Don BodnarDan AloiToday’s automobiles have a wide variety of RF systems with antennas on them for Sirius and XM radio, collision avoidance radars, the Global Positioning System (GPS) and other systems. Conventional test facilities can only perform terrestrial directed pattern measurements of the antenna on the automobile. Special test facilities are required when the automobile must communicate with a satellite as well as other ground systems. Table 1 provides a partial list of satellite-based wireless systems below 2.5 GHz in frequency. This paper discusses a system specially designed for making antenna measurements from the zenith to the horizon. In addition, some of the issues involved in making satellite band measurements such as Sirius/XM and GPS and terrestrial band measurements such as CELL800 and CELL1800 are reviewed. 

MI Tech-Table1

Test System
A telematics measurement system must be able to measure the antenna pattern over the upper hemisphere when the antenna is installed on the vehicle since the vehicle can deteriorate the pattern if the antenna is installed in the wrong location [1]. A typical system for performing these measurements is shown in Figure 1. A turntable mounted flush with the ground rotates the vehicle 360° in azimuth while a gantry arm positions the measurement probe from 0° to 90°. This turntable-gantry arm combination allows the probe to measure the antenna pattern over the entire upper hemisphere.

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The installed antenna can induce currents to flow on the vehicle and these currents can corrupt the desired pattern from the antenna. A theoretically exact pattern from the installed antenna is obtained the using the spherical near-field measurement technique [2]. This method uses the probe to sample the near-field on the measurement hemisphere at uniformly spaced azimuth and elevation angles. The measured near-field data is then processed using the near-field algorithm, the effects of the probe are removed and the data converted to a far-field pattern. Alternately, the far-field pattern can be measured directly without near-field processing if the interaction of the antenna with the vehicle is fairly localized. This latter approach is commonly used since it allows rapid pattern measurements. The instrumentation for the measurements consists of a measurement system, here a MI-3000, which automatically positions the turntable and gantry to the required locations as well as controlling a vector network analyzer that make the RF measurements at specified frequencies. The data is automatically collected, processed and displayed.

Illustration of GPS Antenna Pattern Measurements
An example of the radiation pattern performance for a GPS antenna mounted on a vehicle’s roof at a single frequency is presented in Figures 2-4. The measurement setup collected data spatially over the hemisphere from 0°to 80°in zenith angle in 10°increments and from 0°to 359°in azimuth angle in 1°increments resulting in 3240 spatial points. The spatial data were collected at 3 distinct frequencies. Figure 2 shows the three-dimensional RHCP gain pattern for the roofmounted GPS antenna. The peak RHCP gain is maximum at zenith (i.e. ?=0°) and minimum and the antenna horizon (i.e. ?=80°).

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Range Performance Issues:
Many factors impact the accuracy and repeatability performance of antenna range. A few of these issues are briefly mentioned. Transmit antenna characteristics such as the half-power beamwidth and the polarization purity are important traits that will impact range performance when out of tolerance. The short term stability of the radio frequency power link is important as typical gain calculations are typically specified relative to the absolute gain of reference antenna such as a dipole or horn antenna. The measurement instrumentation that transmits and receives the radio frequency signals should possess adequate dynamic range so that the received power levels have adequate margin above the noise floor. The purity of the radio frequency environment for outdoor antenna ranges is important as interference increases the noise level in the gain calculation measurement. Also, the antenna range operating procedures for aligning test antennas and post-processing the collected data can impact range performance if done with little attention to detail.

1 Mazen Alsliety and Daniel N. Aloi, “A Study of Ground-Plane-Level and Vehicle-level Radiation Patterns of GPS Antenna in Telematics Applications,” I.E.E.E. Antennas and Wireless Propagation Letters, Volume 6, 2007, Page(s): 130-133.
2 J. E. Hansen, editor, “Spherical Near-Field Antenna Measurements,” Peter Peregrinus, 1988.