While your building a j-pole you might consider just making it collinear. It adds a few feet to the height of the antenna but gives you a few dB gain over the basic designs you often see. Here is some instructions on how to do this-
I would highly recommend you take weather into consideration before you attempt this. If you have high winds this might not be the best idea...
[URL="http://NorthJeffcoARL.org"]North Jeffco Amateur Radio League[/URL]