Harmonic Analysis of a Half-Wave Dipole and Director Parallel to a Reflecting Plane

Abstract

When substantial harmonic power exists at the input of a radiating antenna, a lobe structure will result that may create a substantial interference problem. In addition, the privacy of the radiating source may be severely hampered. These harmonic fields are generally dependent upon: 1) the order of the harmonic relative to the geometry of the antenna, 2) the impedance the higher harmonic sees at the antenna input, 3) the power available at the given harmonic. For purposes of providing a measure of the seriousness of this problem, a harmonic analysis of a particular type of antenna configuration is examined, namely, that of a center-fed, thin half-wave dipole with director parallel to a reflecting plane. The active dipole is assumed λ0/4 away from the reflecting plane and the director λ0/8 away from the active dipole. As a criterion to determine the worst case situation, the antenna is assumed matched at each odd harmonic examined, and equal power is assumed fed at the input. The resulting field patterns show: 1) The presence of the director results in the introduction of sidelobes at the fundamental frequency in azimuthal plane. 2) The presence of the director plays less of a role the higher the harmonic. 3) In the meridian plane the sidelobes become more intense exceeding the forward-lobe intensity as the order of the harmonics is increased. 4) In the azimuthal plane, the sidelobe intensity approaches but does not exceed the forward-lobe intensity.