A talk I prepared with the late Alan Hobbs, G8GOJ, for the RSGB Data Symposium in 1987. Presented as three short articles with plenty of illustrations. Extra pictures added and existing ones made bigger November 1999. Revised text with additions & corrections & newer, bigger pictures 2014.
These articles © 1987-2014, AG Hobbs & SM Hallas. Pictures © Sam Hallas except where source acknowledged