On Mon, 29 Aug 2005 21:12:58 -0700, dansawyeror
wrote:

When you drive a dipole with a coax, the exterior conductive path of
the shield (a separate circuit from the interior conductive path of
that same shield) is in parallel with one arm of the dipole. This
means you have a third radiator that has a length and termination that
is undefined.

In the case where a mistuned dipole is being driven directly from coax there is
radiation from the coax feed. This can only happen from current in the shield.
Is this what you are referring to in the second paragraph?

Hi Dan,

Hmmm, The dipole is mistuned by the third conductor, the coax's
shield's exterior; otherwise, the dipole would be suitably matched
(this is the presumption, of course).

The source of the current on the coax's shield's exterior comes from
the excitation voltage seen across the dipole drive point (to which
the shield is common to one of the arms). The arm of the dipole that
is not attached to the shield, sees both its opposite arm, and the
undefined length of the shield's exterior path. This additional load
both unbalances, and mismatches. It is the unbalance that gives rise
to the Common Mode current, the mismatch simply comes for free.

Of course, you could fall into the condition where the dipole would
not normally be tuned, but through luck and happenstance, the addition
of the third radiator creates a match - this is strictly opportunistic
and sometimes the source for glowing reports of an otherwise horrible
antenna design. And this is the genesis of favorable accolades for
many of the mythic antennas that go by initials: CFA, EH, and so on
down the line. The "inventors" have simply contrived to tune the
driveline to their "inventions." Their aversion to discussing
driveline isolation is a hallmark of their "science." Their
insistence that choking the driveline is unnecessary or an impediment
to the design's utility, is further evidence of a generous thumb on
the scale of proof.

The addition of the choke gives its Z to snub this Common Mode
current. As both interior paths (that of the line's center wire, and
the interior of the shield) driving the dipole pass through the same
loops, their magnetic fields are unperturbed and see no additional
impedance. However, the "return" path of the shield exterior sees
these loops alone, and thus the Z is inserted into series with it.

If you think in terms of the W2DU style BalUn, the interior
current/magnetic lines both transit THROUGH the beads, whereas the
exterior shield current/magnetic lines CUT the beads - hence the
choking action is more apparent in this configuration.

73's
Richard Clark, KB7QHC