worse as logic speeds increase, because fast logic edges are capable of generating harmonics across the
UHF range which are then radiated effectively by the PCB tracking. In extreme cases system range can
be reduced by a factor of 3 or more. To minimise any adverse effects, situate the antenna and module as
far as possible from any such circuitry and keep PCB track lengths to the minimum possible. A ground
plane can be highly effective in cutting radiated interference and its use is strongly recommended.
A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same
regardless of whether the microcontroller or other logic circuitry is running or in reset.
Two types of antenna are recommended for use with the TX3H:
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.
RF
SMT inductor may be used if reqd.
Base-loaded whip
Fig. 5: 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, TX3H Data Sheet
page 5
