[sci.electronics] Help: Optimal Coil Geometry for Metal Detector

G.Moretti@massey.ac.nz (Giovanni Moretti) (12/20/90)

Hi
I'm making a metal detector for my son's birthday - you know the sort
of thing that it's obvious  how it works and you just whip one up in
an evening :-)  Well I did that and I have a few questions ...

I'm using the two oscillators at about 300KHz mixed together and
listening for the beat note.  One of the oscillators is fixed and the
other has the search coil as its inductance, so that its frequency
will vary when metal is bought close to it.  

I've prototyped it last night and it seems to work but it's not as
sensitive as I'd like.  When the oscillators are about zero beat (or
close) you can detect (" denote INCHES):

(The round coil has about the same inductance as the oblong one - I
 didn't count its turns)
                                        COIL Shape and Turns
                             -----------------------------------------
     OBJECT                   15Turns 4" x 6"        4"diameter 
3"*3" PC bare PC board           5"                      4"
Bean Tin 5" diameter             7"                      5"
Coin 1.5" diameter               2"                      1"

The coil shape obviously has a marked difference on the sensitivity -
what is the optimal shape and screening (ie should the coil be round,
square, oblong, a spiral to  encompass a lot of area, shielded)?  

I'm aiming to be able to detect objects at greater distances that the
above.  Obviously the type of metal has an effect but there must be
some general guidelines.  Do you need different shaped coils for
maximum  sensitivity with different sized object?  One magazine
article suggested that the sensitivity was related to the ratio of the
object size to the coil size, but didn't give any further guidelines :-(

The frequency I'm using is sort of mid-range,  I didn't want to go
too high in case the metals are too lossy - I know this sounds sort
of vague, but I know for example that you don't use a nail as core for
RF coils :-)  Would a higher frequency be better?

The other sort of metal detector I know of has two coils are right
angles to minimize the coupling between them, until some metal comes
along.  Is this type notably more sensitive?

A friend has just come in and suggested using a phase comparator as in
4046 VCO chip instead of listening to the beat note - Seems as though
this might be too sensitive ???

Gian, my son is only seven and would be happy with the current
version, but _I_ don't like not understanding the things I build :-)

Any suggestions would be welcome.

Thanks
Giovanni ZL2BOI


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