Radio Antenna Engineering is a free introductory textbook on radio antennas and their applications. See the editorial for more information.... 
Home LowFrequency Antennas Antenna Reactance Introduction  
Search the VIAS Library  Index  
Antenna ReactanceAuthor: Edmund A. Laport
Lowfrequency 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 highpowerantenna systems to reduce antenna potential and to design properly the insulation and the conductors. Reducing antenna reactance also reduces tuningcoil 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 heavyduty 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 transmissionline analosgy, the reactance of an opencircuited line of characteristic impedance Z_{0} and electrical length G is
With capacitive top loading of a vertical radiator of electrical height Gν and characteristic impedance Z_{0} with a fiattop of reactance X_{1} at a frequency f_{1}, the reactance at the feed point would be
Both X_{1} 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.


Home LowFrequency Antennas Antenna Reactance Introduction 