COMMS HARDWARE ANTENNA DESIGN
Figure 3: Pi
Matching Network
example
Author details:
Geoff Schulteis is
Senior Antenna
Applications
Specialist with
Antenova
Figure 4: A slim
design with the FPC
integrated inside
and cable route.
6mm
Trace lines (or transmission lines)
carry the RF signals to and from the
antenna and the characteristics of
these lines typically yield low levels of
resistance when designed properly.
The power transfer and return loss of
an antenna can be as high as 50% in
a poorly matched design.
Trace lines operate at 50, so
RF traces and other elements of the
RF system - such as transceivers or
power ampli ers – must therefore
match this impedance.
I would suggest the use of
grounded coplanar waveguides
(GCPW) for embedded antennas, as
the antenna can be mounted on the
component side of a PCB and achieve
better performance levels. They
also remove the need for backside
manufacturing processes for the PCB.
The quality of the impedance
match is measured as the VSWR
(Voltage Standing Wave Ratio) or
Return Loss. This is one of the
most critical parameters in antenna
integration, as a well-matched
circuit can provide good levels of
Cable route FPC
90mm
PCB
I-PEX connection
Wi-Fi module
35mm
SR4G013
Feed
Gnd
Gnd
Gnd
1 2 3 4
C1
L1
C2
GNSS antenna
1
Feed
of 50. At Antenova, we recommend
that the transmission lines should be
kept as short as possible, and that
any other parts of the RF system like
transceivers, power ampli ers, etc.,
should also be designed to have an
impedance of 50 .
The thickness of the PCB and the
dielectric constant of its material is
important. When the material for the
PCB has been chosen, a coplanar
transmission line can easily be
designed using one of commercial
software packages for transmission
line design. These calculate the
appropriate transmission line width
and the correct gaps to ground to
allow on either side of the feed for
the thickness of the PCB, copper and
substrate.
FPC antennas are supplied with
I-PEX MHF (U.FL) type connectors
to connect them to the host PCB or
wireless module, which often has
the mating I-PEX MHF connector to
join directly, with the cable in varying
lengths. This gives the designer further
options for positioning the antenna
within the device.
Matching
An SMD antenna requires matching
to the other RF components in the
design to optimise its performance for
the bands of interest in situ. Antenna
matching is the process of aligning the
impedance of the antenna chip with
the other RF circuitry in the design,
and it increases the performance
of the antenna by optimising the
impedance for the bands of interest.
performance, even when its ef ciency
is inhibited.
Using Pi matching circuits between
the antenna terminal and the 50
input ports to optimise the power
transfer in the bands of interest, is
recommended.
If the device is likely to be
used in the hand or close to the
human body as in the case of
trackers and wearables devices, the
antenna should be matched and
tuned according to the operational
environment.
With the FPC antenna, very little
tuning or matching is required.
The production process may
affect your choice of antenna. Chip
antennas are compatible with the pick
and place machines, while exible
antennas are xed in place by a
peel-back adhesive strip on one side.
FPCs are a good solution for smaller
production runs and often require
hand assembly.
Summary
In the end the nal choice of antenna
will depend on the shape of the
device, the space available on the
PCB, and the eventual manufacturing
process.
Chip antennas function very
well in small devices provided
there is enough space for them to
perform correctly and they are very
popular. They require space for a
ground plane, as speci ed by the
manufacturer, and need to be placed
away from components that create
“noise”.
If the space on the PCB is limited
the FPC may be the way forward. The
FPC option is also good for devices
with a longer chassis.
FPCs, being larger, will probably
not be your rst choice for smaller
devices, but they can work well for
projects that suit them. Folding
the antenna and inserting it inside
the casing of the design helps to
keep it away from other, potentially
interfering, components.
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