Expected range
Predicting the range obtainable in any given situation is notoriously difficult since there are many
factors involved. The main ones to consider are as follows:
Type and location of antennas in use (see below)
Type of terrain and degree of obstruction of the link path
Sources of interference affecting the receiver
“Dead” spots caused by signal reflections from nearby conductive objects
Data rate and degree of filtering employed (see page 5)
Assuming the maximum 64kb/s data rate and ¼-wave whip antennas on both transmitter (@ 5mW) and
receiver, the following ranges may be used as a rough guide only:
Cluttered/obstructed environment, e.g. inside a building
Open, relatively unobstructed environment
:
:
50-75m
200-300m
It must be stressed, however, that range could be much greater or much less than these figures. Range
tests should
always
be performed before assuming that a particular range can be achieved in any given
application.
Antenna considerations and options
The choice and positioning of transmitter and receiver antennas is of the utmost importance and is the
single most significant factor in determining system range. The following notes are intended to assist
the user in choosing the most effective arrangement for a given application.
Nearby conducting objects such as a PCB or battery can cause detuning or screening of the antenna
which severely reduces efficiency. Ideally the antenna should stick out from the top of the product and
be entirely in the clear, however this is often not desirable for practical or ergonomic reasons and a
compromise may need to be reached. If an internal antenna must be used, try to keep it away from
other metal components and pay particular attention to the “hot” end (i.e. the far end), as this is
generally the most susceptible to detuning. The space around the antenna is as important as the
antenna itself.
Microprocessors and microcontrollers tend to radiate significant amounts of radio frequency hash,
which can cause desensitisation of the receiver if its antenna is in close proximity. 868MHz band is
generally less prone to this effect than lower frequencies, but problems can still arise. Things become
worse as logic speeds increase, because fast logic edges are capable of generating harmonics across the
UHF range which are then radiated effectively by the PCB tracking. In extreme cases system range can
be reduced by a factor of 3 or more. To minimise any adverse effects, situate the antenna and module as
far as possible from any such circuitry and keep PCB track lengths to the minimum possible. A ground
plane can be highly effective in cutting radiated interference and its use is strongly recommended.
A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same
regardless of whether the microcontroller or other logic circuitry is running or in reset.
Two types of antenna are recommended for use with the BiM3B:
Whip (¼-wave).
This consists simply of a piece of wire or rod connected to the module at one end. The
lengths given below are from module pin to antenna tip including any interconnecting wire or tracking
(but
not
including any 50Ω coax or microstrip connection). This antenna is simple, cheap, easy to set up
and performs well. It is especially effective when used with a ground plane, which in practice is often
provided by the main PCB or by a metal case.
Base-loaded whip.
In applications where space is at a premium a shortened whip may be used, tuned
by means of a coil inserted at the base. This coil may be air-wound for maximum efficiency, or a small
SMT inductor can be used. The value must be empirically chosen to tune the particular length of whip
for best results “in situ”, making this antenna more difficult to set up. Radiated power will generally be
slightly less than that obtained from a ¼-wave whip.
Radiometrix Ltd, BiM3B Data Sheet
page 7
RADIOMETRIX [ RADIOMETRIX LTD ]
