I didn't need another project -

[Ferric Toes]

I seem to remember that in the '40's -50's, hypodermic needles had a larger metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

[WM6]

Attached is a description of the workings of the UHF detector. [ATTACH]53357[/ATTACH]
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.

It is not best possible copy, yet good enough for overview.
If it was of a grater value for me; i would pass it through the OCR and make proper
fine document... but is not.
OCR is not perfect and sometimes demands additional labor.
That's why i processed only part of the last page, which interests WM6 mostly.

Electronic Metal Locators
(Continued from page 42)
U.H.F. Type
This metal locator has several unique operating features. Fig, 6 shows the block diagram. The u.h.f. locator
is capable of detecting either metallic or non-metallic objects and is able to discriminate between these objects
and the normal clutter of rock discontinuities. Operation is somewhat similar to the induction-balance locator,
except that the operating frequency is 400MHz and the loops are replaced in a search array consisting of
inductively loaded dipole antennas.
Two transmitting dipoles are used with a receiver dipole between them. A figure-eight pattern is produced in the absence
of any target, resulting in balanced voltages that nearly cancel in the receiver dipole. The presence of any object of uniformly
different conductivity and dielectric constant from the surrounding medium upsets
the balance and produces an output signal. There are two modes of operation, the "Search" mode and the "Point" mode.
In the "Search" mode, all of the spurious return is averaged out by a long time constant a.g.e. loop, while any sudden changes
in the field patterns are greatly amplified by an expander circuit, indicating the edge of a target direct below the search array.
In the "Point" mode, the receiver output is amplitude-sensitive and the instrument may be used to outline
the buried object.
The u.h.f. locator is principally used by the military for the detection of metallic and non-metallic mines.

Thanks both.

Despite (needle) resonance way is possible, as we know from from resonant small pole WiFi antennas,
we are here with about 3 - 5cm needle length in couple of GHz band to get resonance , so searching dipole
antennas become very small and unpractical to search some school yards, cause antenna should be put as
close as possible to hidden object (to exclude as much amount as possible of disturbing surrounding medium).
UHF dipole antenna can be increased in stacked antenna way (this add more questions and a lot of research)

Differentiating way (as described in present article) is in fact based on eddy current principle. At very high
frequencies different medium (soil par example) act as ferrite core load with very different characteristic
(par example eddy currents free or absence substances act as hole in surrounding medium). Main problem
here is amount of detecting medium (needle) which we are trying to differentiate from surrounding medium.
This amount is very small, usually to small to get useful distinct for indication.

Regular sewing needles are easily to grab with magnetic broom but hypodermic needles are made from
high grade medical steel and this way immune to magnet.

Probably we need here another approach and start to produce ferromagnetic medical injection needles.

[ivconic]

That's just like when you've rented or bought an expensive item for a one-off job and someone says "I've got one of those that you could have borrowed for free".
I never even thought to look there.

Neither than i.
Last place on Earth to look up for such material!

[Carl-NC]

Does saltwater decay straight line linear time log amplitude, if yes any idea what range of time constants?

Does salt water act the same as a smaller target? With a small target and a 200mm coil, increasing the the distance between a coin and coil from 200mm to 225mm reduces the signal to about half. Is there a ball park distance to move a 200mm coil in or near saltwater to get a reduction in signal to 1/2?

A figure8 Rx coil reduces ground signal, would it be a better at cancelling saltwater?

In wet salt sand I believe the decay is reasonably exponential. I assume the tau depends on salinity but probably on the order of a couple of microseconds. In full submersion it may behave differently, I've never investigated. I would also assume that moving the coil away from wet salt sand will require more distance than a coin to cut the signal in half because the salt "target" is so much larger.

Yes, a Figure-8 will cancel salt effect.

[Qiaozhi]

And it was right here on Geotech the whole time:

https://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=info/lancaster/index.dat

That's just like when you've rented or bought an expensive item for a one-off job and someone says "I've got one of those that you could have borrowed for free".
I never even thought to look there.

[green]

Salt water effect varies based on the volume of the seawater. As you go from hunting wet salt sand to fully submersed the volume changes considerably. A bucket of seawater won't replicate that no matter what you do to it.

Does saltwater decay straight line linear time log amplitude, if yes any idea what range of time constants?

Does salt water act the same as a smaller target? With a small target and a 200mm coil, increasing the the distance between a coin and coil from 200mm to 225mm reduces the signal to about half. Is there a ball park distance to move a 200mm coil in or near saltwater to get a reduction in signal to 1/2?

A figure8 Rx coil reduces ground signal, would it be a better at cancelling saltwater?

[Carl-NC]

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/.

Salt water effect varies based on the volume of the seawater. As you go from hunting wet salt sand to fully submersed the volume changes considerably. A bucket of seawater won't replicate that no matter what you do to it.

[Carl-NC]

Thanks. No wonder I couldn't find it, as it was in the American Electronics World version, not the UK one.

And it was right here on Geotech the whole time:

https://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=info/lancaster/index.dat

[ivconic]

It is not best possible copy, yet good enough for overview.
If it was of a grater value for me; i would pass it through the OCR and make proper fine document... but is not.
OCR is not perfect and sometimes demands additional labor.
That's why i processed only part of the last page, which interests WM6 mostly.

Electronic Metal Locators
(Continued from page 42)
U.H.F. Type
This metal locator has several unique operating features. Fig, 6 shows the block diagram. The u.h.f. locator is capable of detecting either metallic or non-metallic objects and is able to discriminate between these objects and the normal clutter of rock discontinuities. Operation is somewhat similar to the induction-balance locator, except that the operating frequency is 400MHz and the loops are replaced in a search array consisting of inductively loaded dipole antennas.
Two transmitting dipoles are used with a receiver dipole between them. A figure-eight pattern is produced in the absence of any target, resulting in balanced voltages that nearly cancel in the receiver dipole. The presence of any object of uniformly different conductivity and dielectric constant from the surrounding medium upsets the balance and produces an output signal. There are two modes of operation, the "Search" mode and the "Point" mode. In the "Search" mode, all of the spurious return is averaged out by a long time constant a.g.e. loop, while any sudden changes in the field patterns are greatly amplified by an expander circuit, indicating the edge of a target direct below the search array. In the "Point" mode, the receiver output is amplitude-sensitive and the instrument may be used to outline the buried object. The u.h.f. locator is principally used by the military for the detection of metallic and non-metallic mines.

[Qiaozhi]

Don't bother.
https://worldradiohistory.com/Archiv...ld-1966-12.pdf

Thanks. No wonder I couldn't find it, as it was in the American Electronics World version, not the UK one.

[ivconic]

Extracted:

[ivconic]

Don't bother.
https://worldradiohistory.com/Archiv...ld-1966-12.pdf

[Ferric Toes]

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.

I found this on the internet but, as yet, not the article. I can copy mine and send it as it is very interesting considering so long ago. Eric.

Lancaster Donald E.

Electronic Metal Locators, Basic Types and Design Factors

Electronics World

Dec. 1966

pp. 39-42 and 62.

excellent revievs of operating methods (no P.I.), good text, many block diagrams

[ivconic]

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.

C'mon Q !!!
By now you should know me better than that!
There is no negativity on my side at all.
Production paper which is 40-50 years old, discontinued, obscure, abandoned, irrelevant... why not sharing it with community here?
What's the problem?
No negativity. Only confusion.
Incredible interesting phenomena too.

[ivconic]

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.

Eric you really do not understand my point here!?
I don't give a squat about that schematic, really.
I have several versions of that schematic, for years back, probably among the very first schematics that i collected in deep past.
Having it so long ago; i never even payed attention on it nor tried to replicate it.
It is not the point.
Point is in indeed interesting phenomena that NO ONE from the known "great designers" (including you on the first place when we talking about PIs) will ever show good will to disclose any of production papers.
No matter how old the design is. No matter how obscure and irrelevant the design is too.
NO ONE from "officials" will ever show even slightest good mood to share similar documentation. Even if it is dated 50 years back. Discontinued, abandoned etc.
That's my point all the time here. I find this as indeed fascinating phenomena.
I am here since 2006. 14 years and still haven't seen the exception. Except Carl in few rare cases.
I find this as amazing phenomena.
Even with you now. This is fantastic!
You've been circulating one and pretty much the same circuit and design for years. In various shapes, models, brands.
Already disclosed totally. Known. Reverse engineered by many.
No secrets at all. And yet you are resisting hard even on such benign demands!??
I find this amazing! Fascinating!
That is my point here all the time. But you don't understand it.
I think that such kind of "paranoia" kills any chance for this hobby to progress more.
Also makes even wider gap between people like you and majority of hobbyists.
Of course.. ain't nobody else that crazy as me to tell this directly in public.