Vertical Radiators

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More power

One of the most important units in a radio transmitter engineer's daily life is antenna current. This is the measure of the RF current flowing in the antenna. Power is important to the station owner and he always wants more power. The only way to get more power is to increase the antenna current with a given antenna resistance. Without resistance no power is developed. The equation P = I2 Rant calculates the antenna power where I is the antenna base current, or the common point current in the case of a DA.

Rant is the measured antenna or common point resistance, which includes ground system losses. The correct all-inclusive term is radiation resistance. It is commonly referred to as base resistance because that is the place where it is measured. Reference was made to base resistance, the proper term is base impedance because an antenna has inductive or capacitive reactance in addition to its very important resistance.

The most generally used broadcast antenna is the vertical radiator. Although it is commonly called a quarter-wave antenna, it is often several degrees plus or minus a quarter-wave in height. Sinusoidal current distribution is usually assumed and used in most antenna. However, occasionally measured antenna current distribution is required by the FCC in the case of unusual configurations.

An actual quarter-wave antenna will have approximately 37Ω base resistance and zero ohms reactance on its operating frequency. Immediate surroundings, as well as tower width, can have an effect on the base impedance.

Figure 1. The current distribution across a 90-degree radiator decreases along the height of 
the structure.

Figure 1. The current distribution across a 90-degree radiator decreases along the height of the structure.

The single 90-degree radiator actually operates with an imaginary 90-degree radiator below the surface completely separated from it. Figure 1 shows the current distribution at the base of the tower.

The important measure of a transmitter is its field strength at one mile. A 180 degree dipole has a resistance of approximately 73Ω. With 1 amp, two quarter-waves each with half the radiation resistance with 1 amp of RF will produce the same field strength, i.e. 37.4 mV/m. The power required is I2 R or 1×1 ×37 = 37W.

If RF power is doubled, the field strength at a given point will increase by the square root of two. In other words, field strength increases by the square root of the power increase.

In order to match the tower's operating impedance to the 50Ω transmission line, an antenna tuning unit (ATU) is required. This is a network that transforms the 37Ω operating resistance to 50Ω and matches the j0 of the line and the measured antenna reactance. An L or a tee network may be used. I prefer a tee because it gives easier control of the match in my opinion, and this is often very important when tuning a directional antenna.

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