Ultra high frequency

[4] UHF radio waves are blocked by hills and cannot travel beyond the horizon, but can penetrate foliage and buildings for indoor reception.

Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required.

Occasionally when conditions are right, UHF radio waves can travel long distances by tropospheric ducting as the atmosphere warms and cools throughout the day.

UHF wavelengths are short enough that efficient transmitting antennas are small enough to mount on handheld and mobile devices, so these frequencies are used for two-way land mobile radio systems, such as walkie-talkies, two-way radios in vehicles, and for portable wireless devices; cordless phones and cell phones.

Since at UHF frequencies transmitting antennas are small enough to install on portable devices, the UHF spectrum is used worldwide for land mobile radio systems, two-way radios used for voice communication for commercial, industrial, public safety, and military purposes.

The most rapidly-expanding use of the band is Wi-Fi (wireless LAN) networks in homes, offices, and public places.

A second widespread use is for cellphones, allowing handheld mobile phones be connected to the public switched telephone network and the Internet.

There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz, regulated under Title 47 CFR Part 15.

These ISM bands—frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus—are now some of the most crowded in the spectrum because they are open to everyone.