Applications information
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
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
The following are typical examples – but range tests should always be performed before assuming that a
particular range can be achieved in a given situation:
Data rate
1.2kbps
10kbps
10kbps
10kbps
Tx antenna
half-wave
half-wave
helical
Rx antenna
half-wave
half-wave
half-wave
helical
Environment
rural/open
rural/open
urban/obstructed
in-building
Range
10-15km
3-4km
500m-1km
100-200m
helical
Note: The figure for 1.2kbps assumes that the receiver bandwidth has been suitably reduced by utilising an
outboard sallen-key active audio filter and data slicer or similar arrangement.
Antennas
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 antenna type for any given application.
Integral antennas
These are relatively inefficient compared to the larger externally-mounted types and hence tend to be
effective only over limited ranges. They do however result in physically compact equipment and for this
reason are often preferred for portable applications. Particular care is required with this type of antenna to
achieve optimum results and the following should be taken into account:
1. 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.
2. 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. The problem becomes
worse as logic speeds increase, because fast logic edges generate harmonics across the VHF range
which are then radiated effectively by the PCB tracking. In extreme cases system range may be reduced
by a factor of 5 or more. To minimise any adverse effects situate 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.
Radiometrix Ltd.,
HX1 high power transmitter data sheet
Page 5
RADIOMETRIX [ RADIOMETRIX LTD ]
