ANTENNAS FOR THE SHORTWAVE BROADCASTER BY RICHARD R GREENE
No other component in a shortwave broadcast station offers as many options in terms of performance and cost as the antenna system. An overview of the principal antenna types and their costs will help with station planning.
The antenna and feeder system must support the currents and voltages produced by the transmitter at full modulation. The relevant specifications are average power and peak power which are, in turn, a function of the transmitter's rated carrier power and the modulation percentage. With 100% amplitude modulation, average power is 1.5 times carrier power, and peak power is 4 times carrier power. Thus, a 100-kW transmitter requires an antenna rated for 150 kW average and 400 kW peak.
The antenna's target area is determined by its directivity - its beamwidth in the horizontal and vertical planes. Its takeoff angle (TOA), the angle above the horizon at which maximum radiation occurs, may range from low (< 20°) to medium to straight up (vertically incident, ±90°).
Low takeoff signals travel hundreds or even thousands of kilometers before being refracted by the ionosphere back to the earth's surface; narrow vertical beamwidths always accompany low takeoff angles. High takeoff angles are used for local coverage.
The horizontal (azimuthal) beamwidth is defined as the angle across the horizon between the -3 dB points (referenced to gain at boresight).
Antenna gain is a function of directivity. High gain is achieved by narrowing and lowering the radiated beam, making high-gain and wide beamwidth mutually exclusive. Gain is measured in dB's referenced to a theoretical radiator having equal gain in all directions (isotropic radiator = 0 dBi). Only gain at the TOA required to reach the target is of interest; gain at other TOA's is wasted.
Polarization: Vertically polarized antennas are often used in shortwave communications, particularly for marine work, and usually at the low end of the HF band. Today virtually all shortwave broadcast antennas, however, are horizontally polarized.
Efficiency: All modern shortwave broadcast antennas are essentially fully efficient (I2R < 1%). An exception is the rhombic - rhombics require resistive termination and have I2R losses of several dB.
Size, Frequency Range, Cost
The size of a fully efficient shortwave antenna depends on its lowest rated frequency: low frequencies mean large antennas. A 3-MHz model will always be about twice as tall and require four times as much land as a 6-MHz version of the same design. The choice of lowest operating frequency is crucial with respect to cost: as mast height increases, wind loads and other factors increase even faster. For medium- and long-range broadcasting in the temperate zones, 5.9 MHz is the lowest frequency that need be considered.
An antenna's takeoff angle is also a cost factor since TOA varies inversely with radiator height above ground. TOA is often lowered by increasing mast height to enhance long-distance transmission.
Shortwave Antenna Types
Broadcasters using low power can choose from the broad range of antennas used for communications. Communications antenna types include log-periodics, fan dipoles, cut dipoles, spirals, conical monopoles, whips, and many other types. Of these, only log-periodics find widespread use in low-power AM broadcast applications.
Log-periodic Antennas (LPA's): LPA's are available in a wide variety of configurations including short-, medium-, and long-range directional, omnidirectional, and rotatable types. Their most attractive feature is a wide frequency range - large models can operate from 2-30 MHz (almost 4 octaves) without tuning.
The basic LPA curtain has a directional pattern with a 70° to 80° beamwidth and gain of about 11 dBi. TOA can be controlled by adjusting the radiator height above ground. Curtains can be arranged vertically to lower the takeoff angle or horizontally to narrow the beam. A 2-wide/2-high LPA can have a gain of 17 dB and a TOA as low as 10°.
The installed cost of a 5.9-MHz low-power LPA can range from $20K for a short-range model to over $100K for a low-TOA multi-curtain version.
Fan Dipoles: The fan dipole may merit consideration by the low-power broadcaster who needs an omnidirectional radiation pattern. Its frequency range is limited to approximately 4 to 1, and a typical maximum peak power rating is 40 kW (10 kW carrier), but it offers smaller size and lower cost than other types.
Many other antenna types were once used in broadcast applications. Most have been supplanted by log-periodics and dipole arrays, but a few deserve brief mention: Cut Dipoles are inexpensive, bi-directional, and short-range; they are limited to fairly low power and a narrow range of frequencies. Conical Antennas may be used for local omnidirectional ground-wave coverage; they are vertically polarized and radiated power is diminished by ground losses. Rhombics and Sloping-V's are simple traveling-wave antennas that can have significant directional gain over a relatively narrow frequency range; however, they require considerable land space and lose 3 dB or more in terminating resistors. Quads and Corner Antennas are sometimes used for local work over narrow frequency ranges. Loops and whips are generally not suitable for broadcast purposes.
All modern communications antennas have 50-ohm unbalanced inputs. Most are designed for single-sideband or FSK transmission, with curtains designed for 50 kW to 80 kW peak power. The actual power ratings are limited by the drive unit, or balun. A communications antenna equipped with a standard 10-kW balun, for example, can not be used with an AM transmitter running 10 kW of carrier power, because at full modulation that transmitter produces 15 kW of average power. In this case, the balun would require a rating of at least 15-kW ave/40-kW peak. Above 20 kW, AM transmitters produce average and peak powers that exceed the ratings of most communications baluns.
High-power Shortwave Broadcasting
The international broadcaster typically uses a carrier power level of 100 to 500 kW. In this range, antenna choices include dipole arrays and log-periodics, Broadcast antennas are designed especially for the peak powers produced by amplitude modulation. Most have 300-ohm balanced inputs, so connection to a 50-ohm transmitter requires a high-power balun.
Dipole Arrays, also called curtain antennas, are the antenna type most used in international shortwave broadcasting today. Dipole arrays consist of rows and columns of dipole "cages" arrayed in front of a reflecting screen. There are usually two or four columns. A four-column configuration has a beamwidth of about 24° and can be steered, or slewed, typically by ±15 and ±30°, allowing the broadcaster to direct the beam to different targets (refer to Figure 2-C). The number of rows can be 1, 2, 3, 4, or 6. The 6-high case permits the beam to be steered in the vertical plane or switched to a 2-high or 4-high mode; mode-switching alters the takeoff angle and allows the broadcaster to reach targets at varying distances.
Dipole arrays are limited to operation over a maximum 2:1 frequency range. Low-band arrays generally cover the 6, 7, 9, and 11 MHz bands, while high-band arrays cover the 11, 13, 15, 17, 19, and 21 MHz bands (or the 13-26 MHz bands). The extensive switch fields associated with slewing and mode-switching, combined with their large size, make these antennas expensive. The installed cost of a high/low pair of HRS 4/6/.5 antennas (4-wide, 6-high) on three masts with full switching exceeds $1 million.
The high-power LPA can often be cost-effective at 100 kW. The LPA, whose basic characteristics are described above, offers excellent performance over a wide frequency range. Omnidirectional LPA's developed for use in the tropical bands (down to 2.3 or 3.2 MHz) give excellent coverage to 1500 km radius with 100-kW transmitters. Directional LPA's are available for short-, medium-, and long-range broadcasting up to 300 kW. A single antenna typically covers all frequencies from 5.9 to 26 MHz.
A 100-kW LPA can often be installed for less than $200K. From 250 to 500 kW the cost of an LPA approaches that of a dipole array, which may then be more cost-effective owing to its ability to be slewed.
Materials for Shortwave Antennas
Modern shortwave antennas employ materials that are rugged and resist corrosion. Radiators and support elements (catenaries) are generally of Alumoweld wire rope. Alumoweld has a coating of aluminium bonded to a steel core; this cable has the mechanical properties of steel with the conductivity and corrosion resistance of aluminium. Copper cable is weaker and heavier than steel. State-of-the-art antennas do not use copper, fiberglass, or any other materials that can stretch over time or deteriorate in severe environments.
The feeder system for a low-power station may consist only of a run of coax from a single transmitter to a single antenna. At 50 to 500 kW, however, a tapered-line balun is needed to convert the power to 300-ohm balanced, since at this power level the antenna will not include its own "balun-in-a-can". Coaxial line (rigid or semi-flex) will run from the transmitter through the building wall to the balun; balanced transmission line (TML) provides the connection to the antenna. Rigid coax costs more than $300/meter, while TML costs less than $100/meter, including support poles. 皇冠金花下载