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WiFi is from 2400-2500MHz or from 5000-5100. Fm Radio is 87-108Mhz, Phones : 450-3400Mhz But most of the spectrum is "wasted". For eg - Why we don't use above 5Ghz?

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The spectrum is allocated over all useable ranges. There's a lot more going on then what you named. Here's the regulatory allocation chart for the US as an example. It has allocations up to 300 GHz.

https://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf

As for why certain frequencies are used for certain things and not others, that's partly a regulatory judgement call but also related to physics. For example, some frequencies are heavily attenuated by atmospheric water, which make them good for applications where you don't want to signal to travel far (or you want to heat water). Other frequencies correspond to natural phenomena and are reserved for radio astronomy. Others have good penetration depth into buildings. Others require very large or very small antennae, making them good or bad for different applications. Etc.

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