My sticking point exactly Doc, nowhere's in the 21st edition of that book pertaining to the construction of a 2m J-Pole does it mention anything about variables, it's laid out in such a way that the measurements are written in stone, and that's the only way it'll work. Being a rookie at antenna construction, I didn't know any better. Of course I know now after dicking around with two different models over the course of three days.
I can only assume that since the J-pole is a reletively easy project compared to some of the more complex yagi's, beams, trapped verticals, and multi band dipoles, they didn't see a need to include some basic level #1 information. They should have assumed that since it is an easy project, it will be taken on as a first antenna building project by many, and that little tid bit of info would go a long way in keeping people interested in home brewing antennas.
I certainly won't allow this incident to diswade me from future antenna projects, or using the ARRL Antenna Book as my text, I'll just file this as a learning experiance, and remember it for the next project I tackle. I'll also research other designs against those in this book before I start, that way I'll know at least a few variables, and know how to work around them.....
