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LNB for circular polarized signal

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Apr 28th, 2011 at 11:19am  
I am receiving a linear polarized signal but do I have to changed my LNB to receive an RHCP signal?

thanks,

ken
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Eric Johnston
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Reply #1 - Apr 28th, 2011 at 2:06pm  
If you are trying to receive a circular polarisation you need to change it to a linear polarisation before inputting the signal to the LNB.

To do this you need a polariser.  This is a short or long, straight waveguide tube, typically with a square or circular cross section with some delay structure along the inside, oriented at 45 deg.  The structure is typically two bands of slots or screws along opposite sides or a dielectric flat plate or vane set at 45 deg.

At one end of the polariser you have right hand circular polarisation(RHCP) and left hand circular polarisation (LHCP); at the other end you have vertical and horizontal linear polarisation. Whether RHCP converts to horizontal or vertical depends on the orientation of the 45 deg polariser.  At the linear polarisation end you need a normal OMT to separate the two linear polarisations into separate rectangular waveguides.

This page may help explain: http://www.satsig.net/pointing/circular-polarisation-set-up.htm

Note that there is so much confusion between RHCP and LHCP that is quite normal to have to try both until you get it right - on 50% of occasions !  If your antenna has one reflector the polarisation changes during the reflection, so your feed needs to be the opposite to what comes from the satellite.

Circular polarisation is essential at low frequencies, like L band and C band, particularly in the tropics as linear polarisation gets rotated by varying amounts in the ionosphere.  Ku band works fine with linear polarisation, the feed system is simple and cheap but the installer needs significant skill and patience to set the polarisation angle accurately to avoid interference to other services.  At Ka band circular polarisation is being adopted, despite the increased cost of the polariser, as it de-skills the installers job, which I hope will help encourage more people to use systems like Wildblue, Hylas and KA-SAT. Installers either get it right or it does not work at all and there is less scope for them causing interference.
wxw
Best regards, Eric.
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Reply #2 - Apr 28th, 2011 at 2:24pm  
Quote:
I am receiving a linear polarized signal but do I have to changed my LNB to receive an RHCP signal?
More info required.
1. what frequency band? (C-band, Ku-band, Ka-band)
2. what signal type? (video, voice, data)
3. receive-only system? or transmit/receive system?
4. which satellite(s)?

//greg//
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Reply #3 - Apr 29th, 2011 at 1:16am  
Thanks sir eric, another question we have a 9m dish and we are using it to receive an RHCP signal from Intelsat and I did'nt see a polariser in it. The LNB is connected at the side of its feed. How is it possible?

Sir greg we are receiving C-Band video receive only on Intelsat 701.

Thanks,

ken
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Eric Johnston
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Reply #4 - Apr 29th, 2011 at 4:35pm  
The C band downlinks from Intelsat701 are circularly polarised.

If you have a linear polarisation feed system (without polariser) then your LNB will receive half of the signal from right hand circular polarisation and half the signal from the left hand circular polarisation.

If there is no carriers presently on the left hand circular polarisation then your 9m dish may produce good results even as the signal is reduced by 3 dB and there is unwanted extra noise from the left hand polarisation transponder. Maybe your dish is then equivalent to say a 6m dia dish.

If however there is a co-frequency active carrier on the left hand circular polarisation you will receive half of the wanted signal and half the unwanted left hand interfering signal. With C/N=0 dB, operation will not be possible.

Best regards, Eric.
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Reply #5 - May 2nd, 2011 at 9:50am  
Sir is there an LNB that can receive a circular signal even it was on a linear feed?
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Reply #6 - May 2nd, 2011 at 9:58am  
For TV receive only single polarisation, the polariser may comprise a small dielectric vane at 45 deg at the input to the LNB. The LNB is inline with the feed horn waveguide and no OMT is required.   The vane needs to be flipped 45 deg the other way to get the other circular polarisation.

To receive both circular polarisations simultaneously you need a feed horn, polariser, OMT with two receive ports and 2 LNBs. There are alternatives such as a septum polariser splitting a square waveguide into two rectangular guides or multiple (2 or 4) dipole probes into a circular waveguide with external 3 dB hybrid couplers feeding two LNBs.

Best regards, Eric.
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Reply #7 - May 4th, 2011 at 6:05am  
Sir,

Where can I buy a polariser? Do I have to be specific on C- and Ku- Band before purchasing?

Thanks,

ken
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Reply #8 - May 4th, 2011 at 6:20am  
Sir,

I saw a circular LNBF on the net can I use that one instead of a polariser?

ken
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Eric Johnston
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Reply #9 - May 4th, 2011 at 8:03am  
"LNBF" refers to an LNB with Feed horn attached.  If it is specified to work with circular polaristion there will be a polariser device internally.

Find out how to change from LHCP to RHCP and check your indoor receiver is compatible.

Best regards, Eric.
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Reply #10 - May 4th, 2011 at 9:25am  
Thankyou sir.. Is there any rule on using LNBF? Like on my part I will remove the  old feedhorn of my 4.5m mesh dish and replace it will lnbf, will it work?

thanks,

ken
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Eric Johnston
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Reply #11 - May 4th, 2011 at 1:59pm  
The feed part of the LNBF needs to be suitable for the F/D ratio of the dish.
For example:
...
The feed on the left has a very wide radiation pattern to suit a circular, axial-symmetric, dish with the LNBF in the middle on equal length support legs and relatively close to the dish center.  The feed on the right (conical horn) has a narrow beam and is suitable for an offset design with relatively long focal length to diameter ratio.

Best regards, Eric.
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Reply #12 - May 26th, 2011 at 7:43am  
Sir,

I was able to lock a circular polarized signal by putting a fiber glass inside the feed. But the problem is we were unable to see any program on the receiver. Please note that the program we are receiving is a free to air. Any suggestion on how we could decode the video signal?..

thanks,

ken
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Reply #13 - May 26th, 2011 at 11:35am  
By "fiber glass", I presume that you are referring to a dielectric polariser plate, oriented at 45 deg.

What frequency and symbol rate does your receiver show for the carrier that you are locked to ?

What signal level, signal quality or bit error rate is displayed ?

Best regards, Eric.
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Reply #14 - May 27th, 2011 at 1:21am  
Actually sir it is a fiber glass plate which we cut from the board of our old basketball ring, half of the plate is covered by teflon tape. I was able to get a 90% signal intensity and 72% signal quality, 90% is the maximum of my receiver.

I am receiving Express AM33 DF: 3675 MHz and 33483 sym/sec, please note sir that I was able to lock on other frequency of Express AM33 with 90% SI and 90%SQ. Only on transponder 6 (3675 MHz) which has a FTA program I am receiving 90%SI and 72%SQ.

thanks,
ken
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