Inductive coupling power design

[Guest]

I have an application where I need to inductively couple 10 mW of power
across 10cm of air. However, to make things difficult, my receive coil
must be small enough to fit inside 1 cm^3. I have no restrictions on the
transmit coil design, other than FCC power/frequency rules.

Does anyone have any ideas to share regarding inductive coupling? Every
design I have come across for inductive coupling works only for coil
separation distances of 1cm or less.

I am currently coupling just under 1mW from a 20 watt source.

Take care,
-Chris

 

[Phil Hobbs]

I have an application where I need to inductively couple 10 mW of power
across 10cm of air. However, to make things difficult, my receive coil
must be small enough to fit inside 1 cm^3. I have no restrictions on the
transmit coil design, other than FCC power/frequency rules.

Does anyone have any ideas to share regarding inductive coupling? Every
design I have come across for inductive coupling works only for coil
separation distances of 1cm or less.

I am currently coupling just under 1mW from a 20 watt source.

Take care,
-Chris

If you want to do it inductively (i.e. not radiation), you're fighting a
1/r**3 falloff due to the dipole field. This is hard--it would require
a very high-Q LC resonator with no other sources of loss nearby. If
you're doing it by radiation, your field falloff is less, but you're
still going to be stuck intercepting 1/(40*pi) or ~2% of the radiation,
unless you can make it highly directional.

Cheers,

Phil Hobbs

 

[Phil Hobbs]

I have an application where I need to inductively couple 10 mW of power
across 10cm of air. However, to make things difficult, my receive coil
must be small enough to fit inside 1 cm3. I have no restrictions on the
transmit coil design, other than FCC power/frequency rules.

Does anyone have any ideas to share regarding inductive coupling? Every
design I have come across for inductive coupling works only for coil
separation distances of 1cm or less.

I am currently coupling just under 1mW from a 20 watt source.

Take care,
-Chris

If you want to do it inductively (i.e. not radiation), you're fighting a
1/r**3 falloff due to the dipole field. This is hard--it would require
a very high-Q LC resonator with no other sources of loss nearby. If
you're doing it by radiation, your field falloff is less, but you're
still going to be stuck intercepting 1/(40*pi) or ~0.8% of the
radiation, unless you can make it highly directional.
Cheers,

Phil Hobbs

 

[John Larkin]

If you want to do it inductively (i.e. not radiation), you're fighting a
1/r**3 falloff due to the dipole field. This is hard--it would require
a very high-Q LC resonator with no other sources of loss nearby.

So just make the transmitting coil *big*.

John

 

[Walter Harley]

I have an application where I need to inductively couple 10 mW of power
across 10cm of air. [...]

Does it have to be inductive? Could you use a light and a photovoltaic
cell?

Or, does it have to last forever? Could you use a lithium battery?

 

[Joerg]

Hi Chris,

Depending on the legal circumstances (and of course safety) in you neck of the
woods you may be able to create a substantial field strength on 13.56 MHz. This is
an ISM frequency that is reserved for such purposes. It's easy to do since amps at
this frequency can be made inexpensively. There are some lower ISM frequencies if
that's more practical.

Most of all, scrutinize the circuitry on the receiving side. Every microamp in
current consumption of your electronics counts.

Regards, Joerg