What happens when antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?

For maximum energy transfer between two line-of-sight antennas, their polarizations must match. If a transmitting antenna is vertically polarized and the receiving antenna is horizontally polarized, the mismatch creates "cross-polarization attenuation." This can drastically reduce the received signal strength by as much as 20 to 30 decibels (dB). Inversions of sidebands or echo effects are related to modulation settings or multipath propagation, not polarization. As such, matching polarization is a fundamental rule for reliable point-to-point VHF/UHF communications.