You Asked:
How does a dish receive a signal from a satellite? — Manish, Muzaffarpur, India

Marshall Answered:
Most of the electronic devices that we use have stick antennas. Your car probably has one for the radio. Your cell phone may have one. Walkie-talkies and radio-controlled devices have them. These are called omni-directional antennas. They receive radio signals equally well from just about any angle. That is important in a portable device.

But there is a cost to omni-directional antennas. The transmitter has to use more power. This is why we end up with 50,000 watt AM radio stations.

In the case of TV satellites, there are three reasons why omni-directionality is not a good idea:

1) the satellite is 24,000 miles above earth in geosynchronous orbit. This is a huge distance compared to anything happening with radios on earth. A terrestrial TV station might transmit at most 50 miles.

2) there is no way to put a 50,000 watt transmitter on a satellite. It would weigh far too much and the solar panels would have to be immense to provide that much electricity.

3) there is no reason to use an omni-directional antenna. The satellite is located at a fixed position in the sky and your house is located at a fixed position on the ground. Your house can use a directional antenna pointed at the satellite.

So they lower the transmitting power of the satellite, and use dish antennas on the satellite and on your home. A dish antenna is directional. It only receives radio waves from a particular direction, but handles that direction very well. The radio waves reflect off the metal dish much like light waves reflect off a mirror. The dish has a parabola shape so the radio waves concentrate on a point. At that point a small stick-like antenna receives the concentrated radio waves. For more info see How Radio Works