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LA000577C © 2006 Navman New Zealand. All rights reserved. Proprietary information and specications subject to change without notice.
11
Design of 50 ohm microstrip antenna connection
When designing the signal track from the antenna connection to the antenna input on the
module, a controlled impedance microstrip with a characteristic impedance of 50 ohms must
be used.
The PCB parameters that affect impedance are as follows:
1. Track width (W)
2. PCB substrate thickness (H)
3. PCB substrate permittivity (ε
r
)
4. To a lesser extent, PCB copper thickness (T) and proximity of same layer ground plane.
Figure 2-6 shows a representation of the PCB microstrip and its parameters.
Figure 2-6: PCB microstrip dimensions
Table 2-4 shows typical track widths for an FR4 material PCB substrate (permittivity ε
r
of 4.3
at 1.5 GHz) and different PCB thickness. The effect of track thickness (T) can be ignored for
the short track lengths associated with this design.
Substrate
material
Permittivity
ε
r
Substrate
thickness H
(mm)
Track width
W (mm)
FR4 4.3
1.6 2.0
1.2 1.8
1.0 1.6
0.8 1.4
0.6 1.2
0.4 0.7
0.2 0.4
Table 2-4: PCB substrate thicknesses v track width
Notes:
1. If a multi-layer PCB is used, the thickness is the distance from signal track to nearest
ground plane.
2. If the antenna connection is routed under the module, the track width should be
approximately halved for that section only.
It is recommended that the antenna connection PCB track should be routed around the
outside of the module outline, kept on a single layer and have no bends greater than
45 degrees. It is not recommended, for production reasons, to route the track under the
module.
However, if the track has to route under the module, it should have a modied track width and
solder mask to avoid short circuits to the underside of the module.
To minimise signal loss and reduce the requirement for vias, it is not recommended to place
the signal track on an inner layer of a multi-layer PCB.
The PCB track connection to the RF antenna input must:
• have a characteristic impedance of 50 ohm
• be as short as possible