Current distribution and impedance of an antenna in a parallel-plate region

Closed-form expressions for the current distribution and impedance of an antenna in a parallel-plate region have been obtained by an integral-equation technique. The current distribution reveals a logarithmic singularity in the neighbourhood of the driving point because of the current emanating from the idealised delta-function generator. The effect of the feed-point singularity on the antenna characteristics has been investigated both theoretically and experimentally. Contrary to assumptions made by previous workers, it has been shown that the logarithmic term makes a marked contribution over a significant length of the antenna; a large empirical `gap correction¿ is also shown to be necessary to bring the admittance of the theoretical model into good agreement with the experimental results. The implications and limitations involved in assuming an idealised delta-function generator for driving the antenna are discussed. The paper also accounts satisfactorily for certain anomalies in the behaviour of the antenna at resonance, which were noticed earlier by Lewin. Extensive experimental measurements have been made on the antenna, and a description of the apparatus is included in the paper. The experimental results agree well with theory.