Large Mono Coil Shapes

[Infamy]

Hi, I have a large mono coil, 1m sq. It is actually too large for my current project and I'm wondering if it can be made into a different shape I'm thinking of a 600 mm diameter. Any ideas on the impact of making it into a figure of eight or a 4 leaf clover type shape, that is 4 partial circles in the north south east and west orientation. I don't have anything to plus it into at the moment due to a damage chip.
Just in wondering mode.

[Infamy]

Has anyone built a large spiral mono coil, in the 25 - 36 inch range?
If you did how did it perform.

Another construction type I'm looking at is verticle winding with secondary layer unconnected. Any ideas on performance of such a build.

I don't need to physically carry these coils, so moving such large coils about is not a constraint..

[bbsailor]

Has anyone built a large spiral mono coil, in the 25 - 36 inch range?
If you did how did it perform.

Another construction type I'm looking at is verticle winding with secondary layer unconnected. Any ideas on performance of such a build.

I don't need to physically carry these coils, so moving such large coils about is not a constraint..

Infamy,

If you post the following information, forum members could better jump in with their input.

1. Coil resistance
2. Coil inductance
3. TX pulse frequency or range of frequencies
4. Primary targets sought, metal type and size
5. Minimum delay required
6. Depth of your primary sought targets
7. Type of ground you will use the coil on
8. Any size, weight or balance restrictions

Identify the type of test equipment you have access to for measuring coil characteristics.

Joseph J. Rogowski

[Infamy]

Hello Joseph.

"Identify the type of test equipment you have access to for measuring coil characteristics."

Ok I have,

1 FY6900 DDS Function/Arbitrary Waveform Generator
1 Philips PM 3055 60 MHz oscilloscope
Several multifunction testers.

"8. Any size, weight or balance restrictions"
The PI mono coil will be towed on a barge.
Large Iron targets knock out in magnetometer.

"7. Type of ground you will use the coil on"
Over salt water 6-12" deep, then estuarry mud.

"6. Depth of your primary sought targets"
Single target, depth more than 1 foot less than 4 feet if its not in a channel or hole, which it probably is!

"5. Minimum delay required"
?

"4. Primary targets sought, metal type and size"
Silver possibly gold.

"3. TX pulse frequency or range of frequencies"
?

"2. Coil inductance"
?

"1. Coil resistance"
?

Joseph I read through the first ten Geotech1 pages last night on the Vallon VMH3 mine detector, its a good example on how people close doors on possibilities (300-500uH). I want to keep as many doors open as possible and I'm interested in stuff people have done for small coils but never applied it to the size of coil I'm looking at.

My skills are more in ability to build precisely and in novel ways than electronic design, although I can fault find and solder well. I have the Art of Electronics on my desk to try and understand this invisible confusing magical world.

Be interested in what you have to say about the construction of large coils, using the form as part of the coil, the covering process etc. I intend to use plywood for the coil base, may use carbonfibre hollow rods as stiffeners.

Thanks

Stephen

[GeoMax]

Hi,
why do you want to use plywood for your coil base? Simply use water pipes. These pipes and joints you can find easily in every hardware store and on top, they are very cheap too.
There are many construction pictures and videos for large frame coils in the Internet. Once I saw a frame coil construction, where a motor boat was towing a large frame coil. Just google it. Maybe the images are still floating around in the Internet.

Geomax

[Infamy]

Hi,
why do you want to use plywood for your coil base? Simply use water pipes. These pipes and joints you can find easily in every hardware store and on top, they are very cheap too.
There are many construction pictures and videos for large frame coils in the Internet. Once I saw a frame coil construction, where a motor boat was towing a large frame coil. Just google it. Maybe the images are still floating around in the Internet.

Geomax

Hi,

Cable coil and pipes, I already have that, 1mx1m.

Plywood can be shaped,
It's rigid,
Can be the form too,
Is as cheap as pipe,
Has favourable stealth and aero dynamic characterists,
You can make it look like something else,
Better aesthetic possibilities,
Allows more freedom in design,
Easier to attach to barge.
More stability options.
It floats.

I did some test in 2019, I'm not a fan of the pipe option.

[Infamy]

I went the local secondhand shop to see what materials they have. I'm looking at a canvas painting of Jesus thinking thats about the right size. I can stick a spiral coil on it, glue on a couple of foam hulls and I have the spiral barge coil, with added bonus that I can say to anyone that asks, I'm spreading the word of Jesus to the marine life.

[GeoMax]

Hi,
share some images... would be interesting to see.

Geomax

[Infamy]

Hi,
share some images... would be interesting to see.

Geomax

And the title of this piece is death by cocktail sticks!
Probably be a few months before I start on that.
You don't happen to have a Korean paper on spiral coils do you? I ve put in a request on Reasearchgate but haven't had a reply. I think its five coils of different sizes they tested.

[GeoMax]

Hi,
do you mean this article?
https://www.semanticscholar.org/pape...39166c640f6024

This one I do not have...
In case someone has this article please upload it.

GeoMax

[GeoMax]

Hi,

After I posted the reply I googled the article I mention above. It took me less than 2 min to find the paper for download.
Here the download link:
https://www.hobbielektronika.hu/foru....php?id=270863


Geomax

[Tinkerer]

Hi,

After I posted the reply I googled the article I mention above. It took me less than 2 min to find the paper for download.
Here the download link:
https://www.hobbielektronika.hu/foru....php?id=270863


Geomax

Thank you for posting the interesting article.
Spiral coils are interesting because they can be manufactured with high accuracy. The coil interwire capacitance is low.
The problem arises with the shielding. The coil wire area is large, so the capacitance between the shield and the winding is high.
One way to counteract the high capacitance potential, is to increase the spacing between the winding and the shielding. Like about 10mm, both top and bottom.
The shielding can also be made with a specific pattern to reduce the effective capacitance. Remember, the less capacitance, for a given inductance, the higher the SRF of the coil and therefore the delay to the first sample.

The fill factor for a bundle wire is subject to similar rules. If I wind 100 turns of AWG 22 magnet wire, I get about 4000uH. If I use an insulated fine strand, tinned wire (better for the skin effect) for the same 100 turns, I obtain about 3200uH. (200mm diameter TX coil)
Both coils have 100 turns, but the fill factor of the first one is about 80% while the fill factor of the insulated wire coil is about 25%.
Intuitively I know, that with a greater fill factor and the same amount of turns, I will obtain a stronger magnetic field, because the proximity of the wire turns increases the interwire inductance.
It also increases the interwire capacitance.
The increased capacitance increases the SRF. This means that this coil produces a stronger magnetic field, like 100turns x1A=100 Ampere turns, but it will be more effective on larger targets than on very small targets like a very small gold nugget with a TC of 1us.

It could be interesting to actually calculate all this and find the sweet spot for a coil.

[GeoMax]

Hi Tinkerer,

in the case, your front-end circuit design is capable to automatically removes ground noise and triggers only if a
possible target is near the coil, the sampling circuit. Is it than necessary to shield a mono PI coil too?

GeoMax

[Infamy]

Hi,

After I posted the reply I googled the article I mention above. It took me less than 2 min to find the paper for download.
Here the download link:
https://www.hobbielektronika.hu/foru....php?id=270863


Geomax

Thats the one, thank you.

[Tinkerer]

My first PI detector design (1986) was designed for divers. It was running at 3000 PPs and had a delay of 30us. In the water there was no need for shielding.
Recent designs with 10,000 PPs and a delay of 3us are much more sensitive. Without shielding they see my hand as a target.
My hand is full of blood and tissue that are conductive. So I consider it normal that the detector sees it as a target.

However, I live in a region with high static during dry weather. The static can be strong enough for sparks to fly from my fingers. This means several thousands of volts.
I design the shielding for this static and test it with a PVC pipe which I charge by rubbing with a cloth until I hear the crackling of the static electricity discharging and it picks up pieces of plastic or styro foam.
If I can approach this charged PVC pipe to the detector coil without noticing any reaction from the detector, I consider the coil well shielded.
I notice that the shielded coils reduce environmental EMI to a great extend.
The coil shielding adds capacitance to the coil and therefore reduces the SRF. Leaving a minimum of space of 3mm between the coil wires and the shield helps. Leaving 10mm of space reduces the capacitance a lot.