On Dec 28, 12:36*pm, Richard Fry wrote:
"Art Unwin" wroteThe antenna compendium states that an assumption
is made with MoM programs that an assumption is made that current
in a radiator is sinusoidal where as we all know that the current
degrades in aplitude dependent on radiator length.

_____________

Art, the current distribution along even the shortest fractional
wavelength, constant OD radiator also is ~sinusoidal.

Current always is near zero at the open end of a linear radiator of
every
physical length. The shape of the current wave formed along a very
short
radiator appears to be very close to triangular. *But in fact that
"triangular" current distribution is just a very short section of a
sinusoidal waveform.

N.B. that MoM programs show exactly this for radiators that are very
short
in terms of electrical wavelengths. *This also is proven
mathematically in
the antenna engineering texts of Kraus, Balanis, Johnson & Jasik,
etc.

RF

O.K. have it your way. At the end of a radiator voltage is a maximum
as current is zero
ie the curves of current and current crosses each other. We can then
use the absolute standard
equatiion of E = I R. Using this formula for understanding conditions
at the end of a radiator
we can state that E, I and R equals zero ala a non closed circuit.
Kraus used four travelling waves in his analysis of the helical
antenna an analysis that was not corrobarated by
following examiners or the application of the NEC (MoM) programs where
disturbing differences was never resolved.
You introduce wavelength as if it was a standard without considering
the velocity factor and where a transmission line analogy
does not satisfy a helical antenna because of slow wave created in a
similar way to cavitation as explored by Bernoulle
or by the addition of sharp corners encountered by current flow
As far as what has been proven in text books they are only reflect the
conditions placed on the problem but also assumption of correct theory
applied.
This is why history shows the evolution of science is a series of
broken theories whose value is measured by their resistance to attack
over time.
I would remind you that the metric of time has NOT stopped. But as I
stated earlier you can have it your way without objection from me
Art

The "Method of Moments" (MOM) makes no assumptions about
current distribution on a radiator; it computes the current distribution.
The radiated field is then calculated based on the current distribution.

Frank

Exactly correct. EZNEC (including demo) users can get a good
illustration of this by looking at the pattern from the example file
Cardioid.ez which consists of two identical elements with perfect
spacing and perfectly ratioed base currents. Notice that the front/back
ratio is about 35 dB (dropping to about 31 dB with more segments for
more accurate results), while it should be essentially infinite. The
small back lobe is caused by modification of the current distribution on
the two elements caused by mutual coupling -- although the base currents
are perfectly equal in magnitude and 90 degrees out of phase, the fields
from the two elements aren't, due to their having slightly different
current distributions. When I first saw this back lobe with perfectly
ratioed base currents, I searched through the program code looking for
what I was certain was a bug. I finally realized that the result was
correct and that the lobe was caused by the altered current distribution
which the program had correctly calculated. For more about this, see
http://eznec.com/Amateur/Articles/Current_Dist.pdf.

The assumption of sinusoidal current distribution is strictly true only
with straight, isolated conductors which are infinitely thin, although
it's a reasonably good assumption in many other cases. Mathematical
analyses of antennas done before computers were pretty much limited to
cases where sinusoidal distribution was assumed, because a more accurate
determination of current distribution was virtually impossible to
calculate. The ability to determine the actual current distribution is
one of the very important advantages of computer analysis.

Roy Lewallen, W7EL

Art wrote:
"We can then use the absolute standard equation of E = I R."

For ac (RF) that`s not true. The formula is E=IZ, where Z includes
reactance and resistance in quadrature. I`m not piling on but some
readers may believe Art.

Best regards, Ricxhard Harrison, KB5WZI

On Dec 28, 4:12*pm, (Richard Harrison)
wrote:
Art wrote:

"We can then use the absolute standard equation of E = I R."

For ac (RF) that`s not true. The formula is E=IZ, where Z includes
reactance and resistance in quadrature. I`m not piling on but some
readers may believe Art. *

Best regards, Ricxhard Harrison, KB5WZI

Yes you are correct but the original equation was E=IR
which preceeds the implication of impedance which is a derivative
of my equation and came about with the addition of A.C. technology.
If the impedance is totally resistive then my statement is not untrue
Now to avoid the nitpicking are you saying that E=IZ cannot be used
for calculations at the end of an antenna and if so" WHY "
Art

Art wrote:
"Now to avoid nitpicking are you saying that E=IZ cannot be used for
calculations at the end of an antenna and if so "WHY"?"

It is complicated by multiple currents. Like an open-circuited
transmission line, electrical conduction stops at the end of the
conductor. Current then becomes a phasor problem.

Collapse of conduction current induces a voltage which combined with the
incident voltage almost doubles the total voltage at this spot in many
cases. This reverses the direction of current in the conductor. Due to
capacitance at his high-voltage spot with the iniverse, displacement
current flows into free space from open-circuited antenna ends. It is
usually smaller than the conduction current.

Best regards, Richard Harrison, KB5WZI

On Dec 28, 3:32*pm, Roy Lewallen wrote:
The "Method of Moments" (MOM) makes no assumptions about
current distribution on a radiator; *it computes the current distribution.
The radiated field is then calculated based on the current distribution..

Frank

Exactly correct. EZNEC (including demo) users can get a good
illustration of this by looking at the pattern from the example file
Cardioid.ez which consists of two identical elements with perfect
spacing and perfectly ratioed base currents. Notice that the front/back
ratio is about 35 dB (dropping to about 31 dB with more segments for
more accurate results), while it should be essentially infinite. The
small back lobe is caused by modification of the current distribution on
the two elements caused by mutual coupling -- although the base currents
are perfectly equal in magnitude and 90 degrees out of phase, the fields
from the two elements aren't, due to their having slightly different
current distributions. When I first saw this back lobe with perfectly
ratioed base currents, I searched through the program code looking for
what I was certain was a bug. I finally realized that the result was
correct and that the lobe was caused by the altered current distribution
which the program had correctly calculated. For more about this, seehttp://eznec.com/Amateur/Articles/Current_Dist.pdf.

The assumption of sinusoidal current distribution is strictly true only
with straight, isolated conductors which are infinitely thin, although
it's a reasonably good assumption in many other cases. Mathematical
analyses of antennas done before computers were pretty much limited to
cases where sinusoidal distribution was assumed, because a more accurate
determination of current distribution was virtually impossible to
calculate. The ability to determine the actual current distribution is
one of the very important advantages of computer analysis.

Roy Lewallen, W7EL

If current distribution is calculated correctly as stated then the
answer with respect
to the route taken of the current of a fractional wave antenna should
be available and beyond doubt
as the program is derived from Maxwells laws. The distributed current
should be DC based if current flow is thru the center
of the radiator. As far as the resistance encountered on reverse flow
on the outside of a radiator the figure provided by computor programs
should be rather interesting as I have never encountered in print
suggested figures. In accordance with some engineers the radiation
resistance but be stated as the radiation impedance to avoid sniping
Art

On Dec 28, 5:35*pm, (Richard Harrison)
wrote:
Art wrote:

"Now to avoid nitpicking are you saying that E=IZ cannot be used for
calculations at the end of an antenna and if so "WHY"?"

It is complicated by multiple currents. Like an open-circuited
transmission line, electrical conduction stops at the end of the
conductor. Current then becomes a phasor problem.

Collapse of conduction current induces a voltage which combined with the
incident voltage almost doubles the total voltage at this spot in many
cases. This reverses the direction of current in the conductor. Due to
capacitance at his high-voltage spot with the iniverse, displacement
current flows into free space from open-circuited antenna ends. It is
usually smaller than the conduction current.

Best regards, Richard Harrison, KB5WZI

You skated over the difference between an open circuit of the
transmission
line compared to the end of an antenna.
The analogy is flawed and will be shown when the resistance in the
center of a radiator is disclosed via the
computor programs. You never did supply the information needed to
justify the values of E,I and R when
the current value crosses the zero line on a graph. You can ofcourse,
declare that none of the given factors
can ever be equal to zero by jumping the datum line !!!!! By the way,
could you state a situation where the
displacement current is LARGER than the conduction current so I may
review it in the light of Newtonian laws?
Art
Art

"Roy Lewallen" wrote in message
. ..

The "Method of Moments" (MOM) makes no assumptions about
current distribution on a radiator; it computes the current
distribution.
The radiated field is then calculated based on the current distribution.

Frank

Exactly correct. EZNEC (including demo) users can get a good illustration
of this by looking at the pattern from the example file Cardioid.ez which
consists of two identical elements with perfect spacing and perfectly
ratioed base currents. Notice that the front/back ratio is about 35 dB
(dropping to about 31 dB with more segments for more accurate results),
while it should be essentially infinite. The small back lobe is caused by
modification of the current distribution on the two elements caused by
mutual coupling -- although the base currents are perfectly equal in
magnitude and 90 degrees out of phase, the fields from the two elements
aren't, due to their having slightly different current distributions. When
I first saw this back lobe with perfectly ratioed base currents, I
searched through the program code looking for what I was certain was a
bug. I finally realized that the result was correct and that the lobe was
caused by the altered current distribution which the program had correctly
calculated. For more about this, see
http://eznec.com/Amateur/Articles/Current_Dist.pdf.

The assumption of sinusoidal current distribution is strictly true only
with straight, isolated conductors which are infinitely thin, although
it's a reasonably good assumption in many other cases. Mathematical
analyses of antennas done before computers were pretty much limited to
cases where sinusoidal distribution was assumed, because a more accurate
determination of current distribution was virtually impossible to
calculate. The ability to determine the actual current distribution is one
of the very important advantages of computer analysis.

Roy Lewallen, W7EL

Thanks for the info. A very interesting link. I have experimented
with NEC models of phased arrays, and found the same problem
with a small back lobe. I followed the procedure in the ARRL
Antenna book, which involved calculating the elements in a 2 X 2
complex Z matrix, for a 2 element phased dipole array. The
results appeared to be very good, but I never actually built it.

Frank, VE6CB

Art wrote:
"could you state a situation where the displacement current is LARGER
than the conduction current so I may review it in the light of Newtonian
laws?"

I have difficulty in imagining current between the plates of a capacitor
exceeding the current through the capacitor`s leads.

Values of voltages and currents anywhere along an antenna primarily
depend on the impedance of the antenna at that point and then are
dictated by the phasors of the incident and reflected totals at the same
point. Arnold B. Bailey in Fig. 7-28 on page 368 of "TV And Other
Receiving Antennas" shows current distribution on a half-wave dipole
which smoothly varies from zero at its ends to maximum at its center.

Experience shows that a quarter-wave back from a maximum impedance
point, a minimum impedance point is created by incident and reflected
phasors.

Best regards, Richard Harrison, KB5WZI

In article , Roy
Lewallen wrote:


The assumption of sinusoidal current distribution is strictly true only
with straight, isolated conductors which are infinitely thin, although
it's a reasonably good assumption in many other cases. Mathematical
analyses of antennas done before computers were pretty much limited to
cases where sinusoidal distribution was assumed, because a more accurate
determination of current distribution was virtually impossible to
calculate. The ability to determine the actual current distribution is
one of the very important advantages of computer analysis.

Roy Lewallen, W7EL

Hello, Roy, and while the above is certainly correct, you're probably
wasting your time. Many folks like to fashion their own "reasonable"
explanations even when they're completely off track (should I mention the
CFA again?). Of course they're always right and it's the rest of the
world who's wrong. Certain folks on this ng appear to be in constant need
of validation. Persons without some knowledge of the underlying physics
and applied math are destined to reach the wrong conclusions IMO. Of
course that's not going to stop some from building a CFA because as we all
know it's the standard comms antenna used at the Groom Lake facility to
keep in touch with Klaatu. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337