Table of Contents

OTA signal stability

Standing waves create signal fluctuation vs. time

Without proper absorption in a metal enclosure, signal levels can fluctuate as a result of reflections that add together creating standing waves. In-phase additions result in signal peaks. Out-of-phase additions result in nulls.

In some competing RF enclosures our engineers measured power gradients as high as 10 dB when changing the DUT position by less than an inch. Such extreme power gradients are common in RF enclosures with inadequate absorption material and can significantly compromise the integrity and repeatability of measurements.

For controlled test conditions, it is necessary to create anechoic environment that ensures signal level stability.

Design for signal stability – absorptive walls

Signal reflections from the metal walls of the octoBox are absorbed by specialized absorptive foam. This eliminates standing waves and the fluctuations they cause in the signal, making the OTA signal in the box steady and measurements repeatable.

The job of the absorptive foam is to match the low impedance of the metal walls of the enclosure to the high impedance of air, thereby minimizing reflections due to impedance discontinuity.

To achieve proper absorption the foam needs several layers with monotonically increasing impedance – lowest towards the metal wall of the box; highest towards the air.

Design for signal stability – positioning the DUT

Extremely important for measurement accuracy and repeatability is to consider the 3-dimensional aspect of the antenna field. OTA measurements could be presented as follows:

3-dimensional plot
Integrated over a sphere
Performed through a stable lobe in the antenna field (e.g. for fast production testing)

octoScope has developed a patent pending process of stabilizing the 3-dimensional measurements for fast and accurate production testing.

Please inquire about our consulting services to customize the DUT shelf for stability of measurements.

octoBox II, which will be available in Q4 of 2011, will offer automated 3-dimensional antenna field measurements.