Answer

Antennas in a product that implements antenna diversity have their antennas mounted at a distance of at least ¼ wavelength apart. This amount of spatial separation improves the probability that at least one antenna is NOT in a deeply faded signal condition. Another typical recommendation is ½ wavelength antenna distance, however, it can result quite large board sizes at low frequencies (e.g. for 434 MHz frequency ½ wavelength is 34,5 cm).