J-Pole coax center to short or long element?

[conve36]

I have seen different designs for the j-pole and cant understand why they are all basically the same EXCEPT some designs specify to attach the center of the coax to the short stub and some say to attach it to the longer part of the antenna?? Comments?

 

[nanZor]

Go about 3/4 of the way down this page to see the effect as modeled in EZnec, where not only is the reversed shield/center conductor connection modeled, but also takes into account the common-mode current of the feedline:

End-fed vertical j-pole and horizontal zepp I-max 2000 vertical

 

[conve36]

anyone else comment on this? thanks!

 

[nanZor]

For most people, attaching the center conductor to the "long" side is the way to go. The elevation angle is lower than if you attach it to the short side. Matching is typically the same either way, so the issue is elevation angle.

Yet, there might be cases where you actually want a higher elevation angle - if you are in a canyon, have a mountain in your backyard etc. In this case, having the main lobe higher to catch knife-edge refraction off the top might be desired. In some cases, I've actually pointed yagis upwards to get this on VHF.

But typically, most need the lower angle, so the center conductor to the long side is the way to go.

You may also want to investigate choking near the feedpoint with ferrites to help ensure that you are indeed getting the lowest angle possible by not having the common-mode of the coax becoming part of the antenna.