Home » Antennas » 40 On VHF and UHF bands, polarisation of the receiving antenna is important in relation to the transmitting antenna, but on HF it is relatively unimportant because:   a.   the ionosphere can change the polarisation of the signal from moment to moment   b.   the ground wave and the sky wave continually shift the polarisation   c.   anomalies in the earth's magnetic field profoundly affect HF polarisation   d.   improved selectivity in HF receivers makes changes in polarisation redundant