Single loop coil , Power transfer from target

[Tec]

Hi,

Two questions for your genius:

1.

If a target is a low impedance, low inductance, single loop of current...
Would its' power best be transferred to a low impedance, low inductance single-loop coil ?

http://electronicdesign.com/communic...atching-part-1

i.e for detection of the target, would a single loop be more sensitive ? ( and use a transformer between antenna and amplifier like a radio receiver)

2.

If the magnetic field strength is basically (Current x Loops) The target can't tell the difference between 100 Amps in a single loop, and 1 Amp in 100 loops ?


Cheers !

[WM6]

i.e for detection of the target, would a single loop be more sensitive ? ( and use a transformer between antenna and amplifier like a radio receiver)

Based on Gardiner detectors, which use such solution, not.

[ODM]

Transformers by themselves can be used to match impedances, so target impedance doesn't matter in that sense. Most MD frontends are rather high Z compared to usual targets, as well.

You basically answered the first one with the second.

[Davor]

I don't know about the genius part, but from a certain perspective it is not that complicated.

1.

If a target is a low impedance, low inductance, single loop of current...
Would its' power best be transferred to a low impedance, low inductance single-loop coil ?

http://electronicdesign.com/communic...atching-part-1

i.e for detection of the target, would a single loop be more sensitive ? ( and use a transformer between antenna and amplifier like a radio receiver)

For the Rx part you can observe two kinds of matching, noise and impedance, and being in a world of excellent approximation with lumped element, and no trace of any transmission line effects, a great deal of RF reasoning and impedance matching is off the table.

Things you can match for are noise and minimum influence on target eddy current decay. You can't do much about source (coil) noise, unless you are willing to make a coil very heavy, and you also have a coil inductance that complicates frequency response if supplied to a low impedance, your best option for Rx front end is a voltage sensor with low input noise voltage. Ideal front end would exhibit very large impedance, and noise matching the noise of a coil resistance. As voltage noise density across a 50 ohm resistor is expressed as 1nV/sqrt(Hz), and most expensive op amps having input noise right about there - there you have it. The best possible matching would be making the equivalent noise source resistance of a Rx front end close to the coil resistance.

Making heavy Rx coils make little sense with front ends being capable of only down to about 1nV/sqrt(Hz) noise.

So for influence on eddy current decay, ideal coil would not add anything to a target, as decay tau is expressed as L/R and in case of impedance matching the "R" part would increase, the decay time would shorten, and it would also be related to distance, hence a voltage sensor is thing you need, as it prevents any kind of impedance matching.

2.

If the magnetic field strength is basically (Current x Loops) The target can't tell the difference between 100 Amps in a single loop, and 1 Amp in 100 loops ?

True, but you must also check your resources, and also influence on your targets and detection. What you have at hand is usually voltage, and the current is a result of a coil being exposed to voltage. That being said, you have to realise that the coil is made of the very same material as your targets, and you are after targets - you know that your coil is there by visual inspection. So you wish to ensure that coil participates in eddy current response as little as possible, by reducing the cross section of Tx coil wires. This kinda contradicts with the single loop idea, but also not necessarily so - a single loop can be made of multiple strands of thin wire, and as far as the mutual insulation goes - they do not have to be insulated that well. Tinned strands could do.

[Flash1940]

Maybe not a direct answer to your idea....but maybe this insert may give you something to ponder on. The RX coil or group of coils should be as high Q as possible so that any shift of phase or frequency being reflected from the target will be at a maximum when sensed by the RX coil/s. A high Q tuned circuit has a steeper slope.....a higher degree of selectivity or also a narrower bandpass. The killer of Q is the resistance in the wire. Use heavier gauge wire when selecting wire for your coil. Wouldn't be a bad idea to go and review the theory of resonance. You probably already know about this....but a review won't hurt.

Flash