antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

All,

This is an attempt to add loaded radials to a loaded vertical. The plan is to
make a large efficient loading coil for the radials and offset the variable coil
in a 'screwdriver' vertical segment.

The antenna is about 4 meters overall with about an load at about 1.5 meters
from the base. The antenna is raised about a meter.

The loaded radials are two 1 inch copper tubes about 1.67 meters long. The
radial loading coil is connected from the coax shield to the radial coil.

The feed is 50 Ohm coax, the shield is connected to the loading coil and the
center to the base.

4nec2 predicts an impedance of between 10 and 20 ohms depending of the assumed
values of R in the impedance loads. The actual antenna measures 36 Ohms at the
feed.

The question is: What is the basis of the difference between predicted and
measured values?

Below is a representation of the nec code used to simulate the antenna.

Thanks - Dan

CM 75 m Vertical 12 ft high
CM base 3 ft up - two radials
CM copper conductivity
CE
GW 1 11 0 0 1 0 0 5 0.025
GW 2 7 0 0 1 0 1.67 1 0.025
GW 3 7 0 0 1 0 -1.67 1 0.025
GE 0
LD 4 1 1 1 5 1500
LD 4 1 6 6 8 600
EX 0 1 2 0 1 0
GN 2 0 0 0 13 5.e-3
FR 0 1 0 0 3.74 0
EN

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"dansawyeror" wrote in message
...
All,

This is an attempt to add loaded radials to a loaded vertical. The plan is
to make a large efficient loading coil for the radials and offset the
variable coil in a 'screwdriver' vertical segment.

The antenna is about 4 meters overall with about an load at about 1.5
meters from the base. The antenna is raised about a meter.

The loaded radials are two 1 inch copper tubes about 1.67 meters long. The
radial loading coil is connected from the coax shield to the radial coil.

The feed is 50 Ohm coax, the shield is connected to the loading coil and
the center to the base.

4nec2 predicts an impedance of between 10 and 20 ohms depending of the
assumed values of R in the impedance loads. The actual antenna measures 36
Ohms at the feed.

The question is: What is the basis of the difference between predicted and
measured values?

Below is a representation of the nec code used to simulate the antenna.

Thanks - Dan

CM 75 m Vertical 12 ft high
CM base 3 ft up - two radials
CM copper conductivity
CE
GW 1 11 0 0 1 0 0 5
0.025
GW 2 7 0 0 1 0 1.67 1
0.025
GW 3 7 0 0 1 0 -1.67 1
0.025
GE 0
LD 4 1 1 1 5 1500
LD 4 1 6 6 8 600
EX 0 1 2 0 1 0
GN 2 0 0 0 13 5.e-3
FR 0 1 0 0 3.74 0
EN

Interesting Dan, I get the same results as you, using your code. At 3.74
MHz the input z is 16.8 +j133. The antenna is resonant at about 3.55 MHz.
With your average ground the gain is about -9 dBi.

The only question I have is how certain are you of the accuracy of your test
equipment? Did you use a 1:1 balun at the feed point? You may be getting a
large current on the outside of the coax.

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
. . .
The question is: What is the basis of the difference between predicted
and measured values?

1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
4nec2 predicts an impedance of between 10 and 20 ohms depending of the
assumed values of R in the impedance loads. The actual antenna measures
36 Ohms at the feed.

The question is: What is the basis of the difference between predicted
and measured values?

Does 4nec2 include the ground losses in the feedpoint impedance?
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"Cecil Moore" wrote in message
t...
dansawyeror wrote:
4nec2 predicts an impedance of between 10 and 20 ohms depending of the
assumed values of R in the impedance loads. The actual antenna measures
36 Ohms at the feed.

The question is: What is the basis of the difference between predicted
and measured values?

Does 4nec2 include the ground losses in the feedpoint impedance?
--
73, Cecil http://www.qsl.net/w5dxp

4nec2 does have Sommerfeld/Norton. Incidentally my NEC2 gives 15.7 + j
110.6. The 15.8 + j133 comes from NEC 4.1.

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy,

No I have not decoupled the feed from the antenna. I will try that tonight. I
have wound a 2:1 balun for testing. (anticipating at least a 25 ohm input
impedance)

I have measured the antenna with two different instruments. One is an Autek
analyzer at the antenna, the second is with an 8405a at the end of 100+ feed of
cable. They both show the same results.

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

. . .
The question is: What is the basis of the difference between predicted
and measured values?


1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"dansawyeror" wrote in message
...
Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a 25
ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan

Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

The first thing I'd try, then, would be to put a 1:1 balun (common mode
choke) at the feedpoint and, when using feedline, a second one about a
quarter wave down the line. The balun can be constructed by winding the
coax (if you're using RG-58 or smaller) 8 - 10 turns on a type 43
ferrite core. Or you can clamp or thread a few large cores over the
coax. You can use the Autek to measure the core impedance -- shoot for
500 - 1000 ohms total -- the angle of the impedance isn't important.
Unlike the 2:1 balun, this won't disturb your basic measurement in any
way, it'll just reduce any common mode current.

If you still get the same result, then there are only two other possible
causes I can think of. One is the modeling of the inductors. I've found
that a lumped model of an inductor isn't good if there's any appreciable
current change in the real inductor from one end to the other due to its
physical length. The solution is to model the inductor as a helix.
You'll have to add some extra R to the model, however, if the turns are
spaced closer than a couple of wire diameters, since the program doesn't
account for proximity effect.

The other possible cause is that there's some source of loss you're not
accounting for in your model. The inductors and coupling to nearby lossy
objects are the most obvious candidates.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a
25 ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

. . .
The question is: What is the basis of the difference between
predicted and measured values?


1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank's wrote:

Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

Hm, if the Autek measures only the magnitude of the impedance, how does
Dan know the resistance? The model shows about 133 ohms of reactance,
which is much greater than the resistance.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
All,

This is an attempt to add loaded radials to a loaded vertical. The plan
is to make a large efficient loading coil for the radials and offset the
variable coil in a 'screwdriver' vertical segment.

The antenna is about 4 meters overall with about an load at about 1.5
meters from the base. The antenna is raised about a meter.

The loaded radials are two 1 inch copper tubes about 1.67 meters long.
The radial loading coil is connected from the coax shield to the radial
coil.

The feed is 50 Ohm coax, the shield is connected to the loading coil and
the center to the base.

4nec2 predicts an impedance of between 10 and 20 ohms depending of the
assumed values of R in the impedance loads. The actual antenna measures
36 Ohms at the feed.

The question is: What is the basis of the difference between predicted
and measured values?

Below is a representation of the nec code used to simulate the antenna.

Thanks - Dan

CM 75 m Vertical 12 ft high
CM base 3 ft up - two radials
CM copper conductivity
CE
GW 1 11 0 0 1 0 0 5
0.025
GW 2 7 0 0 1 0 1.67 1
0.025
GW 3 7 0 0 1 0 -1.67 1
0.025
GE 0
LD 4 1 1 1 5 1500
LD 4 1 6 6 8 600
EX 0 1 2 0 1 0
GN 2 0 0 0 13 5.e-3
FR 0 1 0 0 3.74 0
EN


Dan;

I can't answer you question except to note that this is why they call
antenna design an ART not a SCIENCE. Other comm enters have good
suggestions. Let us know what happens.

Dave WD9BDZ

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Are you aware that a coax cable will change the impedance
from the antenna feedpoint impedance in a spiral to 50
ohms in the limit?
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy,

Several weeks back, and confirmed by frequency sweep model runs, you indicated
that minimum impedance is close or equal to the resonance point. I tuned the
antenna to the frequency of interest and then used the Autek to verify the
resonance point. That minimum value was 36 Ohms. I am assuming this is or is
very close to the resonance point for the antenna system.

What does your running of the model show for resonance frequency? At resonance
my running of the model shows close to 20 Ohms for the relatively large values
of R used in the model.

Thanks - Dan

Roy Lewallen wrote:
Frank's wrote:


Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible
to find any accuracy specifications. The 8405A is an excellent
instrument, but assume you calibrated it -- short/open/load -- at the
end of the 100 ft cable. This calibration should also be carried out
on the antenna side of your isolation transformer when you install
it. Curious as to what kind of directional coupler you are using for
HF. I remember using a small HP coupler for HF, but cannot remember
its model number.

Frank


Hm, if the Autek measures only the magnitude of the impedance, how does
Dan know the resistance? The model shows about 133 ohms of reactance,
which is much greater than the resistance.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm loads.
For these two values it is very close. (It is battery level sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan

Frank's wrote:
"dansawyeror" wrote in message
...

Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a 25
ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan


Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"dansawyeror" wrote in message
...
Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm
loads. For these two values it is very close. (It is battery level
sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan

Thanks Dan, I had forgotten about Mini-Circuits. Their price is hard to
beat. I may pick up one of the "PDC" series dual directional couplers.
Incidentally your code indicates resonance occurs at 3.54 MHz. I wonder how
the Autek behaves when subjected to a reactive load does it actually get
close to the magnitude?

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
Roy,

Several weeks back, and confirmed by frequency sweep model runs, you
indicated that minimum impedance is close or equal to the resonance
point. I tuned the antenna to the frequency of interest and then used
the Autek to verify the resonance point. That minimum value was 36 Ohms.
I am assuming this is or is very close to the resonance point for the
antenna system.

Yes, that should be correct.

What does your running of the model show for resonance frequency? At
resonance my running of the model shows close to 20 Ohms for the
relatively large values of R used in the model.

NEC-2 shows resonance (and minimum SWR) at 3.55 MHz, where R = 16.12
ohms; NEC-4 says resonance is at 3.51 MHz., where R is 16.08 ohms. (I'm
using EZNEC implementations of both.) Although small, I don't usually
see that much difference between NEC-2 and NEC-4. I suspect it's because
of the very low height above ground -- the two programs implement the
Sommerfeld ground somewhat differently. An average gain test shows good
average gain, indicating that NEC isn't having numerical difficulties.

I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Thanks - I will try to figure you how to create a non lumped model for the
inductors. Right now that is 'undiscovered country'.

Dan

Roy Lewallen wrote:
dansawyeror wrote:

Roy,

Several weeks back, and confirmed by frequency sweep model runs, you
indicated that minimum impedance is close or equal to the resonance
point. I tuned the antenna to the frequency of interest and then used
the Autek to verify the resonance point. That minimum value was 36
Ohms. I am assuming this is or is very close to the resonance point
for the antenna system.


Yes, that should be correct.

What does your running of the model show for resonance frequency? At
resonance my running of the model shows close to 20 Ohms for the
relatively large values of R used in the model.


NEC-2 shows resonance (and minimum SWR) at 3.55 MHz, where R = 16.12
ohms; NEC-4 says resonance is at 3.51 MHz., where R is 16.08 ohms. (I'm
using EZNEC implementations of both.) Although small, I don't usually
see that much difference between NEC-2 and NEC-4. I suspect it's because
of the very low height above ground -- the two programs implement the
Sommerfeld ground somewhat differently. An average gain test shows good
average gain, indicating that NEC isn't having numerical difficulties.

I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Good question. I will play with that. That said, based on Roy's comment at
minimum the load should appear nearly pure resistive.

I just tested the Autek with 6.25, 12.5, and 25 Ohm loads.
25 read 26
12.5 read 12 - 13 - 12 etc.
6.25 read mostly 7 with an occasional 6.

I would say for non-reactive loads it is pretty close.

Tomorrow - will be to experiment with non lumped inductors. That will be a
challenge.

Thanks - Dan


Frank wrote:
"dansawyeror" wrote in message
...

Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm
loads. For these two values it is very close. (It is battery level
sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan


Thanks Dan, I had forgotten about Mini-Circuits. Their price is hard to
beat. I may pick up one of the "PDC" series dual directional couplers.
Incidentally your code indicates resonance occurs at 3.54 MHz. I wonder how
the Autek behaves when subjected to a reactive load does it actually get
close to the magnitude?

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
Thanks - I will try to figure you how to create a non lumped model for
the inductors. Right now that is 'undiscovered country'.

EZNEC v. 4.0 users should use Wires Window/Create/Create Helix. You'll
get many choices, including position, orientation, various ways of
specifying the pitch and number of turns, twist direction, and so forth.
(EZNEC demo users can create any size helix to see how it works, but
won't be able to run a calculation unless the helix is extremely
simple.) In NEC, use a GH 'card'.

There should be at least a wire diameter of air space between turns,
preferably several. (That is, the center-center distance between the
wires in one turn and the wires in adjacent turns should be at least two
wire diameters, preferably more.) If air spacing is less than 2 or 3
wire diameters, the calculated loss will be somewhat lower than reality
because NEC (or EZNEC) doesn't account for proximity effect.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy Lewallen wrote:
I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Over on qrz.com, W8JI reported that he measured a 60 degree phase
shift through a 100 uH coil at 1 MHz. He also asserted that the
flux density is highest in the middle of a coil. Since the current
is proportional to flux density, that means the current in the
middle of the coil is higher than at the ends. These things are
perfectly consistent with what EZNEC reports when the distributed
network helical coil inductor is used instead of the lumped circuit
load inductor.

Essentially the only time the currents at each end of the coil are
equal is when it is installed near a standing-wave current maximum
point where the slope of the current is already close to
zero whether it be in a wire or in a coil. The phase of the standing-
wave current is relatively constant whether it be in a wire or
in a coil. (The standing-wave current doesn't rotate like a normal
phasor.) The phase shift caused by the coil happens in the forward
and reflected currents, not in the standing wave current which is the
sum of the forward current and reflected current. Not much
changes when part of a wavelength of wire is replaced by a large
loading coil. The current waveform, though warped somewhat by the
high fields inside the coil, still very roughly follows the classic
cosine shape of a wire. After all, no matter what, the current at
the tip of an antenna is zero whether it be a wire or a coil. If
a coil is placed at a standing-wave current node, the phase at each
end of the coil will be opposite, i.e. current is either flowing in
both ends at the same time or out both ends at the same time. Such
is the nature of distributed networks.
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy Lewallen wrote:
EZNEC v. 4.0 users should use Wires Window/Create/Create Helix.

And the detailed results are quite different from the lumped
circuit load inductor.
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"Roy Lewallen" wrote in message
...
dansawyeror wrote:
Thanks - I will try to figure you how to create a non lumped model for
the inductors. Right now that is 'undiscovered country'.

EZNEC v. 4.0 users should use Wires Window/Create/Create Helix. You'll get
many choices, including position, orientation, various ways of specifying
the pitch and number of turns, twist direction, and so forth. (EZNEC demo
users can create any size helix to see how it works, but won't be able to
run a calculation unless the helix is extremely simple.) In NEC, use a GH
'card'.

There should be at least a wire diameter of air space between turns,
preferably several. (That is, the center-center distance between the wires
in one turn and the wires in adjacent turns should be at least two wire
diameters, preferably more.) If air spacing is less than 2 or 3 wire
diameters, the calculated loss will be somewhat lower than reality because
NEC (or EZNEC) doesn't account for proximity effect.

Roy Lewallen, W7EL

As I understand NEC; large errors can be introduced by junctions of
dissimilar wire diameters, and in particular when the wires are at 90 deg.
Therefore, when you have designed your "GH" inductors, the rest of the
antenna should by constructed of the same diameter wire. This may be
difficult since Dan is using two coils of significantly different Qs. I
guess you could overcome this problem by varying the conductivity of the
inductor to obtain the desired Q. Also, since segmentation tends to be
relatively high in a helix, should segment length tapering be applied to
those segments adjacent to the helix?

Frank, VE6CB

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank wrote:

As I understand NEC; large errors can be introduced by junctions of
dissimilar wire diameters, and in particular when the wires are at 90 deg.
Therefore, when you have designed your "GH" inductors, the rest of the
antenna should by constructed of the same diameter wire. This may be
difficult since Dan is using two coils of significantly different Qs. I
guess you could overcome this problem by varying the conductivity of the
inductor to obtain the desired Q. Also, since segmentation tends to be
relatively high in a helix, should segment length tapering be applied to
those segments adjacent to the helix?

Frank, VE6CB

It's difficult to give an absolute answer to these questions, but some
general comments and guidelines should help.

First, the error introduced by NEC-2 when wires of dissimilar diameter
are connected is generally small, unless the wires are grossly
different. This error can be minimized by making the segments as *long*
as possible adjacent to the junction, which of course is contrary to the
general principle that more segments are better. Even a small error can
cause major changes in the pattern when the dissimilar diameter wires
are in a parasitic element. EZNEC and a number of other programs have a
built-in method of avoiding this problem for certain antenna types, but
plain NEC-2 doesn't. NEC-4 is relatively free of this problem, but it's
quite expensive for hobby use.

The Q of an inductor is determined by the inductance and the loss. The
loss is a function of the dielectric, wire resistance, and radiation
(which isn't really loss, but lowers Q as though it were). NEC type
programs automatically account for the radiation, and it's easy to
include wire loss. So assuming negligible dielectric loss, the programs
should predict Q fairly accurately -- except for proximity affect.
Proximity effect could be modeled in NEC by increasing the resistivity
of the wires in the coil. EZNEC currently allows only a single wire
resistivity for the whole model (although this will probably change in
the next version). However, since the overall loss will be dominated by
the inductors, the higher resistivity could be specified for the whole
model without sacrificing significant accuracy. Alternatively, a number
of resistive loads could be inserted in the inductors.

Segment length tapering usually isn't necessary with NEC based programs,
unless there's a source near a place where the segment length changes.
An average gain check should be run to determine if there's a problem.
If there is, segment length tapering is one tool which can be tried in
improving the average gain.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"Roy Lewallen" wrote in message
...
Frank wrote:

As I understand NEC; large errors can be introduced by junctions of
dissimilar wire diameters, and in particular when the wires are at 90
deg. Therefore, when you have designed your "GH" inductors, the rest of
the antenna should by constructed of the same diameter wire. This may be
difficult since Dan is using two coils of significantly different Qs. I
guess you could overcome this problem by varying the conductivity of the
inductor to obtain the desired Q. Also, since segmentation tends to be
relatively high in a helix, should segment length tapering be applied to
those segments adjacent to the helix?

Frank, VE6CB

It's difficult to give an absolute answer to these questions, but some
general comments and guidelines should help.

First, the error introduced by NEC-2 when wires of dissimilar diameter are
connected is generally small, unless the wires are grossly different. This
error can be minimized by making the segments as *long* as possible
adjacent to the junction, which of course is contrary to the general
principle that more segments are better. Even a small error can cause
major changes in the pattern when the dissimilar diameter wires are in a
parasitic element. EZNEC and a number of other programs have a built-in
method of avoiding this problem for certain antenna types, but plain NEC-2
doesn't. NEC-4 is relatively free of this problem, but it's quite
expensive for hobby use.

The Q of an inductor is determined by the inductance and the loss. The
loss is a function of the dielectric, wire resistance, and radiation
(which isn't really loss, but lowers Q as though it were). NEC type
programs automatically account for the radiation, and it's easy to include
wire loss. So assuming negligible dielectric loss, the programs should
predict Q fairly accurately -- except for proximity affect. Proximity
effect could be modeled in NEC by increasing the resistivity of the wires
in the coil. EZNEC currently allows only a single wire resistivity for the
whole model (although this will probably change in the next version).
However, since the overall loss will be dominated by the inductors, the
higher resistivity could be specified for the whole model without
sacrificing significant accuracy. Alternatively, a number of resistive
loads could be inserted in the inductors.

Segment length tapering usually isn't necessary with NEC based programs,
unless there's a source near a place where the segment length changes. An
average gain check should be run to determine if there's a problem. If
there is, segment length tapering is one tool which can be tried in
improving the average gain.

Roy Lewallen, W7EL

Thanks for the information Roy, all remarks noted and saved. Will see what
I can do to generate some realistic helical models.

Frank VE6CB

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

I observed by playing with the relative inductor values on the vertical segment
and the radial elements that it was possible to move the relative feedpoint.
This supports tuning the antenna by either inductor.

Reg's c_poise program predicts a 75 uH loading coil.

I am excited with prospect of coil models.

Thanks - Dan

Frank's wrote:
"Roy Lewallen" wrote in message
...

Frank wrote:

As I understand NEC; large errors can be introduced by junctions of
dissimilar wire diameters, and in particular when the wires are at 90
deg. Therefore, when you have designed your "GH" inductors, the rest of
the antenna should by constructed of the same diameter wire. This may be
difficult since Dan is using two coils of significantly different Qs. I
guess you could overcome this problem by varying the conductivity of the
inductor to obtain the desired Q. Also, since segmentation tends to be
relatively high in a helix, should segment length tapering be applied to
those segments adjacent to the helix?

Frank, VE6CB

It's difficult to give an absolute answer to these questions, but some
general comments and guidelines should help.

First, the error introduced by NEC-2 when wires of dissimilar diameter are
connected is generally small, unless the wires are grossly different. This
error can be minimized by making the segments as *long* as possible
adjacent to the junction, which of course is contrary to the general
principle that more segments are better. Even a small error can cause
major changes in the pattern when the dissimilar diameter wires are in a
parasitic element. EZNEC and a number of other programs have a built-in
method of avoiding this problem for certain antenna types, but plain NEC-2
doesn't. NEC-4 is relatively free of this problem, but it's quite
expensive for hobby use.

The Q of an inductor is determined by the inductance and the loss. The
loss is a function of the dielectric, wire resistance, and radiation
(which isn't really loss, but lowers Q as though it were). NEC type
programs automatically account for the radiation, and it's easy to include
wire loss. So assuming negligible dielectric loss, the programs should
predict Q fairly accurately -- except for proximity affect. Proximity
effect could be modeled in NEC by increasing the resistivity of the wires
in the coil. EZNEC currently allows only a single wire resistivity for the
whole model (although this will probably change in the next version).
However, since the overall loss will be dominated by the inductors, the
higher resistivity could be specified for the whole model without
sacrificing significant accuracy. Alternatively, a number of resistive
loads could be inserted in the inductors.

Segment length tapering usually isn't necessary with NEC based programs,
unless there's a source near a place where the segment length changes. An
average gain check should be run to determine if there's a problem. If
there is, segment length tapering is one tool which can be tried in
improving the average gain.

Roy Lewallen, W7EL


Thanks for the information Roy, all remarks noted and saved. Will see what
I can do to generate some realistic helical models.

Frank VE6CB

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Dan, I have been trying to replicate your inductors with NEC. It is a bit
tedious, just guessing wire gauge, coil diameter etc., to obtain the desired
inductance and Q. What are the physical dimensions of your inductors?

Have you had any luck with GH? You may have a problem with 4nec2 since you
must have a GM card along with the GH.

Frank


"dansawyeror" wrote in message
...
Good question. I will play with that. That said, based on Roy's comment at
minimum the load should appear nearly pure resistive.

I just tested the Autek with 6.25, 12.5, and 25 Ohm loads.
25 read 26
12.5 read 12 - 13 - 12 etc.
6.25 read mostly 7 with an occasional 6.

I would say for non-reactive loads it is pretty close.

Tomorrow - will be to experiment with non lumped inductors. That will be a
challenge.

Thanks - Dan


Frank wrote:
"dansawyeror" wrote in message
...

Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm
loads. For these two values it is very close. (It is battery level
sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan


Thanks Dan, I had forgotten about Mini-Circuits. Their price is hard to
beat. I may pick up one of the "PDC" series dual directional couplers.
Incidentally your code indicates resonance occurs at 3.54 MHz. I wonder
how the Autek behaves when subjected to a reactive load does it actually
get close to the magnitude?

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

A 'target' coil is 300mm x 300mm of 5/16 copper tubing. It is about 50 feet of
tubing, a .5 pitch, and should be close to 75 uH. This should be close. From
there simulation should show the best performance between varying the coils.

Thanks - Dan


Frank wrote:
Dan, I have been trying to replicate your inductors with NEC. It is a bit
tedious, just guessing wire gauge, coil diameter etc., to obtain the desired
inductance and Q. What are the physical dimensions of your inductors?

Have you had any luck with GH? You may have a problem with 4nec2 since you
must have a GM card along with the GH.

Frank


"dansawyeror" wrote in message
...

Good question. I will play with that. That said, based on Roy's comment at
minimum the load should appear nearly pure resistive.

I just tested the Autek with 6.25, 12.5, and 25 Ohm loads.
25 read 26
12.5 read 12 - 13 - 12 etc.
6.25 read mostly 7 with an occasional 6.

I would say for non-reactive loads it is pretty close.

Tomorrow - will be to experiment with non lumped inductors. That will be a
challenge.

Thanks - Dan


Frank wrote:

"dansawyeror" wrote in message
...


Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm
loads. For these two values it is very close. (It is battery level
sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan


Thanks Dan, I had forgotten about Mini-Circuits. Their price is hard to
beat. I may pick up one of the "PDC" series dual directional couplers.
Incidentally your code indicates resonance occurs at 3.54 MHz. I wonder
how the Autek behaves when subjected to a reactive load does it actually
get close to the magnitude?

Frank

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"dansawyeror" wrote in message
...
Frank,

A 'target' coil is 300mm x 300mm of 5/16 copper tubing. It is about 50
feet of tubing, a .5 pitch, and should be close to 75 uH. This should be
close. From there simulation should show the best performance between
varying the coils.

Thanks - Dan

Having trouble producing a good model Dan. NEC 2 indicates Qs which are
5000, but no warnings or errors. I can run it in NEC 4.1, single, and
double precision, but I get over 400 warnings in the NEC output file; such
as:

SEGCHK: WARNING - SEGMENTS 1 AND 271 CROSS AT A MIDPOINT WITH SEPARATION
LESS THAN THE SUM OF THEIR RADII

SEGCHK: WARNING - THE CENTER OF SEGMENT 1 IS WITHIN THE VOLUME OF SEGMENT
271

These warnings are particularly strange since, for example, absolute
segments 1 and 271 are almost 12" apart in the model. Will see what I can
do to correct the error.

73,

Frank

The code I am using is as follows:

CM Inductor Q Calculation
CE
GH 1 300 1 12 6 6 6 6 0.3125
GW 2 5 6 0 12 0 0 12 0.3125
GW 3 10 0 0 12 0 0 0 0.3125
GW 4 5 0 0 0 6 0 0 0.3125
GS 0 0 0.025400
GE 0
EX 0 3 5 00 1 0
FR 0 5 0 0 3.7 0.02
LD 5 1 1 320 5.7001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank wrote:
"dansawyeror" wrote in message
...
Frank,

A 'target' coil is 300mm x 300mm of 5/16 copper tubing. It is about 50
feet of tubing, a .5 pitch, and should be close to 75 uH. This should be
close. From there simulation should show the best performance between
varying the coils.

Thanks - Dan

Having trouble producing a good model Dan. NEC 2 indicates Qs which are
5000, but no warnings or errors. I can run it in NEC 4.1, single, and
double precision, but I get over 400 warnings in the NEC output file; such
as:

SEGCHK: WARNING - SEGMENTS 1 AND 271 CROSS AT A MIDPOINT WITH SEPARATION
LESS THAN THE SUM OF THEIR RADII

SEGCHK: WARNING - THE CENTER OF SEGMENT 1 IS WITHIN THE VOLUME OF SEGMENT
271

These warnings are particularly strange since, for example, absolute
segments 1 and 271 are almost 12" apart in the model. Will see what I can
do to correct the error.

73,

Frank

The code I am using is as follows:

CM Inductor Q Calculation
CE
GH 1 300 1 12 6 6 6 6 0.3125
GW 2 5 6 0 12 0 0 12 0.3125
GW 3 10 0 0 12 0 0 0 0.3125
GW 4 5 0 0 0 6 0 0 0.3125
GS 0 0 0.025400
GE 0
EX 0 3 5 00 1 0
FR 0 5 0 0 3.7 0.02
LD 5 1 1 320 5.7001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

NEC-2 and NEC-4 have different formats for the GH 'card'. This is the
NEC-2 format, which will be interpreted differently by NEC-4. See your
NEC-4 documentation for the correct NEC-4 format.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

NEC-2 and NEC-4 have different formats for the GH 'card'. This is the
NEC-2 format, which will be interpreted differently by NEC-4. See your
NEC-4 documentation for the correct NEC-4 format.

Roy Lewallen, W7EL

Thanks Roy, now you mention it I do remember that there are some
differences. Should have checked with my manual before running it instead
of just cutting and pasting NEC 2 code. Made the appropriate correction and
it is now working with a reasonable correlation with NEC 2. Heck, now I
just noticed I had entered the wire diameter instead of its radius!

Frank, VE6CB

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

I observed by playing with the relative inductor values on the vertical
segment and the radial elements that it was possible to move the relative
feedpoint. This supports tuning the antenna by either inductor.

Reg's c_poise program predicts a 75 uH loading coil.

I am excited with prospect of coil models.

Thanks - Dan

Dan, I have done some minor approximations with your coil. I took the
length and diameter to be 12", rather than 300 mm (11.8"). The coil copper
pipe diameter is, as specified, 5/16" (0.3125"). I was a little confused
with your use of the term "Pitch" as 0.5". In the sense of a screw thread
pitch is the distance between adjacent thread peaks, but I took it to mean
the actual distance between the outer walls of the pipe; in which case the
actual pitch is 0.8125". If this is the case the total pipe length is just
over 47 ft. The inductance calculates to 54.2uH, and the Q = 2990. I have
not yet run the program in NEC 4, for greater accuracy, since I would like
to get the model as close as possible in NEC 2.

If I have gotten the pitch definition wrong then the model dimensions will
violate the NEC criteria of the minimum distance between adjacent turns.

The code for this preliminary run is shown below. Some of the odd-ball
dimensions are just to approximately equalize segment lengths.

Despite some of the weirdness of 4nec2, concerning "GH" cards, you should be
able to run it.

Frank

CM Inductor Q Calculation
CE
GH 1 300 0.8125 12 6 6 6 6 0.15625
GW 2 3 0.72322 -5.95625 12 .35542 0 12 0.15625
GW 3 6 .35542 0 12 .35542 0 0 0.15625
GW 4 3 .35542 0 0 6 0 0 0.15625
GS 0 0 0.025400
GE 0
EX 0 3 3 00 1 0
FR 0 5 0 0 3.7 0.02
LD 5 1 1 312 5.7001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

Thank you. Your assumptions were correct. The only difference is 54 uH vs 75.

The model below runs on nec2. I tried a quick load it into 4nec2 without
success, it seems to be confused by the GH and GS cards. I will have to pick
this up tomorrow.

Thanks again - Dan

Frank wrote:
Frank,

I observed by playing with the relative inductor values on the vertical
segment and the radial elements that it was possible to move the relative
feedpoint. This supports tuning the antenna by either inductor.

Reg's c_poise program predicts a 75 uH loading coil.

I am excited with prospect of coil models.

Thanks - Dan


Dan, I have done some minor approximations with your coil. I took the
length and diameter to be 12", rather than 300 mm (11.8"). The coil copper
pipe diameter is, as specified, 5/16" (0.3125"). I was a little confused
with your use of the term "Pitch" as 0.5". In the sense of a screw thread
pitch is the distance between adjacent thread peaks, but I took it to mean
the actual distance between the outer walls of the pipe; in which case the
actual pitch is 0.8125". If this is the case the total pipe length is just
over 47 ft. The inductance calculates to 54.2uH, and the Q = 2990. I have
not yet run the program in NEC 4, for greater accuracy, since I would like
to get the model as close as possible in NEC 2.

If I have gotten the pitch definition wrong then the model dimensions will
violate the NEC criteria of the minimum distance between adjacent turns.

The code for this preliminary run is shown below. Some of the odd-ball
dimensions are just to approximately equalize segment lengths.

Despite some of the weirdness of 4nec2, concerning "GH" cards, you should be
able to run it.

Frank

CM Inductor Q Calculation
CE
GH 1 300 0.8125 12 6 6 6 6 0.15625
GW 2 3 0.72322 -5.95625 12 .35542 0 12 0.15625
GW 3 6 .35542 0 12 .35542 0 0 0.15625
GW 4 3 .35542 0 0 6 0 0 0.15625
GS 0 0 0.025400
GE 0
EX 0 3 3 00 1 0
FR 0 5 0 0 3.7 0.02
LD 5 1 1 312 5.7001E7
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Frank,

Thank you. Your assumptions were correct. The only difference is 54 uH vs
75.

The model below runs on nec2. I tried a quick load it into 4nec2 without
success, it seems to be confused by the GH and GS cards. I will have to
pick this up tomorrow.

Thanks again - Dan

No problem Dan, I find all this very interesting. You could change all the
dimensions to metric, and drop the GS card. I don't know why it is having
trouble with GH. If you run some of the inductance programs, such as:
http://www.captain.at/electronics/coils/, where I have used the number of
turns as 14.7, and length and diameter 12"; the inductance calculates to
44.71 uH.

Minimizing the proximity effect, with #18 AWG, NEC computes z = 3.3 + j1274.
Therefore Q = 433, and L = 60.8 uH; an even greater difference that the
inductance programs, but closer to your requirement of 75 uH. I have tried
to run the program on NEC 4.1 to see if it agrees with NEC 2, but am having
trouble with connecting the helix since the end points don't appear to be in
the correct position.

Frank