On Oct 3, 9:59 am, Cecil Moore wrote:
So a 0.2WL
matching stub is not a good feed design for a 5/8WL
monopole and will generate common-mode problems unless
there is a ground plane into which the current can flow.
Best to stick with the standard 1/2WL J-Pole design. I
apologize for my fuzzy thinking - it made sense until
I woke up. And it would work for a 5/8WL monopole if
it already had ground plane radials.
--
73, Cecil http://www.w5dxp.com

It's not the matching scheme that is the real problem..
A single 5/8 radiator over no radials is going to be a real
dog no matter how you match it.
And if you do use radials and a 5/8 radiator, you might
as well feed it with a simple series loading coil.
I've never been a great fan of J-poles, but if I were to
build one, it would be the standard 1/2 wave version.
If one is going to build a copper J pole using 5/8
elements, they should use two and build it as a
collinear. And even in that case, there should be a
decoupling section added for the best performance.
My favorite "cheap and easy" antenna for VHF is
not the j pole.. It's the 1/4 wave ground plane with
sloping radials. It's easier to build, needs no matching,
and the gain should be very close to most 1/2 wave
j poles.
MK