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Antenna Reactance

Author: Edmund A. Laport

The impedance of a low-frequency antenna at the operating frequency must be known to determine the power input and the antenna potential. The resistance portion of the impedance has been discussed separately. Prior to actual construction and measurement (or lacking measurements on other similar systems), the total resistance can only be estimated. From this the order of the antenna current for a given power input can be deduced. The antenna potential can then be found approximately if the reactance is known. The size of the tuning inductance must also be determined by the antenna reactance.

Low-frequency antennas encounter very high potentials normally. The power input to an antenna is limited by potential due to corona, pluming, and flashover from the conductors as well as the insulators. The cost of insulation may be a large portion of total antenna cost, and great care is required in the design of medium- and high-power-antenna systems to reduce antenna potential and to design properly the insulation and the conductors. Reducing antenna reactance also reduces tuning-coil losses by decreasing the amount of reactance required for tuning.

Antenna potential is reduced by minimizing antenna reactance at the working frequency. This in turn is accomplished by minimizing the characteristic impedance of the antenna system and by top loading it with as much capacitance as can be afforded. Once the antenna potential gradients are under the limiting values which produce corona and pluming, the designer must carefully avoid excess costs which arise from a large and heavy aerial requiring several heavy-duty supporting structures. Once the potential gradients are reduced below a certain critical value for the maximum anticipated power input, insulation is cheaper than steel and copper.

Applying the transmission-line analosgy, the reactance of an open-circuited line of characteristic impedance Z0 and electrical length G is


With capacitive top loading of a vertical radiator of electrical height Gν and characteristic impedance Z0 with a fiat-top of reactance X1 at a frequency f1, the reactance at the feed point would be


Both X1 and Gν are functions of frequency, so that a computation of X α can be made over a frequency band. At the fundamental frequency of the antenna, X α = 0, under which condition the numerator in this equation must be equal to zero.

Last Update: 2011-03-19