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

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

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

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

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

"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

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

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