Stub detuners for free-standing towers

A standard broadcast AM radio station´s antenna pattern has minima in the directions of other cities with stations on the same frequency. However, scattering can raise the field strength in a certain direction to unacceptably high levels. The scatterer must then be identified and modified or detuned so that it no longer scatters the field. Several detuner designs are presented for installation on a powerline tower for the suppression of the scattered field. The emphasis is on the assessment of the bandwidth over which scattering is suppressed, and on how the detuner can be modified to increase the bandwidth . The performance of detuners is studied by computing the scattered field from a type V1S power line tower using the Numerical Electromagnetics Code. The uses of a detuning skirt or detuning stub are also investigated. It is shown that the bandwidth can be improved by increasing the stub length, as in the elbow stub design. It is further shown that a large increase in bandwidth can be achieved by separating the stub significantly from the tower, thus increasing the energy loss in the resonant system. The effectiveness of the proposed stub designs is investigated by the measurement of the base impedance of the scale model tower detuned with each stub design. The measured impedances bear out the conclusions that the bandwidth of a capacitively tuned stub is improved by increasing the stub length, that the bandwidth of a stub can be increased by separating the stub wire from the tower body, and that the wire stub designs offer as good or better performance than do the capacitively tuned stub designs