No.

A "non-radiating" feedline is one on which there is no net current
(i.e., no common mode current). In the case of coax, this translates to
zero current on the outside of the shield; for twinlead feedlines, it
means that the currents in the two conductors are exactly equal in
magnitude and opposite in direction.

Pattern distortion is caused by current being induced in a conductor by
an impinging field. That current, in turn, creates a field which adds to
the impinging field, resulting in pattern distortion. (This is, in fact,
the way a Yagi functions.) If there is current induced in a feedline by
the field, it's a radiating conductor (because the current causes
radiation which interferes with the impinging field). If there is no
current induced in the feedline by the impinging field, it creates no
field of its own (i.e., it's non-radiating) and therefore causes no
interference.

A transmission line placed symmetrically with respect to a dipole won't
have any current induced in it, although current can be conducted via a
direct connection. A transmission line asymmetrically placed will have
current induced in it and will distort the pattern. The feedline of a
ground plane or J-Pole likewise has induced current which distorts the
pattern. The amount of common mode current flowing in a transmission
line can be reduced by introducing an impedance to the common mode
current. It's desirable to do this without disturbing the differential
mode transmission line operation. That's the function of a balun. The
amount of induced current depends strongly on the orientation and length
of the parasitic conductor, and might be large or small in a particular
case.

Roy Lewallen, W7EL

Richard Fry wrote:
Post below from May 20, 2004

Wouldn't a "non-radiating" feedline in the field of a radiator also distort
the patterns of that radiator? Any conductor can do that, even if it is not
a feedline. NEC-2 models of FM broadcast transmit elements (and test range
patterns) show this clearly.

Paper 6 at http://rfry.org shows the free space patterns that the element
arms of a rototiller FM broadcast transmit antenna develop if they could be
driven from internal power sources -- and then the effects of adding the
element stem, mounts, feedline, and some nearby tower structure. The
patterns can get very skewed, even though the only radiators getting power
via a metallic path from the tx are the element arms themselves.

RF
____________

"Roy Lewallen" wrote

With a typical ground plane antenna, the feedline can radiate
significantly, distorting the pattern. This effect could easily be
different for the different antennas. Modeling indicates that two baluns
are often needed to suppress the current on the outside of the feedline.
A model which includes the feedline might give some insights as to why
the antennas behave so differently.

A balun can reduce common mode feedline currents due to the power supplied
to the line by the transmitter, but if the feedline is immersed in an
asymmetric radiated field, how does the balun reduce/remove the resulting,
unpredictable differential current on the feedline, and its contribution
toward producing the net radiated pattern?

RF
_____________

"Roy Lewallen" wrote:

A "non-radiating" feedline is one on which there is no net current
(i.e., no common mode current). In the case of coax, this translates to
zero current on the outside of the shield; for twinlead feedlines, it
means that the currents in the two conductors are exactly equal in
magnitude and opposite in direction.

A transmission line placed symmetrically with respect to a dipole
won't have any current induced in it, although current can be conducted
via a direct connection.

The amount of common mode current flowing in a transmission
line can be reduced by introducing an impedance to the common
mode current. It's desirable to do this without disturbing the differential
mode transmission line operation. That's the function of a balun.

Richard Fry wrote:
A balun can reduce common mode feedline currents due to the power supplied
to the line by the transmitter, but if the feedline is immersed in an
asymmetric radiated field, how does the balun reduce/remove the resulting,
unpredictable differential current on the feedline, and its contribution
toward producing the net radiated pattern?

It simply provides an impedance to the common-mode currents. Whether
that impedance is high enough to be effective depends upon the system
parameters and configuration.
--
73, Cecil http://www.qsl.net/w5dxp



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The radiated field in which a feedline is immersed produces a common
mode, not differential, current on the feedline(*). The balun creates a
high impedance to the flow of common mode current. The result is that
much less common mode current is induced than would be the case if the
balun were absent. AM broadcasters put insulators periodically in tower
guy wires. Baluns have the same effect, although they're of course not
as perfect as the fully open circuit created by the insulator.

I'd also like to add that the induced current isn't unpredictable, as
you stated. It has to follow rules like all other physical phenomena, so
it's entirely predictable. I have to qualify this, though, by noting
that in many or most amateur installations, the path from the antenna
along the outside of the feedline to the rig and from there to the Earth
is often not well known. And precise predictions of feedline current
can't be made without knowing this path.

This effect is easily observed in models, as well as being
experimentally verifiable with a small amount of effort. For example,
make a simple wire Yagi with wire elements. Slip a few ferrite cores
over one of the elements and see what happens to the pattern. What
you've done is to reduce the current in the parasitic element, which is
immersed in the field from the driven and other elements, thereby
reducing its contribution to the total field. Cutting the element
(analogous to the broadcasters' insertion of an insulator) has the same
effect, although it'll be even more profound than the high but finite
impedance of the cores. Quanitative measurements aren't hard to make,
either. You can find details of current measurement devices in Chapter 8
of the ARRL Antenna book and other references. When placed over a coax
feedline, or over both conductors of a twinlead feedline, they'll
measure the common mode current.

(*) I'm assuming here that we're dealing either with coax, or with
twinlead whose wire spacing is small in terms of wavelength and whose
spacing from the antenna is large compared to the wire spacing. If you
put twinlead in a field such that the field seen by one conductor is
significantly different than the field seen by the other, you will get
an induced differential, as well as common mode current. This won't
happen with coax. Unlike common mode current, an induced differential
current will affect the properties of the line as a transmission line
(that is, change its apparent Z0 and velocity factor). But this would be
an unusual condition in an amateur installation, and I'm not considering
it here.

Roy Lewallen, W7EL

Richard Fry wrote:
A balun can reduce common mode feedline currents due to the power supplied
to the line by the transmitter, but if the feedline is immersed in an
asymmetric radiated field, how does the balun reduce/remove the resulting,
unpredictable differential current on the feedline, and its contribution
toward producing the net radiated pattern?

RF
_____________

"Roy Lewallen" wrote:

A "non-radiating" feedline is one on which there is no net current
(i.e., no common mode current). In the case of coax, this translates to
zero current on the outside of the shield; for twinlead feedlines, it
means that the currents in the two conductors are exactly equal in
magnitude and opposite in direction.


A transmission line placed symmetrically with respect to a dipole
won't have any current induced in it, although current can be conducted
via a direct connection.


The amount of common mode current flowing in a transmission
line can be reduced by introducing an impedance to the common
mode current. It's desirable to do this without disturbing the differential
mode transmission line operation. That's the function of a balun.

As a discussion point, consider 1/2-wave parasitic radiators sometimes
positioned within a foot or two of FM broadcast transmit elements, to
"shape" their patterns. This is the common technique used in "sidemount"
antennas that must meet FCC requirements for directional FM broadcast
assignments.

A parasitic itself is suspended mechanically in space by a non-metallic
support. It has no direct connection to the transmitter, and no conductive
physical path to any part of the antenna, its feedline, mounts, or
supporting structure.

Such parasitics do affect the net radiation pattern(s) of the array.

Isn't a "non-radiating" feedline with a balun just an arbitrary length of
conductor, but now with a metallically conductive path to the driven
element(s), as well?

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?

RF
______________

"Roy Lewallen" wrote in message
...
The radiated field in which a feedline is immersed produces a common
mode, not differential, current on the feedline(*). ETC

Richard Fry wrote:
As a discussion point, consider 1/2-wave parasitic radiators sometimes
positioned within a foot or two of FM broadcast transmit elements, to
"shape" their patterns. This is the common technique used in "sidemount"
antennas that must meet FCC requirements for directional FM broadcast
assignments.

A parasitic itself is suspended mechanically in space by a non-metallic
support. It has no direct connection to the transmitter, and no conductive
physical path to any part of the antenna, its feedline, mounts, or
supporting structure.

Such parasitics do affect the net radiation pattern(s) of the array.

Isn't a "non-radiating" feedline with a balun just an arbitrary length of
conductor, but now with a metallically conductive path to the driven
element(s), as well?

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.

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. 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.

Based on past experience, nothing I say will sway you from the way
you've chosen to interpret observed phenomena. And I believe I've done
enough explaining so that any other readers, who are open to learning
some fundamentals, can come away with a better understanding. So that's
enough for now.

Roy Lewallen, W7EL


RF
______________

"Roy Lewallen" wrote in message
...

The radiated field in which a feedline is immersed produces a common
mode, not differential, current on the feedline(*). ETC

Just curious, Roy. Have you used EZNEC to model an FM broadcast transmit
array, feedlines and adjacent tower -- as in the models in Paper 6 at
http://rfry.org ? If so, and your pattern results are appreciably different
than those, for those same scenarios -- I'd truly appreciate learning from
you the reason(s) you might pose for that. Maybe I'm doing it wrong.

I'll be glad to send you the intermediate results: radiators alone, complete
elements with feedlines, complete elements with feedlines and tower.

RF

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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