The LNB decides what polarity to use for each channel.
The Sat receiver feeds either 13 or 18 volts to the LNB depending on the channel.So like channel 100 is 13 volts,channel 101 is 18 volts and so on up through the channel range.They have always been a digital signal since the small type dishes came out.
The RG59 was never designed to carry voltage and the higher MHZ signal.
Thats also why you can not split any cable runs from the dish and feed 2 receivers.If you do both will only work on the 13 volt side or the 18 volt side at the same time.Thats why they designed the multi switch,2 leads from the LNB into the switch,one for 18 volt and the other line for 13 volts.The multi switch then feeds the needed signal to each receiver depending on what channel its on.
Its hard to explain but i had about 7 years training trouble shooting commercial sat. systems.I still have all my certifications and try to read up on it.
Any digital signal will either have picture or no picture at all. It looks the same at 100% signal strength that it does at 40% Until it gets so low it pixelizes and freezes.
Rain fade is mostly from out of adjustment dishes except for extremely heavy rain that just blocks the line of sight totally from the satellite.
The cable sheild is more critical to cable tv signals to keep bleed over from other signal types with analog.
jlinder is pretty well on target but the sat systems are a little more complicated...