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 7)
Assuming the maximum 64kbps data rate and unobstructed
transmitter and receiver, the following ranges may be used as a rough guide only:
¼-wave whip antennas on both
1) Cluttered/obstructed environment, e.g. inside a building
2) Open, relatively unobstructed environment
:
:
25-75m
100-300m
It must be stressed that range obtained in practice may lie outside 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
apply particularly to integral antennas and 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/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.
900MHz region 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.
Radiometrix Ltd, TX3A & RX3A Data Sheet
page 9
RADIOMETRIX [ RADIOMETRIX LTD ]
