Why would running the coil cable straight for ~10 inches affect metal detector sensitivity?

Question

Metal detecting enthusiast here.

I have been reading various posts pertaining to sensitivity of metal detectors. It is has been suggested that instead of immediately wrapping the coil cable around the stem to instead run the cable straight up the stem for about ten inches and then begin to wrap. What affect would this have on the detecting characteristics of a detecting coil? Would that really do anything? I don't know enough about EE to really understand what's occurring. Can someone explain it?

Answer 1

It isn't really a question of sensitivity per se, but rather an issue relating to whether the connecting cable interacts with the coil and detunes it or reduces its Q.

Two wires interact magnetically if they are parallel and close to each other. Separating them or changing the angle reduces their interaction, and if they're perpendicular, there's no interaction at any distance.

Therefore, if you run the cable along any straight path that intersects the axis of the coil, you'll minimize any adverse effects it might have on the operation of the coil. By the time you're at least 10" away (probably determined empirically), any possible effects from wrapping the cable are too small to matter.

Answer 2

It's probably old wives tales mixed with something someone did once who thought he (or she) knew what the issue was. It seems to me that the "idea" behind this is to avoid creating "rogue" windings that might induce voltages in the search head receive coils that might desensitize the receiver circuits or cause false triggering.

The meat of my answer: -

There might be 2 to 200 transmit coil turns inside a metal detector that creates the penetrating magnetic field and, the current that flows in that transmit coil will be significant because of the use of parallel tuning capacitors local to it. The current that then flows down the cables to feed the search head will be significantly smaller in amplitude (due to parallel tuning capacitors keeping the bigger currents localized to the search head coil).

That significantly smaller current AND the fact that the cable is likely "balanced" means that extraneous magnetic fields from the cable will be very small.

Even if the cable forms several turns that are directly sat on top of the search head, the magnetic fields emanating will be synchronous with signals picked up in the receiver circuits and may only serve to move the "balance" point slightly. Most metal detectors produce audible tones and the effect of a slight change in balance can be heard and accommodated.

So, it's possible that some slight imbalance would occur that slightly favours discrimination towards gold and away from ferrous metal (or vice versa) but I seriously doubt that it makes anything other than really cheap and badly designed metal detectors have problems.

Answer 3

Mathematical theories and explanations that look good on paper, are never the sam as tests in the REAL PHYSICAL WORLD.

An Isotropic antenna being the best example, it can never exist in the real world, only on paper.

I don't care about the mathematical and scientific wxplanations, several of us have tested and compared the straight vs coiled cord, and notice a real improved difference in keeping the cable straight.

Next time, instead of electrical theory from old textbooks, why not just TRY both ways and SEE which works better?

That's what I did.