Just Sharing This What is the best 100cm dish ?

[william-1]

100cm dish test:-

Well I thought my OP Gibertini 100 XP was better than some of the dishes I have tried over the years,

These included a 100cm Andrews & 100cm Orbital,

The Gibertini performed best over the 10.7 to 12.75 Ku band with a Smart Single Universal LNB Titanium Edition; 0.1 dB, Diameter 40 mm

That was a few years ago I have since tried several other Lnb's the best was the Inverto Black Ultra 0.2,
I took a photo of the Gibby with the IBU & the comments came back that I had the Lnb too far forward in the Lnb holder well it worked for me so I left it,

Then a few days ago some Lnb tests with a couple of Lnb caught my eye so I tried them on the Gibby & they did not work very well ?

This was the Golden Media 201+ & the Inverto Premium PLL,

So I thought back about the Lnb position & decided to saw 23mm off the end then bolt the Lnb holder back on well if it did not work I could always add the sawn bit back on ,

But to my astonishment the new Lnb's performed very well.

Signals that were not there before now boomed in,

I then put the Inverto black ultra back on but this time much further back in the Lnb holder & the signals were vastly improved also the rain fade was less as the Lnb was getting more signal in its new position on the shortened Lnb arm.

 

[jeallen01]

TBH, I think we need a "primer" on how to shorten / lengthen dish feedarms as that might be a very good guide as to how to improve SS/SQ levels on various dishes.

 

[Gladstones]

Please provide pic. i done the above and signal gone bad

 

[Channel Hopper]

There have been previous reports on the forum of Gibertini dishes being supplied with the incorrect feed arm for the reflector size.

It is usually the error of the supplier mixing up boxes in storage.

 

[william-1]

The Lnb feed arm is 29 inches / 73.66 cm overall length now after removing 23 mm,
So the original feed arm was 76cm in length nearly 30 inches,

 

[rodscha]

i have 58 cms on an 80/73cms dish..
.is there a formula giving the ideal length of arm versus dish size?
Does using a 13&19E monoblock change this rule?

 

[Lazarus]

i have 58 cms on an 80cms dish...is there a formula giving the ideal length of arm versus dish size?
Does using a 13&19E monoblock change this rule?

Assuming the dish is actually an accurate parabolic section, then fairly basic maths can be applied - there are many references to this sort of calculation freely available via web search - I posted the text of one source then deleted it as it was clearly wrong, so best to look at a few for yourself!

However, it might be that a given dish is not a perfect parabolic section, so there's likely to be a need for an empirical consideration to modify the theoretical result.

As for the Monoblock, the rule applies in the same way for the clamped head. Obviously, only one head can be at the focal point and any offset LNB - whether the unclamped head of a Monoblock, or a separate LNB on a bracket - will result in compromised performance.

 

[rodscha]

i have a strange reception of 16 east,
most TPs fine but Albania 12639H,10833 very very weak,when in the UK it comes loud and clear!
Could it be a length prob? or the LNB not liking this particular TP?

 

[a33]

Assuming the dish is actually an accurate parabolic section, then fairly basic maths can be applied - there are many references to this sort of calculation freely available via web search - I posted the text of one source then deleted it as it was clearly wrong, so best to look at a few for yourself!

Absolutely!
There are three calculators that are OK and faultless, that I know of:
- Parabola6 (in russian or so) by Strannik,
- A33 Satellite dish specs calculator (spreadsheet) by some unknown person called A33 ,
- and a French programm offset_v1_0.xls by F6AGR (using the John Legon equation long before John Legon derived it, if I am not mistaken).

All using height and width of the dish, and depth at the center or depth at the deepest point as input measures, and assuming paraboloid form and flat dish face. My calculators can also manage non-flat dish face (needing four inputs).

Don't trust Parabola2.0 for offset dishes, though, and the satsig calculator. They have annoying faults/imperfections.


When installing a 'new' dish, I usually do a stringtest on it; and measure height, width, and depth at the center to calculate and check the focal point position and the aiming angle of LNB.
For a hobby, that is not a very difficult procedure.
I don't know why others don't usually do this....

Greetz,
A33

 

[rodscha]

Thank you A33,but I still haven t found the way to deduce the arm's ideal length..
My dimensions are
Height 80cms
Width 73cms
Depth 6.2cms
The French programm offset_v1_0.xls by F6AGR only has one page visible,out of three.

 

[Channel Hopper]

Ideal length will be to follow William-1's lead, he has done the homework.

 

[a33]

My dimensions are
Height 80cms
Width 73cms
Depth 6.2cms

Assuming that the 6.20 cm is measured at the center, and that all measures are of only the working area of the dish (so measured without the rim, even for the depth!),
here are the results:



Meaning of the variables:


So topstring length is 818 mm, bottomstring is 491 mm.
With these measures you can check the position for the LNB feedhorn.

Remember the LNB holder usually is about 3.5 cm behind the focal point.

Does this help?

BTW the french spreadsheet expects depth at deepest point, but indeed only has one "feuille" active.

Greetz,
A33

 

[rodscha]

@ A33
Thank you very much!
Night has set in now. I' ll try this out tomorrow,perfect for a sunny lockdown day isn ' t it?
Since my offset dish is oval ,not quite sure where the exact centre is..i took the middle of the widest dimension...
rodolphe in Carcassonne,Zuidfrankrijk.

 

[a33]

Since my offset dish is oval ,not quite sure where the exact centre is..i took the middle of the widest dimension...

Well, I hope you measured the depth exact enough.

I usually mark the top and bottom, and left and right of the working area of the dish, with a dot, thus forming a diamond with equal side-lengths (which can be checked).
Then I mark the exact center of the dish with a dot, and check if the distances to top and bottom are equal, and distances to left and right are equal.
Then I measure depth, if possible even in parts of a millimeter.
And I correct for the height of the dish rim (taking the mean of the dish rim height at the top, and at the bottom; as I measure with a 'wooden plank' that lies from top to bottom).

Did you do the string test, to check if the dish face is indeed flat (and meant as flat)? If the dish is a bit warped, the depth measure is influenced, and then the outcome of the focal position would be heavily influenced.

Greetz,
A33