"Richard Clark" wrote
The null you speak of is exhibited in the far field -
___________

The near-field boundary is located at about 2*(Ant Length)^2 / lambda, which
for a 14 MHz, 1/2-wave dipole is ~32 feet away. The far-field radiation
pattern shape is not well formed inside that boundary, but radiated fields
are non-zero, nevertheless. A coax feedline that does not project on a
radial normal to the dipole feedpoint will have current induced on its outer
conductor by coupling to the dipole -- whether or not a balun is used.

RF

Richard Fry wrote:

"Richard Clark" wrote
The transmission line being orthogonal is in the plane of the
dipole's null - hence zero conduction.

But the nulls of a dipole are off its ends. The t-line connects to the
dipole center, where relative field normal to the longitudinal axis of
the dipole is at a maximum.

The orthogonal part is the important part. The radiation "sees"
the transmission line on edge and doesn't induct (much) energy
to it. The energy transferred from the antenna to the feedline
is a function of the cosine of the angle between them. If the
feedline is hanging down vertically from a horizontal dipole,
for common-mode purposes, the feedline is vertically polarized
and the antenna is horizontally polarized. It is when you bend
the feedline at some angle other than 90 degrees to the antenna
that the cosine of that angle becomes non-zero.
--
73, Cecil http://www.qsl.net/w5dxp


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On Sat, 22 Jan 2005 06:59:07 -0600, "Richard Fry"
wrote:

A coax feedline that does not project on a
radial normal to the dipole feedpoint will have current induced on its outer
conductor by coupling to the dipole -- whether or not a balun is used.

Hi OM,

This is arguable at best, and suitable newsgroup fodder for endless
speculation on the contributions of superposition and the combination
of direct and induced currents.

The classic study of Engineering reveals one principal:
The well defined problem contains its own solution.

The omission of the BalUn/Choke is not revealed as a solution to your
complaint above. It has already been disclosed by others on how the
further application of choking can resolve this crafted failure.
Their discussion and my own comprise a general solution that responds
to the necessary correlative:
What is the degree of coupling?

73's
Richard Clark, KB7QHC

"Richard Clark" wrote
On Sat, 22 Jan 2005 06:59:07 -0600, "Richard Fry"
wrote:

A coax feedline that does not project on a
radial normal to the dipole feedpoint will have current induced on its
outer
conductor by coupling to the dipole -- whether or not a balun is used.

...Their discussion and my own comprise a general solution that
responds to the necessary correlative:
What is the degree of coupling?
__________________

I will email you* a NEC study showing two surface patterns from a 1/2-wave
dipole; one with no feedline, and one having a conductor approaching within
2" of the center of the dipole and not attached to it, and not perpendicular
to the longitudinal axis of the dipole.

The second pattern simulates a dipole fed by a sloping coax cable connected
to the dipole feedpoint through a balun -- a fairly typical scenario for the
coax feedline.

You can judge the affect for yourself.

*and any others wanting to see it

RF