https://www.allaboutcircuits.com/tools/coil-inductance-calculator/
A Practical Application
\(L = μ_0 μ_r {N^2 A \over ℓ}\)
Whilst making a single coil, where I had to consider different permutations of length, turns and wire gauge, I found it useful to work out a block of known values and then play with just one variable, like turns N for example.
And with the following assumptions:
- I am using a drinking straw with a diameter of 5mm.
- I am going to wrap each turn snug up with the next, with no space in between. Hmm probably some!! In fact, doubling the length, halves the inductance of the coil.
- μr of an air core is 1
- Wire gauge ‘d’ is .25mm.
\(L = 1.2566e^{-6} \times \pi(2.5e^{-4})^2 {N^2 \over N \times d}\)
\(L = 2.467e^{-11} \cdot {N \over d}\)
\(L = 9.8696e^{-8} \cdot N\)
So, in my case, I plug in 13 turns and it gives me 1.28μH
I was using trial and error to get to an inductance of just over 1μH, which when coupled with a mid range value of say 25pF of the trimmer cap in the tuned circuit, would get me into the 27MHz range.