Pfthb

I live in Moscow, Russia and use DIY indoor magnetic loop antennas to listen amateur radio. Recently I received quite strong signals from Italy (20m, IV3UTV) and Slovenia (40m, S51CK), both operators are ~2000 km from me. One thing I don't quite understand is that both countries are south-west from me, and the antenna is hanging on a window that faces east.

I see two possibilities. There is a tall concrete building not far away that probably could reflect those signals. Or maybe short waves can just pass through my building? What explanation is more plausible?

(I tried to find the answer on my question using Google. Everyone is telling on how short waves reflect off the ionosphere, but I didn't manage to find the information on how they behave when they meet a concrete building.)

The concrete is relatively transparent to radio waves of such large wavelengths (it attenuates, it doesn't reflect). However, steel bars within concrete typically convert that concrete to a solid reflector from the perspective of a wave with such a large wavelength.

Basically, that effect scales: Just as your microwave oven's front door has a metal plate with holes that are too small for microwave radiation to actually pass through, when you scale up from ca 20 cm microwave oven wavelength to 20 m shortwave wavelength, you can scale up your metal bar spacing, too.

However, your way of thinking "what specifically is it reflected off" isn't 100% right.

When wavelengths are larger or in the same order of magnitude as obstacles (houses, walls, towers, trees, trucks, trains…), you start noticing that ray optics doesn't really represent the way electromagnetic waves work; you get diffuse effects such as refraction, and with a lot of obstacles, even in the optic model, you get more effects like scattering than specular reflections.