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Old May 9th 09, 05:35 PM posted to rec.radio.amateur.antenna
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On May 9, 1:40*am, wrote:
Tom,

I thought I'd quoted some numbers in the related "Dish reflector"
thread - apologies if I did not. Here are some mo

* I modelled a coil as a spiral in EZNEC (40T, diameter=6",
length=12", #14 copper wire)
* I added a 6ft "stinger" and found the frequency where the
combination was resonant: 3.79 MHz
* I checked the feedpoint impedance without the coil present: 0.46-
j2439
* That tells me the "lumped circuit equivalent" reactance of the coil
at 3.79 MHz is +j2439 ohms
* I found the frequency where the coil was resonant with no "stinger":
6.2 MHz

Now I look at what ON4AA's "Corum method" inductance calculator tells
me:

* "Lumped circuit equivalent" reactance at 3.79 MHz: +j2449
* Self-resonant frequency: 6.3 MHz

Unless I'm missing an option, if I want to predict the RF
characteristics of a "bugcatcher" it seems I have *3 choices:

* Use Wheeler's formula
* Build a helical model in EZNEC
* Use the Corum method

Wheeler's formula is inappropriate at frequencies close to a coil's
SRF.

EZNEC and the Corum method give very close results. The Corum formulas
are not difficult to use; even if they were, there is an on-line
calculator which removes the need for any maths. So it seems to me the
Corum formulas would be the more convenient tool to use, at least for
a "first look".

73,
Steve G3TXQ

On May 9, 7:06*am, "Tom Donaly" wrote:



You know, you haven't shown that the Corum model accurately measures
the bugcatcher coil. You have stated - and I have no reason to
disbelieve you - that the Corum model agrees with EZNEC. If that's the
case, it's just as easy to use EZNEC, right or wrong. MoM is a method of
obtaining numerical solutions to integral equations. The only reason to
do that is if symbolic solutions are either too difficult or impossible
to puzzle out of those same integral equations. In other words, some
very deep thinkers decided that MoM would give results superior to
algebraic approximations and hand waving, so they applied it to antenna
analysis. I don't think it's perfect. It's certainly useful. If you
think Corum is good enough for your purposes, though, go for it.




Steve, this is fine for a base loading coil, but I'd suggest you try
your experiment with a loading coil well up the antenna, where the
coil is significantly larger diameter than the straight conductor in
which it's placed. The same size coil you described (though
presumably a different number of turns), placed at least half way up
something like a 15 or 20 foot long thin wire, should illustrate the
point. Is the EZNEC model then in such good agreement with placing a
reactive load at that point in the antenna, where the reactance is
from ON4AA's online calculator?

If I trusted NEC to handle large steps in conductor diameter
accurately, I'd suggest putting a segment in the antenna description
to represent the length and diameter of the coil, with the calculated
reactance placed as a load in that segment. As I understand it,
though, NEC has trouble with large diameter steps.

Cheers,
Tom
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Old May 9th 09, 07:08 PM posted to rec.radio.amateur.antenna
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K7ITM wrote:
Steve, this is fine for a base loading coil, but I'd suggest you try
your experiment with a loading coil well up the antenna, where the
coil is significantly larger diameter than the straight conductor in
which it's placed. The same size coil you described (though
presumably a different number of turns), placed at least half way up
something like a 15 or 20 foot long thin wire, should illustrate the
point. Is the EZNEC model then in such good agreement with placing a
reactive load at that point in the antenna, where the reactance is
from ON4AA's online calculator?


The key to understanding this question and its logical
answer lies in the phase shift that occurs at impedance
discontinuities.

For a base-loading coil, there is only one impedance
discontinuity in the system, a hi-Z0 coil to a low-Z0
stinger. That single discontinuity provides a positive
phase shift at the '+' junction of the coil and stinger.

coil stinger
FP//////////+-------------------

When a straight shaft section is installed under the
coil, it introduces one additional impedance discontinuity
at 'x' in addition to the '+' top of coil to stinger
discontinuity.

base coil stinger
FP-------x////////////+---------

Because the impedance discontinuity between the base
section is a low-Z0 to hi-Z0 transition, the phase shift
is negative, i.e. the antenna *loses electrical degrees*
at that junction.

Therefore, more turns must be added to the inductor
to supply the number of negative degrees lost at the
base section to coil impedance discontinuity.

This might best be illustrated with pieces of transmission
line. Please reference my web page at:

http://www.w5dxp.com/shrtstub.htm

The following concepts apply to the above antennas but
may be easier to understand using transmission lines.

Here is a dual-Z0 stub that is physically 44.4 degrees
long but is 90 degrees (1/4WL) long electrically, i.e.
it is functionally a 1/4WL open-circuit stub.

---22.2 deg 300 ohm line---+---22.2 deg 50 ohm line---

The Z0=300 ohm to Z0=50 ohm transition provides for
+45.6 degrees of phase shift. This is akin to the base-
loaded antenna above.

Here is a dual-Z0 stub with 11.1 degrees (half) of
the 50 ohm line moved to the left. (The words are
abbreviated because of space on the line.)

--11.1 deg 50--+--22.2 deg 300--+--11.1 deg 50--

Who can tell me how long electrically is this stub
using the identical feedlines from the above example?

This reconfigured stub with half of the 50 ohm feedline
moved to the bottom is now electrically only ~80.6 degrees
long. What has happened? The new impedance discontinuity
from the base section at the bottom of the coil has cost
us electrical degrees by providing a *negative phase shift*.

How do we solve the problem? Add some length (degrees) to
the Z0=300 ohm section. If we make the 300 ohm section
38.5 degrees long, the stub will be electrically 90 degrees
long once again.

This is conceptually the same problem we encounter when
we move the loading coil from the base location to the
center location. When we move the coil up the shaft, we
introduce a negative phase shift at the bottom of the
coil. Therefore, we must increase the number of turns
to make the loading coil electrically longer.

Incidentally, w8ji knows about the coil to stinger
positive phase shift and describes it on his web page.
He apparently doesn't know about the opposite negative
phase shift at the bottom of the coil where the shaft
attaches.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
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Old May 9th 09, 09:56 PM posted to rec.radio.amateur.antenna
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Tom,

OK, I tried what you suggested. I put my loading coil midway up a 20ft
vertical wire in the EZNEC model. I reduced the number of turns to
lift the resonant frequency to 5.6MHz. EZNEC predicted that the
magnitude of the current at the top of the coil would be 77% of the
magnitude at the bottom.

Then I removed the coil in the model, replaced it with a straight wire
containing an EZNEC lumped load, and adjusted that load for antenna
resonance at 5.6MHz again. I needed +j1630.

Given the dimensions of the coil, the Corum calculator predicted a
lumped circuit equivalent reactance of +j1573, and it predicted a
current fall-off across the coil of 78%.

73,
Steve G3TXQ


On May 9, 5:35*pm, K7ITM wrote:

Steve, this is fine for a base loading coil, but I'd suggest you try
your experiment with a loading coil well up the antenna, where the
coil is significantly larger diameter than the straight conductor in
which it's placed. *The same size coil you described (though
presumably a different number of turns), placed at least half way up
something like a 15 or 20 foot long thin wire, should illustrate the
point. *Is the EZNEC model then in such good agreement with placing a
reactive load at that point in the antenna, where the reactance is
from ON4AA's online calculator?


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Old May 11th 09, 02:46 AM posted to rec.radio.amateur.antenna
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On May 9, 1:56*pm, wrote:
Tom,

OK, I tried what you suggested. I put my loading coil midway up a 20ft
vertical wire in the EZNEC model. I reduced the number of turns to
lift the resonant frequency to 5.6MHz. EZNEC predicted that the
magnitude of the current at the top of the coil would be 77% of the
magnitude at the bottom.

Then I removed the coil in the model, replaced it with a straight wire
containing an EZNEC lumped load, and adjusted that load for antenna
resonance at 5.6MHz again. I needed +j1630.

Given the dimensions of the coil, the Corum calculator predicted a
lumped circuit equivalent reactance of *+j1573, and it predicted a
current fall-off across the coil of 78%.


Hi Steve,

OK, I'm wondering now exactly what "Corum calulator" you are using
that predices "a current fall-off across the coil of 78%." The
inductance calculator on the HamWaves website that I thought we were
talking about doesn't seem to say anything about "current fall-off" in
coils, though perhaps I'm missing it.

Cheers,
Tom
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Old May 11th 09, 10:26 AM posted to rec.radio.amateur.antenna
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Hi Tom,

I should have been more explicit.

I took the "Axial Propagation Factor" (4.372 rad/m) figure which was
given by the HamWaves calculator and multiplied it by the coil length
(155mm) to find the effective electrical length of the coil (38.83
degrees). Then I took cos(38.83)=0.779 as the fall-off in current
across the coil.

73,
Steve G3TXQ


On May 11, 2:46*am, K7ITM wrote:

Hi Steve,

OK, I'm wondering now exactly what "Corum calulator" you are using
that predices "a current fall-off across the coil of 78%." *The
inductance calculator on the HamWaves website that I thought we were
talking about doesn't seem to say anything about "current fall-off" in
coils, though perhaps I'm missing it.

Cheers,
Tom




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Old May 11th 09, 01:30 PM posted to rec.radio.amateur.antenna
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wrote:
I took the "Axial Propagation Factor" (4.372 rad/m) figure which was
given by the HamWaves calculator and multiplied it by the coil length
(155mm) to find the effective electrical length of the coil (38.83
degrees). Then I took cos(38.83)=0.779 as the fall-off in current
across the coil.


Interesting. W8JI's coil through which he measured a 3 nS
delay was 100t, 2" dia, 10" long, #18 wire.

http://www.w8ji.com/inductor_current_time_delay.htm

Converting everything to metric and entering the data into
the HamWaves calculator at 4 MHz, yields a calculated delay
of 21.5 nS through the W8JI coil and a VF of ~0.04 at 4 MHz.

So which are we to believe? W8JI's measurements or ON4AA's
calculator. There's a 7x difference between 3 nS and 21 nS.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
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Old May 11th 09, 06:56 PM posted to rec.radio.amateur.antenna
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On May 11, 2:26*am, wrote:
Hi Tom,

I should have been more explicit.

I took the "Axial Propagation Factor" (4.372 rad/m) figure which was
given by the HamWaves calculator and multiplied it by the coil length
(155mm) to find the effective electrical length of the coil (38.83
degrees). Then I took cos(38.83)=0.779 as the fall-off in current
across the coil.

73,
Steve G3TXQ

Hi Steve,

OK, so I suppose you are assuming that the current distribution will
follow a cosine along electrical degrees of your antenna, with a
maximum at the base/feedpoint. If that's the case, then would you not
account for the bottom 10 feet of wire, about 20.5 electrical
degrees? If I do that and assume 1 amp at the feedpoint, I should see
about .9367 amps at 20.5 degrees and 0.5101 amps at (20.5+38.83)
electrical degrees. 0.5101/.9367 would then be the ratio of currents
between the ends of the coil, and that's 0.5446, only a 45.54 percent
fall-off.

In fact, it seems to me that the idea of cos(38.83 degrees) = .779
would imply a fall-off of 22.1%... and that tells me that perhaps I'm
still not understanding your model very well. Maybe you are NOT
assuming the current along the electrical degrees of the antenna, up
from the feedpoint, will have a cosine distribution. At this point, I
have to say that I'm just not at all sure what your model really is.
Perhaps you are making different assumptions about the current
distribution...

Also, if you still have the model around, try adding a top hat to the
upper wire. For simplicity, you can just use a simple "T" structure,
where the top horizontal wire is, say, five feet long total. With
such a configuration, what's the current distribution along the
radiating element going to be?

Of course, what I'm suggesting here is that one must be careful to
test ones models at corner cases before putting too much faith in
them, and even then, one must always be wary of cases where the model
may go awry.

Cheers,
Tom

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Old May 11th 09, 09:11 PM posted to rec.radio.amateur.antenna
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K7ITM wrote:
OK, so I suppose you are assuming that the current distribution will
follow a cosine along electrical degrees of your antenna, with a
maximum at the base/feedpoint.


This is a good assumption for horizontal 1/2WL thin-wire
dipoles as presented by Kraus. It doesn't seem to be valid
for loaded vertical antennas where there is an instantaneous
phase shift at the impedance discontinuities. There is a
definite change in the slope of the current profile at
such boundaries.

And there is the nagging current bulge in the loading coil
causing a rise in current in adjacent turns. Normally a
current maximum would indicate a purely resistive impedance
but that doesn't seem to be the case inside a loading coil.

Years ago, I gave up on the current cosine argument for
loaded mobile antenna current in favor of loading the
coil with its characteristic impedance and using traveling
wave current to measure the electrical length of the coil.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
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Old May 11th 09, 09:40 PM posted to rec.radio.amateur.antenna
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Tom,

Firstly, I'm guilty of a "sloppy" choice of words. Whenever I've been
using the phrase "drop off in current" I've meant the current at the
top of the coil as a percentage of the current at the bottom. So when
I've quoted 70% the current will have reduced by 30%. Apologies!

Secondly, you're testing the limits of my understanding with the
overall current distribution from base section, through the coil, to
the top section. However I think the point is that you can't simply
"add electrical degrees" through the various sections when the
characteristic impedances of the sections are so disparate. That was
Cecil's point in the very first posting. We also know that, as
expected, summing the "degrees" for the three sections gets nowhere
near a total of 90 degrees, so clearly you can't assume a cosine
distribution that is contiguous across all three sections.

I'll investigate what happens with a "top hat".

73,
Steve G3TXQ



On May 11, 6:56*pm, K7ITM wrote:

Hi Steve,

OK, so I suppose you are assuming that the current distribution will
follow a cosine along electrical degrees of your antenna, with a
maximum at the base/feedpoint. *If that's the case, then would you not
account for the bottom 10 feet of wire, about 20.5 electrical
degrees? *If I do that and assume 1 amp at the feedpoint, I should see
about .9367 amps at 20.5 degrees and 0.5101 amps at (20.5+38.83)
electrical degrees. *0.5101/.9367 would then be the ratio of currents
between the ends of the coil, and that's 0.5446, only a 45.54 percent
fall-off.

In fact, it seems to me that the idea of cos(38.83 degrees) = .779
would imply a fall-off of 22.1%... and that tells me that perhaps I'm
still not understanding your model very well. *Maybe you are NOT
assuming the current along the electrical degrees of the antenna, up
from the feedpoint, will have a cosine distribution. *At this point, I
have to say that I'm just not at all sure what your model really is.
Perhaps you are making different assumptions about the current
distribution...

Also, if you still have the model around, try adding a top hat to the
upper wire. *For simplicity, you can just use a simple "T" structure,
where the top horizontal wire is, say, five feet long total. *With
such a configuration, what's the current distribution along the
radiating element going to be?

Of course, what I'm suggesting here is that one must be careful to
test ones models at corner cases before putting too much faith in
them, and even then, one must always be wary of cases where the model
may go awry.

Cheers,
Tom


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Old May 11th 09, 09:19 PM posted to rec.radio.amateur.antenna
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On Mon, 11 May 2009 02:26:04 -0700 (PDT), wrote:

I should have been more explicit.

I took the "Axial Propagation Factor" (4.372 rad/m) figure which was
given by the HamWaves calculator and multiplied it by the coil length
(155mm) to find the effective electrical length of the coil (38.83
degrees). Then I took cos(38.83)=0.779 as the fall-off in current
across the coil.


Hi Steve,

I don't often drop into this side-thread as the topic had drifted into
a stagnated intellectual backwater.

On this and one prior posting by you:
On Sat, 9 May 2009 13:56:31 -0700 (PDT),
wrote:
OK, I tried what you suggested. I put my loading coil midway up a 20ft
vertical wire in the EZNEC model. I reduced the number of turns to
lift the resonant frequency to 5.6MHz.


I note how little Corrum really has to offer when you had to take the
same:
effective electrical length of the coil (38.83 degrees)

and change it (to the same effective electrical length? I think not.)
to fit the same available wire, at the same specific frequency - only
at a different height along the available wire.

By my quick read on the stale crisis of current "fall-off" and proving
Corum by EZNEC; it seems quite apparent that EZNEC (the authority) is
driving the coil requirements which are then force fitted by Corum's
inappropriate application.

After all, Corum says nothing of:
1. Application;
2. Base loading;
3. Mid or Top loading;
4. Stinger selection;
and yet all solutions seem to derive from their math with the elegance
of an ad-hoc "missing degrees" provision (that is quickly discarded as
shown above when current becomes the focus).

Corum DOES say that the formula is only applicable for certain
constraints which I note are NEVER observed in the application nor the
breach. All of the commentary proceeds through equation (32) when
every argument is an instance of equation (31).

How much are you willing to accept of that paper (which is another way
of asking how much you are willing to discard)?

I will ask one ace-buster question that I expect no one will answer:
Show me the computation for M (= tau · a)
which would be appropriate for the NON-quarterwave resonance of the
coil in question at 3.85 MHz.

For extra credit:
1. What is the wave number, k for 3.85 MHz?
2. What is the phase velocity for the original (not changed) coil?
3. What is tau for the original (not changed) coil at 3.85 MHz?

Yes, this is intimidating to ask; but seeing there are so many
authorities on Corum; and that these considerations would have been
done by the authors themselves; then their solutions must reside
somewhere in notes or as marginalia for quick reporting (or could be
summoned up through running through the same math as before).

73's
Richard Clark, KB7QHC


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