Satellite Communication - Part 2

Communications by Satellite

Satellite communication "Transponder" operates like REPEATER, which is very similar to Terrestrial Radio Communication.

The term "Transponder" here, in satellite itself, easier for satellite operator to maintain power liniarization if divided into some lot of frequency. Usually,1 transponder consist of 36 MHz in bandwidth (certain satellite use 72 MHz bandwidth).

Main functional similarities between Terrestrial Communication and Communication via / by Satellite are :

• Input signal at antena (F-in = F-Rx) relatively weak and coming from many directions.
• Mixing (F-Rx) with Local Oscilator, therefore results Frequency Translation (F-out not equal to F-in).
• F-out (F-Tx) of Repeater is filtered, amplified, then with relatively very High Power is transmitted by Antenna, for greater distance and wide area coverage (benefit of using satellites)
• Repeater's site is always placed on high surface, structure , etc, to fulfill and guarantee its desired area coverage.
• it has a certain Band-Width and Frequency (MHz) allocations.

Ok, time to explain why F-out and F-in (to satellite) is not equal and therefore occur a Frequency translation. The reasons behind this are :

• A satellite is a repeater in space. It must receive on one frequency, and transmit on another. When it comes to transmitting, it is more efficient to generate high power at a LOWER frequency
• Separate frequency paths guarantee clear transmission. The uplink frequency (Earth to satellite) is of a higher value than the downlink to mitigate the free space spreading losses, and the tropospheric losses (gases, clouds, rain), all of which are related to wavelength and therefore to frequency (wavelength=speed of light in vacuum/frequency). 
• Higher the frequency, means lesser the wavelength. So, high frequency can be received using smaller antenna in satellite, which is more practical (due the effort for throwing the satellite to its orbit), whereas in earth we can have bigger antennas.