We have no connection with the following web sites, but have found them to provide useful tools for designers.
Link L/C Meter IIB and Filter Design program
L/C Meter is a portable instrument for measuring (1% typ) L and C components for RF circuits.
The filter design program has loads of features and is free.
Link Tonne Software - impressive collection of programs for RF experimenters.
Link oh8ro - rf design programs in DOS.
Link g4fgq - many design programs.
Link HamCalc - amateur radio programs in DOS.
Link arrl/qex - lots of download information for rf experimenters.
Link wk0o - rod antenna calculation.
Link micrometals - programs for rf and power conversion using MicroMetals cores.
Link EMCfilt3.zip - useful demo emc filter (low pass) design program.
Link - if you do not have the reader get the large download from Adobe, but it is generally included on cd's attached to pc magazines.