On Fri, 21 Jan 2005 12:06:42 -0600, "Richard Fry"
wrote:

The outside of
the outer conductor of the coax feedline still will be coupled into the
received and/or radiated fields,

Hi OM,

What you fail to bring into this is the "degree" of coupling. The
transmission line being orthogonal is in the plane of the dipole's
null - hence zero conduction. It only supports conduction through
either direct connection (which the BalUn/Choke breaks) or loss of
symmetry (not falling in the plane of the dipole's null or the
environment distorting that electrical plane - an unbalanced dipole).

As an illustration of this, consider the effect of a 1/2-wave dipole
suspended near, and parallel to another 1/2-wave dipole. Only one dipole is
driven.

This, again, reveals the nature of "degree" of coupling. That is, in
your scenario the second dipole MUST be parallel AND broadside. If it
were parallel and online, the coupling would be considerably (10 - 15
dB) less.

So how important is the balun in the total RF system?

What is the "degree" of coupling?

73's
Richard Clark, KB7QHC

"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.

RF

On Fri, 21 Jan 2005 14:50:05 -0600, "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.

Hi OM,

It takes only a moment to visualize a dipole, frozen in time, where
each arm supports the opposite charge. The continuum of forces
between the two, in three-space, shows a distinct plane of response
where a net-zero force is exhibited. This reference plane, a virtual
ground, falls between the poles and is orthogonal.

A common artifice of erecting vertically polarized antennas above
dipoles bears this out. The two are invisible to each other. It also
allows for the use of towers to support beams, but also explains why
guy wires which violate balance (do not fall within the plane) must be
broken up as conductors. The towers have a smaller degree of coupling
than do the guy wires that support them.

Even folded dipoles in commercial installations make use of this
reference plane by providing a mounting point (180 degrees from the
feed) to the support structure. No regard needs to be made for
"shorting" out the loop at this point.

The null you speak of is exhibited in the far field - the utility of
BalUn/Chokes are in the near field. The transmission line may lie
within the reference plane, but its metallic connection to one of the
poles necessarily violates the electrical balance. The BalUn/Choke
isolates this connection.

73's
Richard Clark, KB7QHC

"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

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

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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