Dual polarisation LNBs
The LNB shown above has one wire going into the waveguide to pick up vertical polarisation. If the input waveguide is circular is can support two polarisations and it can be arranged for there to be two input probes at right angles, thus allowing two alternative polarisations to be selected (vertical or horizontal), either one or the other. Dual polarisation LNBs may commonly be switched remotely using two alternative DC supply voltages. e.g. 13 volts makes it receive vertical polarisation and 19 volts make it receive horizontal polarisation.
LNB supply voltages
The DC voltage power supply is fed up the cable to the LNB. Often by altering this voltage it is possible to change the polarisation or, less commonly, the frequency band. Voltages are normally 13 volts or 19 volts. Perfect weatherproofing of the outdoor connector is essential, otherwise corrosion is rapid. Note that both the inner and outer conductors must make really good electrical contact. High resistance can cause the LNB to switch permanently into the low voltage state. Very peculiar effects can occur if there poor connections amongst multiple cables to say an LNB and to a transmit BUC module as the go and return DC supplies may become mixed up and the wrong voltage applied across the various items. The electrical connections at the antennas between the LNB and the BUC chassis are often indeterminate and depend of screws in waveguide flanges etc. Earth loop currents may also be a problem - it is possible to find 50 Hz or 60 Hz mains currents on the outer conductors - so be careful. Such stray currents and induced RF fields from nearby transmitters and cell phones may interfere with the wanted signals inside the cables. The quality and smoothing of the the DC supplies used for the LNBs is important
How to test an LNB
Check with a current meter that it is drawing DC current from the power supply. The approx number of milliamps will be given by the manufacturer. Badly made or corroded F type connections are the most probable cause of faults. Remember that the centre pin of the F connector plug should stick out about 2mm, proud of the surrounding threaded ring.
Use a satellite finder power meter. If you point the LNB up at clear sky (outer space) then the noise temperature contribution from the surroundings will be negligible, so the meter reading will correspond to the noise temperature of the LNB, say 100K (K means degrees Kelvin, above the 0 K absolute zero temperature). If you then point the LNB at your hand or towards the ground, which is at a temperature of approx 300K then the noise power reading on the meter should go up, corresponding to approx 400K (100K +300K).
Note that LNBs may fail on one polarisation or on one frequency band and that the failure mode may only occur at certain temperatures.
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