There is a general belief that a balun should be used where coax connects to
the feedpoint of a 1/2-wave dipole (for example), to avoid RF current on the
outer surface of the coax outer conductor -- which can affect antenna input
match, radiation pattern etc.

But does even a perfect balun fully remove these effects? The outside of
the outer conductor of the coax feedline still will be coupled into the
received and/or radiated fields, even when there is no current flowing into
the coax outer conductor via a metallic connection directly with the dipole
itself.

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. For simplicity of concept, let's say the active RF device (either a
tx or rx) is a physically small unit built in to the center insulator of the
driven dipole, e.g., no feedline. Standard equations, and NEC-2 analysis
show that considerable interaction exists between the two dipoles. The
input match of the driven dipole changes, and the radiation pattern of the
simple dipole is strongly affected. Yet the only coupling between these two
dipoles is by radiation.

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

RF

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


For simple dipoles it makes lots of money and helps out with the
unemployment.

Richard Fry wrote:

There is a general belief that a balun should be used where coax
connects to the feedpoint of a 1/2-wave dipole (for example), to avoid
RF current on the outer surface of the coax outer conductor -- which can
affect antenna input match, radiation pattern etc.

But does even a perfect balun fully remove these effects?

There are two sources for common-mode current. One is
conduction. The other is induction. Since common-mode
currents are somewhat unpredictable, the best way to
ascertain if there is a problem is to measure the
current. A toroidal pickup slipped over the coax will
give one a relative reading and is super simple to
implement.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Richard Fry wrote:

There is a general belief that a balun should be used where coax
connects to the feedpoint of a 1/2-wave dipole (for example), to avoid
RF current on the outer surface of the coax outer conductor -- which
can affect antenna input match, radiation pattern etc.
But does even a perfect balun fully remove these effects?

No, it doesn't, for the reasons that Richard gave, and Cecil summarizes
below.

The reasons why these currents can appear is because the antenna, the
feedline layout and/or the environment is asymmetrical - and often it's
all three.

How much unwanted feedline current you have will depend on the physical
layout, how long the feedline is in wavelengths, and where and how it
may be grounded at the shack end. That's what makes it unpredictable, as
Cecil says below; so whether or not you need a balun will depend
entirely on your particular situation.


There are two sources for common-mode current. One is
conduction. The other is induction.

The 'conduction' component of the current is launched onto the feedline
where it connects to the antenna, and a good choke balun at the
feedpoint can effectively knock it out. The balun at the feedpoint
doesn't have to be perfect; only good enough to make sure that the
conduction current doesn't dominate any more.

That leaves the 'induction' component of the current, which is due to
asymmetrical electromagnetic coupling between the antenna and the
feedline. Unless the feedline is much closer to one side of the antenna
than the other, this induction component will generally be much smaller
than the conduction component was (before you mostly removed it with a
balun at the feedpoint).

If the remaining induction current is a problem, you may need to use
additional chokes at various places along the feedline. Depending on the
feedline length and the grounding arrangements, there will be current
maxima and minima at various places along the feedline. The most
effective place to put a choke is obviously at a maximum, where the
current is trying to be large; the least effective place is at a
minimum, where it's low anyhow.

The last traces of feedline current are very hard to kill completely.
It's rather like trying to squeeze the air out of a long balloon - choke
the current in one place, and a new maximum will try to pop up somewhere
else. Even so, a few chokes in the right places - and always put the
first one at the feedpoint - will almost definitely put you in control
of the situation.

But how do you know how much current you've got, and where the current
maxima are?

Since common-mode
currents are somewhat unpredictable, the best way to
ascertain if there is a problem is to measure the
current. A toroidal pickup slipped over the coax will
give one a relative reading and is super simple to
implement.

Here is a whole page about clip-on current meters, which have the
advantage that you don't have to disconnect the cable to feed it through
a toroid:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm

The clip-on gadget is a bit more trouble to build, but you'll be very
glad you did! With one of these, you can actually *see* how much RF
current you've got, and where it is. Without it, you might as well be
investigating RFI problems with your eyes shut.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

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 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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