How does diffraction affect radio signals?
Same as for light. See below.
Remember radio waves or signals is the same as light waves.
Light is just a small fraction of the entire spectrum of the electromagnetic waves.
In the case of light, diffraction causes it to bend around corners of an obstacle, you would have a similar phenomena woth radio signals, but the "radius" of bend would be far larger on account of the larger wavelengths of the radio signals.