Misplaced Pages

#222777

86-721: In 1906, Jacob B. Blaw created the Blaw Collapsible Steel Centering Company, located in New Jersey, to manufacturer his patented steel form used to construct improved concrete circular tubes for sewers and tunnels. In 1909, Luther Knox created the Knox Pressed and Welded Steel Company to manufacture steel products used in high temperature applications. In July 1917, the firms merged to form the Blaw-Knox Company. Blaw-Knox added

172-425: 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

258-470: A 469-foot (143 m) tall tower in 1948 for WKQI (then known as WLDM) located on Ten Mile Road in the Detroit suburb of Oak Park, Michigan . However, unlike its namesake diamond cantilever form, this structure was built as a conventional four-sided self-supporting lattice tower . Mast radiator A mast radiator (or radiating tower ) is a radio mast or tower in which the metal structure itself

344-504: A ceramic insulator about three-foot wide, shaped like a ball and socket joint. Thus, the tower required guy-wires to hold it upright. The distinguishing feature of the Blaw-Knox tower was its wide diamond (or rhomboidal , rhombohedron ) shape, which served to make it rigid, to resist shear stresses. One advantage of this was to reduce the number of guys needed. Blaw-Knox masts required only one set of three or four guys, attached at

430-402: A concrete base, relieving bending moments on the structure. 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. Radial wire ground systems were also introduced during this era. During the 1930s the broadcast industry recognized the problem of multipath fading , that at night high angle waves reflected from

516-418: 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

602-405: 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 λ {\displaystyle \lambda } is that at slightly above

688-420: 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 λ {\displaystyle \lambda } are not generally used because above this the power radiated in horizontal directions decreases rapidly due to increasing power wasted into

774-408: A hazard to aircraft. Aviation regulations require masts to be painted in alternating strips of international orange and white paint, and have aircraft warning lights along their length, to make them more visible to aircraft. Regulations require flashing lights at the top, and (depending on height) at several points along the length of the tower. The high radio frequency voltage on the mast poses

860-421: 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 2 λ / π {\displaystyle 2\lambda /\pi } = 0.637 λ {\displaystyle \lambda } ) As shown in the diagram, at heights below

946-492: 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 λ {\displaystyle \lambda } , 90 electrical degrees) from

SECTION 10

#1732780136223

1032-506: A length of a half wavelength, so a mast around that length had an input resistance that was much higher than the ground resistance, reducing the fraction of transmitter power that was lost in the ground system, eliminating the need for a capacitive topload. 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 λ {\displaystyle \lambda } (225 electrical degrees). By 1930

1118-418: A little less than a multiple of a quarter wavelength, 1 4 λ , 1 2 λ , 3 4 λ {\displaystyle {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

1204-429: 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. 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

1290-457: A problem for powering the warning lights: the power cable which runs down the mast from the lights to connect to the mains power line is at the high RF potential of the mast. Without protective equipment it would conduct radio frequency (RF) current to the AC power wiring ground, short-circuiting the mast. To prevent this a protective isolator is installed in the lighting power cable at the base of

1376-617: A significant share of the paving equipment business, having purchased the German paving equipment company ABG in 1990, while Blaw-Knox owned a major share of the US paving market (which would grow to 50% of all new pavers bearing the Blaw-Knox brand by 2006). A few days later, in April 1995, Clark accepted a higher $ 1.5 billion ($ 3 billion today) offer from Ingersoll-Rand, who became owners of Blaw-Knox and all other Clark companies. Ingersoll-Rand soon sold

1462-520: A small reduction in horizontal gain. The optimum height is around 190 electrical degrees or 0.53 λ {\displaystyle \lambda } , so this is another common height for 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

1548-403: A stable base on which to erect the other half. A disadvantage of the diamond mast shape was that the current distribution on the tower caused less radio power to be radiated in horizontal directions and more at an angle into the sky, compared to a slender uniform width mast. Since AM radio stations covered their listening areas with ground waves , radio waves that traveled horizontally close to

1634-452: 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

1720-429: 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 300 m (980 feet) are prohibitively expensive and very few have been built;

1806-700: Is no chance that high voltage will be present on the mast when personnel are working on it. A tall radio mast is a convenient structure to mount other wireless antennas on, so many radio stations lease space on their towers to other radio services for their antennas. These are called colocated antennas . Types of antenna often mounted on mast radiators are: fiberglass whip antennas for land mobile radio systems for taxi and delivery services, dish antennas for microwave relay links carrying commercial telecommunications and internet data, FM radio broadcasting antennas consisting of collinear bays of twisted dipole elements, and cellular base station antennas. As long as

SECTION 20

#1732780136223

1892-407: 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

1978-525: Is stabilized by guy wires attached only at the vertical center of the mast, where its cross-section is widest. During the 1930s AM radio broadcasting stations adopted single mast radiator antennas, and the Blaw-Knox design was the first type used. A 1942 advertisement claims that 70% of all radio towers in the United States at the time were built by Blaw-Knox. The distinctive diamond-shaped towers became an icon of early radio. Several are listed on

2064-441: 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

2150-489: 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. Base-fed mast radiators have a high voltage on the base of the mast, which can deliver a dangerous electric shock to a grounded person touching it. The potential on the mast is typically several thousand volts with respect to the ground. Electrical codes require such exposed high voltage equipment to be fenced off from

2236-421: The T antenna or umbrella antenna which can have higher efficiency. 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 of horizontal wires extending radially from the top of the antenna. It acts as a capacitor plate; the increased current in

2322-407: 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

2408-481: The ionosphere interfered with the ground waves, causing an annular region of poor reception at a certain distance from the antenna. 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

2494-411: The longwave band, which limited the vertical height of the radiator to much less than a quarter wavelength, so the antenna was electrically short and had low radiation resistance from 5 to 30 ohms. Therefore, most transmitters used capacitively toploaded antennas like the umbrella antenna or inverted L and T antenna to increase the power radiated. During this era, the operation of antennas

2580-490: The Blaw-Knox expertise in concrete and steel works. As highway construction moved away from rigid concrete construction, Blaw-Knox began making highway pavers for the new method of flexible asphalt paving ( tarmac ). In 1931 Blaw-Knox introduced a form-riding finisher for asphalt paving. By 1932 they released the first self-propelled non-form riding finisher for the placement of stones and asphalt. Until this time it had been common to pave roads using formwork on either side of

2666-450: 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. The transmitter which generates the radio frequency current

Blaw-Knox - Misplaced Pages Continue

2752-689: The UK design and manufacturing division to Babcock , while keeping the North American operations of Blaw-Knox as a stand-alone unit in its road construction equipment division. In February 2007, Ingersoll-Rand accepted an offer of $ 1.3 billion ($ 1.9 billion today) from Volvo for the road construction equipment division, including the Blaw-Knox brand and the ABG brand. In July 2020, Volvo Construction Equipment sold Blaw-Knox pavers to Gencor Industries Inc. Blaw-Knox Tower The Blaw-Knox company

2838-618: The United States National Register of Historic Places , the distinctive diamond antenna design has been incorporated into logos of various organizations related to radio and a very large (scale) replica of the WSM (AM) Blaw-Knox tower has been built into the Country Music Hall of Fame and Museum . The diamond-shaped tower was patented by Nicholas Gerten and Ralph Jenner for Blaw-Knox July 29, 1930. and

2924-435: The amount of power it radiates at different elevation angles, is determined by its height h {\displaystyle h} compared to the wavelength λ = c / f {\displaystyle \lambda =c/f} of the radio waves, equal to the speed of light c {\displaystyle c} divided by the frequency f {\displaystyle f} . The height of

3010-405: The antenna in which reception may be inadequate, sometimes called a "zone of silence", fading wall or mush zone . 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

3096-416: The antenna's radiation pattern. In some Blaw-Knox mast designs (see WBT towers, right) the upper pyramidal section was made longer than the lower, to keep the attachment point of the guys as low as possible, to minimize their interference. Another advantage mentioned in the patent was that the tower could be erected in two parts. Half of the mast could be built, then its wide central section could be used as

3182-448: The antenna. Masts shorter than 0.17 λ {\displaystyle \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

3268-477: 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 (from 2 to 5) at different levels are used to make

3354-414: 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

3440-469: The colocated antennas do not operate at frequencies anywhere near the transmitting frequency of the mast, it is usually possible to isolate them electrically from the voltage on 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

3526-403: The construction cost of a single mast antenna, far more land area, and parasitic currents in the masts distorted the radiation pattern. Two historic papers published in 1924 by Stuart Ballantine led to the development of the mast radiator. One derived the radiation resistance of a vertical monopole antenna over a ground plane. He found that the radiation resistance increased to a maximum at

Blaw-Knox - Misplaced Pages Continue

3612-487: The construction of radio towers to its operation in 1927, and soon developed the distinctive diamond-shaped Blaw-Knox Tower design of vertical, medium wave radio towers. Several are now listed on the US National Register of Historic Places . In 1929, Blaw-Knox purchased the A. W. French & Company paving equipment company, which manufactured machines that produced concrete roads, a natural extension of

3698-647: The diamond towers were built, and several remain; all transmit AM radio signals. The most well-known example in Europe is the Lakihegy Tower , located in Szigetszentmiklós -Lakihegy, Hungary . Several additional diamond cantilever towers were built at stations in the Central Valley of California , but are less well known. These towers were much smaller in both height and cross-section than

3784-404: The disadvantages of the T antenna led broadcasters to adopt the mast radiator antenna. One of the first types used was the diamond cantilever or Blaw-Knox tower . This had a diamond ( rhombohedral ) shape which made it rigid, so only one set of guy lines was needed, at its wide waist. The pointed lower end of the antenna ended in a large ceramic insulator in the form of a ball-and-socket joint on

3870-415: 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

3956-452: 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

4042-404: 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

4128-426: The equivalent of 15-30 degrees of added electrical height. 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

4214-420: 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

4300-489: 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. However, during the radiotelegraphy era before 1920 most long-distance radio stations transmitted in

4386-470: The ground surface, this meant the listening area was smaller. The realization of the nonideal radiation pattern of the design caused the diamond-shaped tower to fall out of favor in the 1940s in radio stations, replaced by the narrow uniform width lattice mast used today. Many Blaw-Knox towers, of both conventional (uniform cross-section) and diamond design, remain in use in the United States . Few of

SECTION 50

#1732780136223

4472-462: The ground. A mast radiator is a form of monopole antenna . 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" (arc) welds across

4558-481: 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. The vertical or monopole antenna was invented and patented by radio entrepreneur Guglielmo Marconi in 1896 during his development of the first practical radio transmitters and receivers . He initially used horizontal dipole antennas invented by Heinrich Hertz , but

4644-399: 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

4730-438: The lightning arrester should go directly to a metal ground stake by the shortest path. The top of the mast should have a lightning rod to protect the top aircraft warning light. The mast should also have a DC path to ground, so that static electric charges on the mast can drain off. Also at the base is a grounding switch, which is used to connect the mast to the ground system during maintenance operations to ensure that there

4816-467: The mast is usually specified in fractions of the wavelength, or in " electrical degrees " where each degree equals λ / 360 {\displaystyle \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

4902-451: The mast required to charge and discharge the topload capacitance each RF cycle increases the radiated power. Since the topload 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. Capacity hats are structurally limited to

4988-452: 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

5074-460: The mast which blocks the RF current while letting the low frequency 50 or 60 Hz AC power pass through up the mast. Several types of isolator devices have been used: At its base, the mast should have a lightning arrester consisting of a ball or horn spark gap between the mast and the ground terminal, so that current from a lightning strike to the mast will be conducted to ground. The conductor from

5160-403: The mast with a node (point of zero current) at the top and a maxima one quarter wavelength down where i ( y ) {\displaystyle i(y)} is the current at a height of y {\displaystyle y} electrical degrees above the ground, and I max {\displaystyle I_{\text{max}}} is the maximum current. At heights of

5246-619: 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 (     1   8 λ ∼ 1   10   λ   {\displaystyle \ {\tfrac {\ 1\ }{8}}\lambda \sim {\tfrac {1}{\ 10\ }}\lambda \ } ). Mast radiators can also be built as free-standing lattice towers , wide at

SECTION 60

#1732780136223

5332-472: The mast. No. 10 gauge soft-drawn copper wire is typically used, buried 10 to 25 cm (4 to 10 inches) deep. For AM broadcast band masts this requires a circular land area extending from the mast 47–136 m (154–446 feet). 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

5418-462: 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

5504-454: The mating flanges. 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

5590-618: The new WCI subsidiary. In April 1994, WCI sold the Blaw-Knox unit, based in Mattoon, Illinois , to Clark Equipment Company for US$ 144 million ($ 296 million today). In March 1995, Ingersoll-Rand offered $ 1.34 billion ($ 2.7 billion today) in a hostile takeover bid for Clark Equipment Company. Clark cited its Blaw-Knox unit as a reason for the US Federal Court to stop the takeover, claiming that Ingersoll-Rand would violate Federal antitrust laws since Ingersoll-Rand already controlled

5676-400: The pavement. These new machines were essentially a self-propelled tailgate spreader that pushed the asphalt truck. In 1943 they introduced the first road widener. In 1954 they introduced the first wheel-driven, rather than track-driven , paver. The paver tires rode on a prepared base -- or often on an older pavement -- and the paver pushed the asphalt truck. The screed was the hindmost part of

5762-426: The paver. The hot mix was dumped into a front hopper, then carried back by conveyors to be dropped in front of the screed, where horizontal augers would distribute it laterally. In 1956, they developed the first auto-grade-and-slope pavers. Blaw-Knox went on to establish a broad line of paving machines for the US and UK markets. The UK employed a distinctly different paving method, applying a deeper mat of asphalt than

5848-448: 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

5934-424: The proportion of power radiated in horizontal directions and allows the mast to be taller than 0.625 λ {\displaystyle \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. The lower limit to the frequency at which mast radiators can be used

6020-451: The public, so the mast and antenna tuning hut are surrounded by a locked fence. Usually a chain-link fence is used, but sometimes wooden fences are used to prevent currents induced in a metallic fence from distorting the radiation pattern of the antenna. 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. Antenna masts are tall enough that they can be

6106-656: 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 (FCC). Plans for a mast must be approved by regulators before building. 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

6192-492: 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. There are several ways of feeding a mast radiator: 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

6278-402: 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

6364-409: 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

6450-414: 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

6536-438: The sky in the second lobe. For medium wave AM broadcast band masts 0.625 λ {\displaystyle \lambda } would be a height of 117–341 m (384–1,119 feet), 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

6622-593: The tallest masts in the world are around 600 m (2,000 feet). 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 λ {\displaystyle \lambda } , 90 electrical degrees) are called electrically short antennas. Electrically short antennas are efficient radiators ;

6708-437: 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 ( parasitic elements ), absorbing and reradiating radio waves from

6794-417: The tower's wide "waist". In contrast, narrow masts require two to four sets of guys, attached at different heights, to prevent the tower from buckling. The advantage of fewer guys was to simplify the electrical design of the antenna, because conductive guys interfered with its radiation pattern . The guys acted as " parasitic " resonant elements, reradiating the radio waves in other directions and thus altering

6880-660: The towers listed elsewhere; only one — KSTN , Stockton  — remains in use for broadcasting. In the following chart:   structure is no longer standing.   structure has changed height.   rebuilt structure. Three other Blaw Knox towers of unknown heights also used to exist but have since been removed for the following stations; WABC in Wayne, N.J., WCAU's in Newtown Square, Pennsylvania and WHO in Des Moines, Iowa. Blaw-Knox also constructed

6966-400: The transmitter control room. The hut also usually contains the power supply for the aircraft warning lights. The ideal height of a mast radiator depends on transmission frequency f {\displaystyle f} , the geographical distribution of the listening audience, and terrain. An unsectionalized mast radiator is a monopole antenna , and its vertical radiation pattern ,

7052-461: Was an American manufacturer of steel structures and construction equipment based in Pittsburgh, Pennsylvania . The company is today best known for its radio towers , most of which were constructed during the 1930s in the United States . Although Blaw-Knox built many kinds of towers, the term Blaw-Knox tower (or radiator) usually refers to the company's unusual "diamond cantilever" design, which

7138-519: Was common in the US, which resulted in a marked reduction in paving speed. The Blaw-Knox UK design and manufacturing division was eventually based at Rochester, Kent , in southeast England, on the former site of the Short Brothers flying boat factory. Blaw-Knox was purchased by White Consolidated Industries (WCI) in May 1968. The Blaw-Knox Construction Equipment Corporation name was used for

7224-431: Was little understood, and designs were based on trial and error and half-understood rules of thumb. The beginning of AM radio broadcasting in 1920 and the allocation of medium wave frequencies to broadcasting stations sparked an increase in interest in medium wave antennas. The flattop or T-antenna was used as the main broadcasting antenna through the 1920s. It had the disadvantage that it required two masts, twice

7310-429: Was not able to communicate further than a few miles. He discovered by experiment that if he connected one terminal of his transmitter and receiver to a vertical wire suspended overhead, and the other terminal to a metal plate buried in the Earth, he could transmit for longer distances. Marconi's antennas, as well as most other vertical antennas through the 1920s, were constructed of wires suspended by wooden masts. One of

7396-405: Was one of the first mast radiators. Previous antennas for medium and longwave broadcasting usually consisted of wires strung between masts, but in the Blaw-Knox antenna, as in modern AM broadcasting mast radiators, the metal mast structure functioned as the antenna. To prevent the high frequency potential on the mast from short-circuiting to ground, the narrow lower end of the tower rested on

#222777