Roy Lewallen wrote:
The feedline (and other metallic structures) adjacent to an FM broadcast
transmit antenna will affect the radiation patterns of the antenna even
though the measured match between the feedline and antenna input is
extremely good (even 1:1 SWR) -- in which case the line should have no
differential current to produce such an effect. What is the explanation for
that, please?

We've been down this path before, and you've shown that you won't
accept the fact that SWR has nothing to do with whether or not common
mode current exists on a feedline, and there's nothing I've been able
to do to convince you otherwise. You also either haven't read or won't
believe that it's common mode, not differential, current that causes a
line to radiate and thereby contribute to the overall pattern.

I agree with Roy that SWR itself is not the *cause* of radiating
feedlines.

However, if the feedline is allowed to carry currents and radiate, it
effectively becomes part of a new and different antenna configuration.
That new configuration will have a different feedpoint impedance, so the
SWR will change.

But the SWR didn't cause the feedline current and radiation; in fact it
was exactly the opposite.

Also, Richard is assuming that unwanted feedline currents will always
change the SWR for the worse. That's often true, but it doesn't have to
be - sometimes the SWR gets worse when the feedline current is choked
off.


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

Roy Lewallen wrote in message ...

No. A "non-radiating" feedline is one which has no significant amount of
common mode current. This can be accomplished by making the feedline a
length such that the induced current is minimal; by inserting a balun or
baluns; and/or by placing the feedline symmetrically with respect to the
antenna. I thought I had explained this -- I don't seem to be
communicating well.



What's up Roy? Long time no see!

Ok, well, I'd like to discuss this a bit.

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).




We've been down this path before, and you've shown that you won't accept
the fact that SWR has nothing to do with whether or not common mode
current exists on a feedline, and there's nothing I've been able to do
to convince you otherwise. You also either haven't read or won't believe
that it's common mode, not differential, current that causes a line to
radiate and thereby contribute to the overall pattern. But hopefully
other readers have learned from this exchange. Once the basic principles
are grasped, these phenomena lose their mystery, and they're no longer
"unpredictable", but readily measured, modeled, and understood.


Ok, so I understand how the common-mode-rejection-ratio works with
an audio amplifier that has an XLR cable input: signals in phase
(common
mode) will cancel each other out when they reach the input transformer
(balun). And although the XLR cable is shielded, the two signal wires
are more like a twin-lead transmission line instead of like coaxial
cable.

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.

This is a discussion he

http://lists.contesting.com/archives.../msg00484.html



Slick

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).

6 turns is probably not enough inductance to do much choking on 160m
or 80m.

If the dipole is non-resonant, 6 turns of coax may have very little
effect on any HF frequency.
--
73, Cecil http://www.qsl.net/w5dxp



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On Fri, 11 Jun 2004 15:39:59 -0700, Roy Lewallen
wrote:

This can be accomplished by making the feedline a
length such that the induced current is minimal; by inserting a balun or
baluns; and/or by placing the feedline symmetrically with respect to the
antenna. I thought I had explained this -- I don't seem to be
communicating well.

Hi Roy, Richard,

Perhaps it is because of the naming convention vs. application.

The term more appropriate to the Topic is RF Choke rather than BalUn
(which performs the choking action by implication, and could easily
fail that implication if the incorrect BalUn design is chosen).
Further, as you have noted in past correspondence Roy, a second Choke
(not BalUn) is often required for these VHF/UHF applications to
completely isolate the line.

73's
Richard Clark, KB7QHC

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

Cecil Moore wrote in message ...
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).

6 turns is probably not enough inductance to do much choking on 160m
or 80m.

If the dipole is non-resonant, 6 turns of coax may have very little
effect on any HF frequency.


What's up Cecil?

ok, you may be right about HF, but for VHF (30-300 MHz), 6 turns
in the coax is enough.

So have you made an inductive coil in coax for HF? If so, how
many turns did you need, and what was the diameter of the turns?

I may need to do this for an AM station one day, but maybe not
because most AM antennas are some sort of end-fed random length
wire with a tuner, where the wire length is greater than 1/4 wavelength.
In other words, i haven't seen any dipole antennas for broadcast band
AM....it would be too long!


Slick

Dr. Slick wrote:
So have you made an inductive coil in coax for HF? If so, how
many turns did you need, and what was the diameter of the turns?

I once made one out of a 2-liter pop bottle about 1/3 full
of turns of RG-8X. It worked pretty well on 40m, my favorite
band, but a thunderstorm destroyed it. I have no idea what
the choking impedance was.
--
73, Cecil http://www.qsl.net/w5dxp



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On Sat, 12 Jun 2004 07:52:50 -0500, Cecil Moore
wrote:

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).

6 turns is probably not enough inductance to do much choking on 160m
or 80m.

If the dipole is non-resonant, 6 turns of coax may have very little
effect on any HF frequency.

It took 27 turn here to get decent choking on 80m. But it finally
worked 6" diamater selenoid wound.
73 Dave KC1DI

P.S. feeding a OFCD cut for 3.6 MHZ. Thru a 4 to 1 Balun.

Richard Clark wrote in message . ..


(Large paragraphs of Nonsensical, pyseudo-technobable snipped)


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.



It seems you are still spouting out words and concepts that you
really don't understand, eh? If things are on shaky ground here,
you certainly ain't helpin'!

This is a reminder that ANYONE can post anything they want here.


Slick
Slick

On 13 Jun 2004 14:51:20 -0700, (Dr. Slick) wrote:
This is a reminder that ANYONE can post anything they want here.
Self-fulfilling prophesy?

Slick
Slick
redundant or hyperbolic comment?

Hi OM,

Again, you haven't the talent to attack on style nor bandwidth to
challenge technical content. If you responded to the issue you are
having difficulty with, then that would be another matter.

Try again. ;-)

73's
Richard Clark, KB7QHC