the relationship between the coil diameter and detect distance in PI metal detector?

[robby_h]

Oh man!,

where are my special friends when I need them? I'm missing them so. They don't motivate me anymore.
I'm missing such like:
"No, it won't work!"
"No, there aren't any benefits!"
"No, you fail!"
"No, it's a copy of ML's IP!"
"No, you can't beat ML!"
"No, forget it!"

What's happened to my special mates?

4212

"No, it won't work!" This is true more times than not.
"No, there aren't any benefits!". Yep, usually just another way to skin the same old cat.
"No, you fail!" Mainly because of a flawed concept or flawed science.
"No, it's a copy of ML's IP!" If it is then it's a bit silly to say it's yours.
"No, you can't beat ML!" I think it's time you proved that you can.
"No, forget it!". I'm sure Minelab already have!!!!!!!!

[Waikiki_Sweep]

I am not sure is it good idea to use twisted pair center tapped for PI detector because in twisted pair you will have extra interwire capacitance. Capacitance is what everyone fighting against in PI design.
Probably you can connect that pair in parallel and use as monocoil or TX.

Regarding of that discussion about what is better monocoil or separate RX and TX. It turns to be a separate version looks better
but there is not much practical implementation and tests because mostly everyone building monocoils last years.
Sure You can try that configuration with big TX and regular RX size. Any input on results is greatly appreciated.

Check what is inside Garrett Infinum LS coil because that detector is one of the best PI machines with separate TX and RX:
26 turns is TX, 40 turns is RX

Also Infinum coil looks balanced or adjusted very close to it. Such configuration gives a lot of benefits because RX receiving mostly targets (and ground) signals and less to nothing from TX decay signal.

[PATCHES JUNIOR]

"No, it won't work!" This is true more times than not.
"No, there aren't any benefits!". Yep, usually just another way to skin the same old cat.
"No, you fail!" Mainly because of a flawed concept or flawed science.
"No, it's a copy of ML's IP!" If it is then it's a bit silly to say it's yours.
"No, you can't beat ML!" I think it's time you proved that you can.
"No, forget it!". I'm sure Minelab already have!!!!!!!!

I agree. Aziz is starting to sound like a drug addict, taking all these pills every day. And all he does is talk and show some color spirlgraph thing from his computer. Doug has him trained well in his hate for Minelab for sure. With all his computer detectors, seems like he would of made a actual working detector by now. But it will never happen as he follows Doug too close for building one that actually works "good".

I guarantee you that Aziz did not have this hatred before meeting Doug. Such a shame.

[Aziz]

Yeah, thanks much! I'm fully motived for 2014 now.

But I have to make two coils (small and large) to be very sure, that I get more EMI noise with the large coil. All I need is a high gain low noise amplifier and a frequency spectrum analyzer to make the real measurement. Both at hand already except the two coils.
Stay tuned.

Cheers,
4212

[Aziz]

Hi all,

just finished the real EMI noise induction measurements. I have built two Al-foil shielded coils.
Small coil: 10 turns, 10 cm diameter, 0.4 mm wire, Al-foil shielded
Large coil: 10 turns, 20 cm diameter, 0.4 mm wire, Al-foil shielded
The large coil has four times the flux surface area. It should induce four times more EMI noise voltage therefore (note: same turns count).

I have made two measurements:
11 dB EMI noise increase (small coil -> large coil)
12.5 dB EMI noise increase (small coil ->large coil)
Average increase approx. 12 dB

12 dB voltage increase is:
Factor = 10^(12 dB/20 dB) = 10^0.6 = 4 (almost 4)
Q.E.D. Yep, the theory proven to be correct.

I will make two coil comparisons therefore:
Without EMI noise (for good days )
With EMI noise
I will add the split TX(10", 20", 30", 40")/RX(10") configurations as well. So give me more time and be patient please.

Cheers,
4212

[green]

If I were to wind two coils. Say 8 inch and 16 inch diameter. The ampere turns would be less for the larger coil if the inductance and wire size were kept the same. Should I change the wire size to keep the ampere turns close to the same? Is there a better option? Or the change in detection distance wouldn't be enough to worry about?

[Waikiki_Sweep]

Not only wire diameter. To rise current you can also make a longer TX impulse.
For example in SurfPI TX impulse 50 uS and TX power consumes 25-50 mA.
In many PI detectors you can see 100, 120, 150 even 200 uS TX impulses
and in non motion big loop detectors power consumed by TX can hit Amperes.

Also if you make a bigger coil it is recommended to set current even higher
then current in small coil because with more area of coil you creating weaker
magnetic field. To keep same strength ob magnetic field you need more current.
So find a high power source, like Li-Ion.

[Aziz]

If I were to wind two coils. Say 8 inch and 16 inch diameter. The ampere turns would be less for the larger coil if the inductance and wire size were kept the same. Should I change the wire size to keep the ampere turns close to the same? Is there a better option? Or the change in detection distance wouldn't be enough to worry about?

It is better to use think coil bundle (i.e. not compact coil bundle) to achieve more winding turns for a specified TX inductance. The receive section benetifs from more turns.

Aziz

[Waikiki_Sweep]

That is move to right direction! More turns - more signal amplitude received. What shape you can recommend to get those more turns? Flat spiral?

Also you can see why separation of TX and RX gives you so many benefits. Without any problems you can make RX inductions higher than TX and have more turns in RX to receive stronger signal.
As example, one of DD coils for PI detector - Garrett Infinum LS has 26 turns for TX and 40 turns for RX when coils have same shape and size.
So RX has 1.5 times more inductance than TX and receiving 1.5 times stronger signal in comparison with monocoil.

[Silver Dollar]

So if we are building a coil with separate Tx and Rx why not make the Rx 100 - 150 turns like a VLF?
Would the mass of metal nearby the Tx slow our pulse down?

[Tinkerer]

The concentric RX coil needs to be damped too. More turns for higher inductance will result as longer time to decay to near 0. With the DD coil, the 2 coils are balanced which results in much smaller inherent coil signal. Inherent coil signal = eddy currents in the coil wire and coil inductance and coil, Mosfet, shield and cable capacitance.

Proper wire for the coils is important.

At present I am testing a 1375uH RX coil.

[Carl-NC]

More turns - more signal amplitude received.

There is no free lunch.

[Silver Dollar]

My question was, if we are making a DD coil to use on a PI can we use any number of windings on the RX
as long as we can balance it? Or does the method of transmitting change something that would make us
want to use a certain number (or range of numbers) of windings...

[Waikiki_Sweep]

Sure not. At some point it will be too slow for that application because extra inductance and internal capacitance will make it decays slower.

But any way you can use more turns in RX than in TX because if in TX you have to decay hundred volts or even more in reasonable period of time
in case of non balanced RX coil you will see only few volts depending how far distance between TX and RX coil wires.

If you using balanced coil so you will need to decays only mili volts.

Anyway you will need dumping resistor on RX and when you will find that your coil needs more dumping that dumping resistor can reduce
all benefits from more turns in RX by shunting RX signal.

So it is some way by experimenting or calculating (if you can) that optimum number of RX turns providing maximum amplification of RX signal
and maximum value of dumping resistor to less shortening RX signal.

Anyway it looks much better than use of monocoil which limited in turns number because of need to decay high voltage TX
and shorted by low dumping resistor for the same reason. You see how much RX signal you loosing with monocoil.

Also probably to make good RX you need same tricks as used for TX or MONO coils: stranded or thin wire, Teflon insulated wires,
some shape that lowering inter wire capacitance (flat spioral, basket, ... ) Teflon or polyethylene spacer between shield and coil.

And please NO FOIL.

[Silver Dollar]

Ahh I see. I forgot we still need a fast coil on RX. I like center tapped coils with no shielding myself.