In the case where the J-pole is isolated (crossbanding HT at the
feedpoint?) there's absolutely a node for the common mode current at
the short, just like there's a node for the current on a wire at the
end where the current can't flow.

If you put a feedline on it, though, the standing wave pattern of
common mode current changes.

The feedline + jpole develops some pattern of standing waves on it.
The common mode current is induced on the entire length of coax and
stub...

The sum of the common mode currents at point B is always as stated
before... but the magnitude of ALL THREE of them depends on the details
of the coax, whether or not there is a choke balun on it, the length
and boundary condition on the other end of the coax.

That is to say the common mode impedance at the top of the stub where
the halfwave is connected depends on the details of the feedline.

Stub vs. Stub + Coax: different common mode standing wave pattern,
different common mode impedance at the top of the stub, different
magnitude of current induced on the stub+coax than on the plain stub...


Dan