help me picture a tx coil pulse

[wyndham]

As a pluse is sent to the coil, who can I picture it in my mind. Is it a sphere that radiates in all directions looking somewhat like 2 bowls rim to rim on each other where it goes out and down til in a either clockwise or counter clockwise manor the cut off of the signal, then collapses when the next tx signal comes in the other direction.
Or is it another geometery that might be more of a vertical plane flipping between rt angles to it's axis.
I'm trying to "see" this in my mind. to understand how the tx signal hits the target.
I have also read that it is the collision between the flipping of fields that causes the target to tx it's signal.
In my mind I see a laser reflecting from 2 mirrored surfaces till the energy state gets high enough to lase.
Do the pulses tend to work in this manor where the energy state must be high enough for the target to produce an eddy current then a signal,
I have a most dificult time understanding formulas and equasions but if I can create an image, the understanding improves.
Also if a target, gold coin on edge 6in-12in down, when a tx signal hits this target, are 2 eddy currents produced, one one the face, who's axis is perpendicular to the coil and another on the edge rim of the coin facing up to, parralell to the coil.
Thanks Wyndham

[Carl-NC]

Hi Wyndham,

The radiated field pattern of a coil looks something like the pic below... a mirror-image field is projected above the coil. This is a "frozen-in-time" view, and shows some arbitrary field strength resulting from some particular coil current. It doesn't matter what kind of signal produces the current, even DC will do. As the coil current increases, the field strength increases, and vice-versa.

The radiated magnetic field couples into nearby metal targets and creates eddy currents in the metal. This only happens if the magnetic field strength is changing... if you put a DC current through a coil, and get a resulting constant magnetic field, it will not induce eddy currents. That's why metal detectors use AC currents, like sinusoid or pulse.

There is no threshold field strength needed to create eddy currents in a target. If you have any AC coil current producing any changing magnetic field, then you will get some level of eddy curtrents in a target if it is close enough... then as you move the target away from the coil the eddy currents will just slowly diminish to zero.

The target eddy currents produce a counter-magnetic field (AC) that either messes up the balance of a TX-RX coil pair (induction balance), or messes up the settling decay of a pulsed coil.

Regardless of whether a coin is flat or on-edge, eddy currents are generated on all the surfaces. However, the strongest eddies are on surfaces parallel to the coil (perpendicular to the lines of flux), so a flat coin is easier to detect.

- Carl

[wyndham]

Thanks Carl. This also answers the switching fields question in my mind. The ac creates the switch, if I understand it right.
As simple a question as it maybe to many here, thanks for the pix.
I created a small coil 8" w/28 turns and 22ga insulated wire to test the LCR meter and it came out almost to the induction calc on the net(130uH).
It was interesting to bring a small 6" speaker paralell to the coil which increased from 130 > 136uH but placing in the coil perpendicular into the coil it went to 123uH.
Like I said, those first steps are interesting, Thanks again for your time. Wyndham