Centre
RTU
config. PC
RTU
dummy
antenna
Fig. 3.1: Application bench test
3.2. Frequency
Often the frequency is simply given. If there is a choice, using the optimum frequency range can make
a significant difference. Let us make a brief comparison of the most used UHF frequency bands.
160 MHz
The best choice when you have to cover a hilly region and repeaters are not an option. The only fre-
quency of the set of options which can possibly make it to a distant valley, 20 km from your nearest
point-of-presence, it can reach a ship 100 km from the shore base. The penalty you pay is tremendous
– high level of noise in urban and industry areas, omnipresent multi-path propagation, vulnerability to
numerous special propagation effects in troposphere etc. Consequently this frequency band is suitable
for low speeds using robust modulation techniques only, and even then a somewhat lower long-term
communication reliability has to be acceptable for the application.
450 MHz
The most popular of UHF frequency bands. It still can get you slightly “beyond the horizon”, while the
signal stability is good enough for 99% (or better) level of reliability. Multi-path propagation can be a
problem, hence high speeds may be limited to near-LOS conditions. Urban and industrial noise does
not pose a serious threat (normally), but rather the interference caused by other transmissions is quite
frequent source of disturbances.
900 MHz
This band requires planning the network in “microwave” style. Hops longer than about 1 km have to
have “almost” clear LOS (Line-of-sight). Of course a 2–5 km link can handle one high building or a
bunch of trees in the middle, (which would be a fatal problem for e.g. an 11 GHz microwave). 900 MHz
also penetrates buildings quite well, in an industrial environment full of steel and concrete it may be
the best choice. The signal gets “everywhere” thanks to many reflections, unfortunately there is bad
news attached to this - the reliability of high speed links in such environment is once again limited.
Otherwise, if network capacity is your main problem, then 900 MHz allows you to build the fastest and
RipEX Radio modem & Router – © RACOM s.r.o.
28
Network planning
