From Jason, NT7S, via the qrp-l.org mailing list:

I thought I would share a nice resource that I found earlier when I
was doing some research at work. Mini-Circuits (the guys who make the
SBL-1 DBM, plus other RF goodies) has a great list of reference
material on their application notes page:

Among some of the best documents on the page:

• Return loss vs. VSWR table
• dBm - volts - watts conversion table
• The Effect of VSWR on Transmitted Power
• Biasing MMIC Amplifiers (e.g. ERA SERIES)
• Understanding mixers
• Definition of mixer terms
• VCO fundementals and concepts
• Phase locked loop fundamentals

If you do any homebrewing, then this site is worth bookmarking.