Near field effective range ~ 1 wavelength

Posted on by crackingcontraptions_3voi0v
On Fri, Oct 31, 2014 at 8:20 AM, Mik Lamming <mik@lamming.com> wrote:

Nordic announce support of some near-field charging standard.

I don’t understand what Nordic have done.

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On Fri, Oct 31, 2014 at 8:56 AM, David Carkeek <dcarkeek@gmail.com> wrote:

There are a bunch of new things in this press release so I don’t understand it either. They say “magnetic resonance” and “spatial freedom” but then talk about pads and surfaces as though the device has to be resting on the surface. It s says “Out of Band” which I don’t understand and “up to eight devices”. Why couldn’t it be nine devices? Why do you need an SDK to make a charging system?

What I kind of think is that there a pad which is a kind of transmitting antenna that emits a signal to which a device resonantly couples over a distance that is not zero for the purpose of transferring energy for battery charging. Each device needs a channel (up to 8 are possible). That would be super cool if true. Need to read about Rezence. I might be completely wrong.

“The nRF51 Wireless Charging SDK is available today on a limited basis to lead customers.”
but “Engineering build of S120 and an updated nRF51 SDK is available today as a download for existing customers of nRF51822.”

Whom do I ask for the link? Not that I would know what to do with it after downloading.

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On Fri, Oct 31, 2014 at 10:46 AM, Mik Lamming <mik@lamming.com> wrote

Following up on I’m assuming that the range is at best centimeters because it is near-field which drops off dramatically after a few wavelengths… doesn’t it?  The frequency is 6.8MHz, so the wavelength is ~5cms?  (Wrong 44.1 meters)

Aha…  I found this.
http://en.wikipedia.org/wiki/Rezence_(wireless_charging_standard)  which contains this useful information sentence:

The interface standard supports power transfer up to 50 Watts,[1] at distances up to 5 centimeters.[2] 

I also found this excellent video, the first 2-3 minutes of which gave me a strong intuitive sense of the characteristics.  https://www.youtube.com/watch?v=r1UT4NuygmQ

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On Fri, Oct 31, 2014 at 11:29 AM, David Carkeek <dcarkeek@gmail.com> wrote:

Too bad it doesn’t work across a room, but still quite interesting. I watched about 5 minutes but I’ll have to finish it later.

I’m somewhat ashamed to say I don’t know much about RF fields, near-field, and the relationship of wavelength to power transfer. The wavelength of 6.8 MHz is 44 meters according to the calculator. I wonder why that frequency was chosen. It must mess pretty badly with nearby radios.
A Tesla coil can work at tens of feet, maybe hundreds of feet.

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On Fri, Oct 31, 2014 at 13:59 AM, Mik Lamming <mik@lamming.com> wrote

It’s only yhe first 5mins that are worth the time. The rest got out of my depth anyway. It just gave me a good feel for the strengths and weaknesses of near-field.

Ah MHz, not GHz! Yes – I agree. Doh.  Good job you check all my math.

c = 300,000 = 3 * 10^5 kms/s = 3*10^8 m/s
6.8 MHz = 6.8 * 10^6Wavelength = (3*10^8) / (6.8 *10^6) = 44.1mIt’s only the first 5mins that are worth the time. The rest got out of my depth anyway. It just gave me a good feel for the strengths and weaknesses of near-field.

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