On Oct 4, 7:40 am, wrote:
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

Amen, bro. Another advantage of the ground plane: the radials
decouple the feedline from the antenna. In a J-pole installation,
either you put some effort into decoupling the feedline, or you accept
that the feedline is going to radiate (and change the pattern). (I
suppose yet another option is to deny the fact that feedlines can
radiate...)

It's easy to build a "quick-and-dirty" 146MHz ground plane with an
SO-239, some 4-40 screws, washers and nuts, and three lengths of 12 or
14 AWG copper wire. A couple half-wave lengths attach to the flange
of the SO-239 with the 4-40 hardware (or just solder them) so there
are 4 1/4 wave wires sticking out from the flange; a 1/4 wave piece
solders into the center pin. The radials can be bent down a bit to
get a match to 50 ohms. You can put a little loop in the top of the
radiator and hoist it into a tree with fishing line or the like.
There are several ways you can attach it to the top of a mast; it
doesn't care a lot about what you do below it.

Cheers,
Tom