[Antennas] J-Pole antennas

[Terry Conboy]

I haven't seen the original design of the J-Pole from the 1940's nor 
details of the approach used in Zeppelins.  Perhaps Clete can comment if 
they used a link coupling to prevent the unavoidable unbalanced currents 
on the 1/4 wavelength stub from flowing (and radiating) or otherwise 
describe the proper feed method.

In all the modern implementations that I've seen, whether feed on the 
bottom shunt or a little above it on the 1/4 wl stub, regardless of 
which side is considered "hot", there will be current in an attached 
conducting supporting structure (and coax shield) if some sort of choke 
balun isn't used.  These designs and the supporting mast are easily 
modeled in NEC2 and the current flow can be significant, with the 
magnitude depending on the length of the conducting support and/or coax 
shield.  For an excellent discussion of this topic, see 
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

73, Terry N6RY

On 2009-11-27 4:36 AM, Clete Whitaker wrote:
> de WB2CPN
> The VHF J-Pole design came about prior to WWII, and was
> used by US Army in the 100 - 156 MHz range.   It was  used
> on HF years before that, but wasn't called a J-Pole.   It was
> called a Zeppelin because that's what it was used on.  It kept
> the high RF voltages away from the airship.  TRIVIA:  There
> have been so many  ideas on what a J-Pole is supposed to
> look like that there is no "Right or Wrong" any more.  Now
> days they are made to look like miniature Zepps that are
> incorrectly fed part way down the 1/4 wave section.  A well
> designed J-Pole shouldn't need a choke balun at the bottom,
> but the ones made these days do, because of the improper
> method of feed.  A highly inflaming subject, we're in here.
> End of Trivia.
> 73  Clete
>
>