Wheeler's Formula for Inductance L(uH) = (r^2) * (N^2) / (9*r + 10*h) where: r = coil RADIUS in inches N = number of turns h = coil height in inches Another useful formula is Medhurst's formula for self capacitance of a coil. Medhurst's formula: C = K x D where: C = capacitance in picofarads K = constant which depends on the ratio of the coil height to diameter x = means multiply K times D D = solenoidal coil diameter in centimeters H = coil height in centimeters H/D K 5.0 0.81 4.5 0.77 4.0 0.72 3.5 0.67 3.0 0.61 2.5 0.56 2.0 0.50 1.5 0.47 1.0 0.46 Now that you know the inductance and self capacitance of your secondary coil you can determine the resonant frequency by applying the resonant frequency formula: 1 f = -------------------- __________ 2 pi / L C f = frequency in cycles per second L = circuit inductance in henries C = circuit capacitance in farads pi = 3.141592654 This is remarkably accurate for most secondary coils. You can also determine your resonant frequency after adding a toroid by adding the toroid's capacitance to the Medhurst value. If you are just interested in computing self-resonant frequencies there is another formulad which I have found useful and generally accurate. Its not quite as accurate as the above method. The formula is: (1/5) 29.85 x (H/D) F = ------------------- N x D where F= self resonant frequency in Mhz of an isolated coil H= coil height in meters D= coil diameter in meters N= total number of turns Make sure the top line reads " (H/D) to the 1/5 power" Mike Hammer