STL Antennas — Too Much Height?
Did you know that microwave STL antennas can
be too high?
1:An STL path with water reflection.
microwave signal can reflect off of water, as shown in the exaggerated drawing
in Fig. 1.
If both antennas are within line of sight of water, and the water is in
an unfortunate location, two signals can arrive at the receiving antenna — one
from the transmitting antenna and one from the reflection off the water. If the
reflected signal arrives out of phase with the direct signal, it can cause the
received signal to be severely degraded or even unusable.
problem can be solved by intentionally blocking one of the antennas from the
over-water reflecting path. Trees are often a good source for intentionally
blocking a path, but terrain and taller buildings can also serve the purpose.
In Fig. 2, we simply lowered the height of the transmit antenna to cause the
trees to block the path towards the water, which will eliminate the reflected
2:STL path using terrain to block
reflective medium does not have to be water. A swamp, a marsh or even a flat
pasture might cause a reflection. Make sure you have enough height to clear the
Fresnel zone, but don’t install the antennas higher than necessary without
Jeffrey Gehman is engineering associate at
Kessler and Gehman Associates in Gainsville, Fla.
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