Free space conditions require a direct line of sight between the two antennas involved.
Consequently no obstacles must reach into the path between them. Furthermore in order to avoid the majority of effects caused by superposition of direct and reflected signals, it is necessary that the first Fresnel ellipsoid is completely free of obstacles :
Consequently no obstacles must reach into the path between them. Furthermore in order to avoid the majority of effects caused by superposition of direct and reflected signals, it is necessary that the first Fresnel ellipsoid is completely free of obstacles :
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| Free Fresnel Ellipsoid |
The first Fresnel ellipsoid is defined as a rotational ellipsoid with the two antennas at its focal points. Within this ellipsoid the phase difference between two potential paths is less than half a wavelength.
The radius (b) at the center of the ellipsoid can be calculated based on the formula:
where b is the radius in m,
d is the distance between RX and TX in km,
and f is the frequency in GHz.
d is the distance between RX and TX in km,
and f is the frequency in GHz.