75m Loading Coil Epic

[N3OX]

"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.
"

Cecil, don't get me wrong: I mean a real physical *measurement* not the model.  You know me and you know I am on board with accepting EZNEC results as correct.  I do not insist on measurement often.

But you also know it's hard to get a converged EZNEC model with small loops and I suppose that you have to fiddle for a while to get an acceptably modeled coil.
What I would like to do is build that thing or similar as exactly as possible out of tubing and wire and measure it with a very low capacitance almost all plastic current meter of small physical size.

That part, and the other experiments are not so much about convincing myself.  They're about gathering evidence to people who are willing to persist in disbelieving the model (even if they usually believe the model) because they know it can be difficult to get a proper model given the geometry.

They're about convincing people who are absolutely 100% convinced that every example of this that's been cooked up in EZNEC is contrived and does not model a good mobile antenna.

"
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"?"

Sort of.  Clearly it's not conduction current.  Those electrical charges have been on the coil the whole time.  But the ENERGY comes from the fields around the coil.     There's no conservation problem: that energy was placed in the electric and magnetic fields some time earlier in the RF cycle and returns later.  Electric and magnetic field energy can cause charges in the coil to move.  You can have more current higher up in the coil due to the spatial distribution of fields.  No problem.  No problem even if you talk about "currents through the parasitic capacitance."  no conduction current ever flows through a capacitor. (below breakdown voltage, and ignoring tunneling).  The electrons pile up on plate 1 and their electric field shoves electrons around on plate 2, causing a current over there.  That's how displacement current flows. The electric field responsible can indeed cause a displacement current to appear to "flow from the environment."  That's just a description but it is not a faulty one.

In a coil, where people presuppose that ALL of the energy storage is in the magnetic field, especially if the coil is a "good loading coil," they often think about the electric field energy storage and flow in terms of "parasitic capacitance."  

Don't get too caught up in thinking that this terminology necessitates fuzzy or incomplete thinking.  "Parasitic capacitance" is a dismissive term with regard to the effects of the electric field but it doesn't absolutely require that the user be thinking incorrectly about how electric fields become important on coils.

"If we were discussing 1/2WL loaded end-fed antennas, the current taper through the coil might be exactly the opposite. "

Sure, I agree with that.  It can also easily be zero taper in terms of the end currents if the coil is a modal wavelength long and the impedances on either side match.   It would still have nonuniform current throughout.

This makes me wonder, do you own a hamstick?  It'd be interesting to run a 40m or 80m hamstick through the Hamwaves calculator and see how long it is in modal wavelengths :-)

This information really has some weird consequences for the "wind some wire on a stick" style of helical antenna building and resonating that some people seem to be fond of.

73
Dan

[W5DXP]

> N3OX wrote: I mean a real physical *measurement* not the model.<

I did just that years ago at my previous QTH. Here's the bench setup I used:

http://www.w5dxp.com/coiltest.gif

The results were posted years ago and are enumerated in a lab book that's still in a box somewhere but they were consistent with an ~33 degree delay through the coil at 4 MHz.

Perhaps an easier example would be better, one that doesn't involve a coil. Set up a 10m dipole and install two current pickups, one 1/3 of the distance from the feedpoint and the other 2/3 of the distance from the feedpoint. One would logically assume that one can measure a 30 degree phase shift between those two points. However, the current phase shift is negligible, in the ballpark of one degree. Question is: How can 30 degrees of wire cause one degree of phase shift in the total current?

The answer is that one is measuring the total current on a standing-wave antenna whose primary equation is:

Itot = Imax*cos(kx)*cos(wt)  current anywhere along x

Ifp = Imax*cos(wt)  feedpoint current at x=0

The phase of the total current anywhere along the line (compared to the phase of the feedpoint current) doesn't change appreciably. This is the basic measurement mistake that most experimenters have made.

Now replace the 30 degrees of wire with an inductor. If one cannot measure the phase shift through a wire, why would one expect a phase shift through a coil?

Since a zero phase shift reinforces the preconceived presumed notions of a lumped inductor, almost nobody questions the validity of the results.

> N3OX wrote: This makes me wonder, do you own a hamstick? It'd be interesting to run a 40m or 80m hamstick through the Hamwaves calculator and see how long it is in modal wavelengths :-) This information really has some weird consequences for the "wind some wire on a stick" style of helical antenna building and resonating that some people seem to be fond of. <

In Dr. Corum's paper is a test to verify that a coil meets the requirements for the Corum approximations. I will run a hamstick coil through that test to see if Corum's approximations apply. I can get the number of turns per inch from my 17m hamstick loading coil.
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73, Cecil, w5dxp.com

[W5DXP]

> N3OX wrote: This makes me wonder, do you own a hamstick? It'd be interesting to run a 40m or 80m hamstick through the Hamwaves calculator and see how long it is in modal wavelengths :-) <

OK, I've got my eyesight back and have made some assumptions about the hamstick.

I assumed 720 turns in 610mm of length with a diameter of 11.4mm, wire diameter of 0.7mm.

The coil models out to be 37 degrees long at 4 MHz with a self-resonant frequency of 8.8 MHz. The Z0 is 4015 ohms and the VF is 0.078.

The coil is about two feet long. With a Z0=4015 ohms, it will resonate on 4 MHz with a whip feedpoint of about -j3000 ohms. EZNEC says the whip would be about 5'4" long (~8 degrees).

The antenna is electrically 45 degrees long with a 45 degree phase shift at the coil/whip impedance discontinuity.
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73, Cecil, w5dxp.com

[K5END]

Oh, good.

This topic is alive again.

Yippee.

I have to go water my cactus now.

[W5DXP]

Here is a brainteaser for everyone who is interested.

Assume a large lossless air-core loading coil used on a standing wave antenna. There are zero losses in the coil, no I^2*R losses, no radiation losses, no losses to the surroundings, ... no losses at all.

The amplitude of the forward current through the coil is one amp at both ends of the coil, i.e. zero forward current drop through the coil.

The amplitude of the reflected current back through the coil is one amp at both ends of the coil, i.e. zero reflected current drop through the coil.

The forward current into the coil at the bottom is 1 amp at zero deg.

The forward current out of the coil at the top is 1 amp at -45 deg.

The reflected current into the coil at the top is 1 amp at +45 deg.

The reflected current out of the coil at the bottom is 1 amp at zero deg.

What is the total current at the bottom of the coil? ________

What is the total current at the top of the coil? ________
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73, Cecil, w5dxp.com