I didn't need another project -

[Qiaozhi]

There will be no schematic until this project is finished!! I am making changes and additions each day, so there is no point. Guidelines for this forum are:- General tech discussions on all types of metal detectors: VLF, PI, 2-box, BFO, off-resonance, PLL, etc. Questions, ideas, and anything else that moves you. It does not say that schematics should be attached to any technical discussion. I have the intellectual property rights to what I am doing for my own interest and it is purely at my discretion as to what is shared and when. Same goes for everybody.

Eric.

Just carry on Eric.
Ignore the negativity of some posts.

I've been searching for that UHF detector article from which you posted an extract. Cannot find it anywhere in the internet archives.

[Ferric Toes]

Still no schematic?
This topic will bust!

There will be no schematic until this project is finished!! I am making changes and additions each day, so there is no point. Guidelines for this forum are:- General tech discussions on all types of metal detectors: VLF, PI, 2-box, BFO, off-resonance, PLL, etc. Questions, ideas, and anything else that moves you. It does not say that schematics should be attached to any technical discussion. I have the intellectual property rights to what I am doing for my own interest and it is purely at my discretion as to what is shared and when. Same goes for everybody.

Eric.

[ivconic]

But than again... i can imagine the rough contours of such design yet.
Since there is one very significant and important factor here; the target is already known, with all its features.
Whenever i think on propagations, hum and "dirt" in the air that surrounds us: i also recall the way how small digital radio packets are functioning.
GSM for example. WiFi too (BT too).
Small power, yet effective, long range and accurate.
Since the target is well known; maybe is possible to "memorize" it in a short radio data packet like that.
The rest of the story is easy to anticipate...
But i don't need to give you all on the plate now...

[ivconic]

Obvious nonsense.
Chasing a fog.
Various propagations (world we are currently living in) will overwhelm any tiny signal coming from such needle in a grass.
Even grass itself will choke it.
Aware of "wet&dry grass effect" at VLF I/B's?
"Doppler-ing" at UHF will be 100x more horror.

[ivconic]

US5936586A

[Ferric Toes]

Interesting remark, Eric.

I never think about UHF solution with hypodermic needle problem, cause UHF antennas are usually
lead over surface at greater distance from soil, than PI or VLF coils.

So we need to rethink near to soil UHF antenna movement and possible exclusion near soil impact.

Your suggestion could be real solution, cause dangerous discarded hypodermic needle in play
and park lots around schools and kindergarten are by rule not buried in soil, but only hidden in
grass and sometimes very shallow covered by sand in sandboxes.

Thanks. So Pi technology is practically excluded, same as VLF/IB in searching for hypodermic needle
detecting solution.

I will rethink suggested UHF way. You mentioned here needle resonance and not GPR like reflection.
Hypodermic needles are of different lengths, which lead to conclusion that UHF antenna should emit
sweeped frequency spectrum to resonate with needle about 3 to 7cm body length.
Here we are talking about 5 to 10GHz frequency sweep to catch most needle resonances.

Attached is a description of the workings of the UHF detector. This is from an article in the Electronics World magazine for December 1966. There is no indication of how small a metal object can be detected and I don't know of any actual devices that work in this way. You would think that developments of this technique had progressed since 1966
The resonance method might be worth investigating and, as you say, a frequency sweep method and a spectrum analyser for a Rx display. Objects at resonance should appear as a spike on the display.

Eric.

[ivconic]

Interesting remark, Eric.
I never think about UHF solution with hypodermic needle problem, cause UHF antennas are usually
lead over surface at greater distance from soil, than PI or VLF coils.
So we need to rethink near to soil UHF antenna movement and possible exclusion near soil impact.
Your suggestion could be real solution, cause dangerous discarded hypodermic needle in play
and park lots around schools and kindergarten are by rule not buried in soil, but only hidden in
grass and sometimes very shallow covered by sand in sandboxes.
Thanks. So Pi technology is practically excluded, same as VLF/IB in searching for hypodermic needle
detecting solution.
I will rethink suggested UHF way. You mentioned here needle resonance and not GPR like reflection.
Hypodermic needles are of different lengths, which lead to conclusion that UHF antenna should emit
sweeped frequency spectrum to resonate with needle about 3 to 7cm body length.
Here we are talking about 5 to 10GHz frequency sweep to catch most needle resonances.

You shall give it up by now... as i recall this is old attempt from several years back.
Tinkerer "poured water in my ears" (local saying) several years ago with same demands, talking about millions of euros for such project.
This would be indeed millions of euros project... if possible to be done.
Ain't no wonder why bustards from weapon industry are using similar needles for their plastic undiscoverable mines.
Such detector would be most sold product on planet. Sad fact.
GPR domain is also explored and exhausted well by now. It has its own limits.

[ivconic]

... Sea water starts to give a large signal once you get below 30uS and in water depths greater than a couple of metres you have to retard the delay even further...Eric.

This is indicative info.
My digital attempt of Barracuda can sample starting at 27-30uS with pretty steady behavior.
I am using ancient Roman brick (IV century) to mimic hard soil and minerals.
...
Still no schematic?
This topic will bust!

[WM6]

Hypodermic needles can't be detected even using a 1uS delay due to them being hollow.

Some other technique needs to be used and UHF radio waves may be a possibility as the needles
must have a resonant frequency, depending on their length.

Attached is a block diagram on one such arrangement.

Eric.

Interesting remark, Eric.

I never think about UHF solution with hypodermic needle problem, cause UHF antennas are usually
lead over surface at greater distance from soil, than PI or VLF coils.

So we need to rethink near to soil UHF antenna movement and possible exclusion near soil impact.

Your suggestion could be real solution, cause dangerous discarded hypodermic needle in play
and park lots around schools and kindergarten are by rule not buried in soil, but only hidden in
grass and sometimes very shallow covered by sand in sandboxes.

Hi WM6, No, that delay range is not good for sub-gram nuggets.
the XL500 is designed for shallow and deep water searching mainly in the sea.
Sea water starts to give a large signal once you get below 30uS and in water depths greater than
a couple of metres you have to retard the delay even further.

For sub gram nuggets you need to be at 10uS delay or even less.
The problem there is that mineralised ground gives more signal at shorter delays and a good GB
system is needed.

Eric.

Thanks. So Pi technology is practically excluded, same as VLF/IB in searching for hypodermic needle
detecting solution.

I will rethink suggested UHF way. You mentioned here needle resonance and not GPR like reflection.
Hypodermic needles are of different lengths, which lead to conclusion that UHF antenna should emit
sweeped frequency spectrum to resonate with needle about 3 to 7cm body length.
Here we are talking about 5 to 10GHz frequency sweep to catch most needle resonances.

[eclipse]

BTW a circuit that monitors where the preamp has settled to a specific voltage level and adjust the delay to start from that point can work in no matter what sea water depth and advantage on the park/fields (sample at 10us etc). What do you think?

[eclipse]

Is there a way to test sea water effect on delay say in a bucket of water?
Something like adding a graphite powder /something else - higher conductive is going to be needed I guess/ step by step will give similar result as sea water /just for testing purposes of course/.

I have a JW Fishers 8X underwater PI and I want to see how it performs if conditions are not ideal /air - regular ground etc/.
Or just go with a bucket full of dirt and black sand /Magnetite/.

[Ferric Toes]

According your experience, Eric, is this delay range still sufficient to detect sub-gram gold nuggets?

Or from another point of view, what would be most adequate delay range (using fast 18-22cm coil),
to detect sub gram gold nuggets?

Asking this I hope to correlation between sub gram gold nuggets and syringe injection needle lost in
parks and playgrounds of schools and kindergarten, made from medical grade stainless steel which
is here my main goal of interest.

Hi WM6, No, that delay range is not good for sub-gram nuggets. the XL500 is designed for shallow and deep water searching mainly in the sea. Sea water starts to give a large signal once you get below 30uS and in water depths greater than a couple of metres you have to retard the delay even further. For sub gram nuggets you need to be at 10uS delay or even less. The problem there is that mineralised ground gives more signal at shorter delays and a good GB system is needed.

Eric.

[Ferric Toes]

Nice progress !
What about STP14NK60ZFP for RFP8N18 replacement?

Rds 0.45 Ohm (vs 0.5 Ohm)
Vds 600V (vs 200+v)
Coss 240pf at 25v (vs 250pf)
Gate treshold 4.5 V max (vs 4V)

Price around $1.5
I dont know how, but I was lucky to get it for free as it came from salvaged tv board. Fate)

If Coss after installing isolation diode play no role, SPA17N80C3 may be superior in Vds and Rds

Thanks Vadim, I have a couple on order although they are STP14NK50ZPF. 500V is enough.

Eric.

[ivconic]

NEXT 1245 pages...

[Ferric Toes]

Thanks for interesting insight bbsailor.

According your calculated specification, I cannot see such available technology in hobby sphere.

Our devices sampling capability end up at 5μS at best.

In my view this remain unsolved problem, not only with PI but with VLF/IB tech also. I tested a lot of
prospecting VLF detector, but no one is able to detect hypodermic needle. Empty hand yes, but not needle.




Thanks green, it is known to me, but I was in hope, that maybe technology advanced somewhat in meantime.

As I see state of the art, it is only PI technology capable to solve this problem. So I ask Eric, first authority in
field of PI solutions.

When I was involved in industrial metal detectors, one of the areas we were asked to investigate was that of detecting surgical instruments, including needles. These are invariably made of high grade stainless steel which is non-magnetic and low in conductivity. Even recently, I have been asked if it is possible to detect titanium bolts that are used to secure artificial joints. The latter I can do at several inches range, but only at a delay of 4uS. Hypodermic needles can't be detected even using a 1uS delay due to them being hollow. Some other technique needs to be used and UHF radio waves may be a possibility as the needles must have a resonant frequency, depending on their length. Attached is a block diagram on one such arrangement.

Eric.