The effect of insulation on small loops
 

In persuit of the BPL issue, I have explored small untuned loops for
field strength measurement.

Comparison of a loop calibrated to an NEC2 model to a commercial loop
with traceable calibration was very encouraging.

Constructors would like to use insulated wire for such a loop, and that
raises the question of the effect of insulation on the Antenna Factor.

My guess is that where the loop is small, current is uniform and the
effect of a slightly different phase shift around the loop due to the
insulation would be very small. That would be less so at frequencies
approaching the first resonance of the loop.

My understanding is that NEC4 might provide a good estimate of the
difference between insulated and uninsulated wire. I don't have NEC4 and
am looking for someone who might be prepared to run a bunch of models for
me in batch mode. I would supply the model decks and a batch file to run
them, my volunteer would need to run the batch file and zip the output
files up and mail them back to me for analysis.

My intention is to produce the same data that is plotted at
http://www.vk1od.net/SmallUntunedSquareLoop/ssulNEC.htm for each loop,
and to plot the dB difference vs frequency to infer some general guidance
on loop sizes where conductor insulation becomes significant to Antenna
Factor.

Comments / volunteers?

Owen

owen at vk1od.net

The effect of insulation on small loops
 

Richard Clark wrote in
:

On Tue, 20 Mar 2007 22:22:40 GMT, Owen Duffy wrote:

My understanding is that NEC4 might provide a good estimate of the
difference between insulated and uninsulated wire. I don't have NEC4
and am looking for someone who might be prepared to run a bunch of
models for me in batch mode. I would supply the model decks and a
batch file to run them, my volunteer would need to run the batch file
and zip the output files up and mail them back to me for analysis.

EZNEC, the free version, supports insulated wire - unless you want to
bury it.


Thanks.

No, I did not want to bury the loop.

My version of EZNEC (a pay for version) appears not to model insulation.

Owen

The effect of insulation on small loops
 

On Tue, 20 Mar 2007 22:22:40 GMT, Owen Duffy wrote:

My understanding is that NEC4 might provide a good estimate of the
difference between insulated and uninsulated wire. I don't have NEC4 and
am looking for someone who might be prepared to run a bunch of models for
me in batch mode. I would supply the model decks and a batch file to run
them, my volunteer would need to run the batch file and zip the output
files up and mail them back to me for analysis.

EZNEC, the free version, supports insulated wire - unless you want to
bury it.

The effect of insulation on small loops
 

Owen Duffy wrote:

My version of EZNEC (a pay for version) appears not to model insulation.

You have v. 3.0. Wire insulation was one of the features added at v. 4.0.

Roy Lewallen, W7EL

The effect of insulation on small loops
 

Roy Lewallen wrote in news:130125qpgle3i23
@corp.supernews.com:

Owen Duffy wrote:

My version of EZNEC (a pay for version) appears not to model insulation.

You have v. 3.0. Wire insulation was one of the features added at v. 4.0.

Yes, that is correct.

Where V4 uses the NEC2 engine, is insulation modelled the same way as in
NEC4? If not, is the accuracy comparable?

Owen

The effect of insulation on small loops
 

The EZNEC insulation modeling method is basically similar to that used
by NEC-4, but different. The accuracy is comparable. Both programs
(EZNEC and NEC-4) are accurate only for thin layers of material with
relatively low conductivity and permittivity, i.e., like typical wire
insulation. But since the typical change in effective wire length is
only on the order of 3% or so, high accuracy isn't required.

I believe I recall your mentioning a multi-turn loop in an earlier
posting. The insulation feature in these programs doesn't model the
effect of insulation between wires, which might be an important factor
for a multi-turn loop. It only models the effect of the insulation on
the wire's self impedance. So use with care.

Roy Lewallen, W7EL

Owen Duffy wrote:
Roy Lewallen wrote in news:130125qpgle3i23
@corp.supernews.com:

Owen Duffy wrote:
My version of EZNEC (a pay for version) appears not to model insulation.
You have v. 3.0. Wire insulation was one of the features added at v. 4.0.

Yes, that is correct.

Where V4 uses the NEC2 engine, is insulation modelled the same way as in
NEC4? If not, is the accuracy comparable?

Owen

The effect of insulation on small loops
 

Roy Lewallen wrote in
:

The EZNEC insulation modeling method is basically similar to that used
by NEC-4, but different. The accuracy is comparable. Both programs
(EZNEC and NEC-4) are accurate only for thin layers of material with
relatively low conductivity and permittivity, i.e., like typical wire
insulation. But since the typical change in effective wire length is
only on the order of 3% or so, high accuracy isn't required.

I believe I recall your mentioning a multi-turn loop in an earlier
posting. The insulation feature in these programs doesn't model the
effect of insulation between wires, which might be an important factor
for a multi-turn loop. It only models the effect of the insulation on
the wire's self impedance. So use with care.


Thanks Roy.

My interest is in small single turn untuned loops loaded with 50 ohms.
IMHO, the accuracy of modelling practical multi turn loops becomes an
issue because of the distributed capacitance and possibly proximity
effect.

A small test was done recently where one of my loop designs with Antenna
Factor predicted by an NEC2 model was compared on BPL emission
measurement over a range of HF frequencies with a calibrated R&S active
loop, and the median of the difference was around 0.5dB from memory,
InterQuartileRange was less than 1dB... so that provides some confidence
that the models are sound.

I think I recall responding to someone else's post about peformance of
multiturn loops.

Owen

The effect of insulation on small loops
 

Roy Lewallen wrote in
:

The EZNEC insulation modeling method is basically similar to that used
by NEC-4, but different. The accuracy is comparable. Both programs
(EZNEC and NEC-4) are accurate only for thin layers of material with
relatively low conductivity and permittivity, i.e., like typical wire
insulation. But since the typical change in effective wire length is
only on the order of 3% or so, high accuracy isn't required.


Roy, I went back and re-read Cebik's notes about the way in which the IS
card is processed vs using and LD card to add inductive loading. He
passes on an explanation:

===quote
In calculating the E-field for each segment in a model NEC calculates a
"cosine" component, a "sine" component, and a "constant" component. NEC
then sums the three fields to arrive at a total field value for each
segment.

LD commands modify the "constant" component of the E-field calculation,
whereas IS commands (in NEC-4) modify the "cosine" component. As a
consequence, any workaround formulation will result in satisfying only a
limited range of geometries.
===equote

It was with this in mind that I chose to head down the NEC4 path.

Owen

The effect of insulation on small loops
 

Owen Duffy wrote in news:Xns98FAA23A9BF6nonenowhere@
61.9.191.5:

Roy Lewallen wrote in
:

The EZNEC insulation modeling method is basically similar to that used
by NEC-4, but different. The accuracy is comparable. Both programs
(EZNEC and NEC-4) are accurate only for thin layers of material with
relatively low conductivity and permittivity, i.e., like typical wire
insulation. But since the typical change in effective wire length is
only on the order of 3% or so, high accuracy isn't required.

I believe I recall your mentioning a multi-turn loop in an earlier
posting. The insulation feature in these programs doesn't model the
effect of insulation between wires, which might be an important factor
for a multi-turn loop. It only models the effect of the insulation on
the wire's self impedance. So use with care.


Thanks Roy.

My interest is in small single turn untuned loops loaded with 50 ohms.
IMHO, the accuracy of modelling practical multi turn loops becomes an
issue because of the distributed capacitance and possibly proximity
effect.

A small test was done recently where one of my loop designs with
Antenna
Factor predicted by an NEC2 model was compared on BPL emission
measurement over a range of HF frequencies with a calibrated R&S active
loop, and the median of the difference was around 0.5dB from memory,
InterQuartileRange was less than 1dB... so that provides some
confidence
that the models are sound.

I think I recall responding to someone else's post about peformance of
multiturn loops.

When I make loops (usually out of LMR400), I use my antenna analyzer to
get them to resonate properly and to match them. I really don't care
what the insulation is doing because the analyzer will tell me anyway.

BTW, a 1-turn LMR400 loop is about as good as an 80m hamstick, but a
whole lot more narrow!


--
Dave Oldridge+
ICQ 1800667

The effect of insulation on small loops
 

Dave Oldridge wrote in
9:

....
When I make loops (usually out of LMR400), I use my antenna analyzer to
get them to resonate properly and to match them. I really don't care
what the insulation is doing because the analyzer will tell me anyway.

BTW, a 1-turn LMR400 loop is about as good as an 80m hamstick, but a
whole lot more narrow!



Thanks, but that approach cannot provide the information that I need.

Owen

The effect of insulation on small loops
 

I'd appreciate and welcome reports from anyone who finds a significant
difference in insulated wire results between EZNEC and NEC-4, in the
region where both should be accurate (typical wire insulation values).

Roy Lewallen, W7EL

Owen Duffy wrote:

Roy, I went back and re-read Cebik's notes about the way in which the IS
card is processed vs using and LD card to add inductive loading. He
passes on an explanation:

===quote
In calculating the E-field for each segment in a model NEC calculates a
"cosine" component, a "sine" component, and a "constant" component. NEC
then sums the three fields to arrive at a total field value for each
segment.

LD commands modify the "constant" component of the E-field calculation,
whereas IS commands (in NEC-4) modify the "cosine" component. As a
consequence, any workaround formulation will result in satisfying only a
limited range of geometries.
===equote

It was with this in mind that I chose to head down the NEC4 path.

Owen

The effect of insulation on small loops
 

Roy Lewallen wrote in
:

I'd appreciate and welcome reports from anyone who finds a significant
difference in insulated wire results between EZNEC and NEC-4, in the
region where both should be accurate (typical wire insulation values).


Roy,

My comment is not intended to impune EZNEC in any way. I don't know how
EZNEC v4 models insulation, and cannot comment on it.

I have read a description on Cebik's page about modelling the effects of
insulation by loading all segments with a constant value of L which
apparently makes a insulated wire half wave dipole model in NEC2
calibrate well with NEC4' IS model. It would seem that the amount of the
inductance was chosen empirically, a curve was fitted to the experimental
model data. It occurs to me that the results depend to an extent on the
current distribution on the conductor, and might be different for a small
loop where the current is almost uniform.

The reason for the study is to be prepared to fend off criticism from
opponents (the BPL industry, the FCC etc) who might criticise performance
modelled on bare wire when everyone wants to make the loops from
insulated wire. I can defend a NEC4 IS model, but if there are questions
about the accuracy of LD loading of a small loop based on empirical data
from half wave dipoles, I am in a weaker position and possibly worse than
NEC2 models of bare wire.

Modelling the insulation is probably splitting hairs. If I address that,
the next question will be over using stranded wire vs single core.

Owen

The effect of insulation on small loops
 

Owen Duffy wrote:
. . .
I have read a description on Cebik's page about modelling the effects of
insulation by loading all segments with a constant value of L which
apparently makes a insulated wire half wave dipole model in NEC2
calibrate well with NEC4' IS model. It would seem that the amount of the
inductance was chosen empirically, a curve was fitted to the experimental
model data. It occurs to me that the results depend to an extent on the
current distribution on the conductor, and might be different for a small
loop where the current is almost uniform.

An underlying assumption in any NEC model is that the current in any
given segment varies only a small amount from one end to the other.
Therefore, any criteria applicable to a loop should also apply to other
geometries. If not, then the other geometry has an insufficient number
of segments. I would, however, use NEC-4 if in your shoes, because the
source code is available for scrutiny. Be careful, though, since there
have been quite a few modifications to NEC-4 since its introduction, and
various compilations can also give slightly different results. None of
the changes have been in the insulated wire calculation that I'm aware
of, however.

The reason for the study is to be prepared to fend off criticism from
opponents (the BPL industry, the FCC etc) who might criticise performance
modelled on bare wire when everyone wants to make the loops from
insulated wire. I can defend a NEC4 IS model, but if there are questions
about the accuracy of LD loading of a small loop based on empirical data
from half wave dipoles, I am in a weaker position and possibly worse than
NEC2 models of bare wire.

Modelling the insulation is probably splitting hairs. If I address that,
the next question will be over using stranded wire vs single core.

It is indeed splitting hairs. The effect will be so small it will be
hard to see in a model (except possibly in the resonance of a very high
Q loop) and probably impossible to discern by measurement of a real
loop. But then your modeling and measurements will show you that.

Stranded wire might noticeably reduce the Q of a very high Q loop, but I
couldn't be sure. Its effect can be seen in coaxial cable, but the field
distribution around the center conductor is very different than for an
antenna wire.

Roy Lewallen, W7EL

The effect of insulation on small loops
 

Roy Lewallen wrote in
:

....

Thanks for the thoughts Roy, appreciated.

Owen