colinear representation in NEC
 

On Tue, 31 Mar 2009 22:27:57 GMT, Owen Duffy wrote:

I will have to revisit the comments in this thread and tie them to the
cogent points of your page.

Constructive comments are always welcome, and appreciated.

Hi Owen,

I can well appreciate the issue of common mode driven by coupling to
the field. The work-arounds I would have expected Roy to have offered
would have been a combination of the TL faculty of NEC for the
differential mode, and an appendix-like wire to support the common
mode contribution. The lack of this discussion where it often appears
in other threads leaves me to wonder if other issues are being
discussed here; hence my problem with topic focus.

As for the modeling of a coaxial transmission line by wires, I have
fairly convinced myself that that approach is thoroughly dead (having
seen no contrary response to my comment about the concept of a Faraday
Shield being unknown to NEC).

By these two, it would seem that modeling coaxial components in NEC is
intractable and claims applied to their use will only be
proven/disproven in the lab or the field.

Proceeding from this last conclusion, I cannot see any purpose to the
comparison of the two colinear representations. You certainly bring
many issues to bear, but except for vague references that are 60 years
old, I don't see any solution to your original questions (which is
where I thought the focus resided).

73's
Richard Clark, KB7QHC

colinear representation in NEC
 

On Tue, 31 Mar 2009 17:08:52 -0500, Cecil Moore
wrote:

OK, Richard, I admit that you caught me asking rhetorical
questions - Congratulations!

Congratulations? In noting the absolute uniform homogeneity for the
technical equivalent of:
"Are we there yet?"

Cheap kudos with the equivalent buying power of shares in Lehman
Brothers.
"Are we solvent yet?"

colinear representation in NEC
 

Jim Kelley wrote:
Cecil Moore wrote:
EZNEC says there is ~3 degrees of phase change in the
current in 90 degrees of monopole. How can that current
be used to measure the delay through 'n' degrees of
monopole?

I have absolutely no idea. Sounds like you've made an error somewhere.

Nope, there's no error. Roy once verified that the total
current in a standing wave antenna, like a dipole, changes
phase very little over the 180 degree length of the 1/2WL
dipole. Yet, he used that same total current with its unchanging
phase to try to measure the delay through a loading coil.

In "Antennas", Kraus' plot of the total current on a dipole,
(Figure 14-2 on page 464 in the 3rd edition) also shows that
same 3 degree phase change in the total current over the 180
degree length of a 1/2WL dipole thus agreeing with EZNEC.

If one cannot detect a phase shift in 25 degrees of 1/4WL
monopole or 1/2WL dipole using the total current, how can
one expect to detect a phase shift in 25 degrees of loading
coil using that same constant phase current?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

colinear representation in NEC
 

Jim Kelley wrote:
According to the plots that I've seen, the standing wave pattern will
show a discontinuous change in amplitude at positions where there is an
abrupt change in phase of the traveling waves.

There's no discontinuity because the 180 degree phase shift
occurs at an amplitude zero crossing, i.e. when the phase
shift occurs, the amplitude is zero. But please note the
phase shift doesn't occur at all on a 1/4WL (or shorter)
monopole.

Since a
standing wave can be considered an amplitude vs phase plot (where both
phase and amplitude vary with position) ...

For the standing wave function, I(x,t)=Io*cos(x)*cos(wt),
the phase at any point x, for a particular (t), doesn't
change phase. Set t=0 and vary x to see what happens.
Only the amplitude changes with x. That's why standing
wave current phase cannot be used to measure the delay
through a loading coil.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

colinear representation in NEC
 

Richard Clark wrote:
On Tue, 31 Mar 2009 22:27:57 GMT, Owen Duffy wrote:


By these two, it would seem that modeling coaxial components in NEC is
intractable and claims applied to their use will only be
proven/disproven in the lab or the field.


Depends on what you want to do with modeling coaxial components. A wire
to represent the (radiating)outside, and an appropriate NT or TL to
represent the (non radiating) inside works fairly well.

If you actually want to model the cable itself (including the fields
inside), I suspect it won't work so well. MoM codes in general often
don't deal with modeling the fields inside closed boxes very well.

I suspect that the cases where it doesn't are basically in the category
of things that MoM codes don't do well with in general, and you need to
go to a different kind of model (FDTD? etc.)

colinear representation in NEC
 

Cecil Moore wrote:
Jim Kelley wrote:

Since a standing wave can be considered an amplitude vs phase plot
(where both phase and amplitude vary with position) ...

For the standing wave function, I(x,t)=Io*cos(x)*cos(wt),
the phase at any point x, for a particular (t), doesn't
change phase. Set t=0 and vary x to see what happens.
Only the amplitude changes with x. That's why standing
wave current phase cannot be used to measure the delay
through a loading coil.

The term x is the phase of the cosine function, Cecil. The phase of the
standing wave function varies by 90 degrees along the length of a 1/4
wave resonant standing wave antenna.

ac6xg

colinear representation in NEC
 

Jim Kelley wrote:
The term x is the phase of the cosine function, Cecil. The phase of the
standing wave function varies by 90 degrees along the length of a 1/4
wave resonant standing wave antenna.

I'm sorry, Jim, that is just not true. The standing-wave
function has a *constant phase* along the length of a 1/4WL
monopole for any fixed (t). Cos(x) is the *envelope amplitude*
function (not phase function) for any fixed (t).

What Gene Fuller said previously is true regarding the
cos(kz)*cos(wt) term in a standing wave:

Gene Fuller, W4SZ wrote:
In a standing wave antenna problem, such as the one you describe,
there is no remaining phase information. Any specific phase
characteristics of the traveling waves died out when the startup
transients died out.

Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be
seen again.

The only "phase" remaining is the cos (kz) term, which is really
an amplitude description, not a phase.

One can ask EZNEC to display the phase of the total current.
When one does that, one will see that the phase is ~constant
for a 1/4WL thin wire monopole over mininec ground. The change
in amplitude is what allows us to calculate the actual delay
through the wire using an ARCCOS function.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

colinear representation in NEC
 

Cecil Moore wrote:
Jim Kelley wrote:
The term x is the phase of the cosine function, Cecil. The phase of
the standing wave function varies by 90 degrees along the length of a
1/4 wave resonant standing wave antenna.

I'm sorry, Jim, that is just not true. The standing-wave
function has a *constant phase* along the length of a 1/4WL
monopole for any fixed (t). Cos(x) is the *envelope amplitude*
function (not phase function) for any fixed (t).

Refer to a table of sines and observe the two things which vary
throughout the period of any sinusoidal wave. One of them is amplitude.
What would you prefer I call the other one?

ac6xg

colinear representation in NEC
 

Jim Kelley wrote:
Refer to a table of sines and observe the two things which vary
throughout the period of any sinusoidal wave. One of them is amplitude.
What would you prefer I call the other one?

Did you not understand what Gene Fuller said?
Io*cos(kx) is the amplitude term. If kx=0 then
the amplitude is Io. If kx=pi/4, the amplitude
is 0.707*Io. If kx=pi/2, the amplitude is zero.
cos(wt) does not vary with (x), only with time.
At any snapshot in time, e.g. t=0, the phase
does not vary at all.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

colinear representation in NEC
 

Cecil Moore wrote:
Jim Kelley wrote:
Refer to a table of sines and observe the two things which vary
throughout the period of any sinusoidal wave. One of them is
amplitude. What would you prefer I call the other one?

Did you not understand what Gene Fuller said?

Somebody once made a claim about answering a question with a question.
He said that it was a means of diversion. Clearly that is the case.

It's no coincidence that the phase of the standing wave varies by 90
degrees along the length of a 90 degree standing wave antenna.

ac6xg