How to calculate depth of satellite dish?

I

italy2006

Archived Member
Messages
43
My Satellite Setup
starsat
My Location
iran
I have an 110 cm offset dish with smaller wide 100.its depth is 9 cm. Now I want to know how many centimeters the depth of a 30 cm dish is.Please help me.
 
A

a33

Specialised Contributor
Messages
470
My Satellite Setup
XSAT410
Clarketech HD4100+
My Location
NL
So you have 110 hight, 100 width and 9 depth (at the middle? or maximum depth?).

30cm = width, I presume?
Then multiply everything with 30/100, so with 0.3:
33 hight, 30 width and 2.7 depth.
That would give you a dish with the same LNB viewing angle (normal LNBF for offset dish).

But of course you could have a 30cm dish with other paraboloid characteristics, which have a different depth.
So there are other possible answers, depending on which is the relevant criterium.

So, why do you want to know?

Greetz,
A33
 
I

italy2006

Archived Member
Messages
43
My Satellite Setup
starsat
My Location
iran
I want to extend bigger width of this dish to 130 and smaller width to 120 cm with aluminum sheet. so I want to know is new depth of this enlaeged dish is a standard and correct depth or not. AS you know new depth certainly will be more than 9 cm.
 
I

italy2006

Archived Member
Messages
43
My Satellite Setup
starsat
My Location
iran
I have an 110 cm offset dish with smaller wide 100.its depth is 9 cm. Now I want to know how many centimeters the depth of a 30 cm dish is.Please help me.
I meant 130 cm dish not 30 cm.
 
A

a33

Specialised Contributor
Messages
470
My Satellite Setup
XSAT410
Clarketech HD4100+
My Location
NL
When you now have measured depth at the middle, and you want to add exactly 10 cm dish surface around the dish, maintaining the paraboloid form of it (Is that what you want to do?),
then the dish doesn't have a 'flat face' anymore, and the 'depth' measure will be different, horizontally and vertically.

Apart from that, the LNB viewing angle will become bigger ~ do you have a matching LNBF of feedhorn for that?
Otherwise there won't be much benefit from the augmenting.

So I still don't understand what it is exactly, that you want to do and achieve, I'm afraid.

Greetz,
A33
 
vma

vma

Regular Member
Messages
276
Location
Portugal
My Satellite Setup
RX: 3 motorized DiSEqC-1.2 dishes (100cm, 110cm, 120cm) + 7 regular dishes.
Main receiver: Vu+ Duo + many others & IRD
TX: Modulators for DVB-S/C/T + Dektec Modulators
Field Meter: Emitor, KWS, Kathrein, Unaohm, Promax, Spaun, Rover Instruments, ...
My Location
Portugal
Hi,

A dish is calculated so that the focal distance is compatible with the beam angle on the LNB feedhorn.

The curvature of the dish is calculated like this:

z = (x ^ 2 + y ^ 2) / (4 * fd)

Where X, Y and Z are the 3D coordinates of the dish surface (a parabolic shape) and FD is the feed distance.

If you want to increase the size of your dish, that would be easy on a prime focus dish, as you would have to just "continue" the curvature of the existing shape in a tangent manner.

But if you have an offset dish, that won't be that easy, because all around the dish you will have different curvatures. An offset dish is actually a cutted shape from a bigger parabolic 3D surface! Imagine taking a sphere and cutting it at an angle with a plane. The resulting "slice" would be your offset dish.

The dish is, however, not a perfect sphere, because it would be too big (in Z - think of "too fat"). This is why one uses a parabolic shape, which gets you a big diameter on a relatively flat dish.

The magic of geometry is that no matter where the beam hits the parabolic surface, it will always be reflected to the focus - the LNB. But the even greater magic is that the distance of each beam to the surface of the dish and then reflected to the LNB will always be the same, which is why the signal is in phase!

If you try to increase the surface of your dish by adding to it on the borders, chances are you will get the wrong deometry and the reflected beams will either be off from the LNB or, if they do hit the LNB, they will be out of phase and as a result you will have a worse reception.

Building big size dishes is actually quite expensive and complex due to the fact that the sheet metal suffers both a plastic and an elastic deformation inside the press tool. The elastic deformation causes the dish to spring back when extracted from the press tool - the geometry will be wrong.

A good (and expensive) dish will be produced by trial and error on the press tool, so that the shape of the press tool is exagerated in order to compensate for the elastic deformation. Or, expensive simulation software is used to calculate the theoretic shape.

Using the above formula I wrote many years a go an application to automatically calculate the 3D surface of an offset dish. You only had to input the desired dish size and the tool would calculate and create the surface on the attached CAD software. This was developed for a dish manufacturer.

Regards,
Vitor
 
RimaNTSS

RimaNTSS

Super Moderator
Staff member
Messages
4,787
My Satellite Setup
Some SAT-related hardware.
My Location
N-E from Riga
IMHO best way to increase size of dish is to buy the bigger one. All other methods are not good enough, especially in Ku-band.
 
satesco

satesco

Specialist Contributor
Messages
881
My Satellite Setup
seven antennas,one toroidal with 16 lnbs,
6 satellite receivers,2 Meters,9 PC cards(tbs6983,6903,6590,6522,6209,2603,6909X,6903X,skystar hd2),2 tuners usb-tbs5927,Skystar HD.
Satellite reception between 100.5E-50.0W
My Location
Romania
Why get complicated when SatLex's computer does everything!You have everything there.
 
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