Advice Needed Before I make a complete ass of myself again!

[Channel Hopper]

From space, an offset dish looks round. So a 1.8m dish that is 1.8m horizontally and 2.0m vertically appears as a round 1.8m dish. This is because unlike a prime focus dish, that looks directly at a satellite, an offset dish looks at the satellite at an angle (the offset angle). It's that angle that causes the vertical dimension to look smaller than it is.

I think........

Which only partly explains the reason for having an offset design.

A parabola is a parabola whatever the cross section, any prime focus dish will have an f/d ratio that is set by one formula.

An offset dish is nothing more than a section of a prime focus version. Once you understand this it should all fall into place.

 

[Archive4]

Which only partly explains the reason for having an offset design.

A parabola is a parabola whatever the cross section, any prime focus dish will have an f/d ratio that is set by one formula.

An offset dish is nothing more than a section of a prime focus version. Once you understand this it should all fall into place.

View attachment 92883

Well said, but did you understand it

 

[davemurgtroyd]

From space, an offset dish looks round. So a 1.8m dish that is 1.8m horizontally and 2.0m vertically appears as a round 1.8m dish. This is because unlike a prime focus dish, that looks directly at a satellite, an offset dish looks at the satellite at an angle (the offset angle). It's that angle that causes the vertical dimension to look smaller than it is.

I think........

I don't think so - I think you will find it only appears circular from the lnb

 

[a33]

Well, 'from space' in general (from all directions in space) is incorrect, that's true.

And, good point, from the LNB. That's why one can use an offset dish upside down...

But, 'from the direction of the satellite the dish is aimed at', it is still correct.

That is, it is only correct if the dish is purely parabolid, and has a flat surface when you lay it down. To be precise ;) .
Do I still forget something?

Greetz,
A33

 

[a33]

That's why one can use an offset dish upside down...

Come to think of it: This part of what I wrote is nonsense! It is true that you can use an offset dish upside down, but the one thing has nothing to do with the other...
Sorry!

Well, 'from space' in general (from all directions in space) is incorrect, that's true.
...
But, 'from the direction of the satellite the dish is aimed at', it is still correct.

This part is still highly correct, though, I'm quite sure.

Greetz,
A33

 

[davemurgtroyd]

Come to think of it: This part of what I wrote is nonsense! It is true that you can use an offset dish upside down, but the one thing has nothing to do with the other...
Sorry!


This part is still highly correct, though, I'm quite sure.

Greetz,
A33

An offset dish can only appear truly circular from one direction and that is the lnb - any other angle and it is elliptical. Think about it the forshortening effect is only correct from one angle - any other angle and it is too short or too long vertically - simple physics. The same is true of prime focus dishes although the lnb and the satellite then are in the same direction.

 

[Channel Hopper]

An offset dish can only appear truly circular from one direction and that is the lnb - any other angle and it is elliptical. Think about it the forshortening effect is only correct from one angle - any other angle and it is too short or too long vertically - simple physics. The same is true of prime focus dishes although the lnb and the satellite then are in the same direction.

Untrue
Parabolic reflector - Wikipedia, the free encyclopedia

 

[davemurgtroyd]

Untrue

It is true - forget the fact that the dish is a parabole - the rim of the dish (which is what defines its appearance) is an ellipse - viewing that from different angles can only make it appear circular from one angle ( to be precise two angles both at the same incident angle to the ellipse). Try it with an elliptical piece of paper/card.

 

[a33]

the rim of the dish (which is what defines its appearance) is an ellipse - viewing that from different angles can only make it appear circular from one angle ( to be precise two angles both at the same incident angle to the ellipse).

So what is your point? The two incident angles of an offset dish are 1. from the satellite, and 2. to the LNB; these angles are mirrored.
Why would (with perfect symmetry of the ellipse) only the viewing angle of the LNB be right, then? The viewing angle is mirrored to the angle towards the satellite.
As far as I can see, viewing distance (infinite or near) is not important for this issue.
[Though I cannot really imagine how it would be, if you stand just 10 millimeter before the dish..... My eyes would have a problem seeing the whole dish, I guess.]

Greetz,
A33

 

[Archive4]

Hey Boys! This thread is supposed to stop ME making an ass of myself.......

 

[Channel Hopper]

It is true - forget the fact that the dish is a parabole - the rim of the dish (which is what defines its appearance) is an ellipse - viewing that from different angles can only make it appear circular from one angle ( to be precise two angles both at the same incident angle to the ellipse). Try it with an elliptical piece of paper/card.

Not true, concentrate on the word 'ellipse' rather than forget the fact the dish is a parabola.

The circular paraboloid is one unique example of an infinite number of elliptical paraboloids, each with unique focal point but sharing the same formula.

Elliptic Paraboloid

For a circular paraboloid , a=b.




Whilst I have not studied feedhorn designs of this type, you will see that on many dishes out there, the shape of a matched horn is moulded / cast in a similar elliptical design to maximise the illumination in the required planes, realised by installing at 90 degrees to that of the nearest reflector.

 

[davemurgtroyd]

So what is your point? The two incident angles of an offset dish are 1. from the satellite, and 2. to the LNB; these angles are mirrored.
Why would (with perfect symmetry of the ellipse) only the viewing angle of the LNB be right, then? The viewing angle is mirrored to the angle towards the satellite.
As far as I can see, viewing distance (infinite or near) is not important for this issue.
[Though I cannot really imagine how it would be, if you stand just 10 millimeter before the dish..... My eyes would have a problem seeing the whole dish, I guess.]

Greetz,
A33

That is true for a prime focus dish but for an offset dish we are not talking about mirror images from the dish surface but from the elliptical rim of the dish and the incident angle from that which is not the same. See my reply to CH

 

[davemurgtroyd]

Not true, concentrate on the word 'ellipse' rather than forget the fact the dish is a parabola.

The circular paraboloid is one unique example of an infinite number of elliptical paraboloids, each with unique focal point but sharing the same formula.

Elliptic Paraboloid

For a circular paraboloid , a=b.

View attachment 92902


Whilst I have not studied feedhorn designs of this type, you will see that on many dishes out there, the shape of a matched horn is moulded / cast in a similar elliptical design to maximise the illumination in the required planes, realised by installing at 90 degrees to that of the nearest reflector.

You also are getting confused here. With an offset dish the "cut" across the paraboloid is not at right angles to its axis. The incident and reflected angles of the satellite beam are relative to that axis. The ellipse formed by the dish rim is not at right amgles to the paraboloid axis and hence the incident and reflected angles optically of it are completely different to the microwave reflections of the dish surface and hence the dish would not appear to be circulsr to the satellite (but some 20 odd degrees above the satellite LOS depending on angle that dish is cut from the paraboloid). Remember we are talking "reflection" from the elliptical plane of the dish rim and not the paraboloid surface of the dish. Only a prime focus dissh would appear circular to the satellite.

 

[Lazarus]

Hey Boys! This thread is supposed to stop ME making an ass of myself.......

It's a fascinating adjunct to your main Topic

 

[Channel Hopper]

You also are getting confused here. With an offset dish the "cut" across the paraboloid is not at right angles to its axis. The incident and reflected angles of the satellite beam are relative to that axis. The ellipse formed by the dish rim is not at right amgles to the paraboloid axis and hence the incident and reflected angles optically of it are completely different to the microwave reflections of the dish surface and hence the dish would not appear to be circulsr to the satellite (but some 20 odd degrees above the satellite LOS depending on angle that dish is cut from the paraboloid). Remember we are talking "reflection" from the elliptical plane of the dish rim and not the paraboloid surface of the dish. Only a prime focus dissh would appear circular to the satellite.

The shape of the antenna's parabola is irrelevant in all cases where there is 'shaping' during manufacture, or by machining the edges after cutting from a die to make packaging easier. Not the case with smaller pressed offset dishes where these can be stacked on crates in tens/fifties etc.

All elliptical antennas I have come across (notice the caveat ;) ), where the vertical axis is longer than the horizontal are moulded / pressed to a shape that makes the observation from both the feed and the satellite circular. It is not a coincidence that the feed position of an elliptical dish is designed to be the final position of the incoming signal, hence the more professional antennas use a circular feedhorn without difficulty.

I believe the Orbital 80 and 1m antennas could be flatpacked alternately inverted without damage (however the stamped bolt holes prevent this in reality) .

 

[Archive4]

If a standard 80mm offset dish is mounted with dish face perpendicular to the ground, and an identical 80mm offset is inverse mounted, but also perpendicular to the ground directly opposite at a distance of 10 meters, do you think the will ever get around to seeing they are the same?

 

[a33]

With an offset dish the "cut" across the paraboloid is not at right angles to its axis.
....
Only a prime focus dissh would appear circular to the satellite.

The beauty of a paraboloid dish, with its main axis directed to a satellite, is that ANY (straight) CUT looks circular from the satellite. Even the 'prime focus' cut ;) .

I am glad you added and pointed out that the view from the LNB is also circular; I forgot that for a moment.

For the rest: Your notions are contrary to common knowledge, as far as I know. Would you consider the possibility that you were wrong?

Greetz,
A33

 

[Channel Hopper]

The last page of the Channel Master 2.4 m two piece antenna

Print twice, cut out the reflector using a sharp pair of scissors (ask a parent ) and stick them in the 69 position (oooer missus)

 

[Trust]

Would the prove anything ?
My laserpointer can be adjusted at a angle and rotate by motor .
Pointed at a flat surface its making a 100% circle .
Placed in the lnb holder the spot follows the edge of a PrimeFocus- or Offset dish .

 

[Captain Jack]

That's a very good demo. Could do with one of those!