2 deleted 1 character in body edited Jan 9 '16 at 17:15 SunnyBoyNY 1,49199 silver badges1616 bronze badges Let me shed some light on the physics background here. Inductance of each coil can be calculated as $$\ L = \frac{N^2}{R} \$$. The reluctance term is calculates as: $$\ R = \frac{core-length}{\mu A_c} \$$. Therefore, the inductance is proportional to the permeability $$\ \mu \$$, which is a product of absolute permeability $$\ \mu_o \$$ and the relative permeability $$\ \mu_r \$$, which is 1 for air and ~thousands for magnetic materials. Therefore, if you have two inductors with closed-loop magnetic path close to each other, the leakage fluxes will affect each other very little, well beyondbelow 1%, since all the flux is contained in a low reluctance magnetic circuit . However, if you have two air coils or somehow connect the magnetic circuits of the two individual coils, you will get much stronger mutual coupling since the magnetic flux is not constrained to the magnetic core. Air cores inductors will have their individual inductances unchanged because the reluctance will not change. Magnetic core inductors will see some change or reluctance (decrease) and therefore the individual inductances will decrease as well. This is similar to adding inductors in parallel but the coupling is not done electrically but magnetically. Let me shed some light on the physics background here. Inductance of each coil can be calculated as $$\ L = \frac{N^2}{R} \$$. The reluctance term is calculates as: $$\ R = \frac{core-length}{\mu A_c} \$$. Therefore, the inductance is proportional to the permeability $$\ \mu \$$, which is a product of absolute permeability $$\ \mu_o \$$ and the relative permeability $$\ \mu_r \$$, which is 1 for air and ~thousands for magnetic materials. Therefore, if you have two inductors with closed-loop magnetic path close to each other, the leakage fluxes will affect each other very little, well beyond 1%, since all the flux is contained in a low reluctance magnetic circuit . However, if you have two air coils or somehow connect the magnetic circuits of the two individual coils, you will get much stronger mutual coupling since the magnetic flux is not constrained to the magnetic core. Air cores inductors will have their individual inductances unchanged because the reluctance will not change. Magnetic core inductors will see some change or reluctance (decrease) and therefore the individual inductances will decrease as well. This is similar to adding inductors in parallel but the coupling is not done electrically but magnetically. Let me shed some light on the physics background here. Inductance of each coil can be calculated as $$\ L = \frac{N^2}{R} \$$. The reluctance term is calculates as: $$\ R = \frac{core-length}{\mu A_c} \$$. Therefore, the inductance is proportional to the permeability $$\ \mu \$$, which is a product of absolute permeability $$\ \mu_o \$$ and the relative permeability $$\ \mu_r \$$, which is 1 for air and ~thousands for magnetic materials. Therefore, if you have two inductors with closed-loop magnetic path close to each other, the leakage fluxes will affect each other very little, well below 1%, since all the flux is contained in a low reluctance magnetic circuit . However, if you have two air coils or somehow connect the magnetic circuits of the two individual coils, you will get much stronger mutual coupling since the magnetic flux is not constrained to the magnetic core. Air cores inductors will have their individual inductances unchanged because the reluctance will not change. Magnetic core inductors will see some change or reluctance (decrease) and therefore the individual inductances will decrease as well. This is similar to adding inductors in parallel but the coupling is not done electrically but magnetically. 1 answered Jan 9 '16 at 16:27 SunnyBoyNY 1,49199 silver badges1616 bronze badges Let me shed some light on the physics background here. Inductance of each coil can be calculated as $$\ L = \frac{N^2}{R} \$$. The reluctance term is calculates as: $$\ R = \frac{core-length}{\mu A_c} \$$. Therefore, the inductance is proportional to the permeability $$\ \mu \$$, which is a product of absolute permeability $$\ \mu_o \$$ and the relative permeability $$\ \mu_r \$$, which is 1 for air and ~thousands for magnetic materials. Therefore, if you have two inductors with closed-loop magnetic path close to each other, the leakage fluxes will affect each other very little, well beyond 1%, since all the flux is contained in a low reluctance magnetic circuit . However, if you have two air coils or somehow connect the magnetic circuits of the two individual coils, you will get much stronger mutual coupling since the magnetic flux is not constrained to the magnetic core. Air cores inductors will have their individual inductances unchanged because the reluctance will not change. Magnetic core inductors will see some change or reluctance (decrease) and therefore the individual inductances will decrease as well. This is similar to adding inductors in parallel but the coupling is not done electrically but magnetically.