About document Construction of Coplanar Search Coils, one question (doubt)

[etznab]

Hi, I downloaded the pdf: https://www.geotech1.com/cgi-bin/pag...anar/index.dat

I have a doubt regarding what I read in the document. where it says the following: "Wind six turns of wire right on top of the receiver coil and leave a movable loop made of about a half turn of wire. Move the loop around to get a good null" .
That "Move the loop around to get a good null". What is he referring to get a good null?. I need a osciloscope to make this?. Is there another way to do it without an oscilloscope?

I dont have oscilloscope.

Thanks

[etznab]

I maked a PI Polones (AVT–5196). I upload some attachments

I dont have oscilloscope.

Thanks

[Kev]

Hi,
If you have access to the circuit preamp output you can do a crude balancing with a multimeter as described in part 1 of Don's doc on TGSL construction.
https://www.geotech1.com/forums/file...etch?id=367654

[GeoMax]

Hi Etznab,

Not sure whether you have notice it. The PI Polones is a Pulse Induction (PI) detector and the TDSL is a Very Low Frequency (VLF) detector. This are two different detector design concepts. To make a coil (Mono) for a PI Polones you usually do not need any "nulling" and or move around "loops".

[Kev]

Not an issue, a coplanar coil on a PI would improve sensitivity to small items while maintaining depth on larger items, since the Rx coil inductance can be increased to 4 or 500uH. Easily providing a much reduced sample time, and it may even reduce some ground noise, similar to a DD. A plus if you seek jewelry or small coins.

[GeoMax]

Hi Kev,

yes, you can use a coplanar coil on a PI detector. But for the PI Polones you need to modify the circuit a little bit so that you have a separate RX & TX-coil connections. If you do that, you also do not need "nulling" the coil.

[Kev]

Hi Max,
I doubt you could achieve the full benefits of having a separate smaller Rx coil i.e. earlier sampling (5 or 6us) greater sensitivity to small items, reduced ground response, if you didn't achieve induction balance. Otherwise, even if the bucking coil was ballpark, you would need a powerful damping resistance that would ultimately over attenuate the target signal.
A well balanced Rx coil should only require a resistance of around 1 to 2 kOhms to reach critical damping. Additionally the preamp input R could be reduced to allow even greater sensitivity.

All the same, I'm not familiar with the Polones design but what I've said holds true for others I've tinkered with.

P.S.
I think you are right Max to suggest he build a mono first, I thought he already had and wanted to increase performance by making a coplaner, but looking back that appears to not be the case. Cheers.

[etznab]

Thank you all

I already have an oscilloscope Hantek 6022be and and inductance meter LCR VC6243A

Ok. I made first a mono coil 25cm diameter and 399uH . It works

So who to make a coplanar coil with external coil 25cm, internal coil 12cm ? How much uH should measeure it external and internal? What should I see on the oscilloscope?

Thanks again, regards

[Kev]

Hi,
I would use a Tx inductance of around 399uH if that worked well for you, and for the Rx 430 to 500uh.

You will need a bucking coil which will reduce the inductance value of the coil by approximately 15%.

You can connect the bucking coil to either the Tx coil or the Rx coil. In the instructions Dave wrote that you referenced above, Dave connected the bucking to the Tx and wound it over the Rx.

Use the calculation he provides to estimate the bucking coil turns. I start with a couple more and slowly unwind them while I watch my scope connected across the Rx coil (also fit the Rx coil with a damping resistor, I start with 1K to 820R Ohms)

Once you near a balance point you can decrease your scope voltage as the overshoot reduces, and cut off that bit of excess wire. Eventually you will reach a point where the overshoot inverts, then wind a little bit back on until the signal is at a minimum.

You don't need to watch the preamp output because you wont achieve a very small signal like that on a VLF.

It's a bit of fluffing around but I've found it worth the effort.

Cheers

[etznab]

This video of Surf Detector

It seems simpler and more explained to me to manufacture. In This dont use a bucking coil. What is the difference between putting a bucking coil or not?

https://www.youtube.com/watch?v=vyY3-dSVbQs&t=189s

[Kev]

Hi etznab,
yep, a dual field is much simpler than a concentric and a good step up. I've not built one before but I wouldn't use copper shielding if I was going for small jewelry.
Cheers.

[etznab]

Hi Kev

Finding small jewelry is what I really want! because I have a case circular with two circles 25cm outside and 12cm inside of a clothesline. This fits for both coils and I want it to take advantage of the smaller coil to find small objects.
So, if I make the Surf Detector dual field coil omitting the copper shielding (if I don't put it on), will it work for small objects?
In what terrain does it work best? for example in sea?


Thanks

[etznab]

Hi Etznab,

Not sure whether you have notice it. The PI Polones is a Pulse Induction (PI) detector and the TDSL is a Very Low Frequency (VLF) detector. This are two different detector design concepts. To make a coil (Mono) for a PI Polones you usually do not need any "nulling" and or move around "loops".

Hi GeoMax

You're right, what do you think about what I said before?

Thanks

[Kev]

Hi Etznab,
I think a dual field is a good idea if searching for deep gold rings or diamond ear rings, and being PI it will run good at the beach.
Novel idea using clothesline
You'll still need a shield though.
To get small gold you need a short sample delay so I think copper foil would prevent you getting below 10us
Some sort of carbon or graphite paint or Scotch 24 like bbsailor recommends in his fast coil treatise would be better.

There must be some info on the forum by people who have constructed a dual field, they will know more than me on this topic.

Good luck!

[GeoMax]

Hi all,

I would prefer to construct a dual-field coil rather than a co-planar coil. It is simpler to make. The green cloth hanger is quite nice as a coil housing. I have seen other videos where a Filippino guy used these kind as well for his coil housing.