Vlf better for depth and sensitivity than Pi?

[jackdetect]

Hi Eric.

It is all truth. I am not such expert for Pi-mineral problematic. On my test polygons I have alluvial soil with relatively low mineral content, I believe. Also I have collection of lot of different hot rocks. In past years, I was tested lot of commercial Pi models, like Puls star ll,lll, Lorenz5, Sniffer, DeltaPuls, Superscan etc, and also couple of my experimental design. Most of them, totally ignore hot rocks (probably because they small). But they all are always gives strong response to my ground. Shield, also wasn’t help much. As is impossible, to carry coil always on same distance from ground, Pi units works quite unstable. With such work, weak signals from deep targets become same like ground signal variations. This statement is coming from real field work, not from theory.

Ground and rocks indeed makes serious phase changes in Tx signal from IB coil. But with adjusting of phase angle, is possibly to easily cancel it. And with processing of two or more independent channels, is possible to cancel anything you want. Also, Rx - LC with usually high C (in comparing with Pi), and with good shield are become totally inert from any capacitance changes. With Pi method, is impossible to get such stability for capacitance changes from ground, probably because of very low capacitance of Pi coil.

As I understand, such Pi-Vlf debates, only advantages of Pi detectors is Hi power Tx and large coils, which means possibility of deeper search.

But, Vlf also can work with extreme Tx power, and with large coils.



Zeljko.

[ElectroNovice]

Hi Jackdetect,

As PI only responds to the magnetic lag, and the decay (log t) is much the same for laterite soils, basalts, fired brick etc., it is relatively easy to filter out.

Eric.

Eric I have found that for Australian ferromagnetic soils that the decay is not a simple 1/t decay for Pi.I have found that it depends on the transmit pulse length.I have confirmed largely what Bosnar claims.I find the spectral index varies from .81 to > 2.5.
ElectroNovice.

[JC1]

EE

Hi Electro,

spectral index? what is this? can you explain a bit? please.

Hi Jackdetect,

No!!! you cannot use extreme power for vlf. I had tried/done this. Basically you turn up the power until----front end saturates---because of coil nulling that cannot be made perfect, cannot all be made in the same plane, and strong stiff assembly that doesn't flex even a bit, and then there are signals from the ground which are NOT shieldable and still have a functioning metal detector which will saturate your amps quickly/easily.

PI avoids this problems by saturating the heck out of the amp and then hopefully recover from this condition quick enough to get a good signal. So TX can be much stronger--still have to carry the batteries.

[Ferric Toes]

Eric I have found that for Australian ferromagnetic soils that the decay is not a simple 1/t decay for Pi.I have found that it depends on the transmit pulse length.I have confirmed largely what Bosnar claims.I find the spectral index varies from .81 to > 2.5.
ElectroNovice.

Hi,

Do you have any references for Bosnar's work? I would be interested in learning more.

Eric.

[ElectroNovice]

[QUOTE=JC1]Hi Electro,

spectral index? what is this? can you explain a bit? please.

The decaying voltage induced on Rx decays as V=K/TxAlpha where T = time,Alpha is what I describe as the “spectral index” and is assumed by most people to be Alpha =1 ie a ferromagnetic signals decay as a simple logarithmic decay.

We find Alpha varies from .81 to about 2.35 depending on Tx pulse length for non conductive ferromagnetic targets and have established a relationship between Tx pulse length and Alpha. The relationship is very complex but will enable us to remove completely all ferromagnetic responses using a algorithmic approach without compromising the exponential conductive response and to find targets that have time constants and decay characteristics that are very close to ground which at the moment are being nulled or summed out, e.g. by the Minelab approach..
ElectroNovice

[ElectroNovice]

Hi,

Do you have any references for Bosnar's work? I would be interested in learning more.

Eric.

see bosnars patent 6326791 .He found that the value of x in the expression K/t to the power x where k= constant,that the value of x varied according to the soil properties but was typically x=1.3. This is what lead us to do our work. We find that x=1 only for Tx longer than 1100u secs and that there appears to be a relationship also between magnetic permeability,Tx pulse length and X.
ElectroNovice

[jackdetect]

Hi Jackdetect,
No!!! you cannot use extreme power for vlf. I had tried/done this. Basically you turn up the power until----front end saturates---because of coil nulling that cannot be made perfect, cannot all be made in the same plane, and strong stiff assembly that doesn't flex even a bit, and then there are signals from the ground which are NOT shieldable and still have a functioning metal detector which will saturate your amps quickly/easily.

Really?

I wasn’t known it, because I make and sale about twenty such Vlf detectors (50cm coil), for the last couple of month. They are working perfectly stabile, can go deep in ground for medium non-ferrous metals, with good DISC reject and GROUND balanced systems.

This Vlf works with 2W Tx power, and with a lot of new technology solutions.

[JC1]

EE

Hi Electro,

Thanks for the information on the index. I will take a look.

Hi Jackdetect,

Well now, if you lower the receiver gain then you can increase the transmit power and the net result is the same. Except you have to carry bigger batteries.

By the way, since it appears you have this built and for sale (very good !!!) what are the size, type, and number of batteries your unit uses?

What is Pulse Power Vlf? I'm not sure what this means. I thought there was usually a continuous sine wave on Vlf.
Am I wrong on that too?

Do you have some numbers for detection range on specific targets? Do you have a website that everyone but me knows about?

[jackdetect]

Hi JC1



First, it is very hard to me, to deal with complicated technical discussion on English, because my firs second language, in ex communist Yugoslavia was Russian, and sorry if I make some fatal mistakes.

To work, with such Hi Tx power, I wasn’t decreasing receiver gain (maybe a little, from usual), but with lot of experiments, during a 5years, I was designed very quiet differential coils. Long time ago, I was notice, that present commercial Vlf designs are all the same, and all works with very weakly Tx power. Soils, always with some mineral content, additional make droop in that weakly EM field, and result is catastrophic reduction of location depth. All peoples, who are make test polygons, and tested metal detectors in real conditions, know it very well. I was also notice (on the field), that Pi detectors have much less sensitivity for non-ferrous metals, they suffer from ground capacitance too much, and of course, Pi discriminations are comedy in comparing with Vlf.

I was always ask, what are peoples usually searching under ground? Some ferrous sheets 1m x 1m or even more, or usual things from peoples life in past, like coins, weapons, statues, lamps etc. All those things had ordinary diameter 5 – 30 cm. So, I was select Vlf concept, because he can, much better: locate medium non-ferrous targets, obtain with thousands of ferrous pieces everywhere, and obtain with different ground conditions (read: do not locate ground on any step).



My units using a lead-acid rechargeable battery 12V, 2,2Ah, 18.5cm x 4.4cm x 7cm, in apart a bag, which can put on belt too, and which can be easily replaced with reserve battery.

Pulse Power Vlf is just the name, like Dual Voltage Technology. Power MOSFETs sends intense pulses on Tx, but result indeed is something similar to continuous sine wave.

Detection range is the most falsehood information in world of metal detectors. I was read lot of nonsense from sellers in the past, that I really don’t want to take part in it. I can only say, that my units, because of Hi EM Tx field, 50cm very efficient coil, can go much deeper for medium non-ferrous metals then other Vlf or Pi units whose I tested.

I still wasn’t put this detector on my website, simple because we still cannot produce to many units. We simple cannot cover local market, because units are very cheap. So, right now, units are available only in Serbia. Also, I have lot of work to do about molds and tools for 50cm and 30 cm spider coils. It will be some hybrid from EPS and ABS, because polyester is much heavy for big coils.

Next year, I hope, I will solve all problems about production, finance etc. and will put it on world market.

[JC1]

EE

Hi Jackdetect,

Thank you for the information.

Your english is very good, and I understand your statements well. I agree most commercial units are weak in many ways. I am sure you could make many improvements, and sounds like you have.

2 Watts of power is not so high as to not be believed.
At 20 Volts on the coil roughly 0.1 amp of current peak.
Not as extreme power as PI but stronger than most VLF.
As you know, Vlf receiver is much narrower bandwidth so gain can be higher than PI to help make up for TX power difference.

And I understand your TX generator with the pulses becoming sine current through the coil. And the 12 Volt battery makes sense.

Good Luck, the world can use a better VLF detector!!!

[jackdetect]

Thanks JC1.

When I finish my all developer tasks, I will sent like present, couple of my units, probably to Carl or others, which can make objective performance tests, and put it in public.

Zeljko.

[Largesarge]

I'll be waiting for that day, I agree with JC1 especially if the price can be kept within reason.