How to Produce a Non-polarised EM Field

Thank you for your accurate reply. But by "homogenous" I meant homogenous polarisation, not field intensity. Sorry if that was not clear or correctly phrased.

I was thinking perhaps the field cancellation at the center would direct the flux outwardly in a less polarized form. It has to go somewhere.

Putting the frequency aside for the moment, which is another issue, may I restate my original inquiry as follows?

What configuration of coils, fed with a discrete signal, would provide external to the array the least polarized electromagnetic field?

I was hoping for something better than a triaxial array, if there is any such thing.

What about putting it inside of a steel ball?

Bob Griffin

Ok, so your dimensions are such that you can put it in a steel ball.

How about a segmented steel ball in which the segments (as many as you like and can afford) are capacitor plates, excited by different amplifiers which can be randomly gated etc.? You'd have all kinds of fields coming from all sorts of directions and in a quasi-random way, of sorts.

Probably a silly proposal but just storming my brain,

joe

Hmm, Bob I think you need to distinguish between near field and far field. The dividing line is not exact, but something like the wavelength of the radiation involved. For 10-100kHz the wavelength is 3-30 km. I'm guessing you're thinking of something in the near field. In which case the language or E-M radiation is not really useful. Can you write down, (in an equation) what you would like the magnetic field would look like?

George H.

to the op

2) polarization is usually associated with EM fields, not so much with magnetic fileds. If you are talking about magnetic fields, what do you mean by polarization?

Mark

Homogenous polarization is a term that I've never heard used in RF. However, I can't think of a term that would describe a non-polarized RF field.

It might be possible with a bizarre antenna configuration, but not at VLF frequencies. The problem is that it would be huge, on the order of many wavelengths wide. It might be possible to build such a device at microwave frequencies, but not at VLF where 1 wavelength at 10 KHz is 30,000 meters long.

Building a 2 dimensional antenna that is equally polarized in the X and Y directions is easy. It's called circular polarization. If you rotate a dipole or loop in front of a circularly polarized antenna, it will produce the same signal level at all perpendicular orientations. That is a good indication of being both horizontally and vertically polarized at the same time. However, the 3rd dimension, in the Z axis, is not going to happen. The problem is that antennas radiate as in the classic E-M coupled waves. Unless you can find a way to reverse the direction of the radiation, there isn't going to be an polarization in the Z axis (along the direction of travel). The way you can get polarization in the Z axis is to build a device that produces a static field, that radiates equally from all directions as in a Van Veen loops. Even there, the field appears non-polarized only at the exact center of the loops.

The Van Veen loop is normally used for receive, not transmit applications, usually for EMI/RFI measurement. The outputs from the 3 loops are NOT normally combined into some kind of summing circuit (although this can be done). Instead, the signals from each loop is measured individually, and then summed on a computah.

No. You're avoiding my question. Your contention that naturally generated RF fields are non-polarized is what I am questioning. There are natural sources that arrive with random polarizations, but are still polarized, if only for fairly short periods. At VLF frequencies, the ionosphere to earth interface forms a waveguide that does a tolerable job of filtering signals by polarization. I also question the validity of requiring non-polarization in all 3 axis when natural occurring radiation sources can at best do 2 axis polarization. In addition, the ionospheric bending of vertical or horizontal polarized signals, tends to produce elliptically polarized refracted signals. While not a natural source, the ionosphere will affect the polarization of natural sources in much the same manner.

I can't think of any.

Putting what inside a steel ball? An isotropic radiator? There are various simulations of such radiators, each with their own collection of complications: Such constructs are buildable at VHF, UHF, and microwave frequencies, but at 10KHz, I don't think it's possible. Also, the problem with a sphere is getting the RF to the center of the sphere, without the field distorting effects of a coax cable feed.

Three orthogonal sets of Helmholtz coils would get my vote.

Cheers

Phil Hobbs

Driven all in phase it's hard to see how that's much better than a single coil set. If you only had two you could drive them with a 90 degree phase shift, and make a rotating B-Field. Which is fun, but I don't think that's what the OP wants.

George H.

Sounds like a torus, or maybe a toroidal .

Jamie

Having three allows you to vary the 3 field components individually.

Cheers

Phil Hobbs

OK. I more or less get the picture.

The original objective was to produce a localised EMF that simulates as closely as possible natural background radiation within a partcular bandwidth.

I was hoping for something other than a triaxial coil.

Although the ones wound on a spherical form might be better than three stacked solenoids.

Thank you to everyone who set me straight.

Bob Griffin

"What I am after is a magnetic field that is as homogenous as possible"

Take a Dremel tool and put a magnet on it. Spin it up to 10khz for its rotation. You can measure it from a direction that minimizes polarization. How will you measure the magnetic field?