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Top loading, part 1 of 2
Engineers charged with the job of designing a new AM transmitter facility nearly always try to plan for the tallest tower possible. This is not just an ego trip; it's because the taller the AM radiator, the higher the field strength developed in the listening area with a given power.
Years of experience in AM broadcasting have shown that a 90° tower is an easy value to work with; its efficiency factor meets the FCC's Rules minimum radiation requirements, and usually it is not exceptionally expensive to build.
A half-wave tower is almost ideal, but is much more expensive and requires twice as much ground space for the radials and ground system. Over the years we have found that for various reasons, some related to engineering, but more frequently based on nonsensical local zoning restrictions or available land area, it is not always possible to build a tower that is 90° tall. In the early days of roof-top antennas, a full 90° was also not always possible, and a number of those stations started with 100 watts on the high end of the AM band.
Although low frequencies were never used for broadcasting in the United States, they were very popular in broadcasting's early days. England, France, Poland and several other countries had transmitters in the range of 150kHz to 200kHz. That corresponds to about 6,000 feet for a full-wave antenna. These low frequencies resulted in excellent long-distance ground wave coverage and reduced high-angle radiation, and were generally used by government-controlled broadcasting systems.
Even a quarter-wave tower was 1,500 feet high. This is not so unusual these days for TV or FM, but not many AM broadcasters wanted towers that tall or expensive. Therefore, considerable work was done to find a way of using these valuable low frequencies efficiently with shorter towers. It's interesting to note the definition of a small tower: a tower is small if its largest dimension is less than one-eighth of a wavelength. Antennas less than a quarter wavelength usually have capacitive reactance.
This early need to lengthen towers electrically led to the simple flat-top form of top loading. A flat top is created by connecting cables to the top of the tower like the top bar of the letter T, as in Figure 1. This type of construction was used more for frequencies of 100kHz or less. Broadcasters with government money used tall towers plus some form of top loading.
As the top loading increases, so does the impedance of the antenna. But after a certain point is reached, it begins to decrease. This is because the loading wires tend to shield the antenna, and the effective height is reduced.
This draws a parallel to the ground systems for a roof-top antenna. Because there usually was not enough room for long radials, the ground system for a roof-top antenna typically used a great deal of copper around the base of the tower. Sometimes it was necessary to use a counterpoise ground system to achieve the minimum required radiation efficiency. Often there were also related problems of unexpected reradiation at odd frequencies due to semiconductor effects in the ironwork of the buildings.Coming up short
In the years before World War II, the need to use shorter antennas became more pressing, and many engineers began to give the matter serious attention. A short, vertical antenna suffers from a grave disadvantage. The base resistance is low and the reactance high, and high-angle radiation is also high. Thus, horizontal radiation efficiency is low. Radiation resistance consists of two major components. The first is the base resistance, which is determined by its length (height) and some physical features. The second is the I
If the base operating resistance is 10Ù and the I
Efforts to raise the base operating resistance to a reasonable value of 30Ù or more included a type of top loading in the form of a top hat (see Figure 2). This device took the form of a large aluminum structure, extending as much as ten feet or more in radius, mounted on the top of a short tower. It was connected directly on the top of the tower and served to increase the tower mass and electrical length. It was usually circular and sometimes had two layers or rings about 2 feet apart.
There are several ways of exciting a top-loaded antenna. Generally, the antenna is base fed in the usual way. The greatly increased capacity from tower top to ground reduces the negative reactance measured at the base, and large currents can flow into the top hat. Thus, the current in the tower will remain fairly constant and not reduce to zero as in the case of a regular quarter-wave tower, and the loaded tower's radiation resistance is raised.
A major objective of top loading is to raise the base operating resistance, but current distribution is also important. Sinusoidal current distribution is usually assumed in antenna work, but many times this is not the case, especially when using some forms of top loading.
A tower that is shorter than a quarter wavelength may be top loaded so that the current remains constant over most of the tower length. It can have up to four times the radiation resistance of a similar unloaded tower.
A problem with top-hat loading is susceptibility to wind damage and poor current distribution in the radiator; I once replaced a top hat, damaged by high winds, with a folded unipole. It was more economical and efficient to use a folded unipole
An interesting variation in top hat loading was to insulate the hat from the tower, connect a tuning network between the hat and tower, and drive the antenna at the top. The coaxial line went up, insulated from the tower on the inside, and connected to the hat at the top. This will be discussed in the next part. Another method was shunt feeding, but this is rarely used in new stations today.
E-mail John at email@example.com.
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