•
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
better
multipath
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-
lematic site seems to be usable after all (e.g. it can pass commissioning tests), it will keep generating
problems for ever, hence it is very prudent to do something about it as early as possible.
Note:
Never design hops where a directional antenna is used for a direction outside its main lobe.
However economical and straightforward it may seem, it is a dangerous trap. Enigmatic cases of drop-
outs lasting couple of minutes every other day, over a clear LOS hops were created exactly like that.
They look like interference which is very difficult to identify and , alas, they are caused by pure multipath
propagation, a self-made one. So always use a combiner and another directional antenna if such ar-
rangement is needed. Always.
RipEX Radio modem & Router – © RACOM s.r.o.
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Network planning