Monday, May 28, 2018

crystal set step 3

Now let's tie it together. Reactance is expressed as X. The subscript c or l says capacitive or inductive reactance.

With a fixed coil the capacitor is being adjusted to see the effect. At low frequency the Xl is low and the coil passes the current. At some point Xc=Xl and they cancel. The circuit resistance (impedance = Z) is very high. As the frequency increases Xl becomes high and Xc decreases. The coil blocks current and the capacitor passes current. Ao at our radio station frequency we want Xl = Xc. Now back to the calculator.

The example circuit we are using as our pattern used a 182 uh coil so I went with that. Assuming some stray capacitance I use 50 pf as our low setting on the Vcap. The calculator says our resonant frequency is 1670 KHz. Look to the left and it says both Xc & Xl = 1907 ohms. The point is Xc = Xl so it is at resonance.

With the sane 182 uh coil I change the Vcap to 480 pf. We see Xc & Xl = 615 at 540 KHz. So the 182 uh coil in combination with the Vcap chosen wil cover 540 Khz - 1670 Khz as long as we have no more strays than I allowed in the example. If the strays are less we will have a little more range in the adjustment. The next step is to see how to wind the coil.

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