when frequency is that large doesnt it mean shorter wavelength? power line can travel 10000km because of extremely low frequency doesnt it mean that cell phone signals travel very short distance then? i dont understand this at all please help
Long wavelength implies a long distance with little phase shift. When you try to connect the same signal together at many points, like a power grid, it gets really messy if the waves coming around through the grid via different paths have different phases where they meet again. This causes big circulating energy loops in the grid.
Cell phones do not use a grid of conductors, but make use of waves traveling through space. So the phase is unimportant when the waves reach a receiving antenna. All the various paths add together, and some effective phase results. But any phase will do, since the information being carried by those waves uses many cycles to carry each bit of information.
Yes, high (not large) frequency means a short wavelength. Very high frequencies are used to transmit data between the Earth and space probes which are also many thousands of km distant.
Cell phones need to be compact and short range because they operate in CELLS which are limited geographical areas. They are also low power which helps extend battery life.
Because that is the spectrum God gave them.
You are correct that higher frequencies require more power to propagate an equal distance as say low band VHF. However, there is no room in those bands for cellular phones. Also there is no need with cell phones to reach out that far, (unless you live in South Dakota as I do) Also, Wavelength has a bunch to do with the antenna size. You do not want a 6 foot whip antenna on your cell phone or a large coil.
One reason for using a high frequency is that high frequencies can carry more data per second than low frequencies. (Over to you to think about why this is). So a mobile phone mast can fit many 'virtual circuits' into one frequency band, and talk to lots of phones at once.
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Of course the cables used to transmit POWER have some bearing :-) Whereas the whole idea of the cordless phones is just that, no cables trailing around behind you. The signal (very low POWER) is transmitted from a RF transmitter, there is extremely little actual POWER transmitted to the phones.
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