Finding the LNBF target on offset dish with basic tools

[polgyver]

Proper aiming of LNBF is useful, as it helps obtain maximum Q. Some time ago Titanium planned to make laser device for it. Other forum member, Rima NTSS also did good work, using lasers, too.
Other members suggested computer program, Parabola 2, for needed calculations.
I tried to reach this goal without lasers and computer programs, using just paper, ruler and basic calculator.
Drawing of parabola was needed. Focal length was provided by Rima, in a picture of his post, its value - 725 mm.
For drawing of parabola, only the focal length is needed. Assumed scale 1:1
Pictures follow :

 

[RimaNTSS]

Congratulations polgyver , you've chosen hardest way to go (which is not bad) and used tools many people today do not even know they exist! Good job. And, at the end of the day, you got all needed measurement of dish. However, I am afraid you will still need more sophisticated tools to physically install and adjust antenna and LNB. I have no doubts you can do that well.
I've made spreadsheet another day, included in it data from some of my dishes. Perhaps I will need some of these data for future work and it is better to have it available all the times. Spreadsheet will probably grow horizontally and vertically as time passes. And spreadsheet is living creature, some data can be slightly changed in to make it more perfect.
So, what is this data about (columns):
A - Name of the dish;
B - Horizontal size of the working area (This represents real size of dish- 1200mm means 1,2m dish);
C - Vertical size of the working area;
D - Depth of the dish in the very middle of it;
E - Offset angle of the dish;
F - Opening angle (angle from antenna's focal point and lower and upper points of the dish);
G - Distance from upper point of the working area of the dish to the aiming point of the LNB (sometimes it is not easy to measure distance from lower part as there is obstacle- lower LNB-arm);
H - Distance from the upper point of the dish to the focal point F;
I - Distance from the lower point of the dish to the focal point F;
J - F/D of the dish;
K - Surface of the dish visible from satellite;
L - Angle A (see on screenshot);
M - Angle B;
O - Angle AB/FG;
P - Weight of the dish without feedarms and mount;
Q - Distance from center of the initial parabola (from which offset is made) and lower point of the our offset dish;
R - Distance between holes for fixing dish to it's mount.

Attached:
- screenshot of data-spreadsheet;
- screenshot of profile drawing of dish (example for 90cm Gregorian type);

 

[DVB-S2]

Polgyver, you're a constant inspiration to all of us! I fear, though, that you've made an error here: the height of the CM 1.2 m dish is 1.3 m, not 1.2 m (which is the width - as seen in Rima's spreadsheet).

 

[polgyver]

http://www.john-legon.co.uk/offsetdish.htm
Above, there is a link to an excellent article, written by John A. R. Legon, B. Sc.
Formula for calculating focal length of any offset dish is given there, as well interesting way to find needed dimensions for oval dishes - using water. Mr. Legon granted me permission to use this link here.
DVB-S2, you are right, I should have marked one more spot - 1300 mm, on X axis.
But the main goal was to show, that the location of target spot was closer to the bottom edge.
Had I done it correctly, the respective distances from bottom and top of the dish would have been, approximately, 550 and 770 mm.
On a lighter note, I like the fact that RimaNTSS assigned the letter "G" to the target point for LNBF, on his drawing.
Would it be appropriate to add the word "spot" after the "G" letter - what the moderators would say for that?

 

[fred555]

It is good to see old fashioned Engineering skills put to use.
Most people would not be able to wipe thier own butts without a computer these days.

 

[FaT Air]

If you tend to take the easy road - Parabola Calculator 2.0.** - Just punch in your measurements. [And yes the water pool in the elliptical works]
** use the WIFI tab for offset feed dishes.
Will give focal point distance from top and bottom lip for accurate placement. (there's some experimenting in aim point by RimaNTSS in another thread)
**note** accuracy is dependent on measurement accuracy, so leave some leeway for tweaking

 

[RimaNTSS]

There is my new tool for antenna tuning. Very simple, but should be very helpful as well. It is just piece of metal pipe 40mm in diameter. This pipe will temporary replace LNB and digital protractor will be magneted to it. So, than I will be able to measure angle between LNB pointing line and leaning of the dish with the precision up to 0,1*. In such way it is possible to check whether LNB is located at antenna's focal point.

 

[waylew]

It is good to see old fashioned Engineering skills put to use.
Most people would not be able to wipe thier own butts without a computer these days.

There's an app for that

 

[bpalone]

I just have to do it.

But, but, you didn't drag out the slide rule!

 

[jorgek]

If you tend to take the easy road - Parabola Calculator 2.0.** - Just punch in your measurements. [And yes the water pool in the elliptical works]
** use the WIFI tab for offset feed dishes.
Will give focal point distance from top and bottom lip for accurate placement. (there's some experimenting in aim point by RimaNTSS in another thread)
**note** accuracy is dependent on measurement accuracy, so leave some leeway for tweaking

Have to agree with FAT AIR, re Parabola Calculator 2.0. Its accurate and easy to use.

 

[polgyver]

Have to agree with FAT AIR, re Parabola Calculator 2.0. Its accurate and easy to use.

I am not sure if my MacBookPro can use this program, as I looked at the links provided by FaT Air.
I do not have access to Windows based computer.
Therefore, clinging to old-fashioned ways sometimes is not a matter of choice...

 

[polgyver]

In such way it is possible to check whether LNB is located at antenna's focal point.

Can we say "it is pointed (or, directed) to the dish's G-spot" ?

 

[RimaNTSS]

Can we say "it is pointed (or, directed) to the dish's G-spot" ?

Do you mean this? No, of course no, it is about point, where LNB is aiming on the surface of the dish! You are spoiled!

 

[FTA4PA]

I am not sure if my MacBookPro can use this program, as I looked at the links provided by FaT Air.
I do not have access to Windows based computer.
Therefore, clinging to old-fashioned ways sometimes is not a matter of choice...

You may be able to run Parabola Calculator 2.0 using Wine. I use it regularly on Linux to run Windows programs with very good success. There is support for running Wine (3rd party or compiling it yourself) on OS X. More here: http://wiki.winehq.org/MacOSX
Oracle Virtualbox is also a good solution to running Windows applications on Linux or OS X however you need a spare copy of Windows to install, whereas with Wine you do not.

 

[RimaNTSS]

In reality, you do not need lots of things. You just need 3 measurements: -antenna's big and small axis and it's depths, that is all you need folks! The rest is just talking about nothing! From those 3 measurements you are getting all needed data. I mean, you need nothing more! So, what are we talking about?! Linux, Windows..... does not matter... it changes nothing! Offset dis? is offset dish!

 

[polgyver]

You are spoiled!

Hey, aren't you co-author of it? You did choose the letter, I added the word (I like finding associations between even remote areas of human activity). And the moderators did not intervene after my question at the end of post # 4

 

[polgyver]

Tried it again yesterday and today, with a smaller dish which sits at my porch. Applied Mr. Legon equation. More on attached photos.

 

[polgyver]

You may be able to run Parabola Calculator 2.0 using Wine.

If it can show the height of G-point (as measured from the lower edge of dish), then, it is worth to try to learn it.
Not easy for me, knowing only the very basics of computer.
Cheers, polgyver

 

[RimaNTSS]

2 polgyver: Where do you consider is LNB's Phase center? Somewhere where I put red dot on your picture or somewhere else?

 

[polgyver]

I think your dot shows it perfectly. I gathered from reading on this forum, that LNBF's phase center should be 1/4 - 1/2 inch below plastic front cover.
Also in this area was "axis" of 2 bolts for experimenting with LNBF angle, I think I posted this picture in your other thread.