Abstract
The Earth’s magnetic field causes the ionosphere to be birefringent at radio frequencies, which means that any system using ionospherically propagated radio waves, such as long-distance broadcasting, high-frequency (HF) skywave communications, over-the-horizon radar, and oblique incidence sounders (OIS) will receive pairs of waves with different polarizations. In this article, we develop a model for the polarization of ionospherically propagated radio waves; the polarization is dependent only upon the strength and direction of the magnetic field at the location where the radio wave exits the ionosphere. This leads to the hemisphere of possible incoming directions of arrival to any particular receiver being divided into three distinct regions. We then use this model to predict the response of a polarimetric antenna to a transmitted OIS signal and validate the model against real ionograms, including cases where all three polarization regions can be clearly observed.
Highlights
Due to the Earth’s magnetic field, the ionosphere is birefringent at radio frequencies – there are two canonical propagation modes corresponding to the polarization of the radio waves, termed the ordinary (O) and extraordinary (X) modes
The actual polarizations of these two modes will vary depending on the angle between the magnetic field and the wave’s direction of propagation, as well as the wave frequency and the strength of the magnetic field [1]. This birefringence means that any system using ionospherically propagated radio waves – such as long distance broadcasting, high frequency (HF) skywave communications, and over-the-horizon radar – will receive pairs of waves with different polarizations
Vertical-incidence sounders (VIS) and oblique-incidence sounders (OIS) use the time delay of ionospherically propagated HF radio waves to measure the state of the ionosphere
Summary
The two loops are each fed by a 4:1 passive transformer balun, at the centre of the base of the loops, and are electrically isolated from the central mast using nylon bolts and sleeves, as well as isolated from each other. This is the same antenna design used in the PRIME ionospheric sounder systems [7]
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