I want to get to know about the satellite communication at 8000-10000 km, During that, I come across the VHF and UHF. In that I am facing problem for deciding the frequency band for communicating with the satellites at 8000 to 10000 km height from the earth surface.
Please help me and give some important links which will help me for better understanding of the VHF and UHF, Also suggest me which frequency band is most suitable for this height for uplink and downlink.
Satellite communication works from around 30 MHz to 40,000 MHz. Once you are out of Earth's atmosphere, it doesn't really matter how much higher you get since there isn't anything out there to alter your signal's propagation.
At the lower bound, the same phenomena responsible for making skywave propagation on HF possible also make the same frequencies ill-suited to space communication.
At the upper bound, absorption from atmospheric oxygen and water make the atmosphere sufficiently opaque that communication is not feasible.
Absorption by water gets worse with increasing frequency, making frequencies above 11 GHz or so susceptible to rain fade.
In practice most of these concerns are overshadowed by the limited availability of spectrum. For commercial concerns frequency selection is dominated by cost and availability. Amateur operators of course have to stick to the allocations given to them, with 2m and 70cm being popular choices.
I'm not going to try to address the "Whys" (since I'm not competent to do so) but you may find the following useful. This is from the European Space Agency. There's also a really nice graphic to go with it on the site:
L-band (1-2 GHz) Global Positioning System (GPS) carriers and also satellite mobile phones, such as Iridium; Inmarsat providing communications at sea, land and air; WorldSpace satellite radio.
S-band (2–4 GHz) Weather radar, surface ship radar, and some communications satellites, especially those of NASA for communication with ISS and Space Shuttle. In May 2009, Inmarsat and Solaris mobile (a joint venture between Eutelsat and Astra) were awarded each a 2×15 MHz portion of the S-band by the European Commission.
C-band (4–8 GHz) Primarily used for satellite communications, for full-time satellite TV networks or raw satellite feeds. Commonly used in areas that are subject to tropical rainfall, since it is less susceptible to rainfade than Ku band (the original Telstar satellite had a transponder operating in this band, used to relay the first live transatlantic TV signal in 1962).
X-band (8–12 GHz) Primarily used by the military. Used in radar applications including continuous-wave, pulsed, single-polarisation, dual- polarisation, synthetic aperture radar and phased arrays. X-band radar frequency sub-bands are used in civil, military and government institutions for weather monitoring, air traffic control, maritime vessel traffic control, defence tracking and vehicle speed detection for law enforcement.
Ku-band (12–18 GHz) Used for satellite communications. In Europe, Ku-band downlink is used from 10.7 GHz to 12.75 GHz for direct broadcast satellite services, such as Astra.
Ka-band (26–40 GHz) Communications satellites, uplink in either the 27.5 GHz and 31 GHz bands, and high-resolution, close-range targeting radars on military aircraft.
