# ELF antenna and transmitter design

1. ## ELF antenna and transmitter design

Hi All,

I need to develop an circuit and antenna that will be placed on a person's body, and generate a constant electromagnetic field (for medical purposes).

The definitions I received for this project is to generate a field from 10Hz to 700Hz, with field strength of 0.2 mTesla (that is 2 Gauss).

I'm not an RF engineer and have very limited knowledge about this field, but I was warned that 2 Gauss is not a reasonable field strength for a space limited/ battery operated system.

Is this true? If so, what could be considered a reasonable filed strength for this?

Thanks

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2. ## Re: ELF antenna and transmitter design

What's the intended operation volume over that you want to achieve "constant" field?

Whole body? Only a small zone? The answer greatly affects the required magnet coil size and power demand.

According to the low frequency, the problem can be described as purely magnetic rather than electromagnetic. No relevant far field, no radiated wave. Respectively we are talking about magnet coil, not antenna.

3. ## Re: ELF antenna and transmitter design

Hi FvM,

1) The intended operation is to generate a constant field, between 10Hz to 700Hz (it will eventually be a single frequency but it will be from this range of frequencies).

2) The contact point, or the area of the coil will be about 10MM x 30MM.

3) You mentioned: "we are talking about magnet coil, not antenna", so I think I understand the issue with far field, but would like to make sure that this does not mean that I have to make the "coil" as a closed loop , but something that will be similar to the attached drawing.

4. ## Re: ELF antenna and transmitter design

At these frequencies, your "wire" is only useful to create an electric field. To create your ELF magnetic field, you need a closed loop indeed.

5. ## Re: ELF antenna and transmitter design

Magnetic coil definitely means closed loop. There's no noticeable magnetic field generated by the shown apparatus in the kHz frequency range.

The intended field geometry is still unclear. Consider that the magnetic field of a current loop decreases with the squared distance.

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6. ## Re: ELF antenna and transmitter design

Originally Posted by volker@muehlhaus
At these frequencies, your "wire" is only useful to create an electric field. To create your ELF magnetic field, you need a closed loop indeed.

Ok, so if I understand you correctly, in order to make this work I would need the circuit to be built as in attached block diagram, would that work?

In this case is 2 Gauss an extreme requirement, or is it reasonable?

- - - Updated - - -

The device is intended to be strapped directly on the human body as described in crude diagram here, so the distance would be single milliliters from the target:

so assuming that I design a closed loop coil, that can be 10MM x 30MM in dimensions, would that be sufficient to create a 2 Gauss field, or in this magnitude of order?

7. ## Re: ELF antenna and transmitter design

2 G (it's antique cgs unit, better use SI unit 0.2 mT) isn't particular strong, five to ten times earth magnet field. It can be generated with a few ampere turns, depending on the coil geometry.

The suggested geometry is almost unsuitable, long cylindrical coil has homogenous field inside the coil, some field strength at the faces and little field strength around the cylinder. Better use a short coil or ring on the body surface.

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8. ## Re: ELF antenna and transmitter design

Yes, the magnetic field isn't where you expect it. You need a flat coil.

For field strength, it depends how much power is available. For a battery operated device with continuous operation, it might be a challenge.

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9. ## Re: ELF antenna and transmitter design

Guys,

Thank you for the awesome insights, this is real helpful.

So to summarize:

1) The coil construction should be flat as in Volker's picture.

2) Field strength of 0.2 mT is feasible and not far fetched for the described conditions.

3) I would need to verify the current consumption closely, to choose the correct battery.

Any other comments you might have would be very welcome.

Thanks

10. ## Re: ELF antenna and transmitter design

Basically yes. Generated field strength of simple coil configurations (single turn, multi turn spiral) can be calculated based on elementary physical law, e.g. Biot-Savart.

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11. ## Re: ELF antenna and transmitter design

Originally Posted by FvM
Generated field strength of simple coil configurations (single turn, multi turn spiral) can be calculated based on elementary physical law, e.g. Biot-Savart.
That's a good keyword to find online calculators, this one looks useful to me for estimating a spiral coil configuration:
http://hyperphysics.phy-astr.gsu.edu...ic/curloo.html

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12. ## Re: ELF antenna and transmitter design

Once again thank you all for the great help!

I'll update this thread when I have some real results.

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