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

"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

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

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

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

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