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Network planning
3.4.1. How to battle with multipath propagation?
The first step is the diagnosis. We have to realize we are in trouble and only a field measurement can
tell us that. We should forget about software tools and simply assume that a multipath problem may
appear on every non-LOS hop in the network.
These are clear indicators of a serious multipath propagation problem:
•
directional antennas "do not work", e.g. a dipole placed at the right spot yields a better RSS than
a long Yagi, or rotating the directional antenna shows several peaks and troughs of the signal and
no clear maximum
•
RSS changes rapidly (say 10 dB) when antenna is moved by less than a meter in any direction
•
ping test displays the mean deviation of RSS greater than 6 dB
•
DQ value keeps "jumping" abnormally from frame to frame
Quite often all the symptoms mentioned can be observed at a site simultaneously. The typical "beginner"
mistake would be to chase the spot with the best RSS with an omnidirectional antenna and installing
it there. Such a spot may work for several minutes (good luck), sometimes for several weeks (bad luck,
since the network may be in full use by then). In fact, installing in such a spot guaranties that trouble
will come - the peak is created by two or more signals added up, which means they will cancel out
sooner or later.
The right strategy is to find an arrangement where a single signal becomes dominant, possibly the
most stable one. "Sweeping" a directional antenna around the place (in different heights and with dif-
ferent polarization) can tell us where the signals come from. If individual signals come from different
directions, there is a good chance a long yagi can solve the problem by selecting just one of the bunch.
Finding a spot where the unwanted signal is blocked by a local obstacle may help as well (e.g. installing
at a side of the building instead of at the roof).
TX antenna
Fig. 3.4: Antenna location
When the multiple signals come from about the same direction, a long yagi alone would not help much.
We have to move away from the location, again looking for a place where just one of the signals becomes
dominant. 20–50 metres may save the situation, changing the height (if possible) is often the right
solution. Sometimes changing the height means going down, not up, e.g. to the base of the building
or tower.
We have to remember our hop has two ends, i.e. the solution may be to change antenna or its placement
at the opposite end. If everything fails, it is better to use another site as a repeater. Even if such prob-
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better
RipEX Radio modem & Router – © RACOM s.r.o.
multipath
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