FM T
RANSMITTER
& R
ECEIVER
M
ODULES
. (2
ND
G
ENERATION
)
Antenna requirements
FM-TX2-XXX
FM-RX2-XXX
Three types of integral antenna are recommended and approved for use with the module:
Helical
Wire coil, connected directly to pin 2, 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 pin 2 at a point 20% from the ground end. Loops have high
immunity to proximity de-tuning.
This is a wire, rod, PCB track or combination connected directly to pin 2
of the module. Optimum total length is 17cm (1/4 wave @ 418MHz) 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
helical
**
**
***
**
200m
loop
*
*
**
***
100m
whip
***
***
*
*
300m
Loop
Whip
Ultimate performance
Easy of design set-up
Size
Immunity proximity effects
Range open ground to similar antenna
(for TX2-433-A & RX2-433-A)
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.
Antenna selection chart
Fig.8: antenna configurations
DS000022
Rev D
July ‘99
©1999
REG No 277 4001, ENGLAND.
Page 7
RFSOLUTIONS [ RFSOLUTIONS.LTD ]
