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Isolation Ward From ebony pucks to magic foil, mystical and controversial tweaks. |
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Isolation Ward From ebony pucks to magic foil, mystical and controversial tweaks. |
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In Reply to: Ahhh...... posted by unclestu52 on November 16, 2008 at 21:20:08:
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
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Follow Ups
- RE: Ahhh...... - rick_m 09:17:03 11/17/08 (2)
- RE: Ahhh...... - unclestu52 11:54:29 11/17/08 (1)
- RE: Ahhh...... - rick_m 17:58:09 11/17/08 (0)
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