"Jeff Liebermann" wrote in message
...
On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food
poisoning.org wrote:

I had a question about his j-pole analysis.

snip

I agree with the author (Terry Graves K7FE).

snip

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. This article covers the point (and more):
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
Quoting:
Summary End-feds Without Grounds
ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER
EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE
OR MAST COMMON MODE CURRENTS!

Hi, Jeff,

I can agree with the need for preventing feedline radiation but one thing
you and Terry say may be erroneous. I believe the radiating element of a
J-pole to be a half wavelength long, not a quarter-wave. I looked at
Terry's EZNEC wires list and observed the long side (the radiator or driven
element) of the J is 57 inches and the short side (the stub) is 19 inches.
(These dimensions agree with my idea of a 2m J-pole. I've made a few.)

As I understand the action of the J-pole, net radiation is low or nil from
currents in the lower third of the antenna (bottom third of the radiator and
the adjacent stub). The desired radiation comes from the top two-thirds (38
inches) of the radiator, which is very nearly a half wave at 2m. Allowing
for so-called "end effect," it's almost exactly cut to 146 MHz.

Please check my reasoning and math Your comments are welcome. Thanks.

73,
"Sal"
(KD6VKW)