Minimisation of Inductive Coupling between Coils - Page 1
Forum section: electronics / rf-microwave

author: jamfletch
author_rank: Contributor
timestamp: March 09, 2025, 07:43:06 pm
content: I came across this post in the AllAboutCircuits forum about inductive coupling and positioning 2 coils together for minimum coupling :https://forum.allaboutcircuits.com/threads/positioning-two-coils-so-there-is-no-coupling-between-them.202844/Which led me to read about the Hazeltine minimum coupling configuration for parallel coils. The derivation in Hazeltine's patent makes mathematical sense to me :https://www.radiomuseum.org/forumdata/users/133/PDF/Pat_Magic_Angle.pdfWhat I don't get is quite how physically this works. When we have 2 perpendicular coils, the magnetic flux of either wont "cut" through the other, and induce any EMF.This paper linked below has a figure (1a) which claims that at the angle of 55 degrees, saying "The magnetic dipole field lines are orthogonal to the axis of the dipole". But to me, that picture shows that at that angle there's no orthogonality at all? Am I missing something obvious?https://www.researchgate.net/publication/325908170_Minimum_Coupling_Antenna_ArraysHere's another forum post I found (In German)https://www.radiomuseum.org/forum/freed_eise_hazeltine_winkel_magic_angle.html

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2025, 09:17:13 pm
content: With an appropriate overlap, two co-planar coils will have zero magnetic couplingkdue to the cancellation of "up" and "down" flux linking the second coil's area from current through the first coil.If they are on top of each other, you get maximum coupling in one direction, and if they do not overlap the coupling will have the opposite polarity (decreasing with increasing separation).

author: jamfletch
author_rank: Contributor
timestamp: March 10, 2025, 08:40:43 am
content: Excuse the poor MS Paint drawing, but I think I understand what you mean. I've drawn out that situation here, where like you say the "up" and "down" fluxes of each inductor cancel each other?

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2025, 02:10:54 pm
content: Specifically, it’s when the up and down fluxes from coil 1 hit the area of coil 2.By symmetry, the same will happen in the other direction.Call the flux in the overlap region “up” and the flux outside the area of coil 1 “down”, then follow the math to find the overlap condition that gives net zero.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 11, 2025, 11:57:14 am
content: Quote from: TimFox on March 09, 2025, 09:17:13 pmThe OP isn't asking about overlap decoupling though.Quote from: jamfletch on March 10, 2025, 08:40:43 amYour drawing doesn't seem to describe either overlap coupling or "magic-angle" decoupling.I think I have a more intuitive explanation, see attached figure. On the left side, I show the field lines from a coil, whose center location and orientation is indicated by the blue arrow. The red arrows show places where the direction of the flux from the blue coil is orthogonal to the orientation of its own orientation. These red arrows thus represent locations where I could place a second coil (a red coil) with the same orientation as the blue coil without any flux coupling between the two.So this looks like decoupling via locating the coils at specific x-y coordinates. But if I simply rotate the whole picture such that all red arrows form a roughly horizontal line, it turns into the figure on the right side. We see that another way of looking at it is decoupling through a specific angle of the coils (relative to axis along which the coils are located).Note this is just a cartoon representation, not rigorous at all. I think the exact angle will be a function of the diameter, length, and spacing of the coils as well. Also if this approach is applied to many coils, then it can only achieve perfect decoupling between nearest neighbors, not next-nearest neighbors.

author: TimFox
author_rank: Super Contributor
timestamp: March 11, 2025, 02:46:46 pm
content: The OP mentioned Hazeltine’s parallel coils.When I worked on “surface coils” for MRI (receiving coils mounted near the patient’s surface), we used the partial overlap of co-planar coils to eliminate coupling between adjacent coils.The cancellation of flux is an example of minimizing inductive coupling, since the response of overcoupled resonant circuits was not useful.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 12, 2025, 01:50:17 pm
content: Quote from: TimFox on March 11, 2025, 02:46:46 pmYep, I'm familiarQuoteOverlap decoupling is excellent when the coils act as detectors/antennas and you want overlapping coverage. Based on the OP's links I assumed this isn't their case (instead the coils are just elements in a multistage filter), but maybe I read wrong...

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 02:05:06 pm
content: I was responding to his question about visualizing the coupling minimization.

author: MarkT
author_rank: Frequent Contributor
timestamp: March 26, 2025, 09:28:17 pm
content: I think the magic angle approach is designed to work between any two coils in the line, whereas just having a 90 degree rotation would only cancel flux between coils an odd number of steps away. For 3 coils you can have all at 90 degrees if using x, y and z, but for 4 or more that wouldn't work and the diagonal angle trick might be the best approach.In practice we use aluminium cans to screen coils from each other! Much higher densities can be achieved and dust is kept out too.