Antenna requirements
Three types of integral antenna are recommended and approved for use with the module:
A) Whip
This is a wire, rod ,PCB track or combination connected directly to RF pin of the
module. Optimum total length is 16cm (1/4 wave @ 433MHz). Keep the open circuit (hot)
end well away from metal components to prevent serious de-tuning. Whips are ground
plane sensitive and will benefit from internal 1/4 wave earthed radial(s) if the product is
small and plastic cased
B) Helical
C) Loop
Wire coil, connected directly to RF pin, open circuit at other end. This antenna is very
efficient given it’s small size (20mm x 4mm dia.). The helical is a high Q antenna, trim
the wire length or expand the coil for optimum results. The helical de-tunes badly with
proximity to other conductive objects.
A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot' end and
fed from RF pin at a point 20% from the ground end. Loops have high immunity to
proximity de-tuning.
A
whip
***
***
*
B
helical
**
**
***
C
loop
*
*
**
Ultimate performance
Easy of design set-up
Size
Immunity proximity effects
*
**
***
The antenna choice and position directly controls the system range. Keep it clear of other metal in the
system, particularly the 'hot' end. The best position by far, is sticking out the top of the product. This is
often not desirable for practical/ergonomic reasons thus a compromise may need to be reached. If an
internal antenna must be used, try to keep it away from other metal components, particularly large
ones like transformers, batteries and PCB tracks/earth plane. The space around the antenna is as
important as the antenna itself.
0.5 mm enameled copper wire
close wound on 3.2 mm diameter former
RF
433 MHz = 24 turns
A. Helical antenna
Feed point 15% to 25% of total loop length
track width = 1mm
RF-GND
4 to 10 cm2 inside area
C2
C1
RF
C3
C4
B. Loop antenna
16.4cm
wire, rod, PCB-track or a combination
of these three
RF
433 MHz = 16.4 cm total from RF pin.
C. Whip antenna
Figure 8: Antenna types
Radiometrix Ltd
TDH2 Data Sheet
page 9
RADIOMETRIX [ RADIOMETRIX LTD ]
