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

I had a question about his j-pole analysis. He says "A J-pole, like ANY end
fed antenna, needs radials, a counterpoise or ground plane to work
properly."

I've never seen this anywhere else, even as a suggestion or hint. I can
envision places to fit one but what would it do? As built, both the pattern
and the impedance already seem to be what I want.

Thoughts?

I agree with the author (Terry Graves K7FE).

The rule-of-thumb is that an end fed antenna requires a shield ground
to act as a counterpoise and to keep the coax from radiating. The
J-pole (and Zepp) antennas are not exactly end fed, but are close
enough. A possible exception to the rule is a 1/2 wave end fed
antenna, which allegedly does not require a ground plane. I'm
undecided on that point.

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!
This is true for 5/8th waves, Zepp antennas, R7's, R5's, or
even common J-poles. End-feeding antennas is bad news unless
you have a large well-established ground at the feedpoint.
Even 1/4wl groundplanes have common mode problems. When I
designed a commercial 1/4 wave groundplane with four 1/4 wave
long radials, I had to insulate the radials from the mast and
isolate the coax shield from the mast and radials with a 1/4
wave stub that formed a choke balun. Without the decoupling,
I could change SWR simply by changing mast or feedline grounding.

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