DD-Coil design guide for a VLF detector?

[dingbatca]

I am new to DIY metal detecting. I am good with most electronics, but I need help with coil design and tuning. I have found many ?How to build a search coil? articles that just give basic recipes but none of the underlying reasons, or how to test said coil is performing well.

Still in the building steps of my metal detector. It?s a basic VLF machine. I am going to run it somewhere between 5~20kHz.

I am hunting for someone to explain both how to make a good search coil, and more importantly, how to test that it is working correctly. DD style coil would be preferred. What I would really like to get to is a set of equations that I can plug in my desired kHz and search coil diameter, and solve for the variables. Basically a design guide to give me a good starting point.

Tools in my shop for building an testing said coil(s):
Digital multimeter
OScope
Signal generator
Inductance meter
I can also find the resonant frequency of a coil and capacitor (Tank circuit)

Thanks!

[Qiaozhi]

Have you read Inside the METAL DETECTOR - Second Edition - Published 2015 ?

[Koala]

In my opinion getting the coil right is the most important and difficult part to get right if you want an enjoyable experience

Its fairly easy to get something to work on the desk

Difficult might be the wrong word. There's a few simple parameters. But they all have to come together

A coil isn't so good if it drifts with temperature, sounds when knocked, chatters over wet grass, doesn't discriminate correctly or is too heavy.

The phase between the Rx and Tx
The amount of null
Mechanically rigid but light
Well shielded

I used the instructions from the TGSL 101 for my first IDX coils and just used different parameters for the actual coils

https://www.geotech1.com/forums/show...5456#post95456

then the shape of the DD.

I used several different methods of shielding and the coil shell. There's no one way that better than others.

[Skippy]

I suggest using a suitable commercial search coil. Design your detector around the specs of that coil, get it working how you want. Then try building your own coil to match the commercial one. If you're ultimately sucessful, you could then sell the original coil.
A suggested coil is the one for the Teknetics Gamma/Delta series, Fisher F5 etc; 7.8kHz machines. It's not tuned for 7.8kHz, and works on the 19kHz machines like the Tek G2, Fisher F19. This broad freq range makes it suitable for your suggested choice of freq.

[waltr]

Agree with both Q and Koala.

Coil building falls into two parts:
1- the actual construction of which there is lots of information and many methods that work well.

2- Calculate and wind coils on desired inductance. This is pretty easy and be sure to down load the Coil Cal app in the sticky, it works well.

3- the electronic side which includes the resonant frequency of TX and RX coils and how this reacts with the detector's GB and Disc circuits.

#3 is not easy to understand, I still haven't got it yet. Example is the TGSL (this I have built):
TX coil inductance = 6.0mH, 20Ohm oscillates with 20nF at 14.5kHz.
RX coil = 6.5mH, 20Ohm with a 15nF cap and resonates at 16.1kHz.
Why?? It has to do with obtaining a Phase Shift when different targets enter the coil's field.

Then for the IDX:
TX coil = 580uH, 2.3Ohm and oscillates with a 1uF cap at 6590Hz
RX coils = 35mH, 122 Ohm and resonates with a 33nF cap at 4680Hz.

Why does the IDX use a very different coil inductance verse the TGSL???
Both detectors when field testing preform nearly the same. Main difference is due to the Frequency in which the higher Frequency is better for low conducive targets and the lower frequency better for high conductive targets.

Attached are two papers about this:

Since you are new to building VLF detectors I recommend building a know circuit like the TGSL or IDX. There is lots of info in the forums here about these and about building coils for them. Then you can experiment.

[Koala]

If the question is how do you know if you have built a Good coil.

I have no idea. Until you try it in the field

here's. an example made by me

First coil I made for the IDX I adjusted the Rx capacitor until I was getting over 50cm on a pre decimal penny.

The second I made was the same but left the Rx capacitor standard and tested at 35cm on the same coin.

However over real fields the second coil was far superior and found far more

Even main manufacturers coils or machines don't preform identically. I have seen this for myself both with old Tesoro and the Deus


Manufacturer's mass produce to tolerances so some variation is to be expected. However it would be nice if we could spend the time to max out the performance.

[dingbatca]

This is all wonderful information!

Thanks Waltr, as the documents provided really gives me the deep dive I was looking for.

To your point Koala; I feel there is going to be a lot of field testing involved in this process and probably a large tub of dirt in my shop, for some time.

Skippy, there is only one problem with your idea. I would totally tear apart the commercial search coil in order to understand the build process to the last wire. So not the most cost effective solution. But I will start looking for something on the cheap that I can sacrifice in the name of knowledge.

Qiaozhi. Lets see if I can work with the documentation already provided. But I will keep the book in my Amazon wish list, if I feel I need to go deeper.

[Skippy]

The problems with reverse-engineering a commercial coil are numerous.

If you find some feature, how do you determine its purpose?
If a coil is machine-wound, on a former, and fixed with self-bond wire, you're not going to replicate it.
What about graphite/carbon-loaded plastic for the coil housing ? You can't do that.
Fancy foam resins ... what if the chemicals only come in 25 litre tins, and you want 0.05 litres ? Expensive.
Taking a coil apart may damage it, so you can't then fiddle with it to evaluate it.
Etc

Also, commercial coils are built for mass-production, and to a commercial budget. You may well be able to build a better coil yourself, using more labour-intensive techniques.

[dingbatca]

I think I am getting this. Well, at least half of it. ;-)

I have designed a TX coil. Still waiting on a Capacitance/Inductance Meter to test, verify, and tune. It maths out correctly, lets see what the real world numbers show. Here are the numbers I have hammered out:
10" coil
26 AWG
80 turns (About 209' 4" of copper needed)
8 ohm
L = 4474.76 uH
f = 7.5kHz (Target freq)
C = 0.1uF

Now the part that I am stuck on. The relationship of the RX coil to the TX coil. If this was a simple air core transformer, I would build the RX coil to match 7.5kHz TX coil. After a lot of research, it does not seem anyone can agree on what the RX coil should look like. Some say +100hz. Others go for -2.5kHz. While other have 3X more turns on the RX then the TX, and some have 1/2 the number of turns. Can any one give me some good pointers? Or should I just build up 5~10 RX coils and just try them in a controlled environment? AKA the 5gl bucket of dirt.

[kt315]

guys
you have to remember main rule in the electronics.
if you put a copper (RX coil) in a coil (LC contour) - the resonance frequency decreases.
if you put a ferrite in a coil - the resonance frequency encreases.

YOU HAVE TO REMEMBER THAT LIKE YOUR MAMA NAME.

[dingbatca]

So... What value do I shoot for? 7kHz, 6.5kHz, 1hz?

[Skippy]

This is why people tend to copy existing known designs, it avoids the "why's" of the design element.
I don't have enough experience to answer your question properly ... but I suspect what is relevant is the Q-factor of the receive circuitry, it's likely the high-Q RX is tuned to a smaller difference from the TX, compared with a lower-Q RX, which is further away.

[waltr]

guys
you have to remember main rule in the electronics.
if you put a copper (RX coil) in a coil (LC contour) - the resonance frequency decreases.
if you put a ferrite in a coil - the resonance frequency encreases.

YOU HAVE TO REMEMBER THAT LIKE YOUR MAMA NAME.

Yes, that is most important to remember.

So... What value do I shoot for? 7kHz, 6.5kHz, 1hz?

Frequency would be chosen for the types of Targets you are Most interested in. Higher frequency for smaller or lower conductive targets.

TX coil specs Verse RX coils specs???????????
Did you read and STUDY the documents I posted??????
Did you fully understand what they say??

Did you STUDY a Few existing, working VLF detectors and understand how they work?
Did you Read the ITMD book as suggested?
If not then your have nearly ZERO chance to design one that works from scratch.

Same as Skippy said, I also don't know enough to explain and best to start with a known design and build it.
This can be challenging enough to get a simple VLF detector (TGSL or IDX) working properly the first time.

[dingbatca]

Waltr, yes I read through the docs you posted, a few times. No, I do not fully understand them, yet. I think at this point, I need to build up my first test coil(s) and start playing with the interactions. After that, re-read the documentation you posted. Basically I need some hands on + book knowledge to start understanding the full concept.

My copy of ITMD is still in the mail.

Type of targets... At the end of the day, I would like to build up two, semi functional, metal detectors. One for me, and the other for my 5 year old daughter, who wants to find buried treasure with daddy. For me specifically, I love playing with new tech and learning new stuff. I don't mind investing hundreds of hours in the tech side till I have a good grasp of the underlying tech and can build up something fun.

Yes, I have studied a few different VLF detectors. I understand how the electronics work, up to the coil. The search coil part still seems magic to me.

Off the shelf coil, to use as a reference design? Any suggestions? Preferred cheap! I am just fine with used...

[Carl-NC]

Now the part that I am stuck on. The relationship of the RX coil to the TX coil. If this was a simple air core transformer, I would build the RX coil to match 7.5kHz TX coil. After a lot of research, it does not seem anyone can agree on what the RX coil should look like. Some say +100hz. Others go for -2.5kHz. While other have 3X more turns on the RX then the TX, and some have 1/2 the number of turns. Can any one give me some good pointers? Or should I just build up 5~10 RX coils and just try them in a controlled environment? AKA the 5gl bucket of dirt.

There are no rules for any of this. A 1:1 transformer ratio works (Tesoro uMax, Minelab BBS/FBS), and a 10:1 ratio or higher also works (White's, Fisher). A higher transformer ratio produces more RX signal for the target, but does the same thing for ground and EMI. So it's largely a wash, except for thermal noise. For thermal noise, a high transformer ratio is a little better. For a general-purpose design I would use TX=0.5-1mH and RX=1-4mH. No sense in piling on more weight.

On where to set the RX resonance, try starting without a cap at all. Let it be broadband, some detectors do this. Once you get it running, try adding various tank caps and see what happens.