Indeed, the case of "reflex radio", receiving frequency is simply defined by the resonator consisting of variable capacitor and ferrite bar antenna. Then, if we change ferrite bar with proper inductor resonating shortwave with variable capacitor? Here is the answer.

I just wind wires on several ferrite bar. I have a strong tool of inductance measurement...

And I just replace it with ferrite bar in Nuvistor reflex radio. Yeah, shortwave reception is not so difficult. BUT separation is ultra bad. Indeed, I can listen the mixture of DPRK and China (CRI) and also AFN (810 kHz) simultaneously. Superheterodyne or double tuning in order to get more sharp separation is indispensable for "honest" shortwave radio..