Antenna considerations and options
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 arrangement for a given application.
Nearby conducting objects such as a PCB or battery can cause detuning or screening of the antenna which
severely reduces efficiency. Ideally the antenna should stick out from the top of the product and be entirely in
the clear, however this is often not desirable for practical or ergonomic reasons and a compromise may need
to be reached. If an internal antenna must be used, try to keep it away from other metal components and pay
particular attention to the “hot” end (i.e. the far end), as this is generally the most susceptible to detuning.
The space around the antenna is as important as the antenna itself.
Two types of antenna are recommended for use with the MTX3
Whip (¼-wave): This consists simply of a piece of wire or rod connected to the module at one end. The
lengths given below are from module pin to antenna tip including any interconnecting wire or tracking (but
not including any 50Ω coax or microstrip connection). This antenna is simple, cheap, easy to set up and
performs well. It is especially effective when used with a ground plane, which in practice is often provided by
the main PCB or by a metal case.
Base-loaded whip: In applications where space is at a premium a shortened whip may be used, tuned by
means of a coil inserted at the base. This coil may be air-wound for maximum efficiency, or a small SMT
inductor can be used. The value must be empirically chosen to tune the particular length of whip for best
results “in situ”, making this antenna more difficult to set up. Radiated power will generally be slightly less
than that obtained from a ¼-wave whip.
L
wire, rod, PCB track
or a combination of these
RF
1/4-wave whip
L (mm) = 71250 / freq(MHz)
shortened wire, rod, PCB track etc.
with loading coil.
SMT inductor may be used if reqd.
RF
Base-loaded whip
Fig. 4: Antenna configurations
Other types of antennas are feasible but tend to have drawbacks at these frequencies. Helical and tuned
loop antennas are both very compact but tricky to set up, and can be impractical at 868MHz because of their
very small size. Microstrip patch antennas are relatively large in area, directional, and have gain.
Note: Where the specified antennas are mounted on the PCB and/or in close proximity to metalwork (module
casing, components, PCB tracking etc), the antenna radiation pattern may be seriously affected. Radiated
power may be significantly increased in some directions (sometimes by as much as 10dB) and
correspondingly reduced in others. This may adversely affect system performance where good all-round
coverage is desired.
Care should also be taken to ensure that this effect does not increase the radiated power in any direction
beyond that allowed by type approval regulations. Where this occurs the antenna may need to be relocated.
In extreme cases a resistive attenuator of appropriate value may be required between the module and
antenna.
Radiometrix Ltd
MTX3 Data Sheet
page 8
RADIOMETRIX [ RADIOMETRIX LTD ]
