2.4 Prime focus what will be its equivalence in Offset dish for Ku purposes?

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enb141

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Hi guys I was looking to buy a bigger sat but unfortunatelly prime focus bigger than 1.5 M are so expensive so instead of that I was thinking to get a 2.4 M prime focus for the price of 1.5M offset so I would like to know what would be the equivalent size of this 2.4 Prime focus into offset.
 
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pendragon

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It's 1:1. A 2.4m offset should have a similar gain to a 2.4m prime focus. However there's also real life and you need to compare apples to apples. You need feeds and LNBs matched to the dish in question. Using an offset LNBF on a prime-focus dish is guaranteed to get less gain than the dish is capable of providing. There are Ku feeds for prime focus dishes and they work very well. You also did not specify the construction of the 2.4m you are considering. For Ku-band, you probably want a solid 2.4m instead of a mesh or a perf because surface imperfections can eat up gain for Ku on big dishes.
 
Anole

Anole

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alternatives:

From what I've read, I don't think you would be happy with such a combination.
You need feeds and LNBs matched to the dish in question.
Using an offset LNBF on a prime-focus dish is guaranteed to get less gain than the dish is capable of providing.
There are Ku feeds for prime focus dishes and they work very well.

The F/D ratio of your prime focus dish would be around 0.3 or 0.4.
However, the F/D of the Ku portion of those dual-band LNBFs is more on the order of 0.6 to 0.7.
That's true of all the (relatively) new brands. Not picking on just one.

So, if your LNBf isn't seeing all the dish, you're not getting maximum performance.

There are some special Ku-only LNBFs from several suppliers, which should do much better.
SatelliteAV seems to have dropped their GeoSatPro model from their catalog.
DMSI had one, too. I no longer see it listed on their web site.

Invacom makes an AF-120 feedhorn that will match a BUD.
Combined with a QPF-031 (or any other flange-mount) LNB, that should get the job done.
It's it's KU-only, too.

The only other solutions that come to mind are:
- old style co-rotor
- dual band orthomode feedhorn (uses two C-band LNBs and two Ku-band LNBs) $250..500..or more.
 
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enb141

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Do you think with that Invacom I will get as good results as an 2.4 offset or what would be the results?
 
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pendragon

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I've put Invacom C120 flange LNBs on their ADF-120 feed and the combination is very, very good on prime-focus dishes. Better than anything else I've ever tried for Ku. The ADF-120 is adjustable over a very wide range of f/Ds, and I've even modified mine to try on a low f/D offset (0.5) sometime. With the C120 Invacom LNBs you also have a nice choice between Ku Universal and Ku Linear/Circular, depending on what you targets are. Highly recommended.
 
B.J.

B.J.

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Hi guys I was looking to buy a bigger sat but unfortunatelly prime focus bigger than 1.5 M are so expensive so instead of that I was thinking to get a 2.4 M prime focus for the price of 1.5M offset so I would like to know what would be the equivalent size of this 2.4 Prime focus into offset.

It's 1:1. A 2.4m offset should have a similar gain to a 2.4m prime focus. ....

I know that this is probably negligeable when compared to other parameters, but in theory a prime focus dish "should" be capable of higher gain than an equivalent sized offset dish, assuming that you ignore the signal blocked by the feed. This is because the gain is somewhat related to the cross section area of signal gathered. For a prime focus dish, the surface of the dish is ~perpindicular to the direction it's aiming, so you get maximum signal gathering. However an offset dish tends to be sideways to the signal, so the full area of the dish is not gathering signal. As an illustration, consider if the dish were turned almost parallel to the signal direction, the signal would be looking at the edge of the dish, and the dish wouldn't gather any signal.
Offset dishes are often elliptical (taller than wide, unless designed for multiple feeds), and I think the reason is to make it look circular if viewed from the direction it is actually aiming. I think it would therefore be a good approximation to relate the equivalent dimension of an offset dish to be it's width, since that is still perpindicular to the signal direction. Unfortunately, most offset dishes have their size listed pretty close to an average between width and height, rather than just the width. All bets are off if the dish is wider than tall. I think for wide dishes, even the height would be an over-estimate of it's equivalent gathering power related to prime focus.
But as mentioned, there are so many other factors, such as signal blocked by the feed, the F/D viewing angle of the feed relative to illumination of the dish, surface accuracy, etc, etc, that it seems like the comparison between offset and prime focus is a minor issue.
 
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pendragon

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Offset dishes are often elliptical (taller than wide, unless designed for multiple feeds), and I think the reason is to make it look circular if viewed from the direction it is actually aiming. I think it would therefore be a good approximation to relate the equivalent dimension of an offset dish to be it's width, since that is still perpindicular to the signal direction. Unfortunately, most offset dishes have their size listed pretty close to an average between width and height, rather than just the width. All bets are off if the dish is wider than tall. I think for wide dishes, even the height would be an over-estimate of it's equivalent gathering power related to prime focus.

In my experience I would beg to disagree. The offset dish specs I have come across have always spec'ed the equivalent diameter, which is normally very similar to the width. Any spec claiming an offset dish size to be that of the vertical dimension would be exaggerating. Even toroids seem to be spec'ed on equivalent diameter, in spite of the fact that any dimension one measures is greater than this diameter.

The important piece is almost all offset feeds have a conical illumination angle. You certainly don't want to overscan the dish, as that will only add noise. This means each dimension has to be at least large enough for full illumination in both directions. On the other hand there will be no signal improvement gained by adding more dish material to a particular dimension beyond what a single feed illuminates (the exception being dishes designed for mulitple feeds). That means neither dimension is likely to be larger than what is required. The combination results in the classic taller than wider profile for offset dishes, with the width being essentially the equivalent diameter of a prime-focus dish.
 
SatelliteAV

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It is impossible to predict which for these designs will provide the optimum reception in any situation as the reflector efficiency, feed horn illumination and ground noise rejection all come into play.

An offset design will out perform a prime focus design of similar efficiency by approximately 10% if both are outfitted with an appropriate feed horn and optimized placement. An offset design performance advantage can be noted during reception of low elevation satellites due to the reduced collection of ground noise by the LNB.

Most performance gains when comparing reflector types with similar efficiency and gain will be the result of rejection of ground noise.

If the offset design is designed for single satellite collection, an increased ratio of height to width will provide better rejection of terrestrial noise and an increased ratio of width to height will provide better rejection of adjacent satellites.
 
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pendragon

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It is impossible to predict which for these designs will provide the optimum reception in any situation as the reflector efficiency, feed horn illumination and ground noise rejection all come into play.

Yes, and the feed design will also have a fair impact on the on the efficiency, cross-polarization, gain taper, etc. Unfortunately this is not typically well exploited in the FTA domain.

An offset design will out perform a prime focus design of similar efficiency by approximately 10% if both are outfitted with an appropriate feed horn and optimized placement. An offset design performance advantage can be noted during reception of low elevation satellites due to the reduced collection of ground noise by the LNB.

I'm not sure what the 10% refers to. If it's an improvement in CNR, I am a little skeptical as that would translate to nearly a dB of improvement. Any difference that substantial would probably reflect a poor feed/dish mismatch or a mediocre taper.

In terms of the performance of offsets vs. prime-focus on low-elevation satellites, that can be interpreted in more than one way. As an example for a given, moderate latitude location on a polar mount, the performance of an offset in terms of ground noise rejection is actually best at true south (highest-elevation satellites) when the feed is pointing at its highest. As the offset dish is rotated on the polar mount to the low-elevation extremes, an offset feed will lose this advantage. Conversely, a prime-focus feed has the worst ground noise rejection at true south, but as the dish becomes more vertical, there is less ground illuminating the feed. In fact it can make sense to change the gain taper for dishes that spend more time in one regime or the other. I have done this and it is effective.

Most performance gains when comparing reflector types with similar efficiency and gain will be the result of rejection of ground noise.

Perhaps this actually reflects more the quality of the feed match to the dish, which has ground noise rejection as one of its effects.

If the offset design is designed for single satellite collection, an increased ratio of height to width will provide better rejection of terrestrial noise and an increased ratio of width to height will provide better rejection of adjacent satellites.

I will allow that gain tapers/feed illumination patterns are not a step function, so some of what you say here is true. But if we take a reasonably well-matched feed and dish that is not leaking an appreciable amount of ground noise, adding more material to an offset width or height is going to change things very minimally because this additional area is not being illuminated to any significant degree by the feed.

So this leads to a related question about the GeosatPro 1.2m offset dishes you sell. They have a fairly low f/D (0.5) as offset dishes go. This should translate to great ground noise rejection, but at the expense of gain given that many offset feeds are more intended for higher f/Ds. Was this an intentional trade-off, and does it make sense in view of the 'offset advantage'?

I've always been curious about this. Don't get me wrong, I love my GeosatPro 1.2, its performance is excellent and the quality of its build is perfect for the price. I almost don't want to touch it as I can't imagine it doing much better, but my calculations suggest I would do a little better by matching a feed to its f/D.
 
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enb141

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An offset design will out perform a prime focus design of similar efficiency by approximately 10% if both are outfitted with an appropriate feed horn and optimized placement. An offset design performance advantage can be noted during reception of low elevation satellites due to the reduced collection of ground noise by the LNB.

So basically a 2.4M prime focus with this prime focus F/D optimized LNB will be as good as 2.16M offset dish using an F/D for offset LNB?

Also with this the higher elevation, better for the Prime Focus?
 
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pendragon

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So basically a 2.4M prime focus with this prime focus F/D optimized LNB will be as good as 2.16M offset dish using an F/D for offset LNB?

We're debating whether there is even as much as a 10% effect. And because the area is proportional to the square of the diameter, 2.4m vs. 2.16m is a 20% effect. So the question is whether a 2.27m offset can beat a 2.4m prime focus.

Also with this the higher elevation, better for the Prime Focus?

Actually offsets have a slight edge at higher elevations. But that's all woven into the 10% or less factor.
 
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enb141

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We're debating whether there is even as much as a 10% effect. And because the area is proportional to the square of the diameter, 2.4m vs. 2.16m is a 20% effect. So the question is whether a 2.27m offset can beat a 2.4m prime focus.



Actually offsets have a slight edge at higher elevations. But that's all woven into the 10% or less factor.

Here in my country a 2.4 prime focus is not that expensive, but a 2.4 offset is extremely expensive, let say $300 (prime focus) versus $650 (offset) so to me there's no way to get an 2.4 offset dish is just too expensive.
 
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pendragon

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Here in my country a 2.4 prime focus is not that expensive, but a 2.4 offset is extremely expensive, let say $300 (prime focus) versus $650 (offset) so to me there's no way to get an 2.4 offset dish is just too expensive.

A 2.4m solid, prime-focus will work very well for your purposes. The recent discussions have been for the most part only of academic interest.
 
SatelliteAV

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So this leads to a related question about the GeosatPro 1.2m offset dishes you sell. They have a fairly low f/D (0.5) as offset dishes go. This should translate to great ground noise rejection, but at the expense of gain given that many offset feeds are more intended for higher f/Ds. Was this an intentional trade-off, and does it make sense in view of the 'offset advantage'?

I've always been curious about this. Don't get me wrong, I love my GeosatPro 1.2, its performance is excellent and the quality of its build is perfect for the price. I almost don't want to touch it as I can't imagine it doing much better, but my calculations suggest I would do a little better by matching a feed to its f/D.

The consistent formation of the reflectors and the higher efficiency of the GEOSATpro 1.2M was the main deciding factor when choosing this design over samples tested from other manufactures. We were initially concerned about reduced performance due to potential F/D mismatch, but the performance of the design with readily available LNBFs consistently out performed dish samples with higher F/Ds matched to the feedhorns. So yes, the decision was intentional as the resulting combination provided lower baselines. :D

I agree, purchase the best efficiency 2.4M prime focus design in your price range and a matching feedhorn. The increased cost for a fractional improvement is not worth splitting hairs over!
 
Anole

Anole

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Sep 22, 2005
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I believe part of the price is the difference between a professional offset, and a non-pro prime feed.
The other difference is in the shipping cost of a 2.4m solid! It's hard to transport, and will cost a lot to move.
On the other hand, if you could live with a segmented 2.4m prime feed, not only is the dish less to purchase, but the transportation cost is much less.

Here's a picture of the box for a 2.4m dish from Sadoun, compared to some other dishes.
You might also inquire of SatelliteAV if he can ship you a 2.4m dish.
 
B.J.

B.J.

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Oct 15, 2008
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In my experience I would beg to disagree. The offset dish specs I have come across have always spec'ed the equivalent diameter, which is normally very similar to the width.

OK, I was mainly going by the Fortec dishes. For example, I have a Fortec 36" dish. The specs show that it is 33.5" wide, and 37" tall. If you multiply the 37" height by the cos(24.6), you get 33.6 {24.6 is the offset angle}. Ie, it's clear that they've designed the thing to be a shape that looks circular in the direction of the sat, and it's clear that it would have a cross section equivalent to the 33.5" width, but they call it a 36" dish for some reason.
I don't have the h/w specs of other dishes handy, so perhaps other manufacturers are more realistic with their specs.
 
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