Q. What do you mean by Signal multipath?
Signal multipath occurs with electromagnetic wave propagation via sky wave in the HF range. When the transmitted signal reaches the receiver through multiple propagation paths with different delays, signal fading may result. Both atmospheric noise and thermal noise become the additive noise at high frequencies. It is possible to have ionospheric scatter propagation in the frequency range of 30 to 60MHz, and tropospheric scattering in the range of 40 to 300MHz; but relatively large antennas are needed with a large amount of transmitted power, because of large signal propagation losses.