Why a satellite dish never moves, and why rain can blank the picture
A satellite dish bolted to a wall can stay pointed at one spot forever because the satellite it listens to circles Earth exactly as fast as the planet spins. Parked about 36,000 kilometres above the equator, it seems to hang motionless in the sky, relaying television to homes far beyond any cable network.
Every broadcast starts at an uplink station, where dishes 9 to 12 metres across fire a narrow microwave beam at a chosen satellite. The large size helps with precise aiming and delivers a stronger signal to the spacecraft. On board, devices called transponders catch the signal and send it back down on a different frequency, so the returning broadcast does not interfere with the one still arriving. A typical satellite carries a couple of dozen or more of these transponders.
By the time the signal reaches a home it is very faint. The curved dish gathers it and reflects it onto a small flared horn at its focal point, which feeds a device called a low-noise block downconverter. This amplifies the signal and shifts it to a lower frequency that ordinary, inexpensive cable can carry indoors to the receiver. Early home systems had no such trick: they relied on costly gas-filled cable and heavy connectors, and needed dishes two or three metres wide, earning them the nickname big dish.
Physics sets some awkward limits. Satellites sharing the same orbit must be spaced apart in longitude to avoid interfering with each other, around two degrees for the older C-band and one degree for Ku-band, which caps the number of slots. Ku-band signals suffer in heavy rain because water soaks up microwaves at those frequencies, and ice crystals in thunderclouds are worse still. Occasionally the Sun slips directly behind the satellite as seen from a dish, and its radio glare briefly drowns out the broadcast.
Some channels are free to watch without any decoding, while pay services scramble their signal and need a subscription and a receiver that can unlock it. In remote areas with no cable or land-based transmitters, satellite is often the only way to get television at all.
Source: Satellite television
