Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, satellite phones (S band), and numerous short range terrestrial data links. They are also used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, and to cook food in microwave ovens. Frequencies in the SHF range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations. Microwaves propagate solely by line of sight; because of the small refraction due to their short wavelength, the groundwave and ionospheric reflection (skywave or "skip" propagation) seen with lower frequency radio waves do not occur. Although in some cases they can penetrate building walls enough for useful reception, unobstructed rights of way cleared to the first Fresnel zone are usually required. Wavelengths are small enough at microwave frequencies that the antenna can be much larger than a wavelength, allowing highly directional (high gain) antennas to be built which can produce narrow beams. Therefore, they are used in point-to-point terrestrial communications links, limited by the visual horizon to 30–40 miles (48–64 km). Such high gain antennas allow frequency reuse by nearby transmitters. They are also used for communication with spacecraft since the waves are not refracted (bent) when passing through the ionosphere like lower frequencies.