Just a heads up on making roll up J-Poles from 300 Ohm TV twin lead.
I've made a number of these antennas for emergency services use. They are a great antenna for a grab'n'go kits to say the least. I am fully in favor of having several around not only in the grab'n'go but in the glove box of all your vehicles and even backpacks and briefcases.
However, building one is not as straight forward as many internet sites make it seem. The problem comes with different types of 300 Ohm twin lead. There is the brown poly and milky white poly. There is the low loss foam filled also. EACH OF THESE HAS A DIFFERENT VELOCITY FACTOR! This can DRASTICALLY effect the measurements necessary to bring the antenna into resonance (low SWR) on the band on interest. I have personally experienced as much as 4 inches difference in the overall length. Also the placement of the coaxial cable feed point as will as the point to cut the gap can change from type to type.
I suggest you be prepared to build at least two antennas. The first one to modify by cutting or adding wire, moving the feed point and the gap until you find the correct measurements for the particular twin lead type you are using.
PLEASE don't let this discourage you from building this very useful antenna. Consider the work a learning experience in antenna engineering.
I've not posted instruction on building a twin lead J-Pole because there are many sights with the necessary measurements.
Here's a good starting point.
https://www.arrl.org/files/file/Public%20Service/TrainingModules/jpole-dual-band.pdf
If you intend to use your J-Pole on 2 meters and 70 centimeters be sure to check the SWR on both bands. The dielectric value can also effect the antenna's SWR on 70 centimeters. I have built separate J-Poles for each band to avoid the problem.
