Insulated Wire Velocity Factor: How to . . ??
 

I'm in the habit of using insulated stranded all-copper conductors for
HF wire antennas. Usually U.S. standard #14 Type THHN "house wire"
because of it's easy availability everywhere at low cost. However
insulated wires pose an annoying problem, their specific velocity
factors are not published and vary all over creation depending on a
whole collection of variables.

I've tried to nail down the Vf of my usual #14 THHN by cutting the
lengths of 20M dipoles to one or another of the usual equations. I put
together the antennas, hoist them to various operatng heights and nip
the coax feedline to it's minimum possible length so that I can find
the resonant point with an antenna analyzer while I'm on the ground
directly under the feedpoint. A process which I believe should lead me
to a "correction factor" from which the Vf can be determined.

Modeling a 20M dipole at 35 feet indicates a fairly sharp null at the
resonant point. But that's not what I get when I build the antenna and
try to measure the frequency of it's resonant point with an antenna
analyzer. I get a much flatter SWR curve from the analyzer than I do
from modeling probably because of feedline losses and because of the
low analyzer frequency resolution (MFJ-259B). To the point where the
antenna appears to be resonant over a range of maybe 200Khz. Which in
reality it can't be.

From a practical standpoint this scenario isn't any problem in the case
of a simple dipole. I "cut long", put the antenna up, sweep it with the
analyzer, find what seems to be the center of resonace, do the quickie
numbers, trim it and take it to the airwaves.

The problem comes when trying to accurately model complex, fussy wire
antennas like hex beams when the Vf of the wire is unknown. A one
percent error in conductor length at 14 Mhz is 140 Khz which makes
decent modeling just about useless.

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?

Thanks,

w3rv

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

If you have a MFJ-259, you can get the VF of the wire easily
or
1. put up a 10-meter dipole using bare wire - note the resonant freq
2. duplicate exactly using 14 THHN Str - note the NEW resonant freq.

divide the resonant freq from step 1 by the resonant freq from step 2. (or
is it the other way around)

Anyway, I don't think you have to add your social security number.

or just use .95 as the VF and adjust as needed

On Mon, 28 Mar 2005 20:23:34 -0500, "Hal Rosser"
wrote:

or just use .95 as the VF and adjust as needed

Or buy a copy of Eznec v4

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

Hal Rosser wrote:
If you have a MFJ-259, you can get the VF of the wire easily
or
1. put up a 10-meter dipole using bare wire - note the resonant freq
2. duplicate exactly using 14 THHN Str - note the NEW resonant freq.

divide the resonant freq from step 1 by the resonant freq from step
2. (or
is it the other way around)

I wish. Finding the resoant frequecies is my fundamental problem.

Anyway, I don't think you have to add your social security number.

Heh.

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

"Wes Stewart" wrote in message
...
On Mon, 28 Mar 2005 20:23:34 -0500, "Hal Rosser"
wrote:

or just use .95 as the VF and adjust as needed

Or buy a copy of Eznec v4


or just use .95 as the VF and adjust as needed

On Wed, 30 Mar 2005 18:16:04 -0500, "Hal Rosser"
wrote:


"Wes Stewart" wrote in message
.. .
On Mon, 28 Mar 2005 20:23:34 -0500, "Hal Rosser"
wrote:

or just use .95 as the VF and adjust as needed

Or buy a copy of Eznec v4


or just use .95 as the VF and adjust as needed

Or use .1 and adjust as needed.