Radio masts and towers

Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television.

In 1895–1896 he invented the vertical monopole or Marconi antenna, which was initially a wire suspended from a tall wooden pole.

Because the extreme wavelengths were one to several kilometers long, even the tallest feasible antennas by comparison were still too short, electrically, and consequently had inherently very low radiation resistance (only 5~25 Ohms).

To partially compensate, radiotelegraph stations used huge capacitively top-loaded flattop antennas consisting of horizontal wires strung between multiple 100–300 meters (330–980 ft) steel towers to increase efficiency.

The allocation of the medium wave frequencies for broadcasting raised the possibility of using single vertical masts without top loading.

[2](pp 77–78) In 1924 Stuart Ballantine published two historic papers which led to the development of the single mast antenna.

In a second paper the same year he showed that the amount of power radiated horizontally in ground waves reached a maximum at a mast height of ⁠ 5 /8⁠ wavelength.

This had a diamond (rhombohedral) shape which made it rigid, so only one set of guy lines was needed, at its wide waist.

The first, a 665 foot (203 m) half-wave mast was installed at radio station WABC's 50 kW transmitter at Wayne, New Jersey in 1931.

[4][5] During the 1930s it was found that the diamond shape of the Blaw-Knox tower had an unfavorable current distribution which increased the power emitted at high angles, causing multipath fading in the listening area.

[2](pp 79–81) By the 1940s the AM broadcast industry had abandoned the Blaw-Knox design for the narrow, uniform cross section lattice mast used today, which had a better radiation pattern.

However the newer FM and TV transmitters used the VHF band, in which radio waves travel by line-of-sight, so they are limited by the visual horizon.

This construction type has the advantage that cables and other components can be protected from weather inside the tube and consequently the structure may look cleaner.

In Germany, France, UK, Czech Republic, Slovakia, Japan and the Soviet Union, many tubular guyed masts were built, while there are nearly none in Poland or North America.

The crossbars of these masts are equipped with a gangway that holds smaller antennas, though their main purpose is oscillation damping.

Reinforced concrete towers are relatively expensive to build but provide a high degree of mechanical rigidity in strong winds.

The Katanga TV tower near Jabalpur, Madhya Pradesh, in central India hosts a high-power transmitter for the public broadcasters Doordarshan and Prasar Bharati.

Carbon fibre monopoles and towers have traditionally been too expensive but recent developments in the way the carbon fibre tow is spun have resulted in solutions that offer strengths exceeding steel (10 times) for a fraction of the weight (70% less[9]) which has allowed monopoles and towers to be built in locations that were too expensive or difficult to access with the heavy lifting equipment that is needed for a steel structure.

Overall a carbon fiber structure is 40 - 50% faster to be erected compared to traditional building materials.As of 2022[update], wood, previously an uncommon material for telecommunications tower construction, has started to become increasingly common.

[11] For this reason, some utility pole distributors started to offer wood towers to meet the growing demands of 5G infrastructure.

The North Tower of the original World Trade Center also had a 110-metre (360 ft) telecommunications antenna atop its roof, constructed in 1978–1979, and began transmission in 1980.

When the buildings collapsed, several local TV and radio stations were knocked off the air until backup transmitters could be put into service.

In London, the BBC erected in 1936 a mast for broadcasting early television on one of the towers of a Victorian building, the Alexandra Palace.

Disguised cell sites sometimes can be introduced into environments that require a low-impact visual outcome, by being made to look like trees, chimneys or other common structures.

Even though people increasingly depend upon cellular communications, they are opposed to the bare towers spoiling otherwise scenic views.

Telescopic masts consist of two or more concentric sections and come in two principal types: A tethered balloon or a kite can serve as a temporary support.

Larger structures, which tend to require more frequent maintenance, may have stairs and sometimes a lift, also called a service elevator.

[19][20] There have also been instances of rare birds nesting in cell towers and thereby preventing repair work due to legislation intended to protect them.