The Importance of circular polarization for diversity reception and MIMO in NVIS propagation

Near Vertical Incidence Skywave propagation can provide High Frequency (HF: 3-30 MHz) radio coverage in an area of 300 by 300 km without the need for any intermediate infrastructure. It is therefore very well suited for disaster relief communication. However, electromagnetic waves refracted in the ionosphere are prone to fading caused by multipath propagation and by polarization change in the ionosphere. Historically, different diversity reception systems have been devised to reduce this fading. By contrast, modern HF Multiple Input Multiple Output (MIMO) techniques exploit the multipath aspects of the ionosphere to achieve an increased data throughput or a reduction of the required signal-to-noise ratio. The effectiveness of diversity reception and HF MIMO increases with decreasing correlation between the received signal streams. Electromagnetic waves propagating through the ionosphere consist of two characteristic wave components with circular polarization of opposite sense, the ordinary and extraordinary waves, each following a different path through the ionosphere. Adaptation of the polarization of the receive antennas to these characteristic waves provides two signal streams with high isolation. This is demonstrated with an experiment where both the ordinary and extraordinary wave are received separately. An isolation of at least 13 dB is shown.