I was trying to increase the overall performance of the J-pole, in this design. The diagram provided is a more simplified version of the one I did. These are a few of the modifications I came up with. I added a cap on the top end of the PVC. Mounted a so-239 to a split piece of copper tubing, that took the place of the #14 copper wire. And, I also added a short aluminum mast that fit into the lower end of the PVC.  I mounted the antenna to a 10′ antenna mast and a small tripod on the roof. I tried to add some type of a ground plain but everything I did made the antenna perform poorly. After all my efforts the end result was an antenna that out performs the 1/2-wave colinear copper version, with only one exception. The working model is somewhat narrow banded and still requires more experimentation. Some of the elements must be a little long.
Article originally available at http://home.comcast.net/~buck0/5-8thx2j.htm
Hi Simone, shouldn’t the phasing-stub, in the centre of the radiator, be 1/8 not 3/8 wavelength?
73! Tim