Hi Roy,

Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:
Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:
...
Is NEC capable of modelling the configuration shown at
http://www.vk1od.net/lost/King-22.3b.png (which is the same type
of problem as my figure b)?

A point made by King is that if the three half waves are in phase,
radiation resistance will be quite high (one third current required
for same distant field strength), around 316 ohms against 105 ohms
for three half waves not-in-phase. Presumably these figures are for
free space. . . .

I looked up the section in King, Mimno, and Wing and was pretty
disappointed. It's one of my favorite references, and I usually find
the explanations clear. But the description of that antenna is pretty
vague, with considerable hand waving ("[Operation of coaxial stubs] is
much less satisfactory than that with the open-wire stubs. . ."
without explaining why). And in the explanation of the open-wire

This lesser mortal was encouraged that he noted the difference, but there
really was no explanation. My feeling is that to note the difference but
to be unable to explain it, other than nebuluous conditions like the
coaxial tubes must be large diameter ratio, is incomplete... a problem
yet to be solved.

I have come to the conclusion that the coaxial tubes are not simply a
relocation of a TL as popularly explained. Over the years, I have
accumulated a few projects that were works of art, but didn't work
properly... and they all used coaxial phasing sections.

stubs, the authors seem to state that the wires must carry purely
differential currents. And their models (Fig. 22-4) do show purely
differential coupling from the antenna to the stubs.

I speculate that they really didn't understand how these antennas
worked, had discovered that the coaxial sleeve versions didn't work or
at least didn't work as well -- and didn't show the proper impedance
--, but didn't fully understand why. King, in particular, was and is
one of the giants of antenna theory, and leaves us a lifetime of
brilliant insight and rigorous mathematical analysis. But at least at
the time that book was published, they lacked the modeling tools we
have today.

Understood... but, I think after our discussion on this, NEC is not up to
the task, it may take a more advanced EM field modelling tool.

My suspicion is that NEC's shortfall is that a TL element does not
properly represent the coaxial stub and its interaction with the other
elements near resonance, though well away from resonance, it is possible
that it may be quite ok. King raises the issues of diameter ratios, and
the difference with whether the stub is inboard or outboard of the o/c
end... but it is not resolved quantitatively.


This effect is certainly observable in models using my Fig a) (though
half the respective resistances due to the vertical over perfect
ground).

The feedpoint impedance looks like it might provide a hint as to
whether currents are actually in-phase.

It surely does. Given the currents on and locations of the end wires,
the modification to the center wire can be calculated from mutual
coupling considerations. And I think this is a clue that led King,
Mimno, and Wing to conclude that something was amiss with the coaxial
version.

Exploring that thought, an example (to some extent) of King's Fig
22.3b is the W5GI Mystery Antenna (see
http://www.w5gi.com/images/w5gimster...aschematic.gif ) which
claims to be three half waves in phase at 14.2MHz. It is very similar
to the diagram above in King though I note that the phasing sections
are 105° in electrical length.

The W5GI is fed with a half wave (at 14.2MHz) of 300 ohm line, then
34' of RG8X. W5GI reports impedance looking into the RG8X as
42+/-j18. That suggests the load on the RG8X is 31+j2 or 70-j18. The
feedpoint impedance should be about the same value due to the half
wave of 300 ohm low loss line. Neither impedance is within a bull's
roar of 316+j0, and are so low as to question whether the three half
waves are indeed in-phase. (The highest impedance that would yeild
42+/-j18 on a short length of RG8X would be around 80+j0, closer to
the not-in-phase configuration than the in-phase configuration).

W5GI's reported feed impedance seem inconsistent with three half
waves in phase, and questions whether the phasing arrangement works
as suggested.

Thoughts?

I doubt that it does.

Now W5GI does introduce his antenna with the statement "A multi-band wire
antenna that performs exceptionally well even though it confounds antenna
modeling software".

I know that is almost always a harbinger of bunk, the proverbial "Danger
Will Robinson...", but in fairness, it does appear that one modelling
package, NEC, cannot adequately model the coaxial arrangement near
resonance, though in his antenna, the coax section would be resonant
around 12MHz and King suggests it ought to be much shorter (resonant well
above 14MHz).

That is not to say there aren't other BS warnings in the W5GI explanation
of operation, or claims of performance.

Thanks for your comments, I find this an interesting subject.

Owen

Owen Duffy wrote:
. . .
Understood... but, I think after our discussion on this, NEC is not up to
the task, it may take a more advanced EM field modelling tool.

I don't agree with this.

My suspicion is that NEC's shortfall is that a TL element does not
properly represent the coaxial stub and its interaction with the other
elements near resonance, though well away from resonance, it is possible
that it may be quite ok. King raises the issues of diameter ratios, and
the difference with whether the stub is inboard or outboard of the o/c
end... but it is not resolved quantitatively.

I believe that NEC can do a fine job of modeling any of the variations
we've been discussing. But like all modeling systems, it has to be used
properly -- the transmission line object isn't an adequate model for
either a coaxial structure or an open wire stub, if either is carrying
any common mode current. And in these antennas it is, so you can't
insist on using nothing more than a transmission line object and then
bemoaning that the result isn't correct. The wire stub variation can be
correctly modeled as wires. The coaxial structure can be correctly
modeled as a combination of a wire and transmission line object. In
either case I have high confidence that carefully and accurately
measured results will agree closely with NEC predictions.

Now W5GI does introduce his antenna with the statement "A multi-band wire
antenna that performs exceptionally well even though it confounds antenna
modeling software".

I know that is almost always a harbinger of bunk, the proverbial "Danger
Will Robinson...", but in fairness, it does appear that one modelling
package, NEC, cannot adequately model the coaxial arrangement near
resonance, though in his antenna, the coax section would be resonant
around 12MHz and King suggests it ought to be much shorter (resonant well
above 14MHz).

It doesn't appear this way to me at all. What has led you to the
conclusion that it isn't possible to accurately model it with NEC?
Again, it's certainly impossible if you use only a transmission line
object to represent a structure which has common mode current. There are
many ways to build a model which doesn't accurately represent the
antenna being modeled. But just because it's possible to make a bad
model doesn't mean it's impossible to make a good one.

What is the evidence that results from a properly designed NEC model
disagree with careful measurements of pattern, current, or impedance of
an actual antenna of these types? You've noted that the W5GI antenna
impedance isn't consistent with a correctly phased collinear. I'd be
surprised if the impedance isn't close to what a correct NEC model
predicts -- or that the phases of the currents aren't also what NEC
predicts.

That is not to say there aren't other BS warnings in the W5GI explanation
of operation, or claims of performance.

Thanks for your comments, I find this an interesting subject.

Me too, and thanks for bringing it up. I'd never taken a really close
look at this class of antenna before, and the results have been interesting.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

Owen Duffy wrote:
....
I believe that NEC can do a fine job of modeling any of the variations
we've been discussing. But like all modeling systems, it has to be
used properly -- the transmission line object isn't an adequate model
for either a coaxial structure or an open wire stub, if either is
carrying any common mode current. And in these antennas it is, so you
can't insist on using nothing more than a transmission line object and
then bemoaning that the result isn't correct. The wire stub variation
can be correctly modeled as wires. The coaxial structure can be
correctly modeled as a combination of a wire and transmission line
object. In either case I have high confidence that carefully and
accurately measured results will agree closely with NEC predictions.

Taking the W5GI as an example, here is a deck that models the coaxial
stub section as a conductor of 5mm dia, whilst the wires for the other
sections are 2mm diameter. I have calculated the impedance looking into
16.5' of RG8X (W5GI's specified stub) as 14.5-j179 at 14.2MHz, and
inserted that load in both of the segments where the o/c end of the stub
is located. I have not used a TL element, rather I have separately
calculated the input Z of the stub using the technique used at
http://www.vk1od.net/calc/tl/tllc.php , that should be more accurate than
using a lossless TL element.

The model assumes an effective balun, ie that there is no common mode
feedline current since I have not provided such a path.

CM W5GI Mystery Antenna
CM Extended thin wire kernel used
CM
CE
GW 1 31 -5.033 0.000 10.563 5.033 0.000 10.563 0.001000
GW 2 15 -10.067 0.000 10.563 -5.033 0.000 10.563 0.002500
GW 3 15 -15.100 0.000 10.563 -10.067 0.000 10.563 0.001000
GW 4 15 5.033 0.000 10.563 10.067 0.000 10.563 0.002500
GW 5 15 10.067 0.000 10.563 15.100 0.000 10.563 0.001000
GE
EK
FR 0,1,0,0,14.200
EX 0 1 16 0 1 0
LD 5 0 0 0 5.7E7
LD 4 1 1 1 14.505 -191.739 0
LD 4 1 31 31 14.505 -191.739 0
GN 2 0 0 0 13 0.005
XQ
EN

This model indicates out of phase operation of the antenna, a multi lobed
pattern and feedpoint Z of 115-j179. (Although there is a half wave of
300 ohm line in between, this feedpoint Z would cause VSWR=8 on the 50
ohms line.

I think that I have dealt with the common mode path properly.

Try as I might changing stub lengths etc, I cannot get this configuration
to deliver in-phase operation of the radiator.

I suspect the model is not valid.

Owen

I'm very naive in these matters. Could a coaxial stub be modeled as a
cage of wires around the center conductor? Would the orders of
magnitude difference between shield/center distance and wire lengths
cause problems?

73
Jon LA4RT, Trondheim, Norway

Roy,

I have spent a lot of time exploring different modelling options over
recent weeks.

One view that one might take re my fig a) is that at connection of the
stub with the main vertical, the stub offers low impedance to common mode
current and high impedance to differential current. It leads to thinking
of it as a kind of mode trap that guides the system into in-phase
operation.

I have played around with ways of trying to represent that without using
the wire segments of the stub.

One method was to place a transformer with only one centre tapped
winding. The top and bottom of the winding connect to the upper half wave
and the lower quarter wave respectively, and the centre tap connects to a
horizontal quarter wave. My thinking was that this structure provides low
impedance to common mode current on the horizontal section, but a high
impedance to differential input to the top and bottom of the transformer
winding.

The model achieves reasonably good in-phase operation, but works best
with about 0.35 wave horizontal. I have used an NT card to insert the
transformer windings in the two segments. Here is the deck.

CM
CE
GW 1 15 0 0 0 0 0 5 0.005
GW 2 15 0 0 5 0 0 15 0.005
GW 3 15 0 0 5 7.2 0 5 0.005
GE 1
NT 1 15 2 1 0 0.01 0 -0.01 0 0.01
GN 1
EK
EX 0 1 1 1 0
TL 1 15 2 1 100 0
FR 0 0 0 0 15 0
EN

I then tried changing the horizontal section to two opposed radial wires,
and found that worked well with each radial being about 0.2 wave long.

CM
CE
GW 1 15 0 0 0 0 0 5 0.005
GW 2 15 0 0 5 0 0 15 0.005
GW 3 15 0 0 5 4 0 5 0.005
GW 4 15 0 0 5 -4 0 5 0.005
GE 1
NT 1 15 2 1 0 0.01 0 -0.01 0 0.01
GN 1
EK
EX 6 1 1 1 0
FR 0 0 0 0 15 0
EN

One can achieve similar outcome by wiring an appropriately phased zero
length TL between the segments each side of the horizontal wire.

If these models indicate that the common mode path on the horizontal wire
is important, one loses control of the length of that in the case of the
coaxial configuration because there isn't an o/c end indpendent of the
vertical conductor.

The coaxial construction gives the opportunity to create a high impedance
to differential current between the adjacent segments, but lacks the
ability to create a low impedance common mode path independently of the
vertical structure.

Thoughts?

Owen

Cecil Moore wrote:
Jim Kelley wrote:
The only current flowing on an antenna is the current traveling from
one end to the other.

Let's assume you are correct. Here are a few questions:

1. Given a 90 degree monopole fed against an infinite
ground plane, what would be the phase at the top of the
antenna compared to the phase at the feedpoint for any
instant in time?

2. Why would the feedpoint impedance of a 1/4WL monopole
be more than a magnitude less than the feedpoint impedance
of an infinite monopole?

3. Where does the above current go when it hits the open-
circuit at the top of the monopole?

4. Why is the total energy in the E-field at the top of the
monopole so high?

In what way are any of the questions relevant to, or deterministic of
the assumption?

73, ac6xg

Jim Kelley wrote:
In what way are any of the questions relevant to, or deterministic of
the assumption?

Answering a question with a question is a well known
diversion. Please answer my questions and you will
automatically answer yours.

Here's some mo How can a current that changes
phase by 3 degrees in 90 degrees of wire be used
to measure the EM wave delay through the wire?

How can that current be used to measure the delay
through a coil positioned in the middle of that wire?

How fast does EM wave energy travel through a wire?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand

Richard Clark wrote:
Returning to the process - through sub-optimization by adding
bafflegab, ...

As far as bafflegab goes, Richard, no one can hold a candle
to you. Your posting is a perfect example.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand

On Thu, 26 Mar 2009 08:20:07 -0800, Jim Kelley
wrote:

Cecil Moore wrote:
Jim Kelley wrote:
The only current flowing on an antenna is the current traveling from
one end to the other.

Let's assume you are correct. Here are a few questions:

1. Given a 90 degree monopole fed against an infinite
ground plane, what would be the phase at the top of the
antenna compared to the phase at the feedpoint for any
instant in time?

2. Why would the feedpoint impedance of a 1/4WL monopole
be more than a magnitude less than the feedpoint impedance
of an infinite monopole?

3. Where does the above current go when it hits the open-
circuit at the top of the monopole?

4. Why is the total energy in the E-field at the top of the
monopole so high?

In what way are any of the questions relevant to, or deterministic of
the assumption?

Ah Jim!

You have the essence of Cecil's (r) Sub-optimal Conjugated Hypothesis
Information Transform before you, the SCHIT (c) model.

He has taken the ordinary postulate of current flow, conjugated it
into a new hypothesis through his sub-optimization. By removing
random bytes, it becomes more intelligible (I will take a stab at it
here):
1. a 90 degree monopole fed against an infinite ground plane
2. the feedpoint impedance of a 1/4WL monopole
3. current go[es]
4. the total energy
now makes perfect sense and whitens your teeth at the same time.

Returning to the process - through sub-optimization by adding
bafflegab, the future deconstruction (posts that would follow the one
above and for which I have already deconvoluted) would find Cecil
eventually unwinding the original conjugation, proving he was right by
proving you right - except you were wrong in what you "thought" (the
information transform) because he thought you were wrong.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
You must be flattered (an example of information transformation) at
this imitation of you then (your comment here so unabashedly basking
in the intended conjugate of these congratulations).

Just send me some of what you are smoking and I will die happy. :-)
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
"Government 'help' to business is just as disastrous as government
persecution..." Ayn Rand