FM Radio Transmitter & Receiver Modules
Antenna Design
The design and positioning of the antenna is as
crucial as the module performance itself in
achieving a good wireless system range. The
following will assist the designer in maximising
system performance.
The antenna should be kept as far away from
sources of electrical interference as physically
possible. If necessary, additional power line
decoupling capacitors should be placed close to
the module.
The antenna ‘hot end’ should be kept clear of any
objects, especially any metal as this can severely
restrict the efficiency of the antenna to receive
power. Any earth planes restricting the radiation
path to the antenna will also have the same effect.
Best range is achieved with either a straight piece
of wire, rod or PCB track @ ¼ wavelength
(15.5cm @ 433.92MHz). Further range may be
achieved if the ¼ wave antenna is placed
perpendicular in the middle of a solid earth plane
measuring at least 16cm radius. In this case, the
antenna should be connected to the module via
some 50 ohm characteristic impedance coax
T5 / R5
RSSI Values:
The R5 RSSI output provides a DC output
proportional to the RF input signal. The table
below shows the typical RSSI value depending on
the RF signal strength. The circuit diagram shows
how a carrier detect can be obtained from the
RSSI pin on the receiver module.
RF Signal Strength / dBm RSSI / V
RF Signal Strength / dBm
-120
-110
-100
-90
-80
-70
-60
-50
-40
RSSI / V
1.27
1.33
1.49
1.74
1.99
2.3
2.56
2.63
2.63
Carrier Detect circuit
The R5 RSSI output provides a DC output
proportional to the RF input signal. In order to
generate a Carrier Detect (as a logic level) The
following circuit may be used.
Helical Antenna
RF
5V
34mm @ 433MHz
17 turns equally spaced
∅
= 5mm (inside)
10 K ohm
RSSI OUT
PIN 3
39 K ohm
CD
Whip Antenna
62 K ohm
RF
15.5cm @ 433MHz
GND
Figure 3: Antenna Configurations To Be Used
With The T5 & R5 Modules
DS305-8
Nov ‘05
©2005
REG No 277 4001, England.
Page 6
RFSOLUTIONS [ RFSOLUTIONS.LTD ]
