Specifically, this circuit from Elektor:

circuit diagram

The asterisk denotes that L1 in this 433MHz receiver circuit would be 1.5 turns. If that is accurate, what would the dimensions of the inductor air core be? You would need the coil diameter, wire diameter, and the coil length.

My thought is that 1.5 turns seems inaccurate, so if you have suggestions as to what the inductance should actually be here, that would be great as well. I considered 67.55, but that does not appear to resonate correctly at 433MHz though.

  • \$\begingroup\$ That looks close to me, why not build it to be sure. \$\endgroup\$
    – Gil
    Commented Jun 4, 2021 at 2:30
  • \$\begingroup\$ I did and tried it with a 68nH fixed inductor on a printed board with a ground plane with no luck. Out of curiosity, how would you make an air core at that inductance at 1.5 turns? \$\endgroup\$ Commented Jun 4, 2021 at 2:33
  • \$\begingroup\$ How do you get half a turn? \$\endgroup\$
    – Hearth
    Commented Jun 4, 2021 at 3:04
  • \$\begingroup\$ Hearth: forums.qrz.com/index.php?threads/… Now, can we please get back to talking about Rampart? \$\endgroup\$ Commented Jun 4, 2021 at 3:12
  • \$\begingroup\$ Forget about trying to calculate inductance to hit a desired frequency. Stray capacitance, capacitor tolerance, and reactance added by that "antenna" affect resonant frequency. Typically, a coil inductor is distorted, stretched, scrunched post-construction until oscillations are at the desired frequency. Note that for this super-regen circuit, you can't just measure with a frequency counter. Something like a frequency-calibrated measurement receiver or spectrum analyzer would be used. \$\endgroup\$
    – glen_geek
    Commented Jun 4, 2021 at 13:55

1 Answer 1


To resonate a printed coil or wired coil with 2pF at 433 MHz , you can't use a ground plane, which ends up adding to the 2pF significantly.

You have to decide if you want it shielded, coplanar, spiral formed on a screw (metric or imperial) or square printed on FR4. So you have a lot of options but 1.5 turns sounds right for an air core with ~0.6" diameter

  1. If you want some immunity from crosstalk and magnet interference then you design it into a loose cavity as a shield to make it 2 pF to the shield.

  2. If you wanted a printed square coil with no ground plane then it would be:

  • 3 square turns coplanar 64.65 nH then add 0.8nH/mm for feed tracks that are not paired, less if paired as the inductance cancels.

8mm OD , 3 mm ID 0.5mm W, 0.5mm S

enter image description here

  1. Another method is to use a plastic screw with a wire diameter that fills the thread with an almost perfect helix. enter image description here

  2. Here with no gap, except the magnetic wire coating in a spiral.

enter image description here

  1. Next: With larger solid stamped copper loops, I have seen precision coils inside a 4 walled shield soldered into a PCB as a high Q LC resonant accurate filter, where the tilt angle slightly tunes C (pf) with ~ 3/4 turn


simulate this circuit – Schematic created using CircuitLab

These are 5 options.


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