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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 designed dimension of a 130 offset dish y autocad software plotted on the paper and then put it on aluminum sheet and cut sheet according to plan. after that I connected sheet to 110 cm dish with glue. didnt change lnb position but no signal increase. why such failure?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.
Because it is nearly impossible to get such an extention in perfect shape. At least it is imposible at home. Secondly, when you do extension you also need to change the feedhorn of the LNB, namely to make the illumination angle of the feedhorn wider. It would be much easier and better to change the dish to bigger one.why such failure?
As the focal distance doesn't change by adding dish surface, I don't think you'd get more signal by putting the LNB out of the focal point.... and so you could try extending the feedarm so that the original LNB can now receive signals from the whole of the extended area of the dish (at least, I think that to be the case).
So, as you obviously like experimenting: are you gonna make a matching feedhorn for your augmented dish?Apart from that, the LNB viewing angle will become bigger ~ do you have a matching LNBF
ofor feedhorn for that?
Otherwise there won't be much benefit from the augmenting.
Tnx.I dont have feedhorn. what is best feedhorn for 130 cm dish or how to make it because no feedhorn here in the market.As the focal distance doesn't change by adding dish surface, I don't think you'd get more signal by putting the LNB out of the focal point.
Edit: However, I remember testing LNB distance with an LNB that was OUT of the focal point, and there wasn't much signal difference. So maybe there could be a little improvement of signal by doing that. But never an improvement that would match the surface augmentation.
And it is not ideal, to have the LNB feedhorn out of the focal point....
As I already wrote:
So, as you obviously like experimenting: are you gonna make a matching feedhorn for your augmented dish?
There are no feedhorns for 130cm dish. Main parameter of the feedhorn is it's illumination angle. In ideal situation illumination angle of the feedhorn should be equal to antenna's opening angle. So, first you should know what was your antenna opening angle. Do you know the opening angle of your antenna before modification and after modification? I think it could changed for some degrees, for example 10*. Therefore I would say that changing of feedhorn (if you manage to find or make one) will not change a lot. The main obstacle why you can not get considerable increase of performance of your system is the fact that it is impossible to make perfect extention boards. It is also nerly impossible to attach those boards to the antenna. So, IMHO, I would say that it almost does not matter what kind of feedhorn will you use you will never get much better performance of the system. At least till you get extension boards perfectly curved and attached to antenna.... but that is impossible, IMHO again. Do not forget, it is Ku-band and any unprecision of shape of antenna > than 1mm is no go.Tnx.I dont have feedhorn. what is best feedhorn for 130 cm dish or how to make it because no feedhorn here in the market.