The assumption is that every frequency component comes from one single source. This assumption is always disrupted by the presence of noise, as noise exists over the entire bandwidth.
We can distinguish four noise sources, man-made noise, atmospheric noise, antenna preamplifier noise and receiver noise. Man-made noise can be viewed by the large number of sources as random noise unless it concerns a distinct dominant local source. The noise from both antenna preamplifiers and the receiver noise from both receivers are uncorrelated and can therefore have any random phase difference between 0° and 360°. Atmospheric noise and man-made noise are difficult to distinguish. Both antennas receive this noise with a difference in the time of arrival, and corresponding phase difference. The maximum possible phase difference depends on the distance between the antennas and the wavelength, as is the case with a normal desired signal. Each phase difference is also associated with an arc of directions. We therefore do not see this noise over the entire 360° but with a certain distribution over a part of this, as shown in Figure 5. With the antennas at right angles to one another, man-made noise (ground wave) will be around 0° and 180°. Atmospheric noise can be circularly polarized and have a dominant phase difference around –90°. Linearly polarized atmospheric noise can also be distributed over the entire 360º (variable linear polarization), however.
Last update: September 24, 2006