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Can someone correct these assumptions if wrong? Given a particular frequency, increasing the power requires increasing the amplitude of the signal. I'm assuming that those giant antenna's are giant so that the larger amplitude can be pulled down. Is amplitude the parameter which makes beamwidth larger, or something else?

Thanks, sorry for the nube questions, but it's like drinking from a firehose on wireless information.

tom

Hi dumbmick,

Here's my limited knowledge on this topic:

In SATCOM, it is all about GAIN (amplitude), or the term G/T . That G/T compares the gain of the antenna to that of the noise temperature.

The larger dishes pick up more of the modulated footprint (beamwidth) signal from the satellite in orbit transmitting the RF to the end user. They (dishes) pick up more noise, and must filter that noise with more efficient LNAs ( Low Noise Amplifiers).

Because of the antenna size factors, (G/T) the end users with a larger gain antenna may require less transmit power to close the link with the satellite in orbit.


http://en.wikipedia.org/wiki/Satellite_dish

For more in-depth info on link budgets:

http://www.satcom.co.uk/article.asp?article=21

Sorry for this late post..

The giant satellite antennas are primarily used for focusing the beam, which both increases the gain of the signal as well as reduces interference.

The interference issue is the big one. Geostationary satellites are some 25,000 miles up, and the satellites are spaced together closely, so a ground station that is transmitting a fair amount of power can easily "splash" over into other birds.

And on the downlink side, a small antenna has a wide apeture, which means it will "see" more than one satellite at a time, making it more difficult to reduce interference.

So by having large antennas, the beam becomes very narrow, which increases gain and substantially reduces interference.

Dennis,

Thank you for the great visual explanation. I had no idea that sats
fly at 40,000 km above the earth. Gives me a higher respect for
them when I see them flying overhead.

Tom