I have an air core inductor that I want to use for resonant coupling purposes. This inductor has an inductance value of 500nH. Is it ok to connect another inductor with a value of 100uH in series with the first one, to increase the value of inductance, but to only use first air core inductor for resonant coupling?

In short, I want low inductance value antenna in distance from the circuit and second higher value inductor in series with the antenna.

  • \$\begingroup\$ Sounds like an XY problem so come clean on what you are trying to achieve. \$\endgroup\$
    – Andy aka
    May 12, 2017 at 7:56

1 Answer 1


It's ok in principle. In practice it will be difficult to find such placements for the inductors that the 100uH one will catch nothing, but the 500nH one catches. Enclosing the 100uH inductor into a metallic box helps, but the box must be large to prevent it reduce the inductance. Bringing the 500nH inductor far away with long wires changes the circuit totally due the inductance and capacitance of the wires.

BTW Me and surely many others are curious what you actually are going to do. Any data available?

Addendum due the comment of the questioner:

To make a coil to have a substantial inductance its turns should have a substantial free area for the magnetic field to go through every turn. You have flattened the needed way for the magnetic field to nearly nonexistent. That makes the inductance and also magnetic coupling extremely low when compared to what could be possible with the same amount of copper.

Have circular or square turns. The coil can still be flat.

In circuit boards sub 1uH coils often are spirals. That's not practical if you need 100uH.

An open ferrite core ( a bar, not a ring) reduces radically the needed size. Still today in AM radios the antennas are so called ferrite antennas. It is a coil where the wire is wrapped around a ferrite bar (round or nearly flat). It catches as effectively as a half-a door size air core coil. A long enough ferrite bar literally sucks the magnetic field into the coil.

Ferrite antenna images in Google

  • \$\begingroup\$ I am trying to design flat coil for inductive coupling with no luck so far. I have wrapped copper tape around piece of plastic like this also made one layer coil using enameled wire pic. Both coils have low inductance below 1uH so I used 100uH inductor to rise its value. Sadly I am only able to couple 100uH inductor, both custom made wont couple. \$\endgroup\$
    – J.Doe
    May 11, 2017 at 19:47
  • \$\begingroup\$ @J.Doe you have got new info \$\endgroup\$
    – user136077
    May 11, 2017 at 20:18
  • \$\begingroup\$ I have followed your advice, just now I have wrapped a wire around glue stick to test if I can couple it with my "transponders". The green inductor is 100uH, coil measured with LC meter shows 630nH. I am using it with 450pF cap at a frequency around 750khz. Transponders couple pretty good to 100uH coil but nothing happens when I get them near a coil. \$\endgroup\$
    – J.Doe
    May 11, 2017 at 20:48
  • \$\begingroup\$ @J.Doe why do you insist to have a sub 1uH coil? To make it practical you should have VHF range operating frequency. If 100uH catches ok, use it. Make it small enough by inserting a flat ferrite bar. \$\endgroup\$
    – user136077
    May 11, 2017 at 21:03
  • \$\begingroup\$ In my final design, I want to have flat coil made of conductive fabric, since that kind of coil have a low inductance that's why I stick to that. Do you think that increasing frequency will give me a result I expect? For now, my equipment holds me from rising frequency above 1Mhz that is why I use series inductors to meet mine capabilities. \$\endgroup\$
    – J.Doe
    May 11, 2017 at 21:09

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