Monopoles Loaded With Coils: a Comparison of Measured and Computed Results

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Abstract
Loading coils are useful for turning multi-band and broadband wire antennas. Methods for designing coils of desired inductance and for predicting coil-loaded antenna performance are needed to facilitate realization of such antennas. This paper addresses these needs. Expressions for the inductance of a series of loops of constant current are used for approximating the inductance of helical coils. The electric field integral equation is solved in order to model the antennas, and the loads are treated as delta-gap impedances. The input impedance and voltage standing wave ratio (VSWR) of monopole antennas loaded with helical coils were measured over the frequency range 50-1200 MHz. The corroboration of computed results by measurement is excellent in most cases. However, it must not be taken for granted that any coil may be treated as a lumped-load inductor in the antenna model. For further investigation of the properties of coils for which the lumped-load model results do not agree with measurements, the coil-loaded antenna is modeled with curved-wire integral equation solution techniques.

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