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.

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.

Yes, it distorts the pattern, radiating or not.
How much depends upon the situation,
frequencies, spacing, size of feedline.
It is a conductive surface so it will distort pattern.
Side mounted on a tower changes pattern quite a lot
depending upon spacing from the tower, you can tune
the pattern that way. dB Products used to have a good
Catalogue that showed this.
I would not expect much pattern distortion from a feedline
very small, but would from a tower.



"Richard Fry" wrote in message
...
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.

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:
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.

The key to reducing the unwanted feedline current is that knowledge that
"it has to follow rules". We can't change the rules, but we *can* change
the antenna/feedline configuration so that the rules will work in our
favor.

These are the well-known physical rules about the way that voltages and
currents distribute themselves on wires, for example:

* Antenna wires carry standing waves of voltage and current, and also so
do feedlines if they carry net (common-mode or surface) RF currents.

* A voltage maximum is located at a current minimum, and vice versa.

* Maxima and minima are a quarter-wavelength apart.

* Current goes to zero at the physical end of a wire; voltage goes to
maximum.

Whenever you change the antenna/feedline configuration, the rules will
then force the voltage and current to rearrange themselves into a new
configuration also. So how can we "play the rules" to reduce unwanted
feedline currents?

For the moment, let's talk about coax feedline and unwanted surface
currents, because it's easier to insert chokes and also easier to
understand the results.

The unwanted surface current on the feedline has two components:
* Conducted current, launched onto the feedline from the hard-wired
connection at the feedpoint
* Induced current, caused by the feedline being placed in an EM field.

Wherever you insert a feedline choke along the feedline (by winding the
coax into a coil, or using ferrite loading) then you are creating a high
impedance which *forces* a current minimum at that point. The rules will
then force the current distribution along the feedline to change, so
that it meet this new additional requirement.

A choke balun at the feedpoint is the single most effective change you
can make, because it almost completely prevents conducted currents from
being launched onto the feedline.

That leaves the induced currents to be dealt with. These are typically
less of a problem than the conducted current, but are very hard to kill.
It's rather like trying to squeeze the air out of a long inflated tube.
Squeeze in one place by forcing a current minimum, and a new current
maximum will pop up a quarter-wavelength away.

Your feedpoint choke has defined a current minimum at the top of the
feedline, but by squeezing there you have created an induced current
maximum a quarter-wavelength down the feedline. So place another choke
there too, and the rules will make it impossible for ay significant
currents to build up between those two chokes. By making the rules work
in your favor, you have effectively killed the feedline current between
the two chokes.

The current won't quite go to zero, and a current maximum will certainly
try to pop up somewhere else, if the rules allow... but it probably
won't be as large as before, and you can chase that one down too.

There are many other ways to look at the problem, but I've found this
viewpoint of "playing the rules" to be quite a helpful one. I hope it
chimes with some other people too.


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