I first posted this note some weeks ago with information on the relation between vertical height and radial lengths. However, that information was buried in one of the attachments. To make it easier to find I've extracted the parts relevant to radial length versus height and they are now a separate and much smaller download: Download Vertical height versus radial system .
A couple of weeks back I noticed a question on the TowerTalk reflector about making the radials on a vertical the same length as the height, i.e. use shorter radials on shorter antennas. Something along these lines has been "conventional wisdom" for a long time but I'd never seen any real justification for it. So I did a quick modeling study and up to a point it does seem to support this idea. I wrote this up in:
Download radial_system_design_and_efficiency_in_hf_verticals.pdf