Circular vs Linear

[srinath]

Anyone knows if there is any advantage for one side or another ?

 

[Mr Tony]

Linear is what most of the satellites are

Circular polarity is what a few atlantic satellites use

Basically you need the right type of LNB for the satellite....if you dont you wont get much. I had a linear LNB aimed at 40.5W C-Band which is circular and got like 4 or 5 channels and low quality. If I remember, theres a heckuva lot more channels on there

 

[srinath]

Thanks...

But what I meant is: is Circular a better system than Linear ? Do we get better signal quality ?

 

[Mr Tony]

There is no "better" system. The satellites are either linear or circular
If you mainly stick with KU a linear is all you need
C-Band there is some circular polarity sats out there

or are you thinking of a standard or Universal LNB?

 

[srinath]

No, not really. I was only wondering why we have both systems ...

I already have Universal Ku and a Ku/C band LNBF on a 1.2 Fortec.

I'm getting some Linear C-Band and I will put the dielectric plate and try some Circular birds this week.

 

[FaT Air]

Circular only requires aligning Azimuth an Elevation, Linear requires Skew also. So circular is a slight bit easier to tune up. That's about it. Dnet and Dtv uses circular for that one reason alone. ( along with much higher power )

 

[pendragon]

1. One doesn't have to worry about skew with circularly polarized signals.

2. With linearly polarized signals, the vertical polarization is more vulnerable to rain fade than the horizontal. Because circularly polarized signals are a combination of vertical and horizontal, there is somewhat a composite averaging so both left and right experience the same amount of fade. Less than vertical but more than horizontal.

3. There can be more losses in receiving circularly polarized signals vs. linear. This depends on the LNB and feed design.

 

[cracklincrotch]

Why would it be that vertical signals are more susceptible to fade as opposed to horizontal?

 

[pendragon]

Gravity

 

[guapoharry]

That's wild...

Isn't physics cool?

 

[pendragon]

I was being a little tongue-in-cheek, however in my original post I should have said vertical polarization suffers less attenuation than horizontal but I got it reversed.

There are some complications. The simplest case is viewing your true south satellite with rain falling perfectly vertical. The water droplets are not perfect spheres, as gravity causes them to fall and the air resistance alters their shape into roughly oblate spheroids. This causes a difference in the absorption and scattering between the two linear polarizations because the drops will have different dimensions horizontally and vertically.

As you deviate farther from true south, there will be a skew in the polarizations, which means the effect will change with respect to the angles between the raindrop axes and the the polarization. The same can happen if the raindrops are being driven by wind, causing shifts in the raindrop axes.

The main point is both circular polarizations are affected the same, while the two linear polarizations will not be. I should have also pointed out that at C-band frequencies, the Faraday Effect will cause some change in the polarization angles received for linear, but will not affect circularly polarized signals.

 

[pback]

Jeez. Pen..........is there anything you don't know? J/K, great explanation even a layman can understand it:up

 

[srinath]

I was being a little tongue-in-cheek, however in my original post I should have said vertical polarization suffers less attenuation than horizontal but I got it reversed.

There are some complications. The simplest case is viewing your true south satellite with rain falling perfectly vertical. The water droplets are not perfect spheres, as gravity causes them to fall and the air resistance alters their shape into roughly oblate spheroids. This causes a difference in the absorption and scattering between the two linear polarizations because the drops will have different dimensions horizontally and vertically.

As you deviate farther from true south, there will be a skew in the polarizations, which means the effect will change with respect to the angles between the raindrop axes and the the polarization. The same can happen if the raindrops are being driven by wind, causing shifts in the raindrop axes.

The main point is both circular polarizations are affected the same, while the two linear polarizations will not be. I should have also pointed out that at C-band frequencies, the Faraday Effect will cause some change in the polarization angles received for linear, but will not affect circularly polarized signals.

And that's the answer I was looking for ! :up:up:up

 

[sanjap]

I was being a little tongue-in-cheek, however in my original post I should have said vertical polarization suffers less attenuation than horizontal but I got it reversed.

There are some complications. The simplest case is viewing your true south satellite with rain falling perfectly vertical. The water droplets are not perfect spheres, as gravity causes them to fall and the air resistance alters their shape into roughly oblate spheroids. This causes a difference in the absorption and scattering between the two linear polarizations because the drops will have different dimensions horizontally and vertically.

As you deviate farther from true south, there will be a skew in the polarizations, which means the effect will change with respect to the angles between the raindrop axes and the the polarization. The same can happen if the raindrops are being driven by wind, causing shifts in the raindrop axes.

The main point is both circular polarizations are affected the same, while the two linear polarizations will not be. I should have also pointed out that at C-band frequencies, the Faraday Effect will cause some change in the polarization angles received for linear, but will not affect circularly polarized signals.

This is knowledge. Thanks for knowing more than me.

 

[B.J.]

2. With linearly polarized signals, the vertical polarization is more vulnerable to rain fade than the horizontal. .

I might believe this with terrestrial signals, but I'm having a hard time accepting it relative to sat signals.
Assuming that it's true though, would you expect a difference relative to how close to the horizon a sat was? Reason is that I've observed pretty much the opposite on at least one of my sats that's near the horizon, ie when there is rain around, I lose horizontal first, then vertical. I've always assumed that this was just my LNBF going bad, except I'm seeing it with 2 different LNBFs and also with a polarotor style BUD. I mainly see it on a sat that I'm seeing through the leaves of several trees, so I also thought that the wet leaves might be somehow acting as a diffraction grating or something.

 

[pendragon]

...however in my original post I should have said vertical polarization suffers less attenuation than horizontal but I got it reversed.

BJ: My original post had the polarizations reversed, but I got it right the second time. This would be a case where editing old posts would be a good thing. As I also indicated, this sense only applies for a true south satellite and vertical rain. If the LNB is skewed by 45 degrees and the rain stays vertical, both linear polarizations should roughly the same attenuation. And at the horizon as the skew approaches 90 degrees, 'horizontal polarization' should have more loss than vertical. The total attenuation will also increase because of a longer atmospheric path.