Paths Over Sea or Very Flat Ground

Paths over the sea are subject to a special problem due to the very strong reflection from the water. This reflection can add an anti-phase signal to the direct wave and cancel it out completely. This may not happen all of the time because the effective curvature of the earth changes depending upon the temperature gradient in the atmosphere. This gradient can change and in certain circumstances causes the signal to travel a long way in ducts. The following figure illustrates the problem and the solution, using a PTP 670 bridge:

Propagation Over The Sea

The background of the diagram is shaded to illustrate the changing density and therefore refractive index. The upper antennas are in a signal inversion.

The signals pass from one antenna to the other through two paths. One path is the direct path and the other is reflected from the sea. The mean path loss of the two components is almost identical. The graph adjacent to the mast illustrates the signal level that will occur as an antenna is moved vertically on the mast. In this case the x-axis illustrates the amplitude received while the y-axis illustrates the height.

The polarization selected for the antennas are single V and H polarization on the left and a dual polarized antenna on the right. The two graphs on the right illustrate the signal received on each polarization while on the left the individual antennas will receive the same signal level independent of polarization but instead will only depend upon the height.

There is an optimum vertical spacing of the two antennas on the left which is found from the geometry of the two paths. The important parameters are the length of the path, the height of the right single antenna and to a lesser extent the height of the pair of antennas on the left. An allowance is made for the apparent height of the middle of the path due to the mean radio curvature of the earth (4/3).

LINKPlanner does not adjust the reliability of the link based upon the possible reflection, but a link that suffers reflection can have very bad performance if the mitigation has not been applied.