HF polarization sensor with digital processing

An electrically small polarization sensor for use at HF has been designed and built at Roke Manor. This consists of three mutually orthogonal loops of equal dimensions with a common centre. To reduce intersections between them the loops are square, rather than circular, with the two vertical loops placed along the diagonals of the horizontal loop. The sides of the loops are small compared with HF wavelengths ranging from 10 m (at 30 MHz) to 150 m (at 2 MHz). It was assumed that if the signal direction is given the polarization would be found from the sensor outputs, but in fact it is found that in general (for elliptical polarization) the sensor will give both direction and polarization information, assuming only one significant signal is present. The loop outputs feed three digital receivers with simultaneous digitization at the complex baseband, giving frames, or snapshots, of data of three complex values each. The processing can be carried out on a single frame, or over multiple frames, to give some smoothing (in effect low pass filtering) against noise. Two of these antennas have been constructed, as part of a larger programme investigating superresolution direction finding techniques for HF. They are used as an aid to measuring the dynamic polarization characteristics of incoming skywave signals in the frequency range 2 to 30 MHz