MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
9
Once path is appropiately traced and marked as shown in Fig.#1, we obtain the straight-line distance between
Transmitter point (A) and Receiver point (B), this being in our example exactly 40 Km.
Now we take a 4/3 Profile Sheet. This is a special printed paper having approximately the same curvature radius of
the Earth at frequencies beyond 500 MHz over the horizontal axis. This axis is calibrated in Km (or miles). By the
other hand, vertical axis is calibrated in meters (or feet) to mark the height of terrain at the different points of a
certain path. Curvature of horizontal axis is made indeed in order to represent the propagated beam as a straight
line traced between Transmitter and Receiver points. This virtual radius can considerably change for abnormal
conditions of the lower layers of atmosphere, provoking an undesirable phenomenon known as "beam bending
fading" that deviates beam up or downwards, according with the variation of atmosphere's reffraction index.
In this Profile Sheet, we mark now all heights, from one kilometer to the next one, along our propagation path AB,
as shown in Fig. #2, following the horizontal-axis marks previously made along the propagation path AB in Fig. #1.
The next step is to trace a straight line joining the points A and B in the Profile Sheet, taking into account the height
of antennas over terrain at both terminal points.
As said in the note of Fig. #2, remember that, if it were required to widen horizontal scale for long hops, this scale
can be duplicated,
but always multiplying by four the vertical scale
in order to keep the same proportions in the
paper. Otherwise, the drawn profile is not valid. The same is valid for the profile made in miles and feet.
80
0
700
600
500
500
500
400
300
200
100
100
RIVER
200
300
400
500
600
700
800
900
10
20
30
40 Km
Fig.#1. TOPOGRAPHIC CHART WITH PATH BETWEEN STATIONS MARKED FOR CALCULATIONS
