Whites 6 1/2" VLF concentric coil

[nalag46]

Hi again,
My previous post of November 3rd. "Help with coil measurements" attracted a good number of replies that pointed to my incapability to upload pictures to the thread
but unfortunately no one single answer relating to my coil problem. I have solved the uploading problem so I would like to ask again for the help I need and I am attaching
my own schematic drawing of what I actually have left on the coil casing and what the missing TX coil should be. I could be wrong with the drawing, but basically all I need
to know is how to work out the cable length and feeds for that coil which must be 5 3/4" in diameter, as per groove clearly seen on the foam.
Can anyone please point me in the right direction? Have all I need to build the coil (except inductance meter).

[golfnut]

Carl may be able to tap into aincient history for you? Another approach is find the Tx resonating cap that is in there get the value.. Find out what frequency Tx these were. Then using freq = 1/2PI sqrt LC ( Hz ) to get the L you need. You know the diameter... use Quiazis coil calc to give the turns of a wire diameter to give the L you require at your diameter to give you the right freq with your cap , S

[Skippy]

An accurate bucking coil resistance measurement, combined with precise measurement of the wire diameter used, should allow you to calculate the length of wire in the bucking coil. You should be able to measure the diameter of the coil easily (is it on the outside of the RX?), and hence calculate the number of turns it has. It's likely to be low, 5 - 20 turns, at a guess. Knowing turns and diameter, you can calculate L of the bucking coil. As you know the main TX coil diameter, I think you have enough information there to estimate the total number of turns in the main TX.

[nalag46]

Ok. The RX coil resistance is 2.7 ohms. The diameter of the RX coil (in situ) wire is 32 swg. but I don't know if the TX coil should use the same gauge???
As mentioned previously, the diameter of the TX coil needed is 5 3/4" or 145mm.(according to the space/groove left on the foam after removal). So I still don't know how to calculate the exact number
of turns but my guess is about 9 turns taking into account the above measurements, but a guess only is not goo enough.
Also, there must be two extra windings on the TX coil?? The lead out should go to top of the RX coil which in turn goes to the control box cable screen. If the other end goes to the bucking coil end, where
are the two windings that go to White and Green cables to connect to TX circuit?
Need to learn more about coil design but this particular one is a very old design, may be from late 70s, early 80s and cannot find any data anywhere of Treasuremaster VLF, frequency???

[nalag46]

OK, The capacitance across the top end of the RX coil and the Green cable (to TX circuit) is exactly 72.600pf (0.072uf). I cannot find the operating frequency of this model anywhere, not even on the
Whites Support site. My guess is 5 kHz but it is only a guess. So assuming that as the frequency and 1/2 PI sqrt being 2.4674, will the length required be 4.90 meters? I don't see this very clear, can you
please explain the full equation a bit further?. Many thanks

[Skippy]

This is a reply to your first post:
So you're saying the bucking coil that WAS measured as 5.6 Ohms is actually 2.7 Ohms? We still need the mean diameter of the RX coil. And the diameter you quote for the main TX coil is the mean diameter, (not inner or outer diameter)?
I think you are probably going to have to make an experimental TX coil first, without any tappings on it, just to find out how many turns are needed for induction balance. Feed the TX from a signal generator, see what comes out of the RX coil. I suggest winding several seperate isolated windings, to allow you to tinker with the number of turns. Eg. if we decide 50 turns is probably what you need, wind your experimental coil with 50+4+2+1 turns. You can then make up anything from 43 to 57 turns, this should allow a null to be obtained.

[Skippy]

And this is a reply to the second post:
So the capacitance on the TX is not 0.12 microfarad, it's 0.072 microfarad??
You need to understand the winding that's 5.6 Ohms is NOT the recieve coil, it's part of the transmitter circuit, it's called the 'bucking coil'. It's purpose is to cancel out the magnetic field of the main TX coil, in the vicinity of the RX coil. This is the key to working out the main TX coil characteristics. The TX and bucking coil have an intimate relationship - know the TX spec, you can calculate the bucking coil spec. And vice-versa, which is what we are trying to do here.
As for frequency of operation, it's anyone guess, but Whites commonly used 6.6-ish KHz, though that in no way implies your machine is this freq.

[Skippy]

I won't ask why a new thread was started, but I've just had a look at the other thread, seems the bucking coil was measured as 3.9 Ohms. ....??? But you did say the RX coil was 2 3/4 inches diameter, is that correct, and is this the mean (middle/ avarage) diameter?

[nalag46]

Hi, Sorry about the misleading resistance measurements. Let me explain that I took the readings originally with a very basic multimeter but noting the importance quoted here about the accuracy of these readings, I borrowed a friend's better instrument and the bucking coil does indeed reads 2.7 ohms. The capacitance (there are three mica caps inserted in the foam and connected across the
shield and green leads. They are: one at 102, one at 332 and one at 683 = 72600pf. The centred RX coil measures 65mm. mean measurement (midldle to middle). The two leads out to circuit have a
cap across them of 502.
I have also measured the bucking coil's loose wire and this is 570mm or just under 3 turns.

[Skippy]

Ok, more details, thank you. I calculate bucking coil wire length = 9.4 metres, giving number of turns = 43 T. The TX coil is almost exactly twice the buck coils diameter (common arrangement), and a rule-of-thumb appears to be TX turns = 3.4 x buck turns. So TX needs to be 146 turns, nominally, end-to-end. The tapping details, I 've no idea, you'll need to reverse engineer the oscillator circuit to help crack that one.

[Skippy]

You will have to read up on phase polarities, so you get the buck and TX wired up the right way round, the wrong way will cause the magnetic fields to add up , not cancel.
The end of the buck coil that's closest to the RX (ie. the start) should be at a DC voltage, the other end (the 'hot' end) should have AC voltage on it, to minimise capacitive coupling between buck and RX coils. (there's no screen / shield)
The 50cm of free wire is used to fine-tune the null, bit of a black art, it's wiggled about in various positions until the null is good, then it's glued in place.

[Skippy]

Some interesting info:
I've just calculated the L of a 146 turn TX coil, it works out at 7.9 mH. Add on about 4% for the bucking coil (it's L will be roughly (1/3.4)^2 x 0.5 of the main) gives total L = 8.1mH. Resonate this with C = 72nF and you get Freq = 6.6KHz. Not only is this in the sensible ballpark area (I guess 5K to 10K would be likely) but it's actually the common Whites frequency. I think this is a good sign.

[Qiaozhi]

Some interesting info:
I've just calculated the L of a 146 turn TX coil, it works out at 7.9 mH. Add on about 4% for the bucking coil (it's L will be roughly (1/3.4)^2 x 0.5 of the main) gives total L = 8.1mH. Resonate this with C = 72nF and you get Freq = 6.6KHz. Not only is this in the sensible ballpark area (I guess 5K to 10K would be likely) but it's actually the common Whites frequency. I think this is a good sign.

You need to subtract the inductance of the bucking coil (not add it) as it's connected in anti-phase to the TX.

[Skippy]

They are in antiphase, but in my experience the coupling between them is weak, and they can be treated as two isolated inductors. But...either way, we're only talking about +/- 4% or 5%. There's a lot of estimating already taken place in determining the main TX coil characteristics. The '3.4 factor' is , I think a George Payne quote, I guess it applies to closely coupled machine-wound RX and Buck coils. And small errors in measuring the wire gauge used could easily give 10% errors on the no. of turns on the buck coil.
I'm pretty certain the operating frequency will be the Whites standard 6.6KHz. Sven's Treasurelinx site has some catalogues of similar machines, maybe somewhere on his site this can be confirmed.

[nalag46]

Thanks Skippy for all that useful information. It looks like I am going to have to experiment quite a bit here not least because I still cannot fathom that TX coil in the turns number you quote?
OK you say you have calculated the length to be 146 turns end to end with no tappings. So, if the diameter of my casing's groove where the removed coil was neatly laid measures 145mm = 455m
in circumference X 146=66 meters in total length??? That amount of wiring cannot possibly fit in such a narrow groove that measures no more than 4mm in width and same for depth??
I agree with the Whites standard 6.6khz frequency which most probably applies to this old design but the coil length, what do you think?