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In Reply to: RE: A magnetic thought. posted by rick_m on November 13, 2008 at 12:24:54
But it's not the "field" per se that's the issue. It's the frequency and the wavelength. (Neither of which is a characteristic of a magnetic field.)On subj, recall that Acoustic Revive has demo'd their SF generator in large auditorium, so (close) proximity to listener might not be an issue either.
Edits: 11/13/08Follow Ups:
Acoutic revive recommends that the Schuman generator be placed at ear level.
using the formula Frequency=wavelength/speed of sound the Schumann wavelength comes to 8,847.9 feet.
Stu
but the AR device produces an electromagnetic wave (vice acoustic wave) so a different formula must be utilized.
Cheers, Geoff
The definition fits most any frequency, except for those needing correction in a medium. You could substitute the speed of light, if in a vacuum, and the pulse is purely electromagnetic. That loop antenna in the AR is not any where close to that wavelength, however, since it is coiled in a 6 by 6 square (approximately).
Stu
Stu,
Any size loop can be used for any frequency. The rub is the radiation resistance. In other words if a loop is very small with respect to the wavelength of the signal it won't couple for beans to the ether. I touched on this in some earlier postings with Geoff when I was at the coast. Since I'm home now and have access to my library I can put some numbers to it.
The radiation resistance of a small loop per Kraus, Antennas, 1950 is:
Rr=31,200[n(A/lambda^2)]. Where n=turns,A=loop area,lambda=wavelength.
Running the numbers for F=8.3Hz gives a Rr=5.6E-12ohms. Free space is around 377ohms so there is huge mismatch. That just means that there will be almost no far-field radiation. Near-field, which in this case is about anywhere in the same hemisphere, the field will be predominantly magnetic and as I suggested can be readily calculated using the Biot-Savart law or modeled with FEMM. Easier yet, we've all seen the "lines of force" in iron filings from a bar magnet, imagine one of those sticking vertically through the axis of the loop and you will have a good mental picture of the relative field strength and vectors produced by the loop.
By the way, I used 10 turns in the numeric example above. The inner turns on a PCB loop do essentially nothing because their area is small so I figured that was a reasonable estimate.
So forget the wavelength, it's just a red-herring in this case. All that matters is the magnetic field and the sensitivity of the listener or equipment or whatever to it. As you get further away, it just gets weaker if you stay on the same angle. It will be much stronger on axis than off to the side and drop more quickly as you get away from it. If you really want a dose, put it on top of your head. But I don't recommend that on general principles. I'm with Thurber: "Leave your mind alone."
Rick
as the tiny flicker of light ignites above my head (cognitive impairment and all that stuff, you know).....Maybe I should pick up the book on antennas I have sitting beside me, but I only read the part on radio reception, where you wanted the antennas at a fraction of the received bandwidth.
Certainly since the Schumann generator is roughly half the footprint of a piece of paper and uses a wall wart power supply it can not generate much of a magnetic field (I gotta dig up mu gaussmeter).
The magnetic field should be toroidal since the loop antenna in the unit is in the form of a square pattern on a printed circuit board. I would assume that the field is then most intense when in line with the sides of the unit rather than in the vertical plane, directly above or below the unit. If so then placing the unit at the ear level would be preferable to placing on top of the head and thus the manuals admonition the place the unit at ear level. Sorry, just musing out loud....
Thanks,
Stu
Hi Stu,
I can make you a picture, but let me try to describe it. The magnetic field circles the current path. The mnemonic is the "right hand rule": if you grab the wire with your right hand and have your thumb in the direction of the current, you fingers are aligned with the magnetic field which comes out of your nails.
Thinking along those lines imagine a loop wrapped around a clock face which you are viewing from the center. If you grab it at 3:00 your thumb points up (iffins the clock is on a wall) and you fingers point at you from the right. If you grab it at 9:00 your thumb points towards the floor and your fingers point at you from the left. And you just threw your back out of joint trying to do the demonstration. The punch line is no matter where you grab it, the field is coming towards you from the inside of the loop and going away from you on the outside.
You're right about the field being sort of toroidal, but that is only when very near the conductor. The field mostly cancels out as you get a little ways away from the loop. It's all a matter of symmetry. Along the wire the field coming out on the inside is matched by field going in on the outside. The center of the loop is "special", again due to symmetry. It is where the vectors from the whole loop sum together in a single direction. The matching vectors outside of the loop are at infinity so it can actually escape.
So... As you get a little way from the loop, the strongest signal is on axis, in the center and the polarization is orthogonal to the plane of the loop. Going back to the loop wound around a clock, just rip the hands off and jam a bar magnet in the hole and the field looks about the same.
If the loop works best about head level off to the side, you can get the same polarization and a stronger signal by gluing it to your beanie. Another way to think of it is imagine that you've got the loop off to the side six feet away. How much of the overall field does your head occupy? Bear in mind that near-field magnetic fields are closed so all the "lines" that are are everywhere outside the loop, go through the inside of the loop also, so obviously they are much more concentrated there.
You probably can't measure it with your gauss meter, although you might be able to see the needle wiggle.
You are right again, you do want your loop to be a fraction of the wavelength. These are, just a very small fraction. The smaller the fraction, the worse the coupling and thus the lower the radiation resistance.
I need to look up more info on the Schumann levels, but it probably takes far less of a loop or drive levels to exceed that signal strength than you would suppose because you are very close to your local loop. It's easy to generate either an electric or magnetic field locally at most any frequency, the wavelength is inconsequential as you don't give a hoot about far field.
If you antenna book covers loops, give it scan. Sort of interesting stuff if you've got a bend in that direction.
Rick
Got it: I picture it sort of like those pulsars in space.....of course not quite as intense.
Incidentally, I may have sent you some ERS a long time ago; it was that grey stiff paper.
Stu
Stu,
I don't see it in the bag with the remaining quartz and blu-tak. I also can't cogitate up a mental image of it but you well may have. I'll keep my eyes open, if I got it it's probably in a stack somewhere, as my wife will attest I'm not one to throw stuff out...
I think I've got some available time coming up and among other things will try to do some measurements on the stuff you sent. Nothing fancy but worth a shot. I've been using the core over the power transformer right along.
I'm not sure that having a tame pulsar in my room is a very comforting thought, but I suppose the radiation pattern is similar!
Rick
For purposes of this audio-related discussion, could the Schumann Freq. Generator be a CD (acoustic wave generator)? (or is that snake oil?)
Beats Me.
I suppose so since there were posts a few months ago about that very product... Hmmm, I just did a little search to thoughtfully help you find the information and lo and behold, the information came from YOU back in March.
So... what's the deal, did the CD do anything for you?
BTW, it certainly wasn't my intention to try and pass myself off as knowledgeable about things Schumann as I decidedly am not. But I am interested. On the other hand I may know more about small loops than most folks since I used to use them in designs and that gives me a little something to throw into the collective kettle.
Rick
I played around with the CD that produces the Schumann Freq. using 2 beat frequencies, with speakers and headphones, but didn't obtain results I could hang my hat on...G
Edits: 11/17/08
Thanks, good info.
Have you tried one of the electronic varieties?
No, can't say as I have.
Don't radio waves travel at near speed of light in air?
Could the AR device be producing two higher "beat" frequenices, indirectly obtaining the 7.83 Hz SF?
I admit I'm puzzled by the SF generators...
~ Cheers
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