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
Data formats and range extension
The NiM2 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 1.2V approx. with
the modulation ac-coupled and limited to a maximum of 2V
p-p
to minimise distortion over the link. The
varactor modulator in the NiM2 introduces some 2
nd
harmonic distortion which may be reduced if
necessary by predistortion of the analogue waveform. At the other end of the link the NiM2 RXD output
is used to drive an external decoder directly.
Although the modulation bandwidth of the NiM2 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 NiM2 audio output.
The NiM2 in standard form incorporates a low pass filter with a 5kHz nominal bandwidth. This is
suitable for transmission of data at raw bit rates up to 10kbps.
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.
The following types of integral antenna are in common use:
Quarter-wave whip.
This consists simply of a piece of wire or rod connected to the module at one end.
At 434MHz the total length should be 164mm from module pin to antenna tip including any
interconnecting wire or tracking. Because of the length of this antenna it is almost always external to
the product casing.
Helical.
This is a more compact but slightly less effective antenna formed from a coil of wire. It is very
efficient for its size, but because of its high Q it suffers badly from detuning caused by proximity to
nearby conductive objects and needs to be carefully trimmed for best performance in a given situation.
The size shown in figure 5 below is about the maximum commonly used at 433MHz and appropriate
scaling of length, diameter and number of turns can make individual designs much smaller.
Loop.
A loop of PCB track having an inside area as large as possible (minimum about 4cm
2
), tuned and
matched with 2 or 4 capacitors. Loops are relatively inefficient but have good immunity to proximity
detuning, so may be preferred in shorter range applications where high component packing density is
necessary.
Integral antenna summary:
whip
***
***
*
**
helical
**
**
***
*
loop
*
*
**
***
Ultimate performance
Ease of design set-up
Size
Immunity to proximity effects
Radiometrix Ltd.,
NiM2 transceiver data sheet
Page 7
RADIOMETRIX [ RADIOMETRIX LTD ]
