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LNB CIRCULAR POLARIZATION

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lfaborges
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Dec 18th, 2019 at 11:44am  
Hello friends.

This week I finished assembling my helix antenna (RHCP).

My goal is to try to get the signal from a satellite. I am using an LNB, connected to the antenna by an SMA cable, but I am having a problem.

I was informed that the LNB only "sees" vertical / horizontal polarizations and would have to convert before.

How can I do this without being a waveguide?

Thanks and Merry Christmas!
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Admin1
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Reply #1 - Dec 18th, 2019 at 3:56pm  
An LNB with a circular or square waveguide input will normally be able to receive two linear polarisations, at right angles to each other, using a power supply selection voltage 13v or 19v.  This is normal for satellite TV reception.

An LNB with a rectangular waveguide input (as used for Ku band VSATs) will be able to receive one linear polarisation only. The plane of polaristaion is at right angles to the wider dimension.

...

So this is set for vertical polarisation.  The polarisation is the same as the half dipole pin inside.

An LNB with a coax SMA input has no polarisation. It is simply a low noise amplifier, mixer and IF amplifier.

If such an LNB, with a coax SMA input, is connected to an antenna it is the antenna that defines the polarisation.  If the coax connector is connected to the middle joint of a vertical dipole antenna then the polarisation being received is vertical. If the coax connector is connected to the end of an axial-mode helix antenna then the polarisation being received is circular and the choice of polarisation (clockwise or anticlockwise) is fixed permanently by the physical construction of the helix. This might apply to the reception of maritime satellite L band signals.
...

UHF and L band satellites normally use circular polarisation as the signal polarisation is dynamically rotated in the ionosphere (called Faraday rotation).  If linear was used the signal levels would be varying dramatically.

Most Ku band satellite TV signals are linear polarisation, but there are a few popular systems in the USA that use circular. 

Most 20/30 GHz Ka band satellites use circular polarisation, to make life easy for the installers.  Having a polariser costs a bit more but eliminates a skill requirement for installers, as adjusting linear polarisation angle accurately is difficult and extremely important in transmit/receive VSAT systems.

In both Ku (10-14 GHz) and Ka (20-30 GHz) band the use of a helix feed would be unusual. Normally such reception is done using a circular or square waveguide between the feed horn and the LNB, with the waveguide section containing a polariser which is either a dielectric plate at 45 deg or sets of castellations and slots on opposite sides inside the guide, also at 45 deg. In such cases what receive signals that appears after the polariser and going into the LNB are two linear polarisations, which may be selected using a normal dual-polarisation LNB using 13v or 19v.

If you have a low noise amplifier (LNA) or LNB, with a coax SMA input, and want choice of two circular polarisations you might have two helix antennas (wound with opposite senses) which you manually dis-connect and swap over.

I hope that helps.  Does anyone have any pictures to contribute, for example: Amateur radio?.

Best regards and Happy Christmas, Eric.
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« Last Edit: Dec 19th, 2019 at 8:52pm by Admin1 »  
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lfaborges
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Reply #2 - Dec 18th, 2019 at 4:14pm  
Your explanation was very helpful. It is the third forum in which I try to clear this question and everyone only talks in waveguide but forgets the coaxial part.
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Reply #3 - Dec 19th, 2019 at 9:08pm  
It is worth doing Google internet search on "helix antenna"  or "helix antenna matching" (choose images option).

There seems good consensus on the design of the basic helix, 1 wavelength for a turn and approx quarter wavelength for the turn spacing.

There are many alternative ideas about the exact way to match the lower end of the helix wire to a 50 ohm cable. Many suggestions involve making the wire substantially thicker for the first quarter of a turn.

If anyone wants to add comments please note that the registration and login links for this forum are at the extreme lower left corner of the forum home page. https://www.satsig.net/cgi-bin/yabb/YaBB.pl
It is difficult to find these text boxes, but even so this forum gets attacked by automated spambots. There have been 403 hacking attempts in the last 3 hours. This rate is quite normal but none were successful....


Eric
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lfaborges
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Reply #4 - Dec 20th, 2019 at 3:10pm  
Yes Yes. I figured out how to build this way. There are even some calculators that already tell you all the dimensions.
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