Detuning free-standing towers at MF frequencies

Many commercial AM broadcast antennas must maintain a directional pattern to avoid interference with other stations on the same frequency. Scattering of the station´s signal can lead to field strengths in excess of those stipulated by the station´s licence; free-standing towers scatter the signal effectively if the tower height is comparable to the quarter-wavelength. In the paper, means of suppressing the scattered field from a free-standing power line tower by installing a so-called detuning device on the tower are investigated. Several detuner designs are studied by measuring the base impedance of a scale model tower on a ground plane. The measured results were reproduced with a computer model of the tower and detuner. The computer model was then used to study scattering from the detuned tower to determine the degree of reduction in the scattered field and the bandwidth over which an adequate reduction is achieved. Some standard detuner designs and some novel designs are compared