TF coil inductance (PI)

[Pepe LeMiau]

After reading this topic http://geotech1.com/forums/showthrea...t=triple+field I decided to try Triple Field coil on my PI. Unfortunately I have problems estimating total inductance.

I found calculator for two coils, but can't find one for three or more.
However, I noticed that 240mm diameter, 24 turns coil made from 18m of wire has ~300µH. Dual field, 240 + 160mm, 17 turns each also made from 18m is also very close to 300µH.
Is it safe to assume that if I split the 18-18,5m of wire to 3 coils, I'll also get the total inductance close to 300 µH ?

[IBGold]

Allow about 20.5 meters of wire 15 or 16 turns each coil should be close, I do a dummy one check the inductance then pull apart and build the final coil from what I found with the dummy one.

Regards, Ian.

[Aziz]

Why bother with TF coil, when you can build the DF coil?

Carl spread some bull$hit, that White's own a dual field coil patent. He has not provided the proof for it. If you look in the patent servers, there isn't such a patent.

You can build it for your own purpose (not commercial use), whether there is or isn't a patent for it.

Calculating the DF coil inductivity for the following coil configuration:
L1: outer coil inductivity (bigger coil)
L2: inner coil inductivity (half diameter of L1)
L1, L2 connected serial

L = L1 + L2 + 2*k*sqrt(L1*L2),
where
k approximately = 0.11 for L2 half diameter of L1
Note: L1, L2 same winding direction. If opposite winding direction, then k has a negative sign.

Aziz

[Aziz]

Formula for a TF coil inductance:
L1: outer coil inductance
L2: mid coil inductance
L3: inner coil inductance

k12 = coil coupling coefficient between L1 and L2
k13 = coil coupling coefficient between L1 and L3
k23 = coil coupling coefficient between L2 and L3

L (total) = L1 + L2 + L3 + 2*k12*sqrt(L1*L2) + 2*k13*sqrt(L1*L3) + 2*k23*sqrt(L2*L3)

The coil coupling coefficients are dependent on the diameter relations of two coils:
Approximately coil coupling coefficients between two coils:
1/3: k = 0.06 (smaller coil diameter = 1/3 of bigger coil diameter)
1/2: k = 0.11 (smaller coil diameter = 1/2 of bigger coil diameter, compare to above post)
2/3: k = 0.18 (smaller coil diameter = 2/3 of bigger coil diameter)
3/4: k = 0.23 (smaller coil diameter = 3/4 of bigger coil diameter)

Note: The coil coupling coefficients are rough index.

Aziz

[Pepe LeMiau]

I don't care about patents, since I'll build one, perhaps two coils for personal use. I like experiments.
Why TF ? Because from the graphs that you attached in the linked thread, they look better.
Of course spiral coil is even better but it's also harder to build.

[Pepe LeMiau]

Hopefully I'll test my TF this week. I made the coils and painted them with shellac resin, I'm waiting for it to dry completely before testing.

I also made a DF 0,66/1 (16cm/24cm) from two outer coils prepared for badly calculated TF earlier. It went straight to epoxy, who knows maybe it'll work.

Btw, is that true that epoxy/polyester resin increases the coil's inter-wire and total capacitance dramatically ? I read about it in a few different places (Polish forums mainly), they say that the effect is so bad, the coil may not work even with slow PIs like J-267 (it's 50µs ).
So far I used polyester resins to seal all my coils (as I want them to be able to go underwater), everything seems to be fine, however all my coils were painted/strengthened with shellac first.

[Pepe LeMiau]

Both coils are working. The 0.33/0.66/1 TF measured about 360µH, a bit more than I expected, because I added 1 turn extra to each sub-coil, I was worried that the inductance may be too low.

0.66/1 DF measured about 290µH, I already noticed that it is a bit better than single coil of the same diameter. Now I'm waiting for better weather to do field tests

[Aziz]

Both coils are working. The 0.33/0.66/1 TF measured about 360µH, a bit more than I expected, because I added 1 turn extra to each sub-coil, I was worried that the inductance may be too low.

0.66/1 DF measured about 290µH, I already noticed that it is a bit better than single coil of the same diameter. Now I'm waiting for better weather to do field tests

Hi Pepe,

the DF 0.66/1 (2/3 diameter relation) should be better than the TF as this causes less work. The TF would have minimal and neglectable benefit (disadvantage: more capacitance for less benefit).

Note, the inner diameter coil shouldn't be less than 1/2. A relation of 2/3 to 3/4 would be a good index.

Aziz

[IBGold]

I tend to agree with you Aziz after building multiples of both I can not see any advantages in the TF over the DF.

Regards, Ian.

[Pepe LeMiau]

Why didn't you tell me that earlier
I don't see much difference between these two coils, except that the TF has a funny side effect, near-surface very small targets are signalled 5 times during 1 sweep, when they pass under the center of the coil but also directly under mid and outer windings.

[Aziz]

Why didn't you tell me that earlier
I don't see much difference between these two coils, except that the TF has a funny side effect, near-surface very small targets are signalled 5 times during 1 sweep, when they pass under the center of the coil but also directly under mid and outer windings.

The graphics were showing this.

Anyway, the results were giving a major benefit:
Combination of a large coil with a smaller coil.

Example: 1 m coil in combination with a 0.5 m coil.

Keep attention, that you shouldn't use a magnet wire for the coil. You need an insulation between the coil turn windings to keep the coils interwire capacitance low. Take a tinned litz wire with insulation.

Aziz

[Pepe LeMiau]

I used cheap wire (it's double enamelled high voltage 0.65mm copper wire) because my detector is simple and slow, I doubt that low capacitance coil will make a huge difference.

Yesterday I borrowed signal generator and measured the TF's self resonance freq (air test, very short cable). - it's close to 1100kHz, assuming the inductance meter was correct, the parasitic capacitance is around 60pF - is it that bad ?
I couldn't measure the DF coil yet because it's drying again, I glued the bottom made from a plastic bucket.

[Aziz]

Yesterday I borrowed signal generator and measured the TF's self resonance freq (air test, very short cable). - it's close to 1100kHz, assuming the inductance meter was correct, the parasitic capacitance is around 60pF - is it that bad ?

That is pretty good for a magnet wire.
The coil shielding, coil leads, the transmit cct and the amplifier will add parasitic capacitances too.

Aziz

[Alexismex]

Hello Aziz and forum,
I have discover when i make my 1 mtX 1mt coil that i can reduce for 15 to 20 % the inductance of the coil with this simple change between have all the cables ties and the configuration braid of the cable (down ) in the photo (3X 3 cables )
I put also a five cable braid exemple in the photo.
Soon i will make the same for a 16 inch coil to see how the inductance reduce but more work to braid many cables!!!!

I just want to know your comment Aziz ,thank you