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Old March 31st 05, 04:16 AM
Brian Kelly
 
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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

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Old March 31st 05, 05:46 AM
Hal Rosser
 
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"Wes Stewart" wrote in message
...
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.


NOW ur talkin' :-)


  #13   Report Post  
Old March 31st 05, 09:08 AM
Richard Clark
 
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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
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Old March 31st 05, 09:12 AM
 
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"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.
  #15   Report Post  
Old April 1st 05, 12:04 AM
Mark
 
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build a scale model with insulation at 900 MHz and test it on a network
analyzer in the lab

remove insulation and retest?


Mark



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Old April 1st 05, 12:44 AM
K7ITM
 
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The question that comes first to my mind is, "Why do you care?"
Certainly an antenna does not need to be resonant to work well. I can
imagine you'd like a reasonably low indicated SWR, just so your
transmitter has a reasonable load to drive.

If you really want to know what's going on at the antenna feedpoint,
you'll need to back the effects of the feedline out of your antenna
analyzer readings, or use an analyzer that does it for you. If you
have a reasonable estimate of the feedline loss and know its electrical
length (easy to find if you put a short at the end of the line and look
at the resulting impedances read on the analyzer), then you should be
able to translate your analyzer readings to actual feedpoint impedance.
Do you have the feedline properly decoupled from the antenna so it's
not a significant part of the radiating system? If not, there seems
little reason to bother making the measurements.

I'd expect half-wave dipole resonance to result in lowest SWR on a
50-ohm feedline, but it won't be a very sharp minimum. So is it worth
worrying about?

Another 'speriment to try: build a fairly wide-spaced two wire
transmission line from your wire. Short it at one end, open at the
other, and look for quarter-wave resonance; or short both ends and look
for half-wave resonance. Measure the resonant frequency, which will be
a pretty sharp resonance (much sharper than the dipole). Remove the
insulation and see how much the resonance changes. Try for various
spacings to see what effect the spacing has. (Expect that close
spacings will show more effect than wide.)

Cheers,
Tom

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Old April 1st 05, 03:41 PM
Brian Kelly
 
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K7ITM wrote:
The question that comes first to my mind is, "Why do you care?"
Certainly an antenna does not need to be resonant to work well. I

can
imagine you'd like a reasonably low indicated SWR, just so your
transmitter has a reasonable load to drive.

If you really want to know what's going on at the antenna feedpoint,
you'll need to back the effects of the feedline out of your antenna
analyzer readings, or use an analyzer that does it for you. If you
have a reasonable estimate of the feedline loss and know its

electrical
length (easy to find if you put a short at the end of the line and

look
at the resulting impedances read on the analyzer), then you should be
able to translate your analyzer readings to actual feedpoint

impedance.
Do you have the feedline properly decoupled from the antenna so it's
not a significant part of the radiating system? If not, there seems
little reason to bother making the measurements.

I'd expect half-wave dipole resonance to result in lowest SWR on a
50-ohm feedline, but it won't be a very sharp minimum. So is it

worth
worrying about?


I agree 100%, it's not worth worrying about - if all I wanted is to get
a 20M dipole running on the air. But that's not my point. I'm trying to
use dipoles to determine the velocity factors of insulated wires used
for the radiators.

Another 'speriment to try: build a fairly wide-spaced two wire
transmission line from your wire. Short it at one end, open at the
other, and look for quarter-wave resonance; or short both ends and

look
for half-wave resonance. Measure the resonant frequency, which will

be
a pretty sharp resonance (much sharper than the dipole). Remove the
insulation and see how much the resonance changes. Try for various
spacings to see what effect the spacing has. (Expect that close
spacings will show more effect than wide.)


Now we're cookin', I like it, this approach has definite appeal and I
need to explore it for several reasons. First because of the sharp
nulls, it takes out the coax and it's a simple sort of "bench test" I
can do at ground level. I'll build three identical shorted 20M or 30M
close-spaced quarter wave lines. One with bare #14 stranded wire, one
with the insulated #14 THHN wire I usually use for quick & dirty
dipoles and one with The Wireman's very flexible #544 or #546 #14
insulated PVC jacketed wire I'd use to build a hex wire beam or a quad.


Then I'd use a grid dip meter to find the resonant frequencies of all
three. I'd use an HF rcvr with a digital freq display to listen to the
GDO rather than trust the GDO dial calibration and resolution. Yes?

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?

Thanks,

Cheers,
Tom


w3rv

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Old April 1st 05, 07:47 PM
K7ITM
 
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Brian Kelly wrote:

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the

quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?


OK, I admit to being a spoiled brat when it comes to making
measurements like this. The easiest for me would be to set up a
network analyzer, with the analyzer's source and receiver each coupled
lightly to the line. But there are other ways. You may not even need
a signal generator. You could loosely couple a receiver to the line,
and loosely couple an antenna to it on the other side, and as you tune
the receiver across the resonance, you should notice a sharp peak in
atmospheric noise. With a loaded Q of a few hundred, the peak at 10MHz
would be a very few tens of kHz wide. You could also build an
oscillator which uses the tuned line as the frequency-determining
element, and just count the frequency of the oscillator. A simple
version of a network analyzer could be done by lightly coupling an RF
generator into the line, and putting an RF detector across the line a
small distance up from the shorted end. I'd use either a simple diode
detector, which can have pretty high input impedance, or one of the
Linear Technology or Analog devices RF detectors, but since those are
lower impedance, tap them down very far on the line. -- I'd expect a
10MHz quarter-wave resonator made from 600 ohm line using AWG14 wires
to have an unloaded Q around 350.

You can achieve that loose coupling by calling the center of the short
across the end "ground" and tapping up just one or two percent of the
length of the line from that for the "hot" connection. Or you can
couple in with a loop, say of a diameter about equal to the line
spacing, held next to the shorted end of the line. "Reference Data for
Radio Engineers" shows various coupling schemes in the "Transmission
Lines" chapter. As long as you keep the coupling light (to keep the
loaded Q high), and are consistent in the way you arrange things, you
should be able to measure the resonance to within a fraction of a
percent repeatability, if not absolute accuracy. Relative measurements
should be all you need in this case. And I assume it's obvious that
though the resonant line will not have exactly the same VF as an
antenna, as you increase the wire spacing, it should approach the same
effect as you'll see in the antenna. Then compare with the formulas
Reg posted, and if you see significant differences, try to resolve
what's causing them.

By the way, one place where the "velocity factor" effect might be
noticable is in parasitic elements of an array in which you're trying
to achieve maximum gain. The element tuning will affect phasing among
the elements and therefore gain. If the design is narrow-band,
high-gain, you might actually notice some effect from the insulation.

Cheers,
Tom

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Old April 1st 05, 08:05 PM
Jerry Martes
 
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Tom

Someone might refer to you as a "spoiled brat" regarding your interest in
the details. I consider you to be a carefull thinker.

Jerry


"K7ITM" wrote in message
oups.com...
Brian Kelly wrote:

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the

quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?


OK, I admit to being a spoiled brat when it comes to making
measurements like this. The easiest for me would be to set up a
network analyzer, with the analyzer's source and receiver each coupled
lightly to the line. But there are other ways. You may not even need
a signal generator. You could loosely couple a receiver to the line,
and loosely couple an antenna to it on the other side, and as you tune
the receiver across the resonance, you should notice a sharp peak in
atmospheric noise. With a loaded Q of a few hundred, the peak at 10MHz
would be a very few tens of kHz wide. You could also build an
oscillator which uses the tuned line as the frequency-determining
element, and just count the frequency of the oscillator. A simple
version of a network analyzer could be done by lightly coupling an RF
generator into the line, and putting an RF detector across the line a
small distance up from the shorted end. I'd use either a simple diode
detector, which can have pretty high input impedance, or one of the
Linear Technology or Analog devices RF detectors, but since those are
lower impedance, tap them down very far on the line. -- I'd expect a
10MHz quarter-wave resonator made from 600 ohm line using AWG14 wires
to have an unloaded Q around 350.

You can achieve that loose coupling by calling the center of the short
across the end "ground" and tapping up just one or two percent of the
length of the line from that for the "hot" connection. Or you can
couple in with a loop, say of a diameter about equal to the line
spacing, held next to the shorted end of the line. "Reference Data for
Radio Engineers" shows various coupling schemes in the "Transmission
Lines" chapter. As long as you keep the coupling light (to keep the
loaded Q high), and are consistent in the way you arrange things, you
should be able to measure the resonance to within a fraction of a
percent repeatability, if not absolute accuracy. Relative measurements
should be all you need in this case. And I assume it's obvious that
though the resonant line will not have exactly the same VF as an
antenna, as you increase the wire spacing, it should approach the same
effect as you'll see in the antenna. Then compare with the formulas
Reg posted, and if you see significant differences, try to resolve
what's causing them.

By the way, one place where the "velocity factor" effect might be
noticable is in parasitic elements of an array in which you're trying
to achieve maximum gain. The element tuning will affect phasing among
the elements and therefore gain. If the design is narrow-band,
high-gain, you might actually notice some effect from the insulation.

Cheers,
Tom



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