tuning, DD vs Concentric

[robbor]

Tell me a little about tuning coils, I read somewhere you need a oscilliscope to tune a DD coil is the same true for a concentric coil? This is for a VLF detector.

[bbsailor]

Tuning a coil

DD coils operating on a VLF require that a good mechanical null is achieved by the careful positioning of the TX and RX coil in relationship to each other. Commercial coil builders often make a small 2 turn 1" diameter loop that can be moved to fine tune the coil before sealing it up. A scope is most useful in seeing very small signals to achieve the best null. This DD and other concentric and "fold back" type search coils function as loosely coupled transformer. The presence of a target upsets the null balance. Based on the metal composition, the type of metal can often be identified by examining the phase of the balance upset.

DD coils operating on PI machines work on a different principle. While obtaining a good null can't hurt, it is not as necessary as on a VLF machine. The PI machine DD coil does not work like a loosley coupled transformer, it operates more like a radar dish. A signal is emitted from the TX coil. After the pulse dies down (10 to 20 us later) either the same coil in a mono coil design, or the RX portion of the DD coil senses the residual eddy currents flowing in any nearby targets. A slight unbalance in a PI DD coil should not affect performance.

In any event, a scope is most helpful in examining search coils. There are good deals on e-bay for about $100 to $150. A dual trace, 20 Mhz or above should work very well.

bbsailor

[nncunew]

DD Coils

Do you have the specifications for a coil DD for PI ???

For ejample, the HH can use a DD coil ??? how you can make this coil ???

Need Faraday protection (shield) DD coils ???

Ok, thak you

pd: sorry for my bad english.

[email protected]

[bbsailor]

Making a DD coil for HH

To make DD coil for the HH go to the following link to see how to wire up the TX and RX coils. Reg did a good job of explaining with an illustration.
http://thunting.com/geotech/forums/s...d.php4?t=10729

The TX portion of the DD coil should have the same characteristics as a mono coil taking in account that the DD coils are smaller and will require a few more turns to have the same inductance. Just measure the actual diameter of each coil in a DD coil housing and then wind and shield each coil as a normal mono coil. Make sure there is a small shield gap in each coil shield. Shielding on a DD coil is just as important as on a mono coil.

The RX coil can be made with a little finer wire to make the coil more sensitive to weak signals. You will need to adjust the value of the damping resistors for each of the coils independently as there should be little interaction between them. DD coils can be made to operate at a little faster delay than a mono coil because the TX circuit does not see the RX circuit and it's associated resistive and capacitive loading. Therefore, the damping resistor will be slightly different (probably higher) that what an equivalent mono coil would have.

The mechanical balance between the RX and TX coils for a PI machine is not too critical. Just make sure that at the two crossover points that the shields are insulated from each other and that the coils are lashed together to be secure so it will not move. Wire the shield of each coax back to it's own pin and only combine grounds at the circuit board (4 pin connector). You can use a signal generator on the TX coil and a scope on the RX coil to try to get close to a null, but this is not too critical, however it helps.

You will need a 3 or 4 pin microphone connector and plug. Actually I find a 4 pin easier to use. If you want to go back to using a mono coil just make sure that you make a jumper in the mono coil plug housing to tie the RX and TX circuits back together. See Reg's explaination in the link above.

Mechanical stability is very important to avoid false signals. Make sure that you fill the housing with something light and stable, for land use (or neutral boyancy for water use), but also has a good dialectric constant to keep capacitance low.

It is much easier to use commercial DD coil housing available from Hays Electronics. It comes with a cable strain relief that is waterproof. Use a good quality of S-video (containing two independent coax wires) wire that is round and will easily fit the strain relief. Use only as much S-video wire as you need to keep the cable as short as practicle and the capacitance low.

Get you HH circuit working with a mono coil first then make the TX-RX separation (as shown by Reg) and begin to experiment with the DD coil. Make the inductance of the DD TX coil the same as the mono coil. It will probably have an additional 2 or 3 turns to account for it's smaller diameter. The RX coil can be the same or 1.5 to 2 times the TX inductance and made with thinner wire. Here is where you can experiment for the best results on the targets that you seek.

Radio Shack has good wire; it is AWG 30 wire wrap and comes only in 50 ft lengths. The wire insulation is made from Kynar which has a good dialectric constant. This is good wire for a low power coil operating at higher frequencies. This insulation is much thicker than what is found on magnet wire. It will keep the capacitance lower than using magnet wire.

If you want to build a high power coil you will need to use heaver stranded AWG 22 hookup wire for the TX coil and you can use AWG 24 to 26 wire for the RX coil. You will need to order the AWG 24 or 26 wire from an electronics supply house as Radio Shack does not stock thinner stranded hookup wire.

When you adjust and experiment with the coils set up a temporary table with no metal nearby. A large cardboard box (with no staples) and a piece of plywood makes a good temporary coil setup bench. Keep your test equipment as far away from the coil as is practical. If you pick up noise on the RX coil just rotate and ground some of your test equipment, probably the scope, to minimize the noise.

I hope this helps.

[robbor]

Back to the question, please

Thank you for all the posts, but what I really want to know is if a Concentric coils gets set up the same way on a scope??? I am going to a make a DD coil for my white goldmaster 4b, which they do already make small DD coils for so I am guessing no mods are needed. But I am also thinking about making a large concentric coil at the same time. Or do you just put the 2 coils into a housing and go for a concentric???(i am guessing not) . As for the 2turn 1" loop for tuning, I would love to see a pis to see about positioning and lead length. I am right now waiting on a inductance adapter for my dmm to get started. Personally I know nothing about radio shack wire but can only guess that it is no where near the quality of wire that could be had from an electronics store, as a comparison for stranded wire usually places like radio shack and home depot have wire that has fewer strands that is not as good as having more finer strands. I will have alot more questions soon.

[bbsailor]

Tuning, DD vs Concentric

See the link below about how to make a VLF concentric coil.
http://www.thunting.com/cgi-bin/geot...anar/index.dat

There are three coils.

1 The outer TX
2 The inverted phase inner TX
3 The inner RX

The key is to create a null in the RX with the right number or turns in the outer TX balanced by the right number of turns on the inner TX relatve to the area of the TX coil diamenter vs the area of the RX/inner TX coil diameter.

I have not built one of these coils but remember a little theory from some past posts and the above link. Use the forum search feature on the words "concentric coils". You should find some answers to your question.

bbsailor

[Judge]

Judge

I am constructing a DD coil, and I have come to one of many gaps in my knowledge. How do I calculate the correct number of turns that is required, in the reverse direction segment of the TX coil ( the segment which is wrappped over the RX coil), to achieve a null?

[nncunew]

Good

Is very fine your explanation.

I believe that i understand all about you words.

I am planning make a DD coil, everybody say that this coils are more sensitibity


Thank for all, and remember that i used a bad english

[email protected]

[Charles (Upstate NY)]

DD versus Concentric tuning for VLF

Tell me a little about tuning coils, I read somewhere you need a oscilliscope to tune a DD coil is the same true for a concentric coil? This is for a VLF detector.

Yes you will want a scope to tune either a DD or concentric for a VLF. First its a whole lot easier and second you don't have to be out of balance much for this to effect coil performance.

Balancing DD coils for a VLF is tricky, a tiny nudge where the windings overlap can knock them out of balance by quite a ways. I tune mine at 10mv division. Epoxy curing can also pull them out of balance.

Concentrics are easier to balance since the windings are not overlapped but theres a catch. More on that below. For rough balancing you simply add or take away turns or partial turns of the TX bucking coil. Fine adjustments can be made the same way as with a DD, with a small tuning coil.

To keep the math simple people generally wind RX half the diameter of TX. In "theory" a 10 inch diameter TX would have a 5 inch diameter RX. Estimating the turns of bucking coil needed to balance the coil is then simple, divide the number of TX turns by four e.g. a 40 turn TX would require a 10 turn bucking coil around RX.

Thats the theory, in practice its rarely that neat and tidy. I have a 45 turn 9 inch concentric TX winding on my test bench now. I have wound the 4.5 inch RX and bucking coil three different ways...

#1 - flat wound RX and bunched sloppy bucking coil (looked like a birds nest)
#2 - precision square RX and precision flat wound bucking coil
#3 - vertical RX and precision wound vertical bucking coil

Okay theres three different RX/Bucking coil geometries wound around the same size winding jig. #1 required 10 turns of bucking coil, #2 required 10 turns, #3 required 8.5 turns. I mention that just so you know the 4 to 1 rule is just an estimate to get you close as to the number of turns you will need for the bucking coil.

By the way...

#1 - worked fine though a fair bit of overshoot on RX on the scope.

#2 - refused to work, nothing but a constant overload signal on the detector even when balanced. There was perhaps the ugliest distorted signal on RX I had ever seen. Clearly winding the bucking coil flat was not advisable.

#3 - Here I wound RX vertical e.g. it was about .250 inches tall and maybe .030 inches deep, like a cylinder. I wound the bucking coil around this which resulted in a nice square profile for the bucking coil. Note 19awg Litz for TX and the bucking coil, 31awg solid magnet wire for RX.

The result? #3 had one of the nicest looking RX signals I have seen on the scope to date. Very clean, flat lined nicely when balanced.

Another interesting observation of winding #3, most coils before they are shielded can react your hand if you get close enough, a half an inch or so. #3 above was picking up my hand 3 inches from the coil, that was a first.

THE CATCH - Here's the catch on balancing the concentric coils. For each turn of TX you need roughly 1/4 turn of bucking coil. Ideally you would want both leads of the bucking coil to arrive back near the coil cable inlet but you have a good chance that the bucking coil lead will end up clear across the other side of the coil shell when balanced, or at least 90 degrees off. So depending on the design of the coil shell you might have to adjust the size of TX or RX slightly to bring that back around near the coil cable. For my VLF coils I don't have the option of adding or removing turns from TX or RX. A single turn either way will knock them out of factory specs so far as LCR goes. Speaking of which, a good LCR meter is another item you need.

Charles

[fairdinkum]

DD coils and construction

thanks to BBSailor I found the answer to my question I have been asking and getting no replies, that was about the null point or balancing point on overlapping type DD coils as used in PI detectors, I want to build a DD for my minelab SD I found out I can use graphit powder with epoxy resin as a faraday shield, this is similar I believe to the commercial aquadag used by VLF manufacturers, what I was wondering is that earlier VLF detectors used a unique form of DD that had a small coil wound inside the larger coil but all part of the one coil, then the second winding was a small coil inside the main but overlapping the small coil, phased and balanced it worked extremely well both for depth and small nuggetts, so I wanted to build one and try it out on my minelab.

So thats the story so far can anyone tell me how many turns I would need for the SD2200D coils, or in other words are the PI tuned coils as in the VLF det.
or doesnt it matter?
Appreciate any help please
Fairdinkum from down under.

[Reg]

Hi Fair,

Normally, little need is required for "balancing" a DD coil if it is going to be used on a low powered PI. I have found that on low powered units, one can get quite radical the positioning of the windings and see few negative problems. It does become more important on the higher powered units, on them, finding the right position of the two coils does make a difference on how well the coil reduces the ground signal.

The more common method is to "null" the coil so there is minimum output with no target present. This is what is generally used on the VLF"s.

Now, I have been told that in OZ, the ground is so bad, they try to adjust the DD overlap so it helps minize the ground response when building ML coils, rather than try for a "null" position. I have never done this, but suspect it is done by reducing the overlap zone a little.

You might keep the techniques in mind and experiment with it before sealing the coil. Unfortunately, I can't tell you the best technique or method to use in trying to minimize the ground reponse that is used over there. If I were going to try it here, I probably try a large piece of basalt as a test target.

Reg

[Unregistered]

coil making

Do any of you have some precise instructions and pictures about Probe makingm i resently made a detector , and now i need some advices regarding construction of coils and probe it self.
Thanks

[6666]

Would anybody still have the drawings from post 4 and 6 ?
thanks
6666

[Wiltran]

Small loop

DD coils operating on a VLF require that a good mechanical null is achieved by the careful positioning of the TX and RX coil in relationship to each other. Commercial coil builders often make a small 2 turn 1" diameter loop that can be moved to fine tune the coil before sealing it up. A scope is most useful in seeing very small signals to achieve the best null. This DD and other concentric and "fold back" type search coils function as loosely coupled transformer. The presence of a target upsets the null balance. Based on the metal composition, the type of metal can often be identified by examining the phase of the balance upset.

DD coils operating on PI machines work on a different principle. While obtaining a good null can't hurt, it is not as necessary as on a VLF machine. The PI machine DD coil does not work like a loosley coupled transformer, it operates more like a radar dish. A signal is emitted from the TX coil. After the pulse dies down (10 to 20 us later) either the same coil in a mono coil design, or the RX portion of the DD coil senses the residual eddy currents flowing in any nearby targets. A slight unbalance in a PI DD coil should not affect performance.

In any event, a scope is most helpful in examining search coils. There are good deals on e-bay for about $100 to $150. A dual trace, 20 Mhz or above should work very well.

bbsailor

BBsailor, which coil is the small movable loop put on, - the TX, Bucking coil or RX coil?

[WM6]

BBsailor, which coil is the small movable loop put on, - the TX, Bucking coil or RX coil?

Best from bucking coil, but works from other coils (RX or TX) too. Effectivity of such tunning depend a little of the individual coil construction.