Chapter 8
Mobile Terminal GNSS Antennas
8.1 Introduction
This chapter deals with the limitations of antenna size and shape that are imposed when GNSS functions are to be added to small devices such as mobile handsets and personnel trackers. It is shown how the radiation patterns and polarisation properties of the antenna can be radically changed by factors such as the positioning of the antenna on the platform. The presence of a highly sensitive receiver system imposes severe constraints on the permitted levels of noise that may be generated by other devices on the platform without impairing the sensitivity of the GPS receiver; the chapter will describe the design precautions that must be taken to reduce these to an acceptable level. The case studies provide measured results from real applications, illustrating a number of typical performance defects and indicating possible solutions.
The essential properties of an antenna for a GNSS receiver are that it has a radiation pattern that covers as much of the sky as possible with the user device in its normal operating position, and that it has as high a gain, for an RHCP signal, as is consistent with the wide pattern beamwidth.
Almost all low-cost mobile terminals use only the L1 signal from the GNSS satellites. This has the advantage that the antenna can operate in only one frequency band, although to cover all the available and near-future systems, the L1 signal frequencies differ between systems as shown in Table 1.1.
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