What causes the noise when my coil whacks a rock or stump ?

I have been told to tape my cord to my shaft to stop movement and hence noise. How does that actually cause noise and how does the coil hitting a hard object cause noise ?

If there was no space for the bits in the coil to move, would it stop it making a noise when it hits a solid object ? Would it be possible then to fill a coil with light two pack foam to stop this movement ?

Thanks,

Jigalong.

The lighter coils tend to have a slight amount of flex in them and sometimes just this tiny amount of movement is enough to trigger a response if you accidentally knock your coil against a hard surface. Most often though, it's because the coil shifts slightly where the rod meets the mounting screw. Since this is usually not too tight (to enable to coil to be deliberately shifted slightly by pressing it against the ground), the coil will tilt and this pulls on the coil-cable. The amount of shift is usually quite small. The coil field then senses the slight shift in nearby metal (in the cable) and sends you a false signal. The higher end detectors in the SD & GPX range are very sensitive to small gold so they are likewise very sensitive to the minute shifting of conductive materials (like the coil cable) near the emission field.

You probably won't be able to eliminate coil-shift altogether. But a way to reduce the false sounds triggered would be to tighten the coil-mounting screw and make sure your cable is tied down well. Go buy some Velcro tie-downs from Dick Smith Electronics or Tandy. They use the same mini Velcro straps to tie strands of electrical cables together. Also, be aware that some of the larger coils with an open frame (eg. larger Nugget Finder coils) can flex ever so slightly when struck hard. This is usually just enough shift for the winding inside to 'see itself'.

If your strike signal is loud, it's probably because the coil shifted in relation to the cable.

All coils contain a filling compound to suspend the wire inside it and prevent it from shifting. The Nugget Finder coils are slightly lighter because they contain a similar compound to the expandable foam. This keeps the coil lightweight and "suspends" the wire inside. The Commander coils are ever so slightly heavier due to the space inside being filled with a resin. This stuff is very rigid but it adds a tiny amount of weight compared to other coil brands. On the other hand, I hear the performance of the Commanders allows for an supposed increase in performance (of up to 17%+) over other coils although I don't know if this relates to other factors yet. How true that is will be put to the test shortly as I'm doing a comparison study in the next few weeks. It's very hard to advise people not to bang their coil against rocks though since I do it myself all the time ... it's almost impossible to avoid, especially when you are tired or overheated.

Cheers!

Whilst all above is probably true.
I have another thought on the subject: Lets say the box is consistently interpreting the ground at the steady pace of the usual swing and then it is confused by the sudden halt of hitting or bumping an object ?
From my experience, the more the machine is humming and in tune with the ground the more likely a jolt to the coil will give a signal type response. The good news is that it usually only happens once, unless I keep bumping the same obstacle. All coils seem to do this.
Brett

Nero,
As you are in Sydney, where are you doing your testing?
I know there is a few here that travel for their hobby and maybe that's what you do.
Nice that you take the time to put up lengthy explanations.
Thanks.
Cheers.

Hi echidnadigger,
Yes, I'm in Sydney! I do any testing in the Goldfields out beyond Hill End. Usually have to drag a friend or my wife with me. There's a couple iron-rich areas in that direction that simply slaughter the VLF detectors to the point where they only get to pick up targets a few inches deep ...and that's where I can test for comparative stability with the SD/GP detectors. For actual nugget hunting, I work a little further North or to the West of there. Too much junk otherwise... most of it from campers in the 70s if the ring-pull tabs and endless rusted tent pegs are any indication (LOL). It's about 3.5 hours to the nearest Gold Field from Sydney's outer suburbs so the last 10 trips I've made have just been day trips. I need to drive for about 5 hours each way to get to the spots I like. Again, for safety reasons, I usually bring someone with me.

I really don't intend to write such long winded replies... but I start writing and I'm usually watching TV or watching something online and it just gets sort of gets dragged out a bit if I want to fit in everything that might be relevant. There's actually quite a few very experienced Prospectors on this forum so it's good to see their replies whenever they post.

In relation to this thread subject, I can recall hitting a log with my first detector some years ago and the loud beep that ensued. This happened a couple of times and I thought I might have been harming the detector. It wasn't until I tightened the cable to the stem with Velcro and then tightened the connector screw that I realized some time much later that the coil/cable shift was causing the problem.


Cheers!

Hi Nero,

Thanks for the good answer. Another mystery solved.

I have noticed in various photos that some guys put their shaft onto the front hole in the coil and have the lead coming out at the back right next to the bottom of the shaft. This is then taped straight to the bottom of the shaft. There is no way you could tilt the head once you have set the angle like this, but it must make the cord very rigid and cut down noise. Every thing is a tradeoff I guess.

I usually leave an upward half loop in the cord before the tape, to allow the coil to be angled.

Thanks again,

Jigalong.

Without disagreeing with previous posts which can produce a noise, the most common reason is that the actual coil windings shift a little when hitting an object. This causes a change in the coils inductance and capacitance that produces the noise. I have had several coils with similar problems all I sent back to the manufacture and were replaced. To test this keep the coil stable on the ground and knock the edge of the coil housing with a stick whilst preventing the coil from moving. If it sounds off you have a faulty coil and need to send it back. The other reason you can get a noise when the coil hits an object especially a rock is because the coil does an immediate and quick rise off the ground causing the GB to go off, this is why when doing the above test you need to keep the coil firm otherwise you can get this false response. Hope this helps...

Thanks Ismael. I will test what you say next weekend and I will report back if there is a problem with something loose in the coil.
Cheers,
Jigalong.