MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
10
Fig. # 2. PATH MARKED IN 4/3 PROFILE PAPER SHEET
.
1.4.- Fresnel or Phase Zones.
Wave front propagated by transmitting antenna is going out of phase as propagated wave goes apart from
the beam center more and more in a continuous fashion. The limits of phase shift suffered by this propagated wave
front, as it separates from the beam center, are known as "Fresnel or Phase Zones" being these zones enclosed
between well defined values like
π
/2,
π
, 3
π
/2, etc, and defined as follows:
First Fresnel Zone is those region where wavefront is shifted in phase from 0º (beam center) to 90º. It encloses
about 70% of the total beam RF energy density. It's very important in any hop to get a total clearance, non-
obstructed path for this region. As system frequency drops down, propagated energy density enclosed in this zone
increases, and in the 2 GHz band it's required to release only 70% of the total beam area at First Fresnel Zone in
any obstructed point along the propagation path. For upper frequency bands, starting in 4 GHz, First Fresnel Zone
clearance must be total (100%) in order to assure a nominal signal strength at the receiving end.
Second Fresnel Zone is those region where wavefront is phase-shifted from 90º to 180º. Is obvious that this zone
should be blocked out wherever it's possible along the propagation path, because it contributes to weaken the
signal at receiving end if it's reflected over the center beam line at any point of the path. In the third, fourth, etc
Zones, energy density is very weak and it can be considered as negligible.
