Estimation of direction of arrival and polarisation of radio waves at the HF-band

Communications using HF radio waves is supplementary to using satellite-based communications as HF radio waves can travel great distances. Tactically it can be of military interest to quickly be able to distinguish HF-signals that has been reflected from the ionosphere from those arriving directly from a ground-based transmitter. In a previous FOI project a method for determining polarisation and direction-of-arrival of impinging HF radio waves, using a tri-axial magnetic loop antenna, was developed. The method compared the measured values of amplitude and phase in the antenna elements and compared these to pre-simulated solutions in order to determine direction-of-arrival and polarisation. The pre-simulated solutions were calculated using an electromagnetic simulation program. In order to evaluate the method field trials-in which bearing, elevation angle, and polarisation of the impinging radio wave varied-were carried out. The results of the field trials showed that the method worked. The method was capable of finding a solution for the correct direction of arrival and polarisation for the impinging radio waves. Unfortunately, the method also found other solutions which also agreed with the measured data. The method could probably be improved by increasing the resolution of the simulation but also by changing the antenna configuration.