Receiving Circular Polarized signals on a Linear LNB?
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I think its more to do with where you are in the country and any adjacent satellites.
Some members have reported getting GOL TV but not NASA. But its ALWAYS worth a try!
Found this........"Although a standard linear feedhorn(LNB) can still pick up any signal using circular polarization, half of the available power (-3dB) will be lost"
Some members have reported getting GOL TV but not NASA. But its ALWAYS worth a try!
Found this........"Although a standard linear feedhorn(LNB) can still pick up any signal using circular polarization, half of the available power (-3dB) will be lost"
Seems a little EM theory is needed here...
Radio waves have two components, an electrical field, and a magnetic field. These fields are at right-angles to each other. The polarization is defined by the direction of the electrical field.
An antenna needs to be aligned with the electrical field to pick up the maximum signal -- it is essentially picking up the difference in electrical field (voltage). If you orient the antenna at 90 degrees to the electrical field there is (in a perfect universe) no voltage difference to pick up, so you would get no signal.
Circularly polarised signals rotate the electrical/magnetic field. It rotates 360 degrees for each cycle of the radio wave. The rotation can either be clockwise or anticlockwise. To extract maximum signal the antenna needs to "rotate" in the same direction as the signal. The classical circularly polarized antenna is the helical antenna. The direction in which the helix is wound determines the polarization.
As with linear polarization, in a perfect universe a right-hand antenna will see no signal from a left-hand signal.
However, a linear (normal) antenna will see a signal for a portion of each cycle, seeng absolutely othing ONLY when the rotating signal is at 90 degrees to its polarization, maximum signal when the rotating signal is aligned with it. At other angles between it will see varying signal. The end result is that a linearly polarized antenna will see half the signal power that a correctly oriented, circularly polarized antenna would do.
Similarly, a circularly polarized antenna will see half the signal from a linearly polarized signal.
For our use (satellite reception) there is a drawback (besides the 3db signal loss); Satellites use the fact that a vertical antenna won't see a horizontal signal (and vice-versa) to transmit two signals on the same frequency. You select the signal by orienting the antenna to receive the signal you want. Same applies to circularly polarized signals.
Now, if we use a linearly polarized antenna to pick up circularly polarized signals, we will see RH and LH polarized signals equally. With an analog receiver this is fairly obvious as two pictures will be seen. With digital signals the two will interfere with each other, and the end result is usually a complete failure to get a lock on the digital signal.
So why does it seem to work sometimes?
Simply because circularly polarized signals are only truely polarized at the center of the beam. The further off-axis you get, the further from circular the signal becomes (elliptical polarization). So a linear antenna can end up picking up more of one signal than the other. Also, not all transponders carry both RH and LH signals.
Radio waves have two components, an electrical field, and a magnetic field. These fields are at right-angles to each other. The polarization is defined by the direction of the electrical field.
An antenna needs to be aligned with the electrical field to pick up the maximum signal -- it is essentially picking up the difference in electrical field (voltage). If you orient the antenna at 90 degrees to the electrical field there is (in a perfect universe) no voltage difference to pick up, so you would get no signal.
Circularly polarised signals rotate the electrical/magnetic field. It rotates 360 degrees for each cycle of the radio wave. The rotation can either be clockwise or anticlockwise. To extract maximum signal the antenna needs to "rotate" in the same direction as the signal. The classical circularly polarized antenna is the helical antenna. The direction in which the helix is wound determines the polarization.
As with linear polarization, in a perfect universe a right-hand antenna will see no signal from a left-hand signal.
However, a linear (normal) antenna will see a signal for a portion of each cycle, seeng absolutely othing ONLY when the rotating signal is at 90 degrees to its polarization, maximum signal when the rotating signal is aligned with it. At other angles between it will see varying signal. The end result is that a linearly polarized antenna will see half the signal power that a correctly oriented, circularly polarized antenna would do.
Similarly, a circularly polarized antenna will see half the signal from a linearly polarized signal.
For our use (satellite reception) there is a drawback (besides the 3db signal loss); Satellites use the fact that a vertical antenna won't see a horizontal signal (and vice-versa) to transmit two signals on the same frequency. You select the signal by orienting the antenna to receive the signal you want. Same applies to circularly polarized signals.
Now, if we use a linearly polarized antenna to pick up circularly polarized signals, we will see RH and LH polarized signals equally. With an analog receiver this is fairly obvious as two pictures will be seen. With digital signals the two will interfere with each other, and the end result is usually a complete failure to get a lock on the digital signal.
So why does it seem to work sometimes?
Simply because circularly polarized signals are only truely polarized at the center of the beam. The further off-axis you get, the further from circular the signal becomes (elliptical polarization). So a linear antenna can end up picking up more of one signal than the other. Also, not all transponders carry both RH and LH signals.
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