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Author Topic: 75m Loading Coil Epic  (Read 6294 times)
W5DXP
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« Reply #15 on: October 30, 2009, 05:07:37 AM »

> N3OX wrote: I think you think Tom's a lot wronger than Tom actually is... <

I think it is technically impossible to get a 4 MHz signal through a 10 inch long, 100 uH inductor in 3ns. I have bench measured that 4 MHz delay to be approximately seven times greater than 3ns.

The current on a standing-wave antenna is primarily of the form I = Imax*cos(kx)*cos(wt). The phase of such a current (referenced to the phase of the source current) doesn't change over the length of the 1/4WL antenna. Such a current cannot even be used to measure the delay through a wire, much less through a loading coil. Trying to use standing-wave current to measure phase shift between two points is just about as wrong as can be.

What is the phase shift along 90 degrees of lossless stub? Does that zero phase shift make for zero signal delay, i.e. faster than light speed, from one end of the 1/4WL stub to the other?

Traveling wave antenna current is of the form I = Imax*cos(kx+wt), ideal for measuring the delay through a wire or through a loading coil. This is the current existing in 1/4WL of transmission line terminated in its Z0 characteristic impedance.
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N3OX
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« Reply #16 on: October 30, 2009, 07:30:16 AM »

"I think it is technically impossible to get a 4 MHz signal through a 10 inch long, 100 uH inductor in 3ns. "

It would be easy if it went around :-)

 (or we had a very good magnetic material to change the field configuration), but OK, given the paper and EZNEC results I think you're right.  The phase velocity of mode propagation at 4MHz on ( let's be more specific) a 10 turn per inch 100 turn  18AWG coil 2 inches in diameter should be quite slow compared to c.

This should manifest itself as a partial standing wave on that coil.


The problem is that I'm not the one that you had to convince all this time and even I find myself shuddering at the juxtaposition of "3ns" and "100uH" and "4MHz" all in the same sentence ;-)

The problem that I have is that with all the arguing about "delay" is that the phase velocity on the coil is only important to this problem in the sense of setting up the standing wave pattern.  And the phase velocity is essentially impossible to measure except by the standing wave pattern.  You can't turn on and off a 4MHz wave using only 4MHz energy.  It had to always be there or it had to come to your point on the coil from infinity where it had been on forever.  

So I think the delay discussion is distracting even if you've got a point, because when you say "delay" some people think "group delay" some think "phase delay" but the latter only makes sense to spend time thinking about if you already accept that the phase speed might be slow. and furthermore you can't measure it directly, only indirectly.  I find it straightforward to visualize the delay line made up of "distributed inductance and capacitance" that leads to this.    But I have a hard time figuring out how you can measure the "delay" without measuring the effects of standing waves or terminating the coil (which then raises the objection about phase shift due to the RL circuit, which might be a totally valid objection depending on whether or not that's another thing that's indistinguishable depending on the point of view you take)

My point is that there are *other consequences* and signs of this phenomenon ... and you should pick the ones that avoid triggering people's emotional reactions if you want a mutual agreement.  And you should focus on predictions that the model makes that can't be made another way.

I still think that ultimately, the problem here is the definition of being "close to self resonance" vs. being right on it, and where the self resonances are.

Let's set aside 4MHz for a second.

What *should* a group delay measurement look like on a 10 TPI, 100 turn, 2 inch ID, 18AWG coil ?

Instead of going on about "how you can't use standing wave current to measure delay," why not work on what happens when you do measure the group delay using a network analyzer.  Is there a possible signature of the overall effects we're discussing?  And I don't mean what didn't get measured and what I think probably *has* to be measured using amplitude measurements (the phase velocity).  Let's look at what did get measured.

Here's the data:

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

We know exactly what the coil is physically.  ON4AA has given us a quick and easy way to calculate the properties of coils using the Corums' sheath helix model.  

In particular, what's special about 16.1152 MHz?   it's clearly a self resonant frequency if something so wacky happens there, right?  Conventionally speaking, we know how to put that: the coil self capacitance is resonating with its inductance.

But not all inductance calculators should be able to get that right.  G4FGQ's SOLNOID3 doesn't (that's the other one I use a lot).  What does the hamwaves.com calculator say about 16.1152 MHz if you feed it 50.8mm diameter 254mm long coil with 100 turns of 1.024mm wire?

What about if you estimate the other two frequencies of significant departure from 3ns flat delay?

There's a little wiggle around 9.5MHz and another big resonant peak at 24 or 25MHz.  Are these measurement artifacts?  (no way)  Are they due to stray environmental effects?  (doesn't look like it to me).

What is the "electrical length" of one of these coils at each of those frequencies?

I get pi/2, pi, and 2*pi with the middle one being the best agreement because I actually know the frequency.  At 16.1152MHz, according to the hamwaves calculator, a 10 inch coil is within one percent of having beta*length of pi.  

One value of the sheath helix model, in my opinion, is the quantitative prediction of self resonances without resorting to EZNEC.  I may indeed be wrong and there may be an ad-hoc stray capacitance model that can just tell you all the self resonances.  But I don't think so, because it seems to me that the "stray capacitance" as a function of frequency depends on the detailed electric field distribution around the coil.

It seems like the frequency ratio of self-resonances of a coil is sort of non-obvious and if it's predicted well by the sheath helix model and the physical length of the coil, that seems to suggest that the model might be on to something.  So who cares about 4MHz?  What about 9.5, 16.115, and 24 whatever... ?  

73
Dan
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Dan
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« Reply #17 on: October 30, 2009, 09:15:10 AM »

quote,
"BTW, it seems there are two sorts of definitions of "electrical length" which are conceptually different in the physical world."

Dan, Thanks.

That, in part, is the reason I asked, and you
provided a great explanation--also covering most of my follow up questions. That would almost count as an ESP-QSO, right?

Thanks, Larry
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« Reply #18 on: November 04, 2009, 05:09:02 PM »

> N3OX wrote: What *should* a group delay measurement look like on a 10 TPI, 100 turn, 2 inch ID, 18AWG coil? Instead of going on about "how you can't use standing wave current to measure delay," why not work on what happens when you do measure the group delay using a network analyzer. <

Dan, could you explain what relevance "group delay" has when the only signals are coherent single-frequency forward waves and reflected waves?
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N3OX
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« Reply #19 on: November 04, 2009, 09:27:25 PM »

Darn, I lost this once... hopefully better this time.

"Dan, could you explain what relevance "group delay" has when the only signals are coherent single-frequency forward waves and reflected waves?
-- "

Cecil, I'm not interested in talking about this aspect in this way anymore.  I do not need convincing.  I'm all on board with slow phase speeds on slow coils, the resultant standing waves, and that if you try to measure the phase speed using standing waves you have to look at the amplitude.  

The problem is that to measure the phase speed aspect of the slow coil mode model using monochromatic excitation, you already have to *accept the model* and you have to infer the slow modal phase speed from observing the standing waves.  That's fine, I could do that experiment someday and post some results.  Or I could get my hands on HFSS when I have a moment and make a nice 3D simulation picture.  Whatever.  Maybe I will.  I understand and agree with the reasons for standing waves on helices of the typical sort used to load 75m mobile antennas.  

I don't want to talk about monochromatic excitation anymore.  I reject your insistence on that. I don't want to talk about terminating the coil in its characteristic impedance so that you can measure the traveling wave's propagation time.  The reason why I don't want to talk about these things anymore is that at this point, we agree on this.  

My continued desire to discuss, and my continued reluctance to declare some sort of victory for you is *NOT* because we're on different pages with respect to the phase speed of monochromatic waves on a typical 75m bugcatcher coil.  We're not on different pages with respect to the current taper.

Where we differ comes down to what comes next.  I appreciate that you feel a sense of urgency in correcting  the notion that "coils don't add electrical length" using this information.  However, I don't agree with your interpretation that the electrical length of slow coil loaded things is of a *different, distinct nature* than the electrical length of lumped coil loaded things in the context of loaded objects that interact with free space waves!

I understand and appreciate that you can actually see the consequence of the modal wavelength on real coils.  However, the interaction of a 75m bugcatcher coil with **free space waves** (or those on open wire line) is entirely due to the axial component of the current on it.

The phase shift caused in the current from end to end, whether it's partially due to launching a mode on the coil from one end to the other or entirely due to lumped point phase shift by an inductor, is still just a phase shift in the current on the antenna.  

In terms of the *mode on the coil* there's a real and fascinating physical reality to the "electrical length."  In terms of the original thread back in Elmers that caused the new flare up of this discussion, however, I think you were wrong.  Those who said that series coils in open wire conductors don't add electrical length to a feedline were in fact, correct.  

This is also true of short loaded antennas.  The slow coil modes have consequences.  You can get current taper from end to end.  You can even get phase reversals from end to end.  If you let a mobile coil phase reverse you've built a terrible mobile antenna.

However, what you can't get get from a slow coil loaded antenna, as far as I can tell, is a *different* interpretation of the electrical length of the overall antenna structure.  Lumped or self-consistently calculated distributed coil, the antenna ends up 90 degrees long electrically and has about the same radiation resistance (unless you let the taper get severe, in which case it gets worse).

We can continue discussing why we disagree on this point, but don't try to convince me that slow coils are slow coils anymore.  I understand and appreciate that and its consequences to current taper.  I understand that it leads to standing waves on coils.  I'm not interested in having you hammer on me about it, because I agree with you that it happens.  What I don't agree with is that it makes the slow coil have a "more physical" electrical length in terms of the antenna or transmission line you build with it.

So, let's truncate your question:

""Dan, could you explain what relevance "group delay" has"

I think Tom's measurement actually gives evidence for the correctness of the sheath helix model's electrical length prediction.

We know exactly what the coil is in terms of physical dimensions.  It's probably Miniductor stock,  so it's built to mass produced factory precision.   We know that it's been misterminated, because it's hooked to 50 ohms on either end, so it will support standing waves.

Group delay is defined as the rate of change of transfer function phase with frequency.  Since the coil supports standing waves, the group delay from end to end if you ignore the amplitude only changes at special points.  

Those special points happen when you cross a phase reversal as you increase frequency.  Before you get to a special point, when the coil is short, the group delay changes with frequency like this:

0 0 0 0 0 0 0 0 0 0 0 ....

Because the standing wave is standing.  If you ignore the amplitude, you don't notice anything.

When you pass the half wavelength point, however, you go from supporting a partial standing wave to supporting more than half of one.  You go from having the current at either end *in phase* at different amplitudes to having it *180 degrees out of phase*   ... as you go through that point in frequency, you get a delta function spike in the group delay by differentiating the step function change in phase:

 ... 0 0 0 0 0 0 0 0 0 0 180 180 180 180 ...

Now, this is a coil self resonance.  The first one of those can be used to form an ad-hoc lumped model of the non-ideal inductor.  The self resonant frequency tells you the "stray capacitance"  One might fail to see an issue with this first peak in the S21 group delay.

Now we CONTINUE to increase frequency.  The group delay doesn't change because now the phase is just

... 180 180 180 180 180 180 180 180 180 180 ...

Great, fine.  We've seen one jump in the group delay.  But then... there's ANOTHER one.   You and I know it's this:

... 180 180 180 0 0 0 0 0...

This is when the coil passes through 1 wavelength long.  

The frequencies at which these two special points occur correspond to the resonance spikes in Tom's plot.  The one with a marker on it is within 1% of half-wave resonance.  The next one is roughly where the full wave resonance should be, which is a non trivial result.  It's not the second harmonic of the first one or anything.  And it shouldn't be, according to the sheath helix model.  

The little twiddle a bit above 9MHz is about where the coil is a quarter wavelength long.  I don't know why you get a twiddle there in this plot; maybe it's that the effect of the terminations on the standing waves changes greatly for 1/4 wave structures?

THESE are the interesting things I'd like to discuss.  This is the way in which "coil electrical length" is a real, physical thing  that can be used to predict self resonances based on a standing mode on the coil, and the corresponding phase flips.

But don't conflate the modal electrical length and the resulting modal standing waves with the electrical length defined by the standing waves of the mode on a whole short, loaded antenna.   Even with all this information, the electrical length of the *whole antenna* is the same as it ever was.

73
Dan
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Dan
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« Reply #20 on: November 05, 2009, 08:01:35 AM »

> N3OX wrote: W5DXP posts in a typically inappropriate location <

Dan, until you made this posting and directed me to this 'misc' forum, I didn't even know a 'misc' forum existed. I thought 'articles' and 'elmers' were all there are on eHam.net. Thanks for alleviating my ignorance.

> N3OX wrote: Those who said that series coils in open wire conductors don't add electrical length to a feedline were in fact, correct. <

Of course, a coil doesn't add the same electrical length to a piece of 450 ohm line as a piece of 450 ohm line would add - because the coil is NOT Z0=450 ohms. Just like an antenna, the Z01/Z02 impedance discontinuity adds or subtracts electrical length. A large air-core coil always adds electrical length. The resistance value is never the same at both ends of the coil unless there is an absence of reflected waves or a node exists halfway through the coil.

> N3OX wrote: But don't conflate the modal electrical length and the resulting modal standing waves with the electrical length defined by the standing waves of the mode on a whole short, loaded antenna. Even with all this information, the electrical length of the *whole antenna* is the same as it ever was. <

This subject has a long history with two sides of the argument on the rail. So I will summarize. Assume a resonant base-loaded mobile antenna with a 11 degree long stinger. (I have an EZNEC model of such.)

1. One side says the other 79 degrees comes 100% from the phase shift caused at the impedance discontinuity point between the top of the coil and the stinger, i.e. there is zero phase shift through the coil just as the lumped-circuit model presumes. (This was essentially W8JI's position with a few band-aids attached.)

2. The other side says the other 79 degrees comes 100% from the phase shift through the loading coil. (This was essentially K3BU's position with a few band-aids attached.)

Each side is partially right and partially wrong. The 79 degrees phase shift comes from both of those two sources listed above. Part of the 79 degrees comes from the impedance discontinuity and part of the 79 degrees comes from the phase shift through the loading coil. This is easy to illustrate using a Smith Chart. The number of degrees of phase shift through the coil, at the impedance discontinuity, and through the stinger can be read directly from the Smith Chart, one of the most basic analysis tools.

Somewhere I have a Smith Chart illustrating the above concepts but I can't seem to locate it with one bad eye. :-)
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« Reply #21 on: November 05, 2009, 02:19:54 PM »

"Part of the 79 degrees comes from the impedance discontinuity and part of the 79 degrees comes from the phase shift through the loading coil."

Agreed.

"The number of degrees of phase shift through the coil, at the impedance discontinuity, and through the stinger can be read directly from the Smith Chart, one of the most basic analysis tools."

Yes, I understand that and agree.

My problem is that you can get the same 79 degrees broken up in different proportions depending on the coil you choose.  A toroid, while probably too lossy to be of practical use, is the closest-to-lumped extreme of that.  In that case, nearly all the phase shift comes from the impedance discontinuity.

The other extreme is a self resonant helical antenna, where the whole antenna is coil.  Then it's all coil mode.  There aren't any discontinuities on the antenna.
 

But then there is a continuum of possibilities in between.  And that's why I'm hesitant to draw a distinction between the electrical length of mobile antenna loaded with different types of coil.  The electrical length of the resonant antenna is 90 degrees.  I understand and appreciate how that breaks up into different proportions based on coil geometry.  

"The resistance value is never the same at both ends of the coil unless there is an absence of reflected waves or a node exists halfway through the coil. "

Yes, that's right.  The problem I'm having is that the same could be said for two points *separated in space* on either side of a lumped loading coil on a mobile antenna.  A lumped coil in the middle of a rod or wire can add "electrical length" too and if you pick two points on either side of your loading network you might have a hard time telling what's in between:

http://n3ox.net/files/four_bugcatchers.jpg

What's in the dark gray cylinder doesn't have any effect on the *antenna's* electrical length, if we define electrical length of the antenna based on its resonance.  The thing in the cylinder, if this is a possible picture, even has a minor effect on the radiation resistance and base impedance and efficiency.  

The coil in B is some hefty fraction of a half coil mode wavelength.  The toroid in C is essentially zero.  The coil in D is several modal wavelengths.

Can you find loading coils like those in B,C, and D such that the information you get in A is insufficient to tell you which thing is inside the black box coil housing?

In all cases, the measured impedance at the top and bottom of the housing is differerent.

That's not a specialty of slow coils.

So the reason why I'm still having trouble with this discussion is this:

Does the fact that you usually use the coil in B to make a good bugcatcher really mean that there's something different, in terms of the ANTENNA, about the "extra electrical length" provided by the thing inside the box in B, C and D?

The same goes for a feedline.  I could possibly use slow coils to add a sort of electrical length, but I could use lumped coils somewhere distant from my feedpoint, I could use a balanced L network...

I could make the impedance at the input terminals of a 0.322 wavelength line exactly the same as the impedance of the load transformed through 1/4 wavelength of 450 ohm line.  I could use any number of networks to do it.   But other than at that single frequency, it wouldn't behave like I had the rest of 1/4 wavelength of 450 ohm line in there.

Now, I think I said some time back that you couldn't change the resistance at the input terminals with series coils, even slow ones, in a feedline, and I see now that that is certainly false.   But you can also change the resistance seen at the input using lumped coils some distance from the input terminals.

So I'm still having trouble with this.

You say:

"A large air-core coil always adds electrical length"

but a lumped coil in a two foot stick also adds electrical length, and does it incommensurate with the physical length of the stick.

The details are different but the general effect is the same.  The only place you can't "add electrical length" with lumped coils is right at the feedpoint.  

In that sense, I was wrong to suggest that you were wrong to object to people who were saying "coils don't add electrical length"

But I guess what I'm saying now is that coils ALWAYS add "electrical length" to antennas, no matter what.

And unless you butt lumped ones right up against your source, they can be used to add "electrical length" to feedlines too.  

And we have to remember that feedline "electrical length" and antenna "electrical length" mean two different things: for feedlines refer to both resistance and reactance transformation and for antennas it really just refers to the phase shift between voltage and current at the feedpoint.

Seems like talking about what coils do to electrical length is awfully messy.
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« Reply #22 on: November 06, 2009, 07:54:28 AM »

> N3OX wrote: But I guess what I'm saying now is that coils ALWAYS add "electrical length" to antennas, no matter what. <

Yes, that's true in the real world - unlike the overly-simplified lumped-inductor world that can exist only in a human mind.

> N3OX wrote: And unless you butt lumped ones right up against your source, they can be used to add "electrical length" to feedlines too. <

But remember, in the real world, there is no such thing as a lumped (point-sized) inductor. All real-world coils add "electrical length" even if that electrical length may (or may not) be negligible.

> N3OX wrote: Seems like talking about what coils do to electrical length is awfully messy. <

Yep, that's why there is so little understanding even among some hams who are considered gurus on the subject - gurus who publish false technical information on their web pages (which tends to drive me crazy).

Incidentally, I found the Smith Chart that I mentioned earlier. Here it is, part of a magazine article that I am writing:

http://www.w5dxp.com/SCmobant.GIF

The base loading coil is 19 degrees long and the stinger is 19 degrees long. There's 52 degrees of phase shift at the coil to stinger junction.

Here's a question for you and others: Given that the impedance looking into the stinger is -j1100 ohms, what would the characteristic impedance of the base-loading coil have to be to cause the total antenna to be 90 degrees long with zero degrees of delay through the coil?

Hint: What Z0 value divided into -j1100 ohms will equal zero?

Conclusion: For the current into the coil to be equal in amplitude and phase to the current out of the coil, the characteristic impedance of the coil would have to be infinite - proving that such cannot possibly be true in reality.

Given W8JI's "measured" delay of 3ns through his 100uH coil on 4MHz (a 4.3 degree delay), the characteristic impedance of his coil would have to be around 150,000 ohms, obviously a real-world impossibility.
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N3OX
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« Reply #23 on: November 06, 2009, 10:17:26 AM »

"For the current into the coil to be equal in amplitude and phase to the current out of the coil, the characteristic impedance of the coil would have to be infinite - proving that such cannot possibly be true in reality. "



Cecil,

Usually a "thing that happens at a point discontinuity" has a good mathematical definition as a *limit*

But I think it's probably fruitless to talk about that right now.

"But remember, in the real world, there is no such thing as a lumped (point-sized) inductor. All real-world coils add "electrical length" even if that electrical length may (or may not) be negligible. "

I do *NOT* intend to continue this conversation if we're going to use combination of generalities and specifics to prove your point.

"the characteristic impedance of his coil would have to be around 150,000 ohms, obviously a real-world impossibility. "

Yes, 150,000 ohms is much greater than ~4700 ohms.

Now let's stop talking about Tom's coil and talk about something else.

What's the characteristic impedance of a 28 +/- a few turns of wire wound on an Amidon T400A-2 iron powder toroidal core?  (4 inches OD, 2.5 inches ID, 1.3 inches thick Al = 360 uH/100 turns)

Can you or can you not load the same  mobile antenna to resonance with that coil (adjusting turns if necessary to get the right resonant point) as you could with the coil that Tom actually measured?  

What's the ratio of coil electrical length to antenna electrical length if you use 28 turns of wire on a T400A-2?

What's the analysis say about a toroid?  It seems like the toroid has to have massive characteristic impedance, but I think that's right.  Seems like the ratio of magnetic field energy storage to electric field energy storage *is* completely related to the characteristic impedance.

"All real-world coils add "electrical length" even if that electrical length may (or may not) be negligible. "

What I'm suggesting that even a zero-length infinite characteristic impedance ideal coil adds electrical length.  The impedance transformation properties of a zero-length piece of infinite impedance transmission line might seem to you be undefined, but I think it's probably defined in the proper limit.

I realize that it's not physically realizable, but there's the difference between a physically unrealizable thing with a *defined limit* and one without.

If you have a real world system whose properties are very close to an idealized, but extant, limit, it will work very much the same.

If you have a real world system where there's not a nice limit as you head to the idealized case, something nasty happens and being "close" doesn't count anymore.

Loading coils are the former.  A tightly flux coupled coil with negligible electric field effects will have a huge coil characteristic impedance as defined by the sheath helix model and the hamwaves calculator.  It will have a negligible electrical length in terms of a mode on the coil.  It will be very close to a lumped inductor.

It is NOT an inductor that you can build on an air core to the tune of 100uH.  I accept that.

But if you're going to keep talking about an air core 100uH coil instead of allowing us to switch gears to a more tightly magnetically coupled situation for the purposes of discussion, it's probably time to take another break.

And if you accept my suggestion that 28 +/- turns of wire on an Amidon T400A-2 core *can* be used to load a 75m mobile antenna to resonance, and furthermore agree that such a coil would have a huge characteristic impedance in a transmission line description of the antenna...

but then you insist that something nasty would happen as you transition from that "almost lumped" coil to a "lumped" coil, and that the toroid "adds electrical length" in a different way from its conceptual lumped cousin, then I think maybe this discussion is slowing to a standstill.

73
Dan
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« Reply #24 on: November 06, 2009, 12:57:06 PM »

> N3OX wrote: I do *NOT* intend to continue this conversation if we're going to use combination of generalities and specifics to prove your point. <

Whatever it is that I did to upset you, it was not designed for that purpose, and I apologize for it.

> N3OX wrote: What's the characteristic impedance of a 28 +/- a few turns of wire wound on an Amidon T400A-2 iron powder toroidal core? <

I don't know. The Dr. Corum papers and Hamwaves inductance calculator are limited to single-layer, round-wire, air-core, helical coils. I am not aware of any similar experimentation having been done on toroidal inductors.  Thus, there is a missing piece in my knowledge base between infinitesimally small inductors and physically large air-core loading coils. I could speculate but that would get us nowhere.

> N3OX wrote: but then you insist that something nasty would happen as you transition from that "almost lumped" coil to a "lumped" coil, and that the toroid "adds electrical length" in a different way from its conceptual lumped cousin, then I think maybe this discussion is slowing to a standstill. <

What you are experiencing is the limit of my knowledge, NOT an insistence that "something nasty would happen". I simply don't know the answer.
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« Reply #25 on: November 06, 2009, 04:32:31 PM »

"Whatever it is that I did to upset you, it was not designed for that purpose, and I apologize for it. "

Cecil, what makes me upset is worrying and talking about others' errors and potential errors instead of talking about things we know are true and figuring out the details of things we think are true but aren't sure about.

I know that you see some info on Tom's page and others that seems glaringly in error and urgent, and I probably agree thoroughly about *some* of the things you find in error, but none of that matters much to me until everything is neat and tidy in *our* mutual understanding.   And I think that an important part of that is how the lumped coil fits into the picture.  The lumped coil can not be set aside as a physically unrealistic fiction.

"What you are experiencing is the limit of my knowledge, NOT an insistence that "something nasty would happen". I simply don't know the answer."

OK, that's fine.  I'm sorry I got upset.    I think I do know and I'll try to explain why I think that we still need to consider lumped coils to add electrical length that's no different than real coils.

There's a smooth transition from a coil that is almost but not quite lumped to one that is perfectly, ideally, physically impossibly lumped.

And I don't know why I didn't think of this before, but the difference between a lumped coil and a real, "electrically short" coil is much like the difference between a characteristic impedance *discontinuity* in a transmission line and a real pair of lines:

http://n3ox.net/files/real_junction.jpg

Until you zoom in far enough that the junction impedance taper becomes a tapered line of an "appreciable fraction" of a wavelength, you can ABSOLUTELY use theory that assumes a point discontinuity and get the right answer.  A lumped coil is a pointlike phase discontinuity.  It's all shift, no length.  The *shift does not go away* just because you brought the two discontinuities of the line together.  The lack of line length does not cancel the phase shift provided you ramp up the impedance as you take the line length to zero in just the right way to keep the same total phase shift!  

A lumped coil's transmission line analogue really is an infinitesimal length of infinite impedance line where the infinities resolve to give you a phase shift through a point.  That's not unphysical as a limiting case, and it's not inapplicable to "nearly lumped" coils in the real world.

You could build the same phase shift in the real world with a very short piece of extremely high characteristic impedance line or a longer length of lower characteristic impedance line.

Let's say you actually built a real line with 4 degrees of 150,000 ohm impedance line.   Or 0.00003 degrees with 1 mega ohm impedance line.  Or a microdegree of gigaohm line.  (those don't work out all the same, just for illustration).

The thing that I object to when you say that "all real coils add electrical length," is mostly that you insist on electrical length addition as being a property of real coils.  It's not.  It really does happen with ideal coils in the same way.  A very good approximation to a lumped coil might be like adding a microdegree of gigaohm line to get your phase shift, while a coil poorly described by the lumped model might be like adding 25 degrees of 4700 ohm line.

A lumped coil is the addition of zero degrees of infinite impedance line such that the resultant phase shift is the same as with a microdegree of gigaohm line or 25 degrees of 4700 ohm line.

Just because it can never physically exist doesn't mean the *concept is invalid* in a proper limiting case.  Further, the microdegree of gigaohm line will give you the exact same answer as the lumped model, just like assuming theory developed for a *single* discontinuity in a transmission line will work even in a very short real world smoothed out junction like the one I drew.

The lumped model does not fail abruptly.  It fails smoothly and continuously.  And due to that, a lumped coil, a perfect, idealized, zero length inductor, is *conceptually* as capable of adding "X degrees" of electrical length as any other coil.  And the number of degrees an ideal, lumped coil would "occupy" is going to be very well approximated by the number of electrical degrees occupied by a very close approximation to a lumped coil.

And in a mobile antenna, that number of electrical degrees is the deficit from 90.

I think that you would probably be frustrated if I told you I didn't believe anything you said about your Smith Charts and transmission line discontinuities because there's no such thing as a discontinuity.  

For me to say that would actually be *incorrect* even though I'm technically right that there's not such a thing as a physical discontinuity, because a discontinuity is an excellent mathematical model for a physically short junction between transmission lines.  All of the physics of the real, non spread out, continuously tapered junction are captured by treating the junction as a discontinuity until you get very high in frequency.

And the same goes for the lumped coil.  The non-ideal nature of actual air-wound inductors snuck up on some of the participants of this saga.  It surprised them and that significantly clouded the discussion.  If the Corums didn't make an error and you or I have not made an error of interpretation of that paper and of EZNEC results, a real air-wound 100uH bugcatcher coil made from ten inches of 2 inch ID 18AWG miniductor stock is just not well modeled by a lumped inductor in terms of the current in or out (the phase shift, of course, is always the same

However, I think I have a better handle on the root of my initial objection and why I was upset with your posts in the Elmers forum, and why I want you and I to discuss a toroid loading coil.

I object to what I see as trying to burn the lumped model behind us by suggesting that real coils add electrical length and lumped coils don't.  

I understand that a transmission line series section of a zero length line of infinite characteristic impedance might seem unphysical, but I think it's not as long as you tell someone what the phase shift it causes is.  And likewise, there's nothing unphysical or different about the "electrical degrees" added by a conceptual lumped coil, even if you can't build it.

In a transmission line analogy, truly ideal lumped coils are all discontinuity shift and no length.  Toroid loading coils are mostly discontinuity shift and a little section length.  Slow coils are a good dose of both.  Helical antennas are all section length.

My continued uneasiness and sometimes frustruation is related to the fact that  "electrical length occupied" has nothing to do with the distinction between real coils and ideal lumped coils.  It has everything to do with the total phase shift they cause in the system where they're installed.

As such, we can discuss whether or not it's a fiction to say "coils don't add electrical length" but I think we have a disagreement and a disconnect if we insist that the reason coils add electrical length is because they're real world coils.

That would be like me insisting that there's no such thing as a real world transmission line discontinuity at all, and arguing with others' comments on transmission line theory that didn't actually involve continuous transformations in real geometries.  The first part, saying that there's no such thing as a physical discontinuity in real wires, is true.  But because the theory CAN work well, it is counterproductive to call it into question in *general.*  In specific instances where it fails, we must switch models.  But we absolutely don't reject the model where it's a good approximation.  And I think the participants in the coil discussion have always been too far out on the "is lumped a good approximation" curve to see eye to eye.  

We all have good intuition for how the discontinuity model breaks down for real installations of open wire line and coax because the length involved is never surprisingly short.  The wavelength on regular transmission lines is close to that in free space.  The "spreading" of a discontinuity can be seen and estimated easily.  So if you and I and Tom and Yuri and whoever got together and talked about the "breakdown of the point discontinuity model" at transmission line junctions, we'd probably all clock in in the GHz region with our guesses.

I think with the coils discussion, we all have much less intuition where a particular physical object is on the continuum between lumped coil and helical antenna.  But in the same way, it's still a continuum, and we need to be *very careful* not to say "the lumped model is useless for ham radio loading inductors" or "the lumped model applies to all good ham radio loading inductors" because I doubt either is true.  And while a real distributed treatment might be needed for the particular infamous chunk of coil stock in question, I still think it's important to recognize that all of this is a subtle matter of degree, not a general breakdown of the *concept* of lumped inductance.

This is the case even if it causes gross errors in certain aspects of peoples' conclusions and results.    You have to look at what the error in thinking is to assess the severity of the mistake, not only the error in results.  And I think in this case, there has been a lot of overestimating the others' errors in thinking.  People have done that to you and you've done that to others.  I think that you come on too strong with the "failure of the lumped inductance model" and that shuts people off because they know that the lumped inductance model "only fails when there's a lot of distributed capacitance."

The infamous chunk of coil stock used on such a low frequency lives in a very counterintuitive region that needs to be looked at quantitatively.  A "lot of distributed capacitance" is not present at first glance, but strong electric fields, even on the coil itself in isolation, make it so that the electric field effects, the displacement current effects are strong.  This is something that many people understand rather well as a general principle.  They just don't realize it applies at 3.8MHz to Miniductor stock!  

I think, though, you've tended to frame the failure the lumped model for that object as a GENERAL failure of the lumped model, you will shut people off from bothering to look at the details... I think it is basically a specific failure for describing the current taper on thousands-of-ohms-inductive air core coils which is how this all started, I guess.   But to draw big general conclusions, to discuss the big picture, you need to look at the whole spectrum of things that could be used to load an antenna.  

If the statement "coils don't add electrical length" is incorrect, I think it must be incorrect even for lumped inductors.  Lumped inductors add electrical length.  The fact that their transmission line analogue in antenna occupies zero physical length shouldn't cause a conceptual problem with the electrical length added by a lumped coil.  

73
Dan
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« Reply #26 on: November 06, 2009, 10:23:36 PM »

I think I made an N^2 mistake in my toroid inductor example.  You can wind 100uH on that core but I messed up the number of turns ...
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« Reply #27 on: November 07, 2009, 08:12:34 AM »

> N3OX wrote: I object to what I see as trying to burn the lumped model behind us by suggesting that real coils add electrical length and lumped coils don't. <

I'm not guilty of "trying to burn the lumped model behind us". The primary point I have ever tried to make is that the lumped model does not work for *a 75m Texas Bugcatcher coil used in a 75m mobile antenna*. I have been very careful to give large air-core inductors used on 75m as my only real-world examples. I agree with Dr. Corum that there exists a range where the lumped-inductor model ceases to yield valid results and should be abandoned in favor of the distributed network model.

Dr. Corum says that when phase (delay) is involved, any inductor longer than 15 degrees should abandon the lumped-circuit model. Of course, "15 degrees" is an arbitrary boundary. For our purposes of detailed technical discussion, the difference between 5 degrees and 20 degrees cannot be ignored.

Our only disagreement seems to be semantic. When I say that a lumped-inductor doesn't add electrical length to a system, I am not including the electrical length added by the impedance discontinuity. You seem to be including it. That's a problem with word definitions, not with technical concepts.

In fact, it can be argued that a lumped-inductor does nothing except provide an impedance discontinuity at a point. Note that an impedance discontinuity *always* causes reflections.

I personally, have mentally parsed the three components of electrical length of a base-loaded mobile antenna into:

1. Electrical length of the inductor due to the physical length of the inductor. (A lumped-inductor has zero physical length.)

2. Electrical length of the phase shift at the impedance discontinuity point.

3. Electrical length of the stinger.

For a lumped-inductor, #1 is zero, i.e. "the lumped-inductor has zero physical/electrical length", *by definition*. You seem to disagree with that definition. But the pure lumped-circuit model tells us that the current into the coil and current out of the coil are identical in amplitude and phase - because they both exist at the same point.

You seem to be saying that the electrical length of the lumped-inductor is the sum of #1 and #2. I don't have any problem with that definition except that the distinction between lumped-inductors and large real-world inductors becomes impossible to discuss logically - if there is no difference between those two phase shifts.

It is akin to combining the forward wave and reflected wave and declaring that only the standing wave exists after that combination, i.e. the forward wave and reflected wave cease to exist - which a lot of people do.
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« Reply #28 on: November 07, 2009, 11:32:25 AM »

"Our only disagreement seems to be semantic. When I say that a lumped-inductor doesn't add electrical length to a system, I am not including the electrical length added by the impedance discontinuity. You seem to be including it. That's a problem with word definitions, not with technical concepts. "

Yes, good.  We're on the same page.

"
In fact, it can be argued that a lumped-inductor does nothing except provide an impedance discontinuity at a point. Note that an impedance discontinuity *always* causes reflections. "

Yes, that's right.  I agree.

"You seem to be saying that the electrical length of the lumped-inductor is the sum of #1 and #2. I don't have any problem with that definition except that the distinction between lumped-inductors and large real-world inductors becomes impossible to discuss logically - if there is no difference between those two phase shifts. "

Yes, that's basically the issue.  The distinction between lumped inductors and large real world inductors disappears in terms of the *total phase shift* they cause.

I agree with you that it doesn't disappear in terms of the current in vs. out.  

I think it is interesting and educational to discuss the difference between #1 and #2... sometimes.  And I think failing to recognize that the difference between #1 and #2 leads to observable effects on real coils is a problem when those effects are important.

But I also think that if you don't accept "electrical length" in terms of the sum of #1 and #2, you're *often* going to end up arguing over semantics and not physics.  It is possible to load a theoretical mobile antenna to resonance with a purely lumped inductor, a point phase shift.  At that point it's electrically 90 degrees long.  You agree with that.  Tom agrees with that.  It's just true.  Unbuildable, but true.  So I think when you come up against a stubborn person who insists on "almost lumped" inductors as the only relevant ones, conceding and discussing this point can go a long way to establishing that you're on solid footing.

In the context of **current taper** in mobile antenna coils, it's not an argument of semantics to discuss the difference between discontinuity phase shift and modal length phase shift.  In the context of loading **mobile antennas to resonance**, it is.    I think that's caused some strife and needs to be approached with care.

I think maybe just stripping the discussion of unqualified references to "coil electrical length" might actually be a key to that.

I've been trying to use "coil mode electrical length" and "modal electrical length" to that end.  The coil itself supports a short spatial wavelength standing mode that matches external mode current and voltage at either end.  (of course, the coil mode can be viewed as being comprised of counterpropagating traveling waves).

That's a different "electrical length" that's relevant to the discussion of air coil loaded mobile antennas.  But I think the discussion gets clouded if you refer to the "coil electrical length," because all loading coils, from ideal lumped through toroid to big air coils, all have the same "electrical length,"  the same sum of #1 and #2 in the context of loading the mobile antenna to resonance.

I think drawing a clear, unmistakable verbal distinction between #1 and #2 could go a long way in this discussion.  And you have to be careful that everyone understands what you mean by #1.  A real coil has several different "electrical lengths."   My list is a tad longer than yours, to draw even a distinction between different types of "coil physical length!"

A) It's got an electrical length in terms of the total phase shift it causes  when it's inserted in the region it occupies in the system you install it in.  This is #1 + #2 of your list.

B) It's got an electrical length in terms of the wavelength of the n=0 sheath helix mode.  I think this is #1 on your list.

C) It's got an electrical length in terms of the physical length of the coil in terms of free space wavelengths.  This is not one we've discussed much but I think this one is of *critical* conceptual importance, especially to Tom, and especially in his particular quest against certain antenna misinformation.  This electrical length is of negligible importance compared to the total phase shift electrical length or the separate modal electrical length.  But the "free space electrical length" plus the modal current distribution tells you you how much radiation you get from the coil.  This is ALSO well described as the "physical length of the coil"

D) Just for full conceptual completeness, that it's got an infinite number of other electrical lengths corresponding to higher order coil modes (nonaxisymmetric ones where the current varies around the circumference of a turn too!) that we do not discuss here.  These electrical lengths, just like the n=0 mode electrical length, are non-trivial consequences of Maxwell's equations and the boundary conditions.  I don't know what they all are (Kraus handles a number of them), and they're **probably** not of importance to this discussion (at least I hope not), but I think they may help remind us of all the things that innocuous little spiral can do and help us think about "modes"

- - - -

So we've got these three (+1 set aside) things that we can call the "electrical length," and at some point or other in this discussion and argument, I think most people have picked at most two of them and combined those into their concept of the "coil electrical length".

I think Tom is very concerned about hams understanding the effects of A (phase shift)  and C (free space wavelength) , as they tell you **most** of the picture regarding the radiation resistance and feed impedance of the antenna.   If you look at his general comments and philosophy, you will see that he's very, very concerned about people using anything other than "the coil physical length in free space wavelengths"  in place of C.  

I think it's just too easy to get tangled up in A, B, and C if you don't give them different names.

And I think it's important to appreciate the difference between B and C and see that they're both active in a mobile antenna.  

And furthermore, you must combine information from the the modal electrical length B (current distribution on the coil)  with the coil length in free space wavelengths, C, to figure out how much radiation comes off the coil.  (Ignoring, the azimuthal current component which results in a fed-in-phase, current tapered close spaced stack of teeny tiny horizontally polarized magloops undoubtedly down dozens of dB due to their size)

The modal electrical length, B, matters to current taper.  The current taper matters to the total radiation resistance as the current distribution on the length, C,  and the current into the stinger above the coil.  

Both B and C are rightly considered different types of "physical" lengths.  I think that Tom sees a real conceptual danger in putting anything other than the free space wavelength in *place* of C.  I do too.  I'm happily at peace with the the coil having a modal wavelength B and a free space wavelength C.  I think some of the fierce fighting has centered around a concern that the numerical value of B in modal degrees was being suggested as a replacement for the numerical value of C in free space degrees.

I think if we'd used different names all along, that could have been avoided.  

I also think that while this is important to a high level understanding of coils, there is a real danger of confusing people into thinking B and C are the same thing, and that could really hurt their understanding of how to build a good short antenna.  You've said in the past in these forums that you think helical antennas are no good compared to using a good mid load coil.  I think that's clear from measurement, modeling, whatever.

That's a "full size quarter wave" in terms of the modal length, B.   We don't want people to think "longer modal length" is necessarily better.   What's better depends on the details.     It's better to put a "modal-shorter" coil higher up and get nearly constant current below the coil.  For realistic 75m mobile antennas with air core coils, it seems this requires  a particular solution that's not of vanishing modal length.  So it has current taper.  To build the best mobile antenna, we probably want to minimize that taper, though it depends on whether or not you can get a boost in current like my 2nd picture shows:

http://n3ox.net/files/four_bugcatchers.jpg

I think N7WS's simulation and yours both show a current MAX on the coil, that is, the current on the coil does actually exceed the input current before it tapers to a lower value into the whip, correct?   The devil's in the details as far as using this in designs, because the coil free space length C is a smallish fraction of the total antenna length.  Plus, in all of this, I'm still not sure about *losses* from the analytical model (as opposed to well converged EZNEC model) standpoint.  The EZNEC models should be right, but can only be used to model a very limited subset of what might be built.

There's still a discrepancy between the Hamwaves calculator and Tom's measured coil Q even though I think it gets all the self-resonances right as measured...  getting into that seems daunting at this point.

OK, I'm happy that I know we've reached a physical agreement in the context of this theory if not a completely semantic agreement.

I hope you'll understand if I don't fire off an email to Tom today or next week or next month even to discuss this.  Please understand that I don't feel that I can't do that.  It's just that given the sordid and bitter history of this argument, I want to be absolutely, totally sure through and through about what I find in serious error, what I find could use a better description, and finally whether or not this whole thing is BS anyway :-)

I do not expect the last issue to be a problem, but our entire discussion presupposes that Drs. Corum (and by extension, the Hamwaves calculator) are RIGHT, something that I have not actually checked to my satisfaction.

The nice thing is that the Hamwaves calculator does the heaviest bit of the lifting, it and transmission line analysis can be used to predict a couple successive resonances of a carefully built antenna in an non-trivial way, and then that's easy to test.

I'm adding this to my to-do list.  I can't promise to get to it soon (got a dissertation to write).  And I apologize if you would rather I engage in a different order of operations given that we're in general agreement that it's an error to assume that all decent loading inductors are well described by a lumped model.

The departure from lumpiness is *very subtle* in terms of the effects on the radiation of the antenna and the calculation of the proper loading inductor.  However, some predictions of the Corums' sheath helix model are  testable, even with fairly simple equipment.

However, there's an important PR aspect to the testing, given the historical mess on this topic.

In particular, the test must involve exact construction of the theoretical device.  Preferably, that device would also be a demonstrably "good, short antenna," while still showing a significant current taper in the coil.  

The construction must involve an absolute minimum of  tweaking and fudging.  Coil forms must be almost nonexistent.  Securing a twin of the infamous chunk of coil stock is probably rhetorically useful, though not strictly necessary. Predicting and building short dipoles is vastly better than an actual ground mounted or car mounted antenna in terms of error sources so I might need two coils.  I've got a few around here, including two really identical ones destined for a short hatted 40m dipole project.  They're only about 18 modal degrees long at 7.1 MHz  but perhaps I can work with that.  I'm not sure they've got little enough dielectric given the context of the problem.  Their construction is
practically identical to this one:

http://n3ox.net/files/40mtank_1.jpg

My plan with these coils has been to build a short hatted dipole based largely on "W8JI rules" for building good short antennas.  

Dimensions must be as predicted to within a few mm.   It would be REALLY nice to measure the current up the whip and turn by turn in the coil with a minimally perturbing current meter instead of just current in and out.   The current meter must be of clearly acceptable type, not just what I have laying around (which is almost good enough but I want a smaller meter).

I think testing a prediction of the fundamental and next higher resonance of the antenna would be a good piece of evidence, and easy enough to test, along with the current in and out with a low-perturbation meter.

I have to work through all the details, though, because as you might appreciate after all this time, it's very, very easy to take various pieces of evidence and use them to support the wrong story, especially with systems on the borderline.

And I realize that there are some "easy tests" from your standpoint, things you consider smoking guns, like terminating the coil with its characteristic impedance after laying it over sideways above a ground plane.  And while it seems fine to me,, I think  others may consider it to be too contrived and insufficiently convincing until they do it for themselves... which they won't do because they find it too contrived and insufficiently convincing.

That's the PR problem.  The easy to do, easily interpretable tests are too weird and require a lot of prior acceptance of the model.  The unassailable tests are very hard and must be done with extra special care to avoid contaminating the results.  They must be done also with an eye to the other side of the argument, so that you know  when you've convincingly excluded the alternative explanation.  

The test (S21 group delay) that has already been done is kind of interesting and I think verifies the Corums' predictions about the series of self resonances. I've got a lot of things to think about and work on.

For now, I hope you can be satisfied without me demanding corrections.  I'm hoping that Tom's lurking.

But there's been too much animosity for too much time with too much perceived hazard of confusing others and too many screwed up experiments that found exactly what peoples' opinions suggested they'd find for me to do anything but take it real slow and careful.

73
Dan
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« Reply #29 on: November 07, 2009, 04:59:11 PM »

> N3OX wrote: It's too bad that no one did that yet, but I suppose that, in itself, is interesting. <

Dan, you've said that before and I responded that I had done it and gave examples but you seem to have completely forgotten/ignored that fact.

It's hard to get any more current taper than a 180 degree long loading coil, used by Kraus himself, where equal amplitudes of current are flowing into both ends of the coil at the same time. Where is the current flowing out of the coil? Is that a violation of Kirchhoff's laws?

As before, I will point you to:

http://www.w5dxp.com/test316c.EZ

where the current into the bottom of the coil is 1.3 amps and the current out of the top of the coil is 2.1 amps. Is 0.8 amps of current jumping from the environment into the coil through the "parasitic capacitance"?

Current taper through a coil can be anything that happens in a length of transmission line. That's why the distributed network model is necessary. There's absolutely nothing magic about a 1/4WL antenna even though it is popular. If we were discussing 1/2WL loaded end-fed antennas, the current taper through the coil might be exactly the opposite.
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