Mast radiator
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.
Structural design
Most mast radiators are built as guyed masts. Steel lattice masts of triangular cross-section are the most common type. Square lattice masts and tubular masts are also sometimes used. To ensure that the tower is a continuous conductor, the tower's structural sections are electrically bonded at the joints by short copper jumpers which are soldered to each side or "fusion" welds across the mating flanges.Image:Radio antenna mast bonding strips.JPG|thumb|left|upright=0.8|To ensure that the mast acts as a single conductor, the separate structural sections of the mast are connected electrically by copper jumpers.
Base-fed masts, the most common type, must be insulated from the ground. At its base, the mast is usually mounted on a thick ceramic insulator, which has the compressive strength to support the tower's weight and the dielectric strength to withstand the high voltage applied by the transmitter. The RF power to drive the antenna is supplied by a impedance matching network, usually housed in an antenna tuning hut near the base of the mast, and the cable supplying the current is simply bolted or brazed to the tower. The actual transmitter is usually located in a separate building, which supplies RF power to the tuning hut via a transmission line.
To keep it upright the mast has tensioned guy wires attached, usually in sets of 3 at 120° angles, which are anchored to the ground usually with concrete anchors. Multiple sets of guys at different levels are used to make the tower rigid against buckling. The guy lines have strain insulators inserted, usually at the top near the attachment point to the mast, to insulate the conductive cable from the mast, preventing the high voltage on the tower from reaching the ground.
Even though they are insulated from the mast the conductive guy cables can act electrically as resonant antennas, absorbing and reradiating radio waves from the mast, disturbing the radiation pattern of the antenna. 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.
Mast radiators can also be built as free-standing lattice towers, wide at the bottom for stability, narrowing to a slender mast. The advantage of this construction is the elimination of guy lines and thus reduction in land area required. These towers can have a triangular or a square cross section, with each leg supported on an insulator. A disadvantage is the wide base of the tower distorts the vertical current pattern on the tower, reducing the radiation resistance and therefore the radiated power, so guyed masts are preferred.
A country's national radio ministry usually has regulatory authority over the design and operation of radio masts, in addition to local building codes which cover structural design. In the US this is the Federal Communications Commission. Plans for a mast must be approved by regulators before building.
Electrical design
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 and low frequency 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 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 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 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 next to the mast. The antenna tuning circuit matches the characteristic impedance of the feedline to the impedance of the antenna, 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, 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 : 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 radiation and reduces high-angle radiation. This increases the distance to the mush area where the ground wave and sky wave are at similar strength at night.
Mast height and radiation pattern
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, and the direct and reflected current interfere, creating an approximately sinusoidal standing wave on the mast with a node 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, ... 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 As shown in the diagram, at heights below a half wavelength 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 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, 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.