Gps Antenna Considerations For Automotive Applications | f an AMP company GPS Antenna Considerations for Automotive Applications Application Note GPS01 The design of active GPS antennas involves essentially three components a patch element filter and low noise amplifier LNA . Each of these components forms an important part of the overall antenna assembly and is critical to performance. Antenna Element Microstrip patch technology is often used to create the antenna element for automotive GPS applications. Sizes have been reduced to a 1 inch square puck with thicknesses from to inches. The dielectric constant of the ceramic material in these cases is approximately 20. Several different techniques are used to generate Right Hand Circular Polarization RHCP from a patch element including use of a polarization slot offset feed points and polarization tabs. The choice is at the discretion of the designer. Polarization Tabs Offset Feed o Slot Figure 1 Techniques used to generate RHCP Axial Ratio Regardless of the design approach there are common design parameters that are essential to the performance of the antenna. To provide an antenna that exhibits good performance with respect to RHCP the axial ratio a measure of the antenna s polarization purity is critical. The higher the axial ratio the more elliptical the polarization and the lower the gain with respect to RHCP. This parameter is the result of design and process control. A measurement of the VSWR alone does not guarantee the antenna s axial ratio performance. The correlation between axial ratio and circular gain is shown in the following formula which provides a gain correction factor GCF in transitioning from measured linear gain and axial ratio to circular GCF dB 20 log 1 02 1 10 AR 20 where AR is the antenna axial ratio in dB. It can be seen from this formula that an antenna with an axial ratio of 0 dB has a GCF of dB. This means that the circular gain of the antenna will be dB higher than the peak linear gain of the antenna. An antenna with an 8 dB
