Radio communications depend on antennas to transmit (Tx) and receive (Rx) radio signals. Antennas must be mounted in such a way to ensure they have a clear Line-of-Sight (LOS) between each other. Many radio frequencies have a LOS range meaning they travel in a straight line. For frequencies of 30 megahertz and above, any obstacle between the Tx and Rx antennas will block the signal. Obstructions could include buildings, vehicles, trees, foliage and natural obstacles like hills or mountains. The curvature of the Earth also hampers LOS transmissions.
For a person standing at sea level on flat land, the distance between them and the horizon is about five kilometres/km (three miles/m). This is the same for radio signals. They can hug the ground a short distance from the horizon but will continue in a straight line into the sky and eventually into space. Radio antennas allow a signal to be transmitted a certain height from the surface. The higher the antenna, the further the transmission will go. For example, a two-metre/m (six-feet/ft) high person carrying a radio with a one-metre (three-feet) antenna will have a total height of three metres (nine feet). This will give the radio a LOS range of seven kilometres (4.4 miles). A one-metre high antenna atop a ten-metre building will have a range of 13km (8.1 miles). This explains why tall masts are needed for antennas to broadcast television and radio signals over a large area. For example, Crystal Palace transmitting station in south London has a 219m (719ft) mast. The mast is positioned on a hill, 112ft (367.5m) above sea level. The antenna has a total height of 331m (1,086ft) yielding a 74km (46-mile) range.
By the nature of their need to deploy militaries, particularly land forces, tend to use highly portable communications masts. Telescopic or fold-up designs are popular which can easily equip temporary or permanent bases and provide good LOS ranges due to their height. Increasingly, militaries are also adopting aerostats. These carry clusters of communications antennas several hundred feet into the air providing impressive LOS distances.
Some communications masts are designed not so much to provide a long LOS range, but to accommodate the large antennas necessary for communications at certain frequencies. High Frequency (HF) radio communications need large antennas. In addition to achieving LOS ranges HF transmissions provide intercontinental ranges of thousands of miles. This is because HF radios use the ionosphere as a trampoline to bounce transmissions beyond the horizon. The ionosphere is a naturally-occurring atmospheric layer at altitudes of between 48km (30 miles) and 965km (600 miles) which HF signals cannot penetrate. A general rule of radio engineering is that an antenna must be one-half or one-quarter the wavelength it carries. HF frequencies of three megahertz/MHz to 30MHz have respective wavelengths of 99.9m (327.8ft) and 9.99m (32.8ft). This means HF antennas must be between 49.9m (163.7ft) and 24.9m (81.7ft), and 4.9m (16.4ft) and 2.4m (7.9ft) long. This can necessitate large and complex antenna structures to support HF communications which must still be deployable.
Very Low Frequencies (VLF) of between three kilohertz/KHz and 30KHz demand antennas of lengths between 99.9km (62.1 miles) and 49.9km (31 miles), and 9.9km (6.2 miles) and 4.9km (three miles). Unlike HF, this prevents VLF antennas being transportable to support military deployments. As such, VLF antennas tend to be large, fixed installations on land where they are routinely used for submarine communications.