Spiral PCB coils in series

[F117]

Some time ago, I made 4 identical spiral PCB coils to experiment with my Pickini PI detector.
Oval shaped - 28 x 12 cm - a single coil measures 191 microHenry:



As I was looking for something around 300-400 microHenry, I put 2 of these in series.
I placed them back to back -- copper side up on both sides and connected them in 2 differend ways:
A-C shorted and measuring on B-D
or
B-D shorted and measuring on A-C:



Both ways of connecting them measured around 3x the single coil value instead of the expected double:

with A-C shorted and measuring on B-D: 568 microHenry:




with B-D shorted and measuring on A-C: 572 microHenry:



The 2 coils combined in series worked fine on the detector in both cases.
Could somebody please explain this behaviour to someone who doesn't know anything about coils and in all naivity thought that these 2 coils in series would just double in value ?

Thanks in advance,
- Bernard

[F117]

Some googling taught me that this is probably due to "cumulatively coupling":
http://www.electronics-tutorials.ws/...inductors.html

Lt = L + L + 2M and M = L/2 => Lt = 3 x L

[Qiaozhi]

I was just going to reply to your question, but I see you've already found the answer.
i.e. You had forgotten to account for the mutual inductance between the windings.

[F117]

Thanks, George.
This proves at least that I connected the coils in the right way, so that they accumulate and not work against eachother.

[Davor]

A differential coil, the one working against each other, has advantages in strong EMI and in vicinity of large metallic objects, e.g. fences. In that case you need to keep them at distance.

[F117]

Thanks for the info, Davor !
I am still a total beginner when it comes to coils and learning a lot on this forum.