English: A microwave relay tower on Granite Pass, Mojave National Preserve, California. This is part of the TD-2 AT&T Long Lines system, a microwave relay network created by AT&T in the 1950s to relay telephone calls and other data between cities in the USA. The four angular cone-shaped antennas are called horn-reflector antennas, invented by Albert Beck and Harald Friis in 1941 and developed by D. L. Hogg at Bell Labs in 1961. They operate at C-band frequencies. at 4 and 6 GHz. They consist of a vertical flaring metal horn, fed by a vertical waveguide from the shed at ground level, with a reflector mounted in the mouth of the horn at a 45° angle, so the beam of microwaves is emitted horizontally. The reflector is a segment of a parabolic reflector, so the antenna is equivalent to a parabolic antenna fed off-axis. The advantage of this type of antenna over an ordinary parabolic (dish) antenna is that it radiates very little energy outside of the main beam. This allows the same microwave frequencies to be used by several nearby antennas pointing in different directions without interfering with each other. These KD-15676 antennas have a gain of about 43 dBi and produce a beamwidth (HPBW) of about 1° at 6 GHz. Two antennas are used for each path, one receiving and one transmitting. Each beam carried thousands of telephone calls. (From KD-15676 Horn Reflector Antenna Description, Bell System standards, AT&T