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Chapter 6 Antennas
To get the maximum performance from the module, in addition to the own properties
of the module, one of the important factors is how to select fitted antennas for the
module because the quality of the received signals is determined as soon as the signals
enter the RF section and can not be improved much by the subsequent filters and
amplifiers.
The character of the GPS signal is right hand circular polarized (RHCP). So, for
obtaining good GPS signals without losing too much, it’s better to use the RHCP
antennas. Otherwise, for example, using a simple linear polarized antenna to receive
GPS signals, the received GPS signals will lose at least 3 dB in SNR. In addition, the
size of an antenna also affects the received signal energy or SNR. Usually, the smaller
the size of the antenna, the lower overall gain pattern of the antenna. In other words,
the smaller size of the antenna will result in the lower SNR of the received GPS
signals. As more and more new antenna products emphasize on the size issue because
of more and more GPS related portable devices appeared, there is no way to avoid this
problem (low SNR), even with the aid of an amplifier after the antenna.
Therefore, for retrieving the most information, a large size antennas are preferable,
and even for special applications (e.g. surveying), a special mechanism structure
design is desirable, such as choke ring antenna which is used for mitigating multipath
effect. As a result, an antenna with large size, high power consumption, and high cost
is produced for high precision applications. Furthermore, for high precision
applications with millimeter accuracy in position, it is important to have stable phase
centers (L1/L2) that are exactly known.
6.1 Passive Antennas
Utilizing passive antennas in users’ applications, more attention is needed in the
layout of the RF section. Usually, the passive antenna is placed next to a module as
close as possible because of dB loss and no power amplification. However, the
proximity of antenna to electronic parts will induce the interference on the incoming
GPS signals from the module and the peripheral electronic circuits, even worse the
interference will cause signal jamming. Therefore, more careful considerations on the
layout of RF section should be taken. This selection is only suitable for those who are
familiar with the RF design.
