Round vs Square shaped coils?

[Tinkerer]

Exactly … and this depends on the distance and the ratio of coil and target diameter (plus target angle and more, but to keep it simple let’s assume the target is a small loop parallel to the TX coil). The above expression calculates the TX field strength at a point on the coil axis, but the target has a certain radius. And then for the RX path, this target radius is a different (usually much smaller) ‘R’ in the expression.



We could have a look at contactless power transfer. Usually with spiral coils for both TX and RX, but still useful to get an idea about coil coupling. Example (also dealing with coil misalignment):
Wireless Power Transfer in Loosely Coupled Links: Coil Misalignment Model
http://thesciencedude.com/projects/F...nt%20model.pdf

Let’s heat up the targets ;-)

Thomas

Thank you for the interesting link. There are many similarities.
However, the target response to an IB coil field looks much more complex to me.
Intuitively:
If we consider the total coil field of a detector coil, to be a roughly sphere, the IB coil field will be a distorted sphere, because we have 2 opposing fields of different strength.

But in the RX coil itself, we strive to have near perfect balance, meaning the opposing fields generate opposing currents that null.

If we approach a target within the field sphere, anywhere, we upset the balance. Our target response is the measure of the balance upset.

Since the field sphere is not uniform, the response will not be the same with the target at different places, but the RX coil will sense any upset of balance anywhere within the field sphere.

It looks like a mathematical nightmare to me, but this is what I have observed with my IB coils. They sense targets off to the side. They also sense targets that are not under the RX coil.

I have learned to be careful when approaching very small targets to the coil assembly, because it senses my (magic?) hand at some distance. A long stick is needed to obtain only the target response.

[Carl-NC]

5V? That is surprising. I did a simple test on the pirate detector. Running at 5V, it halves the distance on a coin that 9V gives.

I can see why increasing tx voltage beyond what is practically available by modern batteries is kind of pointless, but using half or a third of whats available seems to be self defeating. Unless you are building a detector that is sensitive to sub gram gold on the surface, but how is 5V strong enough for any kind of depth?

Designing a metal detector is a "whole system" process. I can design a transmitter (coil & driver) to run off 18V, or 5V, or even 1V if I want. I can also run it up to 200V using a 12V battery pack. The strength of the transmitted field is proportional to "ampere-turns" which can be achieved even with a low voltage.

Then why do people keep talking about doubling the distance of the field with 64 times more power?

We don't, we talk about "doubling the detection depth."

Is there ever a point where a coil is maxed out? I mean, reached its max distance? If you keep increasing power by a factor of 64, does its distance keep doubling? Or does it max out distance and just keep adding strength or flux or whatever you want to call it :P

The magnetic field theoretically extends to infinity, but quickly gets so weak it cannot be measured. Sometimes you will see a drawing of coil with a line that represents a magnetic field that extends out to some distance; that does not mean that the field stops there and goes no further. It's likely meant to represent a relative field strength; another similar field line can be drawn further away, to represent a weaker field strength.

Theoretically, you can keep increasing TX current, and the magnetic field and resulting detection depth will continue to increase. There are limitations to how far you can go, both practical and physical.

[brucester]

Awesome. Thanks Carl, that's exactly what I wanted to know. All makes sense now

Cheers