Antennas
For transmission, vertical radiators (masts) are most commonly used. Stations broadcasting with low power can use masts with heights of a quarter-wavelength (about 310 millivolts per meter per kilowatt at one kilometer) to 5/8 wavelength (225 electrical degrees; about 440 millivolts per meter per kilowatt at one kilometer), while high power stations mostly use half-wavelength to 5/9 wavelength. The usage of masts taller than 5/9 wavelength (200 electrical degrees; about 410 millivolts per meter per kilowatt at one kilometer) with high power gives a poor vertical radiation pattern, and 195 electrical degrees (about 400 millivolts per meter per kilowatt at one kilometer) is generally considered ideal in these cases. Usually mast antennas are series-excited (i.e., are insulated from the ground). Shunt-excited masts (i.e., not insulated from the ground, hence "grounded at the base") have fallen into disuse, except in cases of exceptionally high power, 1 MW or more, where series excitement might be impractical.
If grounded masts or towers are required, then cage aerials or long-wire aerials are used. Another possibility consists of feeding the mast or the tower by cables running from the tuning unit to the guys or crossbars in a certain height. Directional aerials consist of multiple masts, which need not to be from the same height. It is also possible to realize directional aerials for mediumwave with cage aerials where some parts of the cage are fed with a certain phase difference.
For medium-wave (AM) broadcasting, quarter-wave masts are between 153 ft and 463 ft high, depending on the frequency. Because quarter-wave masts are so large they can be unnecessarily costly and uneconomic, and other types are more commonly used on medium wave for local broadcast stations of under 5 kW, examples being T- and L-antennas. The design of these smaller antennas uses the technique commonly used on Long Wave to allow smaller masts to be used. In this method the mast size is reduced, producing less radiation resistance and increased reactance, and wires are added, supported by the same mast or masts, to compensate these deficiencies adequately. The details depend on the requirement for grounded or insulated towers and masts. The dimensions can and must be calculated accurately to meet the required specifications in bandwidth, power handling and radiation efficiency.
A popular choice for lower-powered stations is the umbrella antenna, which needs only one mast one tenth wavelength or less in height. This antenna uses a single mast insulated from ground and fed at the lower end against ground. In that sense it is a simple monopole, but at the top of the mast extra wires are connected (usually about six) which slope downwards at an angle of 40-45 degrees as far as about one-third of the total height, where they are terminated in insulators and thence outwards to ground anchors. Thus the umbrella antenna can use the guy wires as part of the antenna.
In some rare cases dipole antennas are used, which are slung between two masts or towers. Such antennas are intended to radiate a skywave. The medium-wave transmitter at Berlin-Britz for transmitting RIAS used a cross dipole mounted on five 30.5 metre high guyed masts to transmit the skywave to the ionosphere at nighttime.
Europe's largest antenna park is placed in Northern Jutland, Denmark. The well-known German DX'er Wilhelm Herbst has constructed and built the antennas. DXers are welcome to use the facilities.
For low-noise reception at frequencies below 1.6 MHz, which includes long and medium waves, loop antennas are popular because of their ability to reject locally generated noise. This class of antennas includes the ferrite-rod antenna, also known as a loopstick antenna, which is by far the most common antenna in use for broadcast reception throughout the world. Transistor radios contain a loopstick as the receiving antenna.
Read more about this topic: Medium Wave