On Oct 2, 12:23 pm, Jim Kelley wrote:
michel wrote:
I don't know what construction technique you have in mind, but I prefer to
build them from 1/2" copper pipe. If that's what you intend to do, then
just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the
rest the same (1/4 wave stub). Attach your coax to the stub section using
hose clamps, and tune by sliding the clamps up or down until you achieve
best SWR. Works great.

ac6xg

Should it be that simple?

It can be.

I understood from other posters that it will not
work with the 1/4 wave stub..

For some posters, a view of the forest is obscured by trees. A j-pole
is a quarter wave stub with a 1/2 wave (or longer) radiator attached
to one side of the open end. The feed is connected nearer to the
shorted end of the stub. Build it and they (the QSOs) will come. Get
it up as high above the roof as you can.

ac6xg

I suppose that since there will be an antenna current on the stub, and
the 5/8 section would show a reactive feedpoint if fed against a
ground plane, things are a bit more complicated than just a resistive
matching section (the 1/4 wave stub). I would expect that (1) the
antenna won't behave quite like a 5/8 wave fed against ground, nor
like a 5/4 wave center fed doublet, and (2) the stub will have to be
adjusted in length as well as in transformation ratio to get a
"perfect" match. In addition, if the feedline is not decoupled from
the antenna, the antenna current on the feedline will change both the
pattern and the feedpoint impedance (match) from what it would be if
the stub+radiator were in freespace.

I'd do some NEC simulating to get an idea of a starting point AND an
idea if the pattern was really an improvement over the normal half-
wave over a quarter-wave stub, before trying to build one; and I'd put
some effort into decoupling the antenna from other nearby metal
(including the feedline)--or at least include other elements in the
simulation. Even with simulating, I'd expect to have to do some fine
tuning (of stub spacing or stub tap point, and possibly of stub
length) _if_ I really cared about a good match.

(Even a half-wave over a nominally quarter-wave stub can benefit from
tuning the stub length to get rid of reactance, if you really care
about a good match [though that in itself is somewhat over-rated].
With the stub attached, it's not a simple half-wave radiator. There
will be antenna current on the stub.)

Cheers,
Tom