September 15, 2016
Engineers traveling down the path to 5G face plenty of roadblocks. To avoid delays in the race to bring 5G products to market, there are a number of design factors to consider, such as antenna measurements. Effectively conducting antenna measurements on next-generation designs requires engineers to rethink their approach to instrument selection and techniques used. For example, selecting a vector network analyzer (VNA) capable of conducting Over-the-Air (OTA) near-field measurements (NFM) will provide greater confidence in 5G antenna designs.
First, however, it is best to explain how 5G will affect antenna design. Current mobile terminals have several built-in antennas supporting various wireless services. The rollout of 5G will probably result in further increases in the antenna count. Implementing a measurement connector for each antenna would cause problems with mobile-terminal size and contradict cost reduction trends. Also, 5G base stations will use microwave and millimeter-wave (mm-wave) Massive MIMO technologies. The end result is that the increasing number of antenna connectors makes providing a measurement connector for each antenna a practical impossibility, so OTA measurements become a necessity.
The basic OTA measurement method is the 3D integration technique using a radio anechoic chamber. In this approach, the Equipment Under Test (EUT) surroundings are measured as a spheroid form. This method has problems with needing a radio anechoic chamber and large-scale measuring equipment. Moreover, since it uses far-field measurement (FFM), the mm-wave band attenuation due to free-space path loss is considerable, causing large measurement errors due to smaller measurement dynamic range.
Benefits of NFM
Table 1 shows the characteristics of NFM compared to FFM. Since NFM is a close range approach, it does not require a radio anechoic chamber or other large scale facilities. The mm-wave measuring instruments are compact and the radiation pattern can be measured using a simple radio anechoic box in a room, which eliminates the high cost and long configuration time associated with a measurement system using a large radio anechoic chamber.
Table 1
Accurate measurement results are obtained because the method measures a region where the free space loss is small. In addition, NFM captures the entire 3D radiation pattern immediately in front of the AUT, whereas FFM requires many measurements to capture the horizontal (H) and vertical (E) planes, thereby capturing only a 2D radiation pattern. Capturing the 3D radiation pattern using FFM requires a complex measurement setup and a longer measurement time. Additionally, NFM captures the amplitude and phase distribution near the antenna. If the radiation pattern cannot be captured due to the antenna design, the designer can use the captured amplitude and phase distribution to diagnose the cause. This is particularly beneficial when measuring a phased-array antenna, such as a Massive MIMO antenna.
Antenna Test Example
To confirm the appropriateness of NFM measurements, we compared the radiation pattern of the main polarization wave measured by NFM and FFM. A standard WR-15 horn antenna was used as the AUT, and Table 2 shows the NFM measurement parameters. FFM was performed using a radio anechoic chamber with six radio-wave absorbent surfaces.
Table 2
Some VNA configurations make it difficult to conduct antenna measurements in the mm-wave. Measurement accuracy is also reduced at higher frequencies with these systems. Both are corrected by the antenna measurement setup shown in Figure 1, which uses tethered measurement modules to eliminate cable issues that cause the aforementioned problems. Additionally, Anritsu developed the first miniature commercial NLTL-based reflectometers that can be used to extend the frequency range of a microwave VNA to 145 GHz.
Figure 1
In addition to their miniature size, these reflectometers provide short/long-term thermal stability due to the vanishing thermal gradient across the modules, high amplitude and phase stability, and raw directivity. Most importantly, placing the sampling directional bridge closest to the AUT/DUT provides long-term amplitude and phase stability. It is these features in particular that lend themselves well to antenna measurements whether they are performed in a near field, far field, or compact range scenario.
VNAs such as the Anritsu ShockLine™ support E-band measurements and can conduct OTA measurements in microwave and mm-wave bands using the NFM method. In addition, the conduits used to reduce the cable complexity provide a framework for extending the length of the cables for far-field antenna measurements.
As demonstrated, the NFM method reduces free-space losses to support measurements with the sensitivity necessary for 5G antenna designs. The system outlined in this post has shown to be suitable for OTA measurements in the microwave and mm-wave bands. To learn more about these solutions and OTA measurements on antennas, download a white paper entitled Overview of Technologies for Millimeter-Wave OTA Measurement.
