Radio broadcasting began with AM radio. AM radio broadcasts require the use of antennas and transmitters to transmit the radio signal on a specific radio frequency. Different types of broadcast antennas broadcast on different frequencies. Modern radio antennas broadcast in a frequency range between 2.3MHz to 26MHz. Broadcast radio antennas vary in size; typically lower frequency antennas will be twice as tall as higher frequency antennas.
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Types of Antennas
There are many types of short-wave antennas that are suitable for use by AM broadcast stations. Types of antennas that have been used in the past include: log-periodic, fan dipole, dipole arrays, cut Dipoles, conical Antennas, rhombics, sloping-V's,quads, corner antennas loops and whips. Log-period and dipole array antennas are the preferred method for AM broadcast because of signal strength, and the width of transmission frequency.
Low-power antennas suitable for AM broadcast radio stations include log periodic, fan dipoles, cut dipoles, spirals, conical monopoles and whips. Log periodic antennas are preferred by AM broadcast radio stations because they offer a wider range of frequency. Log periodic antennas can operate in a broad range between 2 to 30MHz. Log periodic antennas are typically available in seven configurations: short range, medium range, and long-range, with directional, omnidirectional, or rotatable capabilities. Low-powered antennas typically use between 15 kilowatts and 80 kilowatts of power.
High-powered antennas typically used in AM broadcasting include dipole arrays and log-periodic. Dipole arrays are commonly constructed in two-, four-, or six-row rotational columns set in front of a reflection screen. The directional columns of the dipole array allow the signal to be transmitted to different targets. Low frequency dipole arrays can broadcast in the 6MHz, 7MHz, 9MHz, or 11MHz frequencies. High frequency dipole arrays can broadcast in the 11MHz, 13MHz, 15MHz, 17MHz, 19MHz, or 21MHz frequencies. High-powered log periodic antennas are suitable for broadcasting over a wider frequency range. Omni-directional log periodic antennas are designed to broadcast in a frequency band of either 2.3MHz to 3.2MHz. High-powered directional log periodic antennas broadcast in frequencies from 5.9MHz to 26MHz. Dipole arrays and log period antennas use between 100 kilowatts and 500 kilowatts of power when broadcasting.
Materials and Components
Short-wave broadcast antennas are often constructed from materials that are resistant to corrosion, like steel. Catenaries, binding the antenna to the ground, are typically constructed of alumoweld, which is a steel rope coated with aluminium.
Short-wave antennas also require a power supply, or feeder system. This is typically constructed from a coaxial cable, connecting the transmitter to the antenna. A tapered line balun may be required to power an antenna of 50kW to 500kW.
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