When testing wireless products, we often rely on wireless testing methods such as OTA (Over-The-Air) or radiated spurs (RSE). From an accuracy standpoint, these measurements are typically conducted in far-field conditions. However, the question remains: what exactly defines a "near field" and a "far field"? There isn't a universally accepted standard that clearly separates the two, which can lead to confusion in practical applications.
Recently, a colleague from the testing industry asked me if it's possible to measure the radiation spurs of 2.4G products in a 3-meter EMC chamber. From an empirical perspective, yes, it is entirely feasible to perform such measurements in a 3M darkroom. But the deeper question is whether the test setup actually meets the far-field requirements for 2.4G devices.
Looking into the standards, like EN300328, which is commonly used for 2.4G RF products, we find that the standard does define far-field conditions. Specifically, it outlines two criteria that must be met, with the larger of the two being the key requirement.
The first definition states that the far field condition should be based on the wavelength λ of the signal. The second definition requires that the distance between the device under test and the measuring antenna must be significantly greater than D²/λ, where D is the geometric diameter of the antenna.
For a 2.4GHz product, the wavelength λ is approximately 0.125 meters. Using this value, we can calculate the required distance to ensure compliance with the far-field condition. In a 3-meter EMC chamber, the distance between the device and the antenna is already 3 meters, which is more than sufficient for most small antennas.
To further validate this, we need to understand what D represents. D is the physical size of the antenna of the device being tested. If the antenna is much smaller than 0.61 meters, then the 3-meter chamber satisfies the far-field condition defined in EN300328.
By applying both definitions, we can conclude that for most 2.4GHz wireless devices with small antennas, a 3-meter EMC chamber provides suitable far-field conditions for accurate measurement of radiated spurs.
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