Make Gregorian dish

[william-1]

Is this link of any help to you ?
http://www.satellite-calculations.com/Fibo/fibo.htm

 

[RimaNTSS]

william-1! Thanks for link. Of course I know this link and it is very helpful. You can find there technical information about 3 dishes (55cm, 90cm, 120cm) made long time ago. But, unfortunately this link does not give us answers we need to know to calculate our own Gregorian type dishes.

 

[Riverblue]

And now, lets speak about IBU. Can anybody provide it's opening angle and also place on it's body dot which will represent focal point?

Link to inverto black ultra specs, can't see anything about opening angle though!
/http://www.inverto.tv/products/product.php?section=1&id=57

 

[RimaNTSS]

Link to inverto black ultra specs, can't see anything about opening angle though!

Agree with you, but IBU certainly has opening angle! Any suggestions?
And what about location of focal point?

 

[Riverblue]

I have dropped Inverto DE an e-mail, let's see if we get a response. ;)

 

[RimaNTSS]

Before they answer (if they answer at all), I suggest it is 70* and focal point is located where I put the red dot. But here I would like more users to agree or disagree (elaboration).

 

[Riverblue]

Seems that you measured your dish quiet precisely. Program shows offset angle 22,6* (as you mentioned earlier), and opening angle precisely 80*. I circled those numbers red. View attachment 69077 View attachment 69078 Circled green distances from Focus to UP and DOWN points of antenna. And blue is distance from Focus till you know what.... "Le point G"
But, now we will only use opening angle of your antenna, which is 80*.
So, lets draw two crossing lines with 80* between them. Something like this View attachment 69079

Humm, according to your program the measurements for my 1.2m CM are A=1211mm, B=712mm and the opening angle is 80⁰.
My measurements were, A=1230, B=720 and the opening angle was 78,2⁰
My measurements were taken with a quality steel tape measure.
The angle was taken with a stabila electronic angle finder (very accurate 0.0005⁰ ) and I took my time and rechecked all measurements, I placed my straight edge from point A (top of dish face) to the centre of the lnb and measured the angle. then repeated the procedure for point B (bottom of dish face). I then added the two angles together to get 78.2⁰
I used the point arrowed in the picture as the centre of the focus for the lnb and therefore my point for measurements. Anyone have any differing thoughts on this, as I couldn't find any info re this point!


The difference in these two sets of measurements would suggest that the lnb support arms are either holding the lnb in a slightly wrong position too far away from the dish face or possibly the lnb wants pulling forward towards the dish face! The only problem is if I pull the lnb forward, the signal gets weaker?I spent a lot of time when I set this dish up adjusting the position of the lnb and it's skew for it to end up where it is now. But the calcs of the parabola4 program suggest it wants to be moved?
When I have some spare time, maybe mid week, I shall re check all these measurements again!

 

[Riverblue]

Before they answer (if they answer at all), I suggest it is 70* and focal point is located where I put the red dot. But here I would like more users to agree or disagree (elaboration).
View attachment 69083

Can I ask how you determined the opening angle to be 70⁰? Is that based on your experience or did you find a source for the info?

 

[archive10]

Need to look up my maths, but the IBU has an f/D of 0.6, I'm sure this can be translated into opening angle?

 

[RimaNTSS]

The only problem is if I pull the lnb forward, the signal gets weaker?

Gotha! After you move LNB closer/further to dish, you should also adjust elevation of antenna, only then signal level will rise. You can always do one small experiment: By adjusting elevation of your dish and getting maximum signal on, lets say, 7*W, measure also angle of dish surface (should use digital protractor). Than, by knowing precise elevation of 7*W for your location and precise offset angle of your antenna you can calculate angle antenna should be leaning. So, compare calculated and real antenna's leaning angles you can also estimate whether LNB is in Focus.

Can I ask how you determined the opening angle to be 70⁰?

I have heard that someday somewhere (do not remember exactly). Or it was about that IBU is designed to work OK on majority of nowadays dishes, like on "average dish" .
We can assume that IBU has 75* illumination angle.

I do not know how to translate f/D of 0.6 to an angle.

 

[Riverblue]

Sorry to go off topic, but if my 1.2m CM has an opening angle of 80⁰ what are the opening angles of most other offset dishes, would 80ish degrees be a common angle? The only reason I ask is because if an Inverto Black Ultra, the supposedly best rated/performing current lnb, really does have an opening angle of 70⁰ then surely it is massively under illuminating my dish? Or am I not understanding that correctly?

 

[Riverblue]

Gotha! After you move LNB closer/further to dish, you should also adjust elevation of antenna, only then signal level will rise. You can always do one small experiment: By adjusting elevation of your dish and getting maximum signal on, lets say, 7*W, measure also angle of dish surface (should use digital protractor). Than, by knowing precise elevation of 7*W for your location and precise offset angle of your antenna you can calculate angle antenna should be leaning.

Yes, I did do this as well as just adjusting the position of the lnb and it's skew. Finitely adjusting each in turn until my meter readings peaked, then carry on until they drop off, then going back to the peak reading again and repeatedly finitely adjusting all the variables! I'm very confident that it's as close to ideal with regard to making the normal adjustments! I will now, however, have to play with trying to ease the lnb into a slightly different position to see if I can gain anything, or not. ;)

 

[RimaNTSS]

I used the point arrowed in the picture as the centre of the focus for the lnb and therefore my point for measurements. Anyone have any differing thoughts on this, as I couldn't find any info re this point!

When I practiced on 1,8m offset, I used to IBU LNB focus little bit further, like 3-4mm further, from white/black line.

if an Inverto Black Ultra, the supposedly best rated/performing current lnb, really does have an opening angle of 70⁰ then surely it is massively under illuminating my dish? Or am I not understanding that correctly?

You are right, your dish is 80* and would be underilluminated by IBU if IBU was 70*. But we do not know how good your dish is illuminated by IBU and also do not know how to check it.

the lnb support arms are either holding the lnb in a slightly wrong position

This is possible, and never mind it is CM dish.

 

[Trust]

Some time ago i was surching for the best lnb position at a Triax TDS 64 , made a tool to messure the openings angle of a BU .
I dont know its the correct way to messure it but it came out to 55º , also in other brands lnb's . (red lines)
But since there are rumors that the receiving point in the lnb is in the center of a ball shaped aerea of ± 25 mm (according to the KU wave lenght) I draw the bleu lines .

 

[Riverblue]

I received a reply e-mail from a Mr Roche at Inverto;

Dear Sir,

All our LNB for offset antenna are for a F/D ratio of 0.6, so the aperture angle is around 84 deg. at -10dB tapper.

The focal point is a spec. of the dish.

The phase center of the LNB feed horn is at the center of the cap and 5mm behind the cap.

Best regards,

Daniel Roche

Manager Customer Service



Very nice and prompt! It also provides us with the answers to our question.

 

[Riverblue]

With the information provided by Inverto we now know to take our measurements from the point shown below.









The focal point is obviously central

 

[RimaNTSS]

Great! It is obvious Inverto representative knows what he is speaking about and he also taught us small terminology lesson
Of course, antenna has Focal point (Focus) and opening angle but LNB has Aperture angle (Illumination angle) and Phase center.
With this data: Aperture angle 82* and Phase center 5mm behind plastic cap we can continue design of Gregorian antenna.

 

[Riverblue]

This also makes a difference to the measurements I took of my own 1,2m CM, it shortens them by approximately 12mm, making them very close to those of the Parobola4 calcs!

 

[RimaNTSS]

Now you better paint picture of IBU in scale for example 1:1, means 126mm long. Also dot where phase center is located and draw 82* aperture angle. Be prepare to move all those elements as one picture many times without changing proportions and location one element from other.
and so on....

 

[RimaNTSS]

And, in parallel of drawing your nice pictures of IBU, start ti find answer to my previously posted question: How to draw an ellipse if you know locations of its focuses and you also given 2 dots on surface of ellipse? Is that possible at all?