much based on the magnification aspect, ie if I have a long lens, I can resolve objects that I can't resolve with a short lens, but this isn't really do to the theoretical resolution, but instead due to the grain of the film or pixels in the CCD. I was also thinking about the fact that with a camera or telescope, if you stop down say a 1.4 F/D lens to F22, you get an increase in depth of field, which I was interpreting as resolution. What I should have been considering was the angular resolution, not the focus of the raw image on a plane, be it film, CCD or my eye.
Anyway, when I looked up the equation for optical resolution, it is basically identical to the equations used for sat dishes, dependent only on the wavelength and diameter of the reflector, NOT dependent at all, on the focal length or F/D ratio. So I feel a bit dumb for not even looking at the equations for optical resolution. Anyway, this is starting to make more intuitive sense to me finally.
The issue I was mainly troubled by was resolution, and I was looking at the equations for resolution for sat dishes, but only thinking intuitively when it came to cameras and telescopes. Never bothered to even look at the equations there.
When considering telescopes and cameras, I was intuitively, confusing resolution with the magnification/field of view. Pretty much, I was thinking that the resolution of a long focal length lens or mirror is greater than the resolution of a short lens, pretty much based on the magnification aspect, ie if I have a long lens, I can resolve objects that I can't resolve with a short lens, but this isn't really do to the theoretical resolution, but instead due to the grain of the film or pixels in the CCD. I was also thinking about the fact that with a camera or telescope, if you stop down say a 1.4 F/D lens to F22, you get an increase in depth of field, which I was interpreting as resolution. What I should have been considering was the angular resolution, not the focus of the raw image on a plane, be it film, CCD or my eye.
Anyway, when I looked up the equation for optical resolution, it is basically identical to the equations used for sat dishes, dependent only on the wavelength and diameter of the reflector, NOT dependent at all, on the focal length or F/D ratio. So I feel a bit dumb for not even looking at the equations for optical resolution. Anyway, this is starting to make more intuitive sense to me finally.
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