Satellite TV polarization question

[nadabro]

nomenclature for satellite tv

Hi.

1-Satellite TV usually use for KU-Band Linear Polarization (Horizontal/vertical), but what is the name of the phenom that allows to receive both H/V polarization at the same time using a specific rotation of the LNB? is it cross-polarization?

2-If i use a Linear polarized antenna, which it can only receive one polarization at each time (For Horizontal polarization the LNB have to be at a position X; and for Vertical Polarization the LNB have to be at a position Y)..am i going to have dB loss? not between polarizations, but in the same polarization..

Sorry for the confused questions..

Thanks.

 

[FvM]

axial ratio vertical satellite

1. I think you receive both polarizations with 3 dB loss using a 45° rotated LNB.
2. I don't see, why you should have losses in normal operation.

 

[nadabro]

horizontal vertical polarization satellite

So if i use a Horn antenna i will receive 3dB better than a Satellite offset dish..because it can only receive one polarization each time, i´m right?

 

[plasma]

satellite tv polarization

there is also circular polarization Satellite TV
maybe the Satellite polarization is circular right and left and by rotating the LNB you always
receive them both but -3db less if you have linear receiving antenna.

 

[nadabro]

linear polarization satellite tv

Usually circular left/right polarization is for C-Band, in KU-band are H or V..

my confusion is: i´m using a Horn antenna, which only works with H or V polarization..meaning that for receiving the Vertical Polarization the LNB have to be (the element inside) in 0º rotation. for Horizontal polarization the LNB have to be in 90º rotation. At 45º i´ve no signal in H or V.

so i´ve a Linear polarized horn?

But for Offset/parabolic dish, to receive a specifc ku-band satellite, i´ve to rotate the LNB according the satellite specification´s (example 32º), and i only receive signal if i put the LNB in that specific angle, 0º or 90º wont work..

what is the offset/parabolic dish polarization that allow to receive both H/V signal´s at specified angle?

 

[PAF]

h/v satellite switch with offset

Hi,
If the satellite give H and V polar in the same time you can't have both with your horn linear polar antenna (you have -3dB less) and you need to use one with circular polar.
If the H and V polar are alternatif your need to know the frequency and swith your LNB with this frequence.

 

[nadabro]

Hi,
If the satellite give H and V polar in the same time you can't have both with your horn linear polar antenna (you have -3dB less) and you need to use one with circular polar.
If the H and V polar are alternatif your need to know the frequency and swith your LNB with this frequence.

So i cant receive both H and V polar. because i´ll have a loss of -3dB? If i choose a V polar Frequency and put the LNB to receive only Vertical polar frequency´s (at 0º), i will not have any dB loss?

I have the Horn build, but even when i put the LNB to receive only Vertical polar. frequency´s, i think i´m loosing -3dB in comparisation with a Offset Dish or a Conical Horn antenna (circular polarization)..

 

[Azulykit]

you may find this helpful:

h**p://

I would suggest Stutzman's book as a very detailed description about how polarization fits into anetnna performance.

 

[biff44]

I am not sure of all the nomenclature, but typically a satellite transmitter transmits in BOTH polarizations. Dish Network, for instance, has around half of their channels on one polarization, and the other half of the channels on the other polarization. In the LNB there is a circular waveguide device that splits the received signal into two linear polarizations. Then before the input amplifier is a SP2T switch that chooses one or the other polarization. You need a good cross-pol rejection is such a scheme, otherwise you will be simultaneously receiving two completely different TV channels.

 

[nadabro]

Thanks for both answers´s..

So i need a circular polarized antenna to receive both H/V polarizations....if i have a antenna that have a axial ratio less than 1dB is circular polarized..or it should be a perfect 0dB axial ratio?

 

[biff44]

Well, here is a spec for the Dish Network LNB. Apparently is received either right hand or left hand circular polarization, and the circular waveguide mechanism separates it into V or H polarization in a square waveguide output. Then a switch selects either of the two outputs via microstrip probes into the square waveguide. (I took one apart once, and my memory may be fuzzy but that is what I remember).

They need 18 dB crosspol isolation for the system to work.

**broken link removed**

 

[nadabro]

Is this correct?

Satellite (KU band) Linear Pol---> -3dB loss <---- Parabolic ----> 0dB loss <---- LNB

Satellite (KU Band) Linear Pol---> 0dB loss <-- Linear Pol. Antenna --> -3dB loss <---- LNB

 

[nadabro]

Is this correct?

Satellite (KU band) Linear Pol---> -3dB loss <---- Parabolic ----> 0dB loss <---- LNB

Satellite (KU Band) Linear Pol---> 0dB loss <-- Linear Pol. Antenna --> -3dB loss <---- LNB

Nobody to help me? for more i read on the internet, more confused i get :roll:

 

[toonafishy]

Satellites send different information on different polarizations but these signal are on the same frequency. So you must be able to seperate them at the receive antenna or they will jam each other. The DBS TV satellites mostly use circular polarization. One signal arrives with the waves rotating clockwise and the other signal arrives with the waves rotating counterclockwise (or right hand circular polarization - RHCP and left hand circular polarization - LHCP). There are two ways to seperate these signals at the receive antenna (remember they are at the same frequency so you must seperate them). You can use an antenna that responds to circular polarized waves such as a helical. One has the helix wound one way and responds to one signal and the other helix is wound the other way and responde to the other signal. This is commonly used on weather satellite receivers.
But what is usually done is the wave enters a feedhorn. Then there a mechanical device inside that converts the RHCP and LHCP to a linear Horizontal and Vertical component. There are two microstrip probes on the PCB at 90 degrees and one probe picks up the RHCP that has been converted to H and the other probe picks up the LHCP that has been converted to V.
In other cases, some satellites use H and V polarization to begin with. Then your receiver antenna does not need the converter in the horn. The two probes will pick up the H and V directly.
I can't see how you can seperate the circular waves using a rectangular feedhorn. You can do it for H and V with feed probes at 90 degrees but this would be poor for circular waves.

This product can receive both circular and linear polarizations
**broken link removed**

 

[Azulykit]

There are some interesting and also misleading answers above. A linearly polarized antenna will respond to any circularly polarized signal. A circularly polarized antenna will respond to any linear antenna. There will be signal loss in both cases. A linearly polarized (vertical) antenna will not respond to a horizontally polarized signal. Likewise a RHCP antenna will not respond to a LHCP signal.

Run down the reference I cited above to really get all this in hand. There are other references around but that is the most complete.

 

[Bhanumurthy]

Hi,
An OMT(Orthogonal Mode Transducer) is used at the LNA to receive on both polarizations.Try to get more info.abt OMT.
Regards
Bhanumurthy.