# How should you position two flat-wound spiral coils for the best coupling?

I created two flat wound-spiral coils (also known as bifilar coils). The transmitter coil I made with an outer diameter of 10 inches and an inductance of 3 mH. The receiver coil I used a much smaller wire diameter, so it is only 4 inches but also 3 mH. These coils will be used for charging a 12 V battery using a 15 V source on the transmitter, so there is not much room for high losses.

They are going to be transferring power through an air gap of about 2 inches. I am wondering if I should try putting one at an angle to the other to try to get the best through-put. I do not have the rest of the circuitry prepared so I cannot test by trial and error yet. Until then I was wondering if anyone could recommend the best positioning.

I have included an image to describe what I am thinking of doing. Note that in the x-direction, the distance will be 2 inches or greater (I can not get them closer).

I do not take credit for this image; I got it at the website https://www.wibotic.com/wireless-power/

Recommendations would be appreciated. Cheers.

• I would think that having them parallel and as close as possible would maximize the coupling. Feb 4, 2018 at 23:48
• You may find the most efficient transfer with x less than 2" and y = 0, angular range close to 0. A frequency of 10 KHZ (rotary transformers) to 300 KHZ (RFID tags) may work best. Also schottky diodes work very well in this range.
– user105652
Feb 5, 2018 at 0:05

## 1 Answer

The amount of flux entering the receive coil is paramount. If the receive coil is at 90 degrees to the transmit coil then there is zero flux entering the "mouth" of the receive coil and zero coupling.

However, if your coils do not share the same common axis then there can be some benefit in using an angle to optimize the receive coil flux.