Measure far-field patterns indoors
Martin Rowe, Senor Technical Editor -- Test & Measurement World, 5/1/2004
Microwave antennas used for communications systems such as wireless local area networks. The antennas operate at frequencies up to 60 GHz at distances of 2 km to 3 km.
THE CHALLENGEFind a way to test microwave antennas indoors. Testing microwave antennas outdoors is difficult and often impractical because engineers must mount the antennas on buildings, and they must get FCC permission to use the test frequencies. Testing indoors eliminates both constraints.
THE TOOLS- Agilent Technologies vector network analyzer, 50 MHz to 40 GHz; microwave sweep generator, 10 MHz to 40 GHz; and two microwave amplifiers, 20 GHz to 50 GHz; www.tm.agilent.com
- MM Communications two 60-GHz mixers. www.mmcomm-inc.com.
Although antenna patterns behave differently in the near field and the far field, there is a correlation between both distances. Engineers at REMEC (Milpitas, CA) take advantage of the correlation to test microwave antennas in an anechoic chamber rather than outdoors.
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Figure 1. A probe antenna transmits a 60-GHz signal to the antenna under test (AUT), where the VNA measures near-field amplitude and phase. |
The test system (Figure 1) generates a carrier signal at frequencies up to 60 GHz by mixing a 35-GHz signal from a sweep generator (LO) with a 25-GHz IF carrier from a vector-network analyzer (VNA). The antenna under test (AUT) transmits a 60-GHz signal that another antenna, called a probe, receives.
Small absorbent cones surround the probe and prevent reflected signals from reaching it. Thus, the probe receives signals directly from the AUT only. A second mixer reduces the received 60-GHz signal's frequency to 25 GHz, and the VNA uses S-parameters to measure the received signal's amplitude and phase.
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Figure 2. A mechanical stand rotates the AUT in 2° increments and changes its vertical position by 3 m. |
Engineers measure the AUT's output amplitude and phase at five frequencies. Each frequency measurement takes place at 108,000 locations. To attain the antenna positions, engineers use a cylindrical scanner (Figure 2) that produces the test pattern. A motorized mechanical arm rotates the antenna 360° in 2° increments. Another motor changes the AUT's vertical position in 0.5-cm increments over a 3-m range. The distance from the transmitting probe to the AUT ranges from 12 cm to 60 cm depending on the antenna.
A computer controls the sweep generator and VNA through an IEEE 488 bus. After downloading the more than 500,000 data points from the VNA, the computer applies a correlation algorithm.
RESULTSJohn Sanford, VP of technology at REMEC, notes "The test setup lets us make reliable near-field measurements and correlate them to an antenna's far-field performance." The cylindrical pattern lets engineers test the AUT with complete angular coverage of the AUT's field. With this test configuration, microwave engineers eliminate the effort and expense of testing antennas mounted on buildings.
