Mast radiator

This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations.

The actual transmitter is usually located in a separate building, which supplies RF power to the tuning hut via a transmission line.

To prevent this, additional strain insulators are inserted at intervals in the guy cables to divide the line into nonresonant lengths: Usually segments should be limited to a maximum of one-eighth to one-tenth wavelength (

In the medium frequency (MF) and low frequency (LF) bands AM radio stations cover their listening area using ground waves, vertically polarized radio waves which travel close to the ground surface, following the contour of the terrain.

They also can radiate enough power at higher elevation angles for skywave (skip) radio transmission.

Alternatively it is sometimes located at the base of the mast, with the transmitter room surrounded by a Faraday shield of copper screen to keep radio waves out.

[5] This may be located in a waterproof box or a small shed called an antenna tuning hut (helix house) next to the mast.

Without the antenna tuner the impedance mismatch between the antenna and feedline would cause a condition called standing waves (high SWR), in which some of the radio power is reflected back down the feedline toward the transmitter, resulting in inefficiency and possibly overheating the transmitter.

Due to the finite thickness of the mast, resistance, and other factors the actual antenna current on the mast differs significantly from the ideal sine wave assumed above, and as shown by the graph, resonant lengths of a typical tower are closer to 80°, 140°, and 240°.

[9] An ideal monopole antenna radiates maximum power in horizontal directions at a height of 225 electrical degrees, about ⁠5/8⁠ or 0.625 of a wavelength (this is an approximation valid for a typical finite thickness mast; for an infinitely thin mast the maximum occurs at

[6]) As shown in the diagram, at heights below a half wavelength (180 electrical degrees) the radiation pattern of the antenna has a single lobe with a maximum in horizontal directions.

At heights above a half wavelength the pattern splits and has a second lobe directed into the sky at an angle of about 60°.

is that at slightly above a half wavelength, the opposite phase radiation from the two lobes interferes destructively and cancels at high elevation angles, causing most of the power to be emitted in horizontal directions.

[9] Some of the radio energy radiated at an angle into the sky is reflected by layers of charged particles in the ionosphere and returns to Earth in the reception area.

[16] The transmitter power lost in the ground resistance, and so the efficiency of the antenna, depends on the soil conductivity.

Another solution is to increase the number of ground wires near the mast and bury them very shallowly in a surface layer of asphalt pavement, which has low dielectric losses.

An alternate design is to mount the mast on top of the antenna tuning hut, out of the reach of the public, eliminating the need for a fence.

Regulations require flashing lights at the top, and (depending on height) at several points along the length of the tower.

[17][3] Without protective equipment it would conduct radio frequency (RF) current to the AC power wiring ground, short-circuiting the mast.

The transmission lines feeding RF power to the colocated antennas pose much the same problem as the aircraft lighting power lines: they have to pass down the tower and across the base insulator and connect to low voltage equipment, so without isolation devices, they will carry the high mast voltage and can short circuit the mast to ground.

The transmission lines are isolated by low pass filter inductors consisting of helixes of coaxial cable wound on a nonconductive form.

He initially used horizontal dipole antennas invented by Heinrich Hertz, but was not able to communicate further than a few miles.

One of the first large mast radiators was the experimental tubular 130-meter (420 ft) mast erected in 1906 by Reginald Fessenden for his spark gap transmitter at Brant Rock, Massachusetts with which he made the first two-way transatlantic transmission, communicating with an identical antenna in Machrihanish, Scotland.

During this era, the operation of antennas was little understood, and designs were based on trial and error and half-understood rules of thumb.

Two historic papers published in 1924 by Stuart Ballantine led to the development of the mast radiator.

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 0.625

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 200-meter (665 ft) half-wave mast was installed at radio station WABC's 50 kW Wayne, New Jersey transmitter in 1931.

[9] 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.

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.