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 other supplier's offering is supposed to be a higher quality with a range of 15 - 450 pf. How does the math work out?