power is fed to the mast by a wire attached to it, which comes from a matching network inside the "
antenna tuning hut" at right. The brown ceramic insulator at the base keeps the mast electrically insulated from the ground. On the left there is an earthing switch and a
spark gap for lightning protection. inserted to prevent the high voltage on the mast from reaching the ground, and to break the lines into segments with non-
resonant lengths. A single mast radiator is an
omnidirectional antenna which radiates equal radio wave power in all horizontal directions. Mast radiators radiate
vertically polarized radio waves, with most of the power emitted at low elevation angles. 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. Mast radiators make good ground wave antennas, and are the main type of transmitting antennas used by AM radio stations, as well as other radio services in the MF and LF bands. They also can radiate enough power at higher elevation angles for
skywave (skip) radio transmission. Most radio stations use single masts. Multiple masts fed with radio current at different
phases can be used to construct
directional antennas, which radiate more power in specific directions than others.
Feed system The
transmitter which generates the
radio frequency current is often located in a building a short distance away from the mast, so its sensitive electronics and operating personnel will not be exposed to the strong radio waves at the base of the mast. 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. The current from the transmitter is delivered to the mast through a
feedline, a specialized cable (
transmission line) for carrying radio frequency current. At LF and MF frequencies foam insulated
coaxial cable is usually used. The feedline is connected to an
antenna tuning unit (
impedance matching network) at the base of the mast, to match the transmission line to the mast. This may be located in a waterproof box or a small shed called an
antenna tuning hut (helix house) next to the mast. The antenna tuning circuit
matches the
characteristic impedance of the feedline to the impedance of the antenna (given by the graph below), and includes a
reactance, usually a
loading coil, to tune out the reactance of the antenna, to make it
resonant at the operating frequency. 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. From the antenna tuner a short feedline is bolted or brazed to the mast. The other side of the feedline from the antenna tuner is
grounded, connected to a
radial ground system consisting of many bare wires buried shallowly in the ground radiating outward from a terminal near the base of the mast. There are several ways of feeding a mast radiator: •
Series excited (base feed): the mast is supported on an insulator, and is fed at the bottom; one side of the feedline from the helix house is connected to the bottom of the mast and the other to a ground system under the mast. This is the most common feed type, used in most AM radio station masts. •
Shunt excited: the bottom of the mast is grounded, and one side of the feedline is connected to the mast part way up, and the other to the ground system under the mast. The impedance of the mast increases along its length, so by choosing the right height to connect, the antenna can be
impedance matched to the feedline. This avoids the need to insulate the mast from the ground, eliminates the need for an isolator in the aircraft light power line and the
electric shock hazard of high voltages on the base of the mast. •
Folded unipole: this can be considered a variation of shunt feed, above. The antenna mast is grounded and a tubular "skirt" of wires is attached to the top of the antenna and hangs down parallel to the mast, surrounding it, to ground level, where it is fed. It has a wider bandwidth than a single tower. •
Sectional: also known as an "anti-fading aerial", the mast is divided into two sections with an insulator between them to make two stacked vertical antennas, fed in phase. This
collinear arrangement enhances low-angle (ground wave) radiation and reduces high-angle (sky wave) radiation. This increases the distance to the
mush area where the ground wave and sky wave are at similar strength at night. Government regulations usually require the power fed to the antenna to be monitored at the antenna base, so the antenna tuning hut also includes an antenna current sampling circuit, which sends its measurements back to the transmitter control room. The hut also usually contains the power supply for the aircraft warning lights.
Mast height and radiation pattern s of 3 different height monopole mast radiator antennas mounted on the ground. The distance of the line from the origin at a given elevation angle is proportional to the power density radiated at that angle. For a given power input, the power radiated in horizontal directions increases with height from the
quarter-wave monopole (0.25λ,
blue) through the half-wave monopole (0.5λ,
green) to a maximum at a length of 0.625λ (
red) The ideal height of a mast radiator depends on transmission
frequency f, 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 h compared to the
wavelength \lambda = c/f of the radio waves, equal to the speed of light c divided by the frequency f. The height of the mast is usually specified in fractions of the wavelength, or in "
electrical degrees" :G = 360^\circ {h \over \lambda} where each degree equals \lambda/360 meters. The current distribution on the mast determines the
radiation pattern. The
radio frequency current flows up the mast and reflects from the top, and the direct and reflected current
interfere, 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 :i(y) = I_\text{max}\sin (G - y) where i(y) is the current at a height of y electrical degrees above the ground, and I_\text{max} is the maximum current. At heights of a little less than a multiple of a quarter wavelength, {1 \over 4}\lambda, {1 \over 2}\lambda, {3 \over 4}\lambda ...(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 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 2\lambda/\pi = 0.637\lambda) 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°. The reason horizontal radiation is maximum at 0.625\lambda 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\lambda 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\lambda would be a height of , 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.
Anti-fading designs A second design goal that affects height is to reduce
multipath fading in the reception area. 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. This is called the
skywave. At certain distances from the antenna these radio waves are
out of phase with the ground waves, and the two radio waves
interfere destructively and partly or completely cancel each other, reducing the signal strength. This is called
fading. At night when ionospheric reflection is strongest, this results in an annular region of low signal strength around the antenna in which reception may be inadequate, sometimes called a "zone of silence", fading wall or
mush area. However multipath fading only becomes significant if the signal strength of the skywave is within about 50% (3 dB) of the ground wave. By reducing the height of a monopole slightly the power radiated in the second lobe can be reduced enough to eliminate multipath fading, with only a small reduction in horizontal gain. The optimum height is around 190 electrical degrees or 0.53\lambda, so this is another common height for masts.
Sectionalized masts A type of mast with improved anti-fading performance is the sectionalized mast, also called an anti-fading mast. In a sectionalized mast, insulators in the vertical support members divide the mast into two vertically stacked conductive sections, which are fed
in phase by separate feedlines. This increases the proportion of power radiated in horizontal directions and allows the mast to be taller than 0.625\lambda without excessive high angle radiation. Practical sectionals with heights of 120 over 120 degrees, 180 over 120 degrees and 180 over 180 degrees are presently in operation with good results.
Electrically short masts The lower limit to the frequency at which mast radiators can be used is in the
low frequency band, due to the increasing inefficiency of masts shorter than a quarter wavelength. As frequency decreases the wavelength increases, requiring a taller antenna to make a given fraction of a wavelength. Construction costs and land area required increase with height, putting a practical limit on mast height. Masts over are prohibitively expensive and very few have been built; the tallest masts in the world are around . Another constraint in some areas is height restrictions on structures; near airports aviation authorities may limit the maximum height of masts. These constraints often require a mast be used that is shorter than the ideal height. Antennas significantly shorter than the fundamental resonant length of one-quarter of the wavelength (0.25\lambda, 90 electrical degrees) are called
electrically short antennas. Electrically short antennas are efficient
radiators; the
gain of even a short antenna is very close to that of a quarter-wave antenna. However they cannot be
driven efficiently due to their low
radiation resistance. The radiation resistance of the antenna, the
electrical resistance which represents power radiated as radio waves, which is around 25–37
ohms at one-quarter wavelength, decreases below one-quarter wavelength with the square of the ratio of mast height to wavelength. Other electrical resistances in the antenna system, the ohmic resistance of the mast and the buried ground system, are in series with the radiation resistance, and the transmitter power divides proportionally between them. As the radiation resistance decreases more of the transmitter power is dissipated as heat in these resistances, reducing the efficiency of the antenna. Masts shorter than 0.17\lambda (60 electrical degrees) are seldom used. At this height, the radiation resistance is about 10 ohms, so the typical resistance of a buried ground system, 2 ohms, is about 20% of the radiation resistance, so below this height over 20% of the transmitter power is wasted in the ground system. A second problem with electrically short masts is that the
capacitive reactance of the mast is high, requiring a large
loading coil in the antenna tuner to tune it out and make the mast resonant. The high reactance vs the low resistance give the antenna a high
Q factor; the antenna and coil act as a high Q
tuned circuit, reducing the usable
bandwidth of the antenna. At lower frequencies mast radiators are replaced by more elaborate capacitively toploaded antennas such as the
T antenna or
umbrella antenna which can have higher efficiency.
Capacitive toploads In circumstances in which short masts must be used, a
capacitive topload (also known as
top hat or
capacitance hat) is sometimes added at the top of the mast to increase the radiated power. This is a round screen or
radiate crown of horizontal wires extending horizontally from the top of the antenna. It functions as a
capacitor plate, with the ground below being the complementary plate; the increased current in the mast required to charge and discharge the top load capacitance each RF cycle increases the radiated power in the vertical section of the antenna. Since the top load acts electrically like an additional length of mast, this is called "
electrically lengthening" the antenna. Another way to construct a capacity hat is to use sections of the top guy wire set, by inserting the
strain insulators in the guy line a short distance from the mast. For simple masts, capacity hats are structurally limited to the equivalent of about 15-30 degrees of added electrical height;
umbrella antennas exceed this limit by having added structural support for the "hat".
Grounding system For mast radiators the earth under the mast is part of the antenna; the current fed to the mast passes through the air into the ground under the antenna as
displacement current (oscillating electric field). The ground also serves as a
ground plane to reflect the radio waves. The antenna is fed power between the bottom of the mast and ground so it requires a
grounding (Earthing) system under the antenna to make contact with the soil to collect the return current. One side of the feedline from the helix house is attached to the mast, and the other side to the ground system. The ground system is in series with the antenna and carries the full antenna current, so for efficiency its resistance must be kept low, under two ohms, so it consists of a network of cables buried in the earth. Since for an omnidirectional antenna the Earth currents travel radially toward the ground point from all directions, the grounding system usually consists of a radial pattern of buried cables extending outward from the base of the mast in all directions, connected together to the ground lead at a terminal next to the base. The transmitter power lost in the ground resistance, and so the efficiency of the antenna, depends on the soil conductivity. This varies widely; marshy ground or ponds, particularly salt water, provide the lowest resistance ground. The RF current density in the earth, and thus the power loss per square meter, increases the closer one gets to the ground terminal at the base of the mast, so the radial ground system can be thought of as replacing the soil with a higher conductivity medium, copper, in the parts of the ground carrying high current density, to reduce power losses. A standard widely used ground system acceptable to the US
Federal Communications Commission (FCC) is 120 equally-spaced radial ground wires extending out one quarter of a wavelength (.25\lambda, 90 electrical degrees) from the mast. No. 10 gauge soft-drawn copper wire is typically used, buried deep. For
AM broadcast band masts this requires a circular land area extending from the mast . This is usually planted with grass, which is kept mowed short as tall grass can increase power loss in certain circumstances. If the land area around the mast is too limited for such long radials, they can in many cases be replaced by a greater number of shorter radials. The metal support under the mast insulator is bonded to the ground system with conductive metal straps so no voltage appears across the concrete pad supporting the mast, as concrete has poor dielectric qualities. For masts near a half-wavelength high (180 electrical degrees) the mast has a voltage maximum (
antinode) near its base, which results in strong
electric fields in the earth above the ground wires near the mast where the
displacement current enters the ground. This can cause significant
dielectric power losses in the earth. To reduce this loss these antennas often use a conductive copper ground screen around the mast connected to the buried ground wires, either lying on the ground or elevated a few feet, to shield the ground from the electric field. 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. == Ancillary equipment ==