On 28 Mar 2005 11:52:23 -0800, "Brian Kelly" wrote:

So two questions in this regard: Is there a way to measure the Vf of a
wire without having to resort to using 2" Heliax to feed a dipole and
without a lab full of HP and GR test equipment? Second, assuming the
Vf becomes known how does one handle it during the modeling process?
Model the antenna wire lengths at an upward-shifted frequency based on
the Vf?

Hi Brian,

1. Drive the design with power instead of low level excitation;

2. Remove half the transmission line muffling of results by using a
field strength meter to find resonance (another reason for power);

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

4. Use the new EZNEC which allows you to employ insulation over wire
and adjusting the thickness to conform with results found
experimentally with real wire at actual length;

5. Assign these insulation properties to all future designs in the
modeler.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On 28 Mar 2005 11:52:23 -0800, "Brian Kelly" wrote:

So two questions in this regard: Is there a way to measure the Vf of
a
wire without having to resort to using 2" Heliax to feed a dipole
and
without a lab full of HP and GR test equipment? Second, assuming
the
Vf becomes known how does one handle it during the modeling process?
Model the antenna wire lengths at an upward-shifted frequency based
on
the Vf?

Hi Brian,

1. Drive the design with power instead of low level excitation;

Sweep the dipole with a transmitter and an SWR bridge?

2. Remove half the transmission line muffling of results by using a
field strength meter to find resonance (another reason for power);

Same as above but with a field strength indicator? Just might work if I
use a 4-digit DVM and a diode.

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

That would seem to work but I'd expect to still have the flat curves
because of the coax losses. I'm starting to think I should go to a
lower frequency band like 40 or 80M to reduce the problems with the
coax. And to reduce the errors in cutting-to-length.

4. Use the new EZNEC which allows you to employ insulation over wire
and adjusting the thickness to conform with results found
experimentally with real wire at actual length;

.. . . all I gotta do is DO that! "The loop has been closed."

5. Assign these insulation properties to all future designs in the
modeler.

I've been using Nec Win Plus which is OK but it doesn't have the the
ability to handle velocity factors like EZNEC 4.0 can.

I don't have a big problem with scaling antenna dimensions to adjust
for the Vf because I physically model antennas with CAD first to get
the locations of the wire end points in 3D space. Which I can quickly
and easily load into NWP. The CAD program does all the tedious trig for
me. When I have a bare-wire model which "works" in NWP I can rescale
the physical model by 0.98 or 0.95 or whatever the Vf might be to get a
"close enough" fully dimensioned antenna design. But I still need to
find the Vf experimentally and we're back to square one. You fed me
some thinking fodder, I'll try a few things per above and get there one
way or another.

Tnx.

73's
Richard Clark, KB7QHC

w3rv

On 30 Mar 2005 08:08:27 -0800, "Brian Kelly" wrote:

2. Remove half the transmission line muffling of results by using a
field strength meter to find resonance (another reason for power);

Same as above but with a field strength indicator? Just might work if I
use a 4-digit DVM and a diode.

Excellant choice (add a filter cap too with resistive load for
averaging).

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

That would seem to work but I'd expect to still have the flat curves
because of the coax losses.

Hi Brian,

Actually, by using the FSM you entirely remove the transmission line
as disturbance to accurate response readings. Those come from the
external reading which interprets all power being applied AT the
antenna junction. However, it imposes upon you that you be scrupulous
about achieving the same drive levels at all the intermediate
frequencies across the swept band. If you do that, then the
transmission line characteristics for the drive going up to the
antenna junction fall out too.

Careful drive monitoring, and careful response monitoring render the
transmission line transparent to the measurement. Thus response/drive
is the antenna characteristic. Define one point's SWR, and you can
cast that into the suite of readings for a swept SWR curve. Take care
in that "one" SWR determination to anticipate the SWR lowering effect
of transmission line loss.

Then you do the same thing in software, and tailor the characteristic
insulation thickness to match your measurements. Having achieved
that, then you have your standard insulation. This does not give you
Vf until you then remove that virtual insulation and find the native,
bare wire resonance. This last step is satisfying (it answers your
question as to Vf), but the step before is more useful because you can
model other antennas from that standard.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On 30 Mar 2005 08:08:27 -0800, "Brian Kelly" wrote:

2. Remove half the transmission line muffling of results by using
a
field strength meter to find resonance (another reason for power);

Same as above but with a field strength indicator? Just might work
if I
use a 4-digit DVM and a diode.

Excellant choice (add a filter cap too with resistive load for
averaging).

Yup.

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

That would seem to work but I'd expect to still have the flat curves
because of the coax losses.

Hi Brian,

Actually, by using the FSM you entirely remove the transmission line
as disturbance to accurate response readings. Those come from the
external reading which interprets all power being applied AT the
antenna junction. However, it imposes upon you that you be
scrupulous
about achieving the same drive levels at all the intermediate
frequencies across the swept band. If you do that, then the
transmission line characteristics for the drive going up to the
antenna junction fall out too.

"Eureka". You're right. This is the way to go. Or at least to try.

Careful drive monitoring, and careful response monitoring render the
transmission line transparent to the measurement. Thus
response/drive
is the antenna characteristic. Define one point's SWR, and you can
cast that into the suite of readings for a swept SWR curve. Take
care
in that "one" SWR determination to anticipate the SWR lowering effect
of transmission line loss.

Since coax losses don't vary much if at all over any of the individual
HF ham bands a decent inline wattmeter with maybe a 4 inch scale should
allow me to maintain a constant power output over the sweep.

Then you do the same thing in software, and tailor the characteristic
insulation thickness to match your measurements. Having achieved
that, then you have your standard insulation. This does not give you
Vf until you then remove that virtual insulation and find the native,
bare wire resonance.

Agreed.

This last step is satisfying (it answers your
question as to Vf), but the step before is more useful because you
can
model other antennas from that standard.

That's what I need. It'll make a worthwhile weekend project which, if
successful, should result in less futzing around with the cutters and
the soldering iron up the tower. I'll also compare the experimental
results of the bare wire sweeps to the predictions given by the modeler
and "calibrate" the modeler in this respect too. Might lead me to my
own real world ground condx vs. the generic "real ground" in the
modeler which is another big source of modeling non-truths.

73's
Richard Clark, KB7QHC

w3rv

On 30 Mar 2005 19:16:44 -0800, "Brian Kelly" wrote:

Since coax losses don't vary much if at all over any of the individual
HF ham bands a decent inline wattmeter with maybe a 4 inch scale should
allow me to maintain a constant power output over the sweep.

Hi Brian,

I left that unsaid, expecting someone, if not you, would also come to
that conclusion. It does not work across all bands, but within a band
it will suffice. Also, even given the impression of accuracy that
most impart to their power meters, this method demands only "relative
accuracy" which can be exceptional when care is shown (try to maintain
a full scale indication or at least greater than 2/3rds at some
cardinal point on the scale).

At this point, one should reflect that if there is a mismatch, then
power at the feed point will vary somewhat. In other words, the
presumption of constant power (to subtract out the effects of the
transmission line) is violated. However, as a first pass estimation,
the method is still quite productive, and tightening up the method and
the numbers is an exercise left to the experimenter.

73's
Richard Clark, KB7QHC

"Brian Kelly" wrote in message roups.com...

I've been using Nec Win Plus which is OK but it doesn't have the the
ability to handle velocity factors like EZNEC 4.0 can.

You could also try 4nec2 in which a provision was added (as described
by L.B. Cebik) to model insulated wires.

The CAD program does all the tedious trig for
me. When I have a bare-wire model which "works" in NWP I can rescale
the physical model by 0.98 or 0.95 or whatever the Vf might be to get a
"close enough" fully dimensioned antenna design. But I still need to
find the Vf experimentally and we're back to square one. You fed me
some thinking fodder, I'll try a few things per above and get there one
way or another.

It also includes a drawing (drag and drop) style geometry editor and
you can rescale part of or the whole structure.

Arie.