page_title: Mast radiator
section_title: Mast height and radiation pattern
hierarchical_section_title: Mast radiator / Electrical design / Mast height and radiation pattern
context_page_description: A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. 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 conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.
context_section_description: The ideal height of a mast radiator depends on transmission frequency , the geographical distribution of the listening audience, and terrain.   An unsectionalized mast radiator is a monopole antenna, and its vertical radiation pattern, the amount of power it radiates at different elevation angles, is determined by its height  compared to the wavelength  of the radio waves, equal to the speed of light  divided by the frequency .  The height of the mast is usually specified in fractions of the wavelength, or in "electrical degrees"
  where each degree equals  meters.  The current distribution on the mast determines the radiation pattern.   The radio frequency current flows up the mast and reflects from the top, creating an approximately sinusoidal standing wave on the mast with a node (point of zero current) at the top and a maxima one quarter wavelength down
  where  is the current at a height of  electrical degrees above the ground, and  is the maximum current.   At heights of a little less than a multiple of a quarter wavelength,   ...(G = 90°, 180°, 270°...) the mast is resonant; at these heights the antenna presents a pure resistance to the feedline, simplifying impedance matching the feedline to the antenna.  At other lengths the antenna has capacitive reactance or inductive reactance.  However masts of these lengths can be fed efficiently by cancelling the reactance of the antenna with a conjugate reactance in the matching network in the helix house.  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°.
  Ground waves travel horizontally away from the antenna just above the ground, therefore the goal of most mast designs is to radiate a maximum amount of power in horizontal directions.  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; the exact maximum occurs at  = .637)   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 has a second lobe directed into the sky at an angle of about 60°.   The reason horizontal radiation is maximum at 0.625 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.  Heights above 0.625 are not generally used because above this the power radiated in horizontal directions decreases rapidly due to increasing power wasted into the sky in the second lobe.
  For medium wave AM broadcast band masts 0.625 would be a height of 117–341 meters (384–1,119 ft), and taller for longwave masts.  The high construction costs of such tall masts mean frequently shorter masts are used. 
  The above gives the radiation pattern of a perfectly conducting mast over perfectly conducting ground.  The actual strength of the received signal at any point on the ground is determined by two factors, the power radiated by the antenna in that direction and the path attenuation between the transmitting antenna and the receiver, which depends on ground conductivity.  The design process of an actual radio mast usually involves doing a survey of soil conductivity, then using an  antenna simulation computer program to calculate a map of signal strength produced by actual commercially available masts over the actual terrain. This is compared with the audience population distribution to find the best design.
caption: Measured base resistance and reactance of a typical base fed mast radiator vs height.