a) | b) |
| |
| |
| |
| |
| |
| |
| |
| |
-------| |
-------| |||
| |||
| |||
| |||
| |||
| |||
| ---
S S
--------------- -----------------

Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.

The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.

Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.

My questions a

1. To what extent is b) equivalent to a)?

2. How is b) modelled in NEC?

Thanks
Owen

Owen Duffy wrote:
a) | b) |
| |
| |
| |
| |
| |
| |
| |
| |
-------| |
-------| |||
| |||
| |||
| |||
| |||
| |||
| ---
S S
--------------- -----------------

Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.

The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.

Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.

My questions a

1. To what extent is b) equivalent to a)?

I can't answer that question right off, except that at first glance they
look quite similar in operation. I'd build both models with EZNEC, then
take a look at the reported currents in the View Antenna display. You
can get the same information from tabular NEC results, but most people
find the graphical display quicker and easier to interpret.

You can see the significance of the seemingly small common mode current
on the a) model stub by replacing it with a transmission line model stub
which of course has zero common mode current. The results are quite
different than for the wire model stub.

2. How is b) modelled in NEC?

A coaxial line can be modeled as a combination of a transmission line
(for the inside of the coax) and a wire (for the outside of the coax).
Download the EZNEC demo program and look in the manual index under
Coaxial Cable, Modeling. It'll direct you to one of the furnished
example files which illustrates how. Then you can do the same thing with
NEC if you're so inclined.

Roy Lewallen, W7EL

On Sat, 14 Mar 2009 21:33:15 GMT, Owen Duffy wrote:

My questions a

1. To what extent is b) equivalent to a)?

Hi Owen,

To no extent as far as I can tell without modeling. I don't think the
phases are going to equivalent.

2. How is b) modelled in NEC?

This takes some presuming of your intent, and my presumption, given
the symmetry of the two smaller elements (why two otherwise?), is you
are attempting to portray a skeletal sleeve with them. Two is
insufficient by my standards, six are barely worth chasing the numbers
and I typically use 16. One such example, complete with a link to the
design can be found at:
http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm

It deviates only by a small degree, but could prove a useful boost in
adding the longer element after opening the top end of the thick
radiator.

73's
Richard Clark, KB7QHC

Roy Lewallen wrote in
treetonline:

Hello Roy,

Thanks for the response.


....
My questions a

1. To what extent is b) equivalent to a)?

I can't answer that question right off, except that at first glance
they look quite similar in operation. I'd build both models with
EZNEC, then take a look at the reported currents in the View Antenna
display. You can get the same information from tabular NEC results,
but most people find the graphical display quicker and easier to
interpret.

Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are the
same diameter, the same wire in this model.

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

I have a 3/4 wave vertical over perfect ground, and I have inserted a
quarter wave s/c transmission line into the vertical at 1/3 height. I
have shunted the TL with 10k ohm to represent some loss in the stub.

The currents report shows the currents in the top half wave to be
approximately 180° out of phase with the bottom quarter wave.

The question is whether such a construction yields three quarter waves in
phase, or whether the NEC model is correct that they are not in phase.


You can see the significance of the seemingly small common mode
current on the a) model stub by replacing it with a transmission line
model stub which of course has zero common mode current. The results
are quite different than for the wire model stub.


My initial feeling is that the wire model of a) is correct. I have not
yet done as you suggest in the previous par.

2. How is b) modelled in NEC?

A coaxial line can be modeled as a combination of a transmission line
(for the inside of the coax) and a wire (for the outside of the coax).
Download the EZNEC demo program and look in the manual index under
Coaxial Cable, Modeling. It'll direct you to one of the furnished
example files which illustrates how. Then you can do the same thing
with NEC if you're so inclined.

Is my model above what you suggest?

Appreciate your comments Roy, thanks.

Owen

Hello Richard,

Richard Clark wrote in
:

On Sat, 14 Mar 2009 21:33:15 GMT, Owen Duffy wrote:

My questions a

1. To what extent is b) equivalent to a)?

Hi Owen,

To no extent as far as I can tell without modeling. I don't think the
phases are going to equivalent.

Ok. Your view is contrary to common explanation... but of course that
doesn't make it wrong.

NEC models of the wire construction at a) show in phase operation, but a
small distortion of the pattern due to common mode current on the stub...
so they support the common explanation in the phase aspect, but not in
respect of the stub causing phase change with no other effects.

The explanation of b) as a) where the stub is relocated coaxially sounds
appealing, but that explanation might be wrong.


2. How is b) modelled in NEC?

This takes some presuming of your intent, and my presumption, given
the symmetry of the two smaller elements (why two otherwise?), is you
are attempting to portray a skeletal sleeve with them. Two is

You seem to have mininterpreted my ASCII art, and that would be easy to
do. I am describing at b), two coaxial tubes, the lowest tube is 1/4
wave, the longer tube is 3/4 wave. The lower tube ends are connected, and
fed between ground and the bottom of the tube assembly.

See the model that I have posted in response to Roy.

Thanks.

Owen

On Mar 14, 2:33*pm, Owen Duffy wrote:
* *a) * * * * | * * * * * * * * * * * *b) * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * -------| * * * * * * * * * * * * * * * |
* * * * * * * -------| * * * * * * * * * * * * * * *|||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * ---
* * * * * * * S * * * * * * * * * * * * * * * * * * *S
* * * --------------- * * * * * * * * * * * *-----------------

Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.

The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.

Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.

My questions a

1. To what extent is b) equivalent to a)?

2. How is b) modelled in NEC?

Thanks
Owen

Hi Owen,

I suppose that R.W.P. King disagrees with the "common explanation."
He makes it quite clear that there is interaction of the antenna field
with the stub perpendicular to the axis of the antenna wire, and that
the coaxial stub does not interact in the same way and the antenna
performance is therefore different. (Antennas chapter of Transmission
Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
like using a feedline to guarantee the phasing. It can be done by
driving collinear dipoles with equal lengths of transmission line, or
by using an arrangement like the "coaxial collinear," where the
radiating elements are outer conductors of coaxial transmission lines
used to insure that the multiple feedpoints are at least fed in-phase
voltages (and you have to consider that the currents are not exactly
in phase).

Cheers,
Tom

Hi Tom,

K7ITM wrote in
:

....
I suppose that R.W.P. King disagrees with the "common explanation."
He makes it quite clear that there is interaction of the antenna field
with the stub perpendicular to the axis of the antenna wire, and that
the coaxial stub does not interact in the same way and the antenna
performance is therefore different. (Antennas chapter of Transmission
Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
like using a feedline to guarantee the phasing. It can be done by
driving collinear dipoles with equal lengths of transmission line, or
by using an arrangement like the "coaxial collinear," where the
radiating elements are outer conductors of coaxial transmission lines
used to insure that the multiple feedpoints are at least fed in-phase
voltages (and you have to consider that the currents are not exactly
in phase).

That it interesting that Prof King declares that there is more than just
a transmission line action with the external style of stub.

An NEC model of a) works well, showing in phase operation and a nice
pattern. I have played around with two stubs of shorter length on
opposite sides of the vertical and stacked on top of each other, and they
worked fine (ie in phase current distribution with zero near the stubs)
at about 0.15+ wavelenths each... which doesn't fit with a propagation
delay around the conductor path explanation. Interesting!

I am trying to support the common explanation of the coaxial colinear in
my diagram b) using NEC, but I haven't yet been sucessful.

Owen

Owen Duffy wrote:

Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are the
same diameter, the same wire in this model.

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

. . .

Is my model above what you suggest?

No. But I did take the time to see what would be necessary to actually
model it. And what I ended up with is identical to a) except that the
wire stub is replaced by the shorted transmission line model, and the
lower wire has become the outside of the coaxial structure so is
increased in diameter. So those are the two differences between a) and
b). As Tom mentioned and I alluded to, there's some interaction between
the wire stub and the antenna which doesn't exist between the ideal
transmission line and the antenna, so performance is different.

You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one
transmission line conductor always equals the current out of the other,
so if the two are shorted, no more current can go into or out of the
shorted end. Therefore, any external connection to it looks like an open
circuit since no current will flow through the external connection.

What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I have
defines only types 1 - 5.

Roy Lewallen, W7EL

Hi Owen,

I suppose that R.W.P. King disagrees with the "common explanation."
He makes it quite clear that there is interaction of the antenna field
with the stub perpendicular to the axis of the antenna wire, and that
the coaxial stub does not interact in the same way and the antenna
performance is therefore different. (Antennas chapter of Transmission
Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
like using a feedline to guarantee the phasing. It can be done by
driving collinear dipoles with equal lengths of transmission line, or
by using an arrangement like the "coaxial collinear," where the
radiating elements are outer conductors of coaxial transmission lines
used to insure that the multiple feedpoints are at least fed in-phase
voltages (and you have to consider that the currents are not exactly
in phase).

Cheers,
Tom

In most phased arrays, the objective is to get the fields from the
elements to be in some particular ratio. Driving them with currents in
that same ratio doesn't always accomplish the desired field ratio,
though, when elements have different current distributions as they often
do. (See http://eznec.com/Amateur/Articles/Current_Dist.pdf.) The
difference between field ratio and feedpoint current ratio is
particularly great when base feeding half wave elements. As it turns
out, you'll often get better field ratios by feeding with voltages
having the desired magnitude ratio and phase difference than feeding
with properly ratioed currents, when dealing with end fed half wave
elements. The coaxial collinear requires a pretty delicate balance of
outer and inner velocity factors as well as the effects of mutual
coupling, particularly when there are more than a couple of elements. So
I suspect that the current distribution can either help or hinder
depending on how the factors are traded off. I wouldn't be surprised,
though, if ratioing the voltages rather than currents is actually helpful.

As an illustration, open the EZNEC example file Cardioid.EZ. Change the
number of segments to 10 per wire for better accuracy. (It can still be
run with the demo program.) Click FF Plot and note the nice cardioid
pattern. Then change the Z coordinates of End 2 of the two wires to 0.47
m to make them nearly anti-resonant, and click FF Plot again. The
pattern deterioration is due to the elements having different current
distributions. Finally, change the source types from I to V. This will
force the voltages, rather than currents, at the antenna bases to be in
the desired ratio. Run FF Plot again. You still won't have the nice
cardioid back, but it's quite an improvement over the pattern with
"correctly" ratioed base currents. The bottom line is that the element
currents are more closely related to the base voltages than the base
currents, when the elements are near anti-resonance (parallel, or half
wave, resonance).

Roy Lewallen, W7EL

Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:

Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are
the same diameter, the same wire in this model.

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99
0.0001 FR 0 0 0 0 15 0
EN

. . .

Is my model above what you suggest?

No. But I did take the time to see what would be necessary to actually
model it. And what I ended up with is identical to a) except that the
wire stub is replaced by the shorted transmission line model, and the
lower wire has become the outside of the coaxial structure so is
increased in diameter. So those are the two differences between a) and
b). As Tom mentioned and I alluded to, there's some interaction

I think that is what I had done, but I used the same diameter top to
bottom.

Here is a revised deck with different diameters:

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 15 0 0 0 0 0 5 0.015
GW 2 30 0 0 5 0 0 15 0.005
GE 1
GN 1
EK
EX 0 1 1 1 0
TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

In the above, the lower conductor is three times the diameter of the
upper conductor. The TL is wired into the lowest segment of the upper
conductor. Again, I have shunted the TL with 10k R to represent loss in a
real TL.

This model does not show in phase currents in upper and lower parts of
the vertical.

between the wire stub and the antenna which doesn't exist between the
ideal transmission line and the antenna, so performance is different.

You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one

I don't think I did that.

transmission line conductor always equals the current out of the
other, so if the two are shorted, no more current can go into or out
of the shorted end. Therefore, any external connection to it looks
like an open circuit since no current will flow through the external
connection.

What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I
have defines only types 1 - 5.

I have been playing with this in EZNEC and 4NEC2. The deck I offered was
from 4NEC2 as my EZNEC files are binaries and couldn't go inline. The EX
6 is an extension for a current source. It is immaterial in this case,
and the 6 can be changed to a 0.

Thanks.
Owen