A Tribute to Giuliano Toraldo di Francia (Special Issue)

This special issue is intended as a tribute to Giuliano Toraldo di Francia, who died recently. For many of us, he was a great teacher who taught us not only physics but also ethics, honesty, and scientific exactness. All those who knew him had the opportunity to appreciate both his great scientific mind and also his human depth. We shall miss him. The work presented here is a reproduction of a work by Toraldo di Francia. Despite the fact that it was written in 1957, it still appears very up-to-date. Even if the subject treated was published in due time, this specific work is unknown to the general reader because it was published as an internal report of the "Centre for the Study of Microwaves" (which then became "Institute of Research on Electromagnetic Waves" (IROE) and is now known as "Nello Carrara" Institute of Applied Physics" (IFAC)) of the Italian National Research Council.