When testing wireless products, we often rely on wireless measurements to evaluate characteristics like OTA (Over-The-Air) performance or radiated spurs (RSE). From an accuracy standpoint, far-field conditions are typically required for such tests. However, the question remains: what exactly defines a "near field" and a "far field"? There's no universally accepted definition, which can lead to confusion in practice.
Recently, a colleague from the testing industry asked me if it was possible to measure the radiation spurs of a 2.4G product in a 3-meter EMC chamber. From an empirical perspective, yes, it is definitely feasible. But the deeper question is whether the test setup meets the actual far-field condition required by the standard.
To clarify this, I looked into the RF standards for 2.4G products, particularly EN300328. In its appendix, there is a clear definition of far-field conditions. Here’s what it states:
The standard clearly emphasizes that far-field conditions should be used when measuring radiation characteristics. It defines two criteria for far-field conditions, and whichever is greater should be followed.
First, the far-field condition is defined based on the wavelength λ of the signal. Second, the distance between the device under test and the antenna must be significantly larger 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 whether a 3-meter darkroom satisfies the far-field requirements. Let’s look at the second formula: D²/λ. Before plugging in numbers, we need to understand what D represents. D is the physical size of the antenna of the device being tested.
In a standard 3-meter anechoic chamber, the distance between the device and the antenna is 3 meters. With a wavelength of 0.125 meters, the calculation shows that D can be up to 0.61 meters. This means that as long as the antenna of the device is much smaller than 0.61 meters, the 3-meter chamber meets the far-field requirement according to EN300328.
Based on these calculations, for a 2.4GHz wireless product, as long as the antenna size is less than 0.61 meters, the test conducted in a 3-meter darkroom would indeed meet the far-field conditions outlined in the standard. This helps ensure accurate measurement of spurious emissions and other critical parameters.
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