# How to find C and L in maximum current?

1. Dec 8, 2013

### shqie

In a RLC circuit, radio waves with different frequency, f, hit the antenna and exert oscillating electric forces on the electrons on the antenna . Thus they act via antenna as individual batteries, each oscillating at their particular frequency f. Suppose R=1000 ohm. What should L and C be so we get the largest current in the circuits at frequency in 90 MHz.However, at frequency 90.3 current is only 1/10 as large.

Now I know that at Imax, Xl=Xc, but I don't know how to use the other given frequency to find C and L.

2. Dec 9, 2013

### Baluncore

Welcome to PF.

I assume the R is in parallel with the L and C.
Xl = 2*Pi*f*L, Xc = -1/ (2*Pi*f*C), when Xc + Xl = 0 you have resonance.

The width of the resonance peak is determined by the Q which is the ratio of reactance to resistance. See https://en.wikipedia.org/wiki/Q_factor

So find the Q required for 10% at 90.3MHz. Then calculate the reactance needed.
Then solve for the L and C at 90MHz that have that reactance.

3. Dec 9, 2013

### sophiecentaur

Then, if you want to build a suitable resonating circuit, you find that, at 90MHz, nothing behaves quite as you'd expect, because of parasitic effects. There can be a lot of 'suck it and see', when you bolt it into a circuit.