lnb skew vs dish skew

[trev1978]

Hi all!!

This is probably a stupid question but bear with me!!!

Would you get a better signal from a dish set vertical and the lnb skewed or the dish skewed and the lnb set plumb on the dish (i.e. as if it was on a motor).

I ask this for one simple reason- I have two identical 1.1m alsat dishes- one on a motor and a fixed one set on 4.8'E. I have upgraded the lnb on the fixed dish and placed it in a better location with a completely uninterrupted line of sight . However i appear to be still getting a better signal reading from my motorised dish!!!! I just began to think that as the dish is deeper than it is wide that there may be an advantage in the dish being skewed. Is there any merit in this?

Trev

[Robbo]

Are the LNBs identical too?


If so, I think you could be right. The wider than higher dishes have more gain in the azimuth direction which helps to separate satellites that are closer together in orbit.

[nvingo]

In the olden days dishes used to be round. The LNBs were set in the centre (prime-focus) and their feedhorns were round too. No matter what the skew, the LNB would only pick up signals reflected from the dish.

Change offset dish (single LNB arm) and the dish shape was changed to taller elliptical to maintain the coverage. Feedhorn still round.

Sky digital and we now have minidishes, as Robbo says their focusing (near satellite rejection) comes from the width but they'd not be "minidishes" if they kept the typical tall shape.
So they required an LNB with an elliptical feedhorn to use the dish surface optimally. The skew for Astra2 is within a small range across the intended audience.

So unless the dish is wide (Sky-shaped) and the feedhorn elliptical, there should be no effect from the suggested alternative mounting. Particularly at 4.8ºE as that would be near-vertical.

[mikie8]

yes skewing the dish is better than skewing the lnb on an offset dish .

imagine a torch shining on a wall . a prima focas is pointing directly at the bird , so this is the same a a torch pointing at right angle to the wall and a round light will be seen . but an offset recieves the signal at an angle . so pointing the torch up or down by the same offset angle will give you a taller eliptical shape . factor in the azimuth and you are better off skewing the dish .