7.4 Antenna fine tuning
Engineers with experience in radio design and the necessary measurement equipment can
consider tuning the antenna performance by impedance matching. If implemented as per
reference design, smart antenna configuration provides the possibility to tune both internal
and external antenna.
Due to the influence of mounting conditions as metallic objects close to the antenna, or
the characteristics (size, thickness, stack-up, etc.) of the mother-pcb and ground plane,
there might be some detuning of the antenna. The capacitors C7, C9, C10 can be used
in combination to tune the internal antenna. The capacitors C6, C7, C8 can be used in
combination to tune the external antenna (see Figure
Simultaneous connection of internal and external antenna should be avoided.
i.e. capacitor C6 and C9 shall not be assembled together.
7.5 Antenna solutions
There exist several kinds of antennas, which are optimized for different needs. Chip anten-
nas are optimized for minimal size requirements but at the expense of range, PCB antennas
are optimized for minimal costs, and are generally a compromise between size and range.
Both usually fit inside a housing.
Range optimization in general is at the expense of space. Antennas that are bigger in size,
so that they would probably not fit in a small housing, are usually equipped with a RF con-
nector. A benefit of this connector may be to use it to lead the RF signal through a metal
plate (e.g. metal housing, cabinet).
As a rule of thumb a minimum distance of
λ
/10 (which is 3.5 cm @ 868 MHz and 1.2 cm @
2.44 GHz) from the antenna to any other metal should be kept. Metal placed further away
will not directly influence the behavior of the antenna, but will anyway produce shadowing.
Keep the antenna away from large metal objects as far as possible to avoid
electromagnetic field blocking.
The choice of antenna might have influence on the safety requirements.
In the following chapters, some special types of antenna are described.
7.5.1 Wire antenna
An effective antenna is a
λ
/4 radiator with a suiting ground plane. The simplest realization is
a piece of wire. It’s length is depending on the used radio frequency, so for example 8.6 cm
868.0 MHz and 3.1 cm for 2.440 GHz as frequency. This radiator needs a ground plane at
its feeding point. Ideally, it is placed vertically in the middle of the ground plane. As this
Ophelia-I reference manual version 1.0
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