What is the formula needed to compute the capacitor needed for a specific frequency for a crystal radio?

Question

I am trying to eliminate the need for a variable capacitor. What capacitor would be needed for 1550 KHz?

Answer 1

The capacitor size needed is based on the inductance of the coil. It can be calculated by using the formula F = 1 / 2 * pi * sqrt(L*C) and manipulating it to yield C= 1 / (4 * pi^2 * F^2 *L). On the other hand, why not use a 365pF variable capacitor just like everybody else does?

Answer 2

That depends on the coil your using to resonate with it. Keep a couple of easy tips in mind. Most coils are adjustable, so the capacitor doesn't have to be exact. The VariCaps tune typically from 365-10 pf. That corresponds to a frequency range of 530-1700Khz. A little math and 1550Khz works out to about 56 pf. Try that for starters. Hope that helps

Answer 3

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Answer 4

That totaly depends on the circuit. A simple circuit analysis will yield the answer. Event that, you will need to tune the circuit, since there is no such thing as ideal component.
Anyway, here is the formula for the resonat frequency an RLC circuit: F=1/(2*pi*sqrt(LC)).