For VHF and UHF signals over a fixed path, what extra loss can be expected when linearly-polarized antennas are crossed-polarized (90 degrees)?

When two linearly-polarized antennas are used over a fixed path, maximizing signal strength requires them to have matching orientations. If one antenna is vertically polarized and the other is horizontally polarized, they are cross-polarized at 90 degrees. This extreme mismatch results in a severe attenuation of the signal, typically causing a loss of 20 dB or more. This is why standardizing polarization (such as vertical for local FM repeater traffic) is critical for reliable communication. While a 3 dB loss occurs when matching a circularly polarized antenna to a linearly polarized one, crossing two linear antennas causes a much more devastating drop in signal.