ENT1_14 Datasheet, PDF (9/13 Pages) Radiometrix Ltd – 12.5kHz Channel VHF TX / RX

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
Tx antenna
half-wave
Rx antenna
half-wave
Environment
rural/open
Range
10-15km
The ENT1’s TXD input is normally driven directly by logic levels but will also accept analogue drive (e.g. 2-
tone signalling). In this case it is recommended that TXD (pin 14) be DC-biased to 2.5V approx. with the
modulation ac-coupled and limited to a maximum of 5V_p-p to minimise distortion over the link.
Although the modulation bandwidth of the ENT1 extends down to DC it is not advisable to use data
containing a DC component. This is because frequency errors and drifts between the transmitter and
receiver occur in normal operation, resulting in DC offset errors on the receiver’s audio output.
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.,
ENT1/ ENR1 data sheet
Page 9