Monday, May 28, 2018

Making a crystal radio - the new project

I'm going to try a crystal radio design. Step by step with the benefit of a design available on the net and some tools also available on the net. The first step is to download the calculator.
Coil design spreadsheet
Read the note on page 1. The author allows sharing as long as he is credited.

The second step is to choose a capacitor. For this design we need a dual variable air capacitor. You could use this one.

The important thing with the capacitor is its range. The one pictured is 20 - 540 pf which is 540 / 20 = 27. The tuning range will be the square root of 27 or about 5 : 1. The capacitor is at its highest value when the frequency is lowest so if 540 pf = 500KHz 20 pf would equal 5 X 500 KHz = 2500 KHz. This is assuming no stray capacitance in the circuit. In the real world we could expect to see 20 pf of stray capacitance. Allowing for that we would see an actual 560 / 40 = 14 and the square root 14 = 3.7 tuning ratio. In this case if the low frequency is 500 KHz the high would be 1850 KHz. This would cover the MW band so would be good to go.
The other supplier's offering is supposed to be a higher quality with a range of 15  - 450 pf. How does the math work out?

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