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
Application Circuit
The application circuits show how the QFMT5
transmitter and the QFMR5 receiver can easily be
integrated into a system to form a wireless link.
ANTENNA
+5V
+5V
T5
1
2
3
4
5
1
2
3
4
5
6
7
8
9
A0
A1
A2
A3
A4
A5
A6
A7
VSS
HT12E
VDD
DOUT
OSC1
OSC2
TE\
AD11
AD10
AD9
AD8
18
17
16
15
14
13
12
11
10
R
OSC
Figure 4: QFMT5 Transmitter Application
Circuit
ANTENNA
+5V
+5V
1
2
A0
A1
A2
A3
A4
A5
A6
A7
VSS
HT12D
VDD
VT
OSC1
OSC2
DIN
D11
D10
D9
D8
18
17
16
15
14
13
12
11
10
DATA OUT 4
DATA OUT 3
DATA OUT 2
DATA OUT 1
R
OSC
1K5
Ω
Quasar
R5
1 2
3 4 5 6 7
Helical Antenna
RF
3
4
5
6
7
8
9
34mm @ 433MHz
17 turns equally spaced
∅
= 5mm (inside)
Figure 5: QFMR5 Receiver Application Circuit
RSSI Values:
The QFMR5 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.
RF Signal Strength / dBm
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
RSSI / V
tba
Whip Antenna
RF
15.5cm @ 433MHz
Figure 3: Antenna Configurations To Be Used
With The QFMT5 & QFMR5 Modules
DS305_2
April 01
©
2001 REG No 277 4001, England.
Page 5
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