RadioBanter - View Single Post - Non-radiating Feedlines?

June 12th 04, 05:50 PM

On 12 Jun 2004 03:49:05 -0700, (Dr. Slick) wrote:

So for most of the dipole based antennas (including Yagis), you
can use 6 turns of 4" diameter coils in the coax, to make an inductive
loop that is supposed to prevent current from moving down the outer
braid (non-radiating).

Hi OM,

Loop has a loaded meaning here in this group, the appropriate term
would be R.F. Choke. What you describe is adequate for many
applications but needs to be confirmed in practice.

There are three (3) conductors implicit in any discussion of
Transverse vs. Common Mode. They are the two wires and ground (ground
being the conductor in the either the sense of a body of available
charge, or a literal wire). If you have absolutely no reference to a
third source of charge, then there is no issue of Common Mode.

The twin lead model in any real application (be it window line, twin
lead, or your AF cables) has some interaction with ubiquitous ground
and this gives rise to Common Modality issues. These models are
fairly obvious, what follows reveals how you have missed the
translation to coax:

So i'm not sure how to ask this, but coxial cable is obviously a
much different beast than twin lead, so the concept of common-mode
currents radiating from the line is a bit strange because the outer braid
completely encloses the inner radial. But this is weird because coaxial cable is
unbalanced already, while twin-lead (or in the case of the audio XLR,
shielded twin lead) is balanced.

The discussions in this group are often on shaky ground (pun intended)
by lacking discussion of ground. The discussions in this group are
often on that same tremulous substrate when the arguments run to
Kirchhoff, and tectonic when Superposition is given its fanfare.

The Kirchhoff/Superposition failures of discussion revolve around the
normal observation in the reciprocity of load as source. For the coax
model, the two conductors are bound just as tightly as any twin lead
or conductor pair. If we were to terminate the coax at a shielded
load, then the self shielding property of the coax would obviate any
discussion of Common Mode. Aside from leakage (if you choose to use
braided shield), it is totally absent. If you replaced the coax with
twin lead, Common Mode WOULD become an issue (an inverse of
expectations), but I will suspend that to return to the coax issue you
ask to resolve.

The coax terminates in real world loads that are not shielded and THIS
imbalances the system. A current is imparted to the load, and a
voltage is developed across it (Kirchhoff/Ohm). To any other system
component, this voltage is a source (Superposition) potential
(irrespective of where the "energy" comes from). This source, now
translated and extended to the feedpoint, sees the antenna, and it
sees the coax shield which has some linkage to ground. That is, the
exterior of the coax is NOT the same conductor as the interior of the
coax. The interior path is self shielded, the exterior is definitely
not. Superposition allows us to discard the interior wire of the
coax, and to simply view this coax as a third wire whose path is
described by the exterior of the shield.

This load-as-source (Superposition), then sees more than one path to
ground: one, through the load as we know it (typically the antenna);
and two, the coax shield's exterior path. Both offer paths for
current and this current is the Common Mode current - by definition,
as they are referenced to the Common (ground).

Where the load is balanced like a dipole (a seriously flawed
presumption, but usefully accepted for the purpose of argument); then
those current paths are equal, out of phase, cancel, and conform to
the Transverse Mode. However, this balanced load has a reference to
ground (this is where the flawed presumption arises and reveals how
the Common Mode originates). So does the exterior of the coax shield.
Some part of the balanced load is out of phase with the coax shield,
and with ground completing the circuit (as both elements share this
Common) then current flows - Common Mode current.

If you snub the coaxial exterior path (or Choke it); then you will
successfully reduce that Common Mode current. Examples of snubbing
mechanisms are bountifully described in this group.

Twin lead applications suffer from Common Mode for the same reasons
when the "balanced" load is in fact unbalanced (quite simple when the
literal, physical load, the antenna, fails to present equal relations
to ground). This is seen when you observe a dipole with one leg drawn
down at an angle (or up away from ground for that matter). The visual
imbalance is as telling as any method to confirm. If one end of a
dipole is supported in the clear, and the other end runs flat into a
wooded area, there is the obvious visual imbalance that will in some
way force Common Mode currents into the pristine twin lead
configuration. The failure mechanisms are manifold, so further
elaboration is unnecessary.

To stir the controversy, the standard FCC model for an AM service
antenna (ground mounted monopole) is possibly the single best, real
example of a balanced system. ;-)

73's
Richard Clark, KB7QHC