# Basic radio theory, circuits and calculations

## L/C ratio

The resonant frequency of a tuned circuit is fixed by the product of inductance and capacitance, ie L x C. There is hence an infinite number of values of L and C which will tune to a given frequency.

The choice of the ratio of L and C is determined by practical considerations according to the particular use.

For instance, in receiver tuned circuits, in order to achieve a high value of dynamic resistance (required for high gain) the L / C ratio is high. The limitation here is that there is always a minimum value of capacitance inherent in any circuit (this is the 'stray capacitance').

In some applications, it may be required to swamp completely the stray Circuit capacitance, in which case the L / C ratio is low.

The optimum choice of L / C ratio is sometimes difficult. As a compromise, it is convenient to assume that the value of C is 1.5pF per metre of wavelength; thus to tune to 30MHz (ie 10m) a capacitance of about 15pF would be reasonable.

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