It sounds like you're looking for a diffuse field, given your analogy of 'ambient' sunlight. If it is intended for indoor (test chamber) use, then it could be generated by 1 antenna inside a reverberating chamber.
If for outdoor use I'd be happy to learn from the responses of the others.
OK, how about 3 orthogonal antennas and you feed them with 3 sources that are on the same frequency but whose amplitudes (and maybe phases, you think about it and tell me) are randomized. Mark
I am not sure that would be homogenous on three axis.
I don't think the polarization vector can ever lie on the direction Of propagation, so maybe you are asking for unpolarized AND isotropic. So you choose, either 2 or 3 orthogonal antennas fed with sources on the same frequency and randomized amplitudes and or phases. That should do it. Mark
Hi Bob, You want a randomly polarized field? (You need to say a bit more about what you are trying to do.)
10 k to 100 kHz, are you interested in the magnetic or electric field? If you add two sources each each polarized at 90 degrees to each other, the reuslt looks pretty much random. (I think).
Circular polarization also looks "linearly" unpolarized. as someone mentioned.
Of course, E and H are vectors, so you can't ever have a nonzero field that's really unpolarized. The instantaneous fields always have a definite magnitude and direction. Thermal light appears unpolarized because all of the components of E and H vary on a timescale of femtoseconds. That makes it essentially impossible to reproduce. There are various schemes for approximating it, e.g. Cornu depolarizers or contrarotating ground glass discs, but none of them work very well because the modulation bandwidth is way, way too small.
In the OP's case, he'll have plenty of control bandwidth, so it's fairly easy to do a good job on the randomness--it's just the transducers that need figuring out.
At those frequencies, though, it'll be in the near-field limit (i.e. electrostatic/magnetostatic), so depending on what you want to do, E and H may need to be controlled separately, e.g. with plates and coils, rather than with antennas.
In a real propagating field in free space or other lossless medium, E and H are always orthogonal and in phase, which isn't true in the electrostatic/magnetostatic case.
I'm also curious as to what the OP wants to do with this.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
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Three loop antennas arranged in X, Y, and Z planes. By "produce" I'll assume that you want to generate such a field. You'll need a 3 way power divider, which probably should be resistive. The signal is feed to each loop with the same phase. They're commercially available for receive. You're on your own if you want to use it as a "producer" (transmitter???). Google for a "Van Veen Loop".
Something similar is done by Narda STS for their low frequency EMI/RFI measurement equipment. They claim "State of the Art Technology with simultaneous 3 axis Acquisition" which I think decodes into orientation and polarization insensitive.
If you want practical advice, you'll need to disclose what you're trying to accomplish. Asking such questions is like asking for where to get a screw fastener, without disclosing what it will be used for.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
What I am after is a magnetic field that is as homogenous as possible. Light is the obvious analogy, but also various forms of EMR which occur naturally in the environment.
Perhaps it was not clear in my original post, but I need to apply a singular frequency (or signal), not a random mix of same. IOW not noise.
However, I am thinking that noise could be mixed in, at perhaps 10%, to help randomise the signal.
The criteria you mention could be satisfied with multiple emitters and/or signals of the same frequency but at different phases.
What would you suggest as being the minimum number required to produce a meaningful result? How should the coils be arranged?
However ... I am left wondering though if a natrually occuring frequency is actually present in differing phases. Rather, I believe it is the scale of the event that makes the field homogenous. That is what I am trying to simulate.
Is there any type of coil, of single or multiple windings, that emits a less linear field than a solenoid?
Thank you for the suggestion. It reminds me of the tower from the movie "Contact". My budget is not quite that big.
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Here is some further info on these.
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I would prefer however that the field not be enclosed, but instead radiate outward in a uniform (non-polarised) way from a centrally located coil array.
How about something like this with the opposing coils driven anti-phase?
I am trying to simulate a naturally occuring ambient EMF using a defined frequency or signal. The point being that most artificial forms of EMR are polarized.
I know this cannot be done in ideal terms, so I am looking for a compromise. Something a few steps up from just a set of X, Y, Z solenoids. And for this purpose I assume air core is best.
It's not going to happen. Let's ignore polarization for a moment and consider the other half of your problem. You also asked for a "homogenous" field, which I'll assume means a unform field. Projected outside of an antenna structure, the away from the near field, radiation follows inverse square law and falls away exponentially with distance. In common practical antennas, the further away you get, the weaker the field. You might be able to produce a non-polarized field with the 3 loop antenna configuration, but it won't be of uniform field strength, except at the center of the loop.
You'll get cancelation at the center and no field at all. I assume that's not what you want.
Ok, let's do this backwards. If you build such a non-polarized field generator, then I could take a tuned loop antenna, attach it to an RF voltmeter, and detect exactly the same detected RF level, no matter what the loop orientation. In addition, at any fixed radius away from the field generator, at any point about a sphere, the signal strength would be the same. I've played with tuned loops enough to know that it will be a very strange day when I can rotate the loop, and not see a change in signal level. I don't believe this can be done.
Your choice of 10KHz to 100KHz is interesting. Naturally ocurring signals in that range are mostly noise produced by lightning. In the US, it usually comes from Florida. Most of the lightning strikes are vertically polarized (ground to cloud), with a few horizontally polarized (cloud to cloud). It's too high in frequency (300Hz to
20KHz) for whistlers: If not lightning, what form of "naturally occuring ambient EMF" are you trying to emulate?
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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