What is the effect of antenna cross-polarization over a line-of-sight VHF or UHF path?

For maximum signal transfer over a line-of-sight path, the transmitting and receiving antennas must have the same polarization. If one station uses a vertically polarized antenna and the other uses a horizontally polarized antenna, they are cross-polarized. Cross-polarization results in a massive reduction of received signal strength, often by as much as 20 to 30 decibels. This means a perfectly readable signal might disappear into the noise floor completely just because the antennas are oriented differently. It does not cause echoes or invert sidebands; the physical orientation of the elements only governs the alignment of the electromagnetic waves, directly dictating how efficiently the receiving antenna captures the transmitted energy.