GPS antenna considerations for automotive industry

Design of active GPS antennas consists mainly of three components: the patch element, filter and low noise amplifier (LNA). Each of these components is an important part of the overall antenna assembly and is essential for the performance.

Antenna Element

Micro strip patch technology is often used to create element antenna for automotive GPS applications. Dimensions were reduced to 1 inch square puck, thickness from 0.16 to
0.25 inches. Dielectric constant of ceramic material in these cases is approximately the 20th Several different procedures used to generate the right-hand circular polarization (RHCP) from the patch element, including the use of polarizing slot offset feed points, and polarization maps. The choice is estimated designer.

Axial Ratio
Regardless of the design approach, there are common design parameters for the antenna performance. Good RHCP, axial ratio and antenna polarization purity (measure) is important to provide performance antenna. More RHCP axial relationship, more elliptical polarization, the higher and lower profits. This parameter is the result of planning and control process. VSWR performance measurement alone does not guarantee the antenna axial ratio.
The correlation between axial ratio and earnings measured in accordance with linear gain and axial ratio for circular migration gain of the correction factor (GCF) that provides a circular by the following formula as follows:

GCF (dB) = 20 log [(1/Ö2) (1 + 10 - (AR/20))] N.

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 +3.0 dB. This means that the circular

gain of the antenna will be 3.0 dB higher than the peak linear gain of the antenna. An antenna with an 8 dB axial ratio will have a GCF of -0.1 dB, and be 3.1 dB lower than the 0 dB axial ratio antenna.
M/A-COM has developed a measurement process that allows us to verify axial ratio and VSWR in a single measurement. We impose strict requirements on the axial ratio of the ceramic patch antennas fabricated for us, and verify them prior to integration of the patch element into the antenna assembly. We have evaluated the antennas of several other manufacturers and repeatedly found that these parameters significantly deviate from optimized. M/A-COM GPS antennas provide boresight axial ratios of 2.0 dB or better typical on boresight. At 2.0 dB, the deviation in RHCP gain due to polarization inefficiency is only 1.0 dB from a perfect antenna. Other antennas we have measured have specified boresight axial ratios of 4.0 dB, but actually exhibit boresight axial ratios from 6 dB to 13 dB, a gain degradation of 2.5 to 4.5 dB from optimal. It is clear in this case that the controls imposed in M/A-COM designs are not in place elsewhere.
A secondary benefit to the control of axial ratio is multipath rejection. RHCP signals from the satellite that reflect off the sides of buildings, etc., will experience a polarization flip with the first bounce and become LHCP. If the path of the reflected signal is still in the main beam of the GPS antenna, then the only method for rejection of the multipath is polarization purity. An antenna with good axial ratio performance and therefore good polarization purity will have a better response to the direct RHCP signal than an antenna with poor axial ratio, and will also provide for better rejection of the reflected LHCP signal.


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