On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:
However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane.

Sorry, that is not correct. Both a Zepp and a J-pole are 1/2WL antennas fed with a 1/4WL matching section of transmission line containing mostly balanced transmission line currents. Here is a plot of the two currents in the matching section. The difference in those two currents is the common-mode current which indeed does radiate. The point is that the average differential transmission line current is more than 10 times the average common-mode radiating current so the matching section is acting primarily as a transmission line, transferring RF energy to the primary radiating element which is indeed 1/2WL element.

http://w5dxp.com/ZeppCrnt.jpg

Does the matching section radiate? Of course it does. But because the common-mode current is a small percentage of the total current, it does not radiate much and transfers most of the RF energy to the 1/2WL radiating element.. And yes, that small percentage of common-mode current on the matching section indicates that it is also acting as the antenna counterpoise in the above graph. But most of the radiation from the antenna is from the center of the 1/2WL element, just as it is for a 1/2WL dipole. In fact, a Zepp meets the IEEE definition of a dipole because it contains two electrical poles.
--
73, Cecil, w5dxp.com

On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP
wrote:

On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:
However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane.

Sorry, that is not correct.

Agreed. See my followup to my posting where I noticed that I've been
building J-pole antennas with the coax feed connected to the wrong
element. Judging by some of the photos I've found, I'm not alone.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sunday, February 16, 2014 11:12:51 AM UTC-6, Jeff Liebermann wrote:
On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP

wrote:



On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:

However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven

element is a 1/4 wavelength long, and therefore DOES require a ground

plane.



Sorry, that is not correct.



Agreed. See my followup to my posting where I noticed that I've been

building J-pole antennas with the coax feed connected to the wrong

element. Judging by some of the photos I've found, I'm not alone.


I never built J-poles myself. Never was that fond of the design
for some reason. I always preferred the "ringo" method of feeding
a base fed half wave. IE: a flat single turn tapped coil, and coax
capacitor. I used them on 10m mostly. They work decently well in
most cases with no decoupling section. But they work even better
with decoupling. I used a 1/4 wave length of coax to a union which
was attached to the mast, and had a set of three radials.

But... The 5/8 ground planes were always better than the 1/2 waves
on distant 10m local stations. Even the decoupled version.
And being low angle space wave stuff, it's a pretty good test.
Both antennas were at about 36 ft up at the base.

According to a modeling exercise I did once, the best way to
run a 5/8 GP is with 5/8 radials. I've also used 3/4 wave radials,
which seemed to work well. But according to the modeling, the 5/8
radials will give more gain. Starts to resemble a dual 5/8 collinear.
And naturally, you would want to use a decoupling section for the
best performance.
This is pretty old, but compares the different lengths for 10m
use.
http://home.comcast.net/~nm5k/acompari.htm