The division of "near field" and "far field" of 2.4G wireless products

We generally use wireless test for wireless product characteristics such as OTA or radiated spurs (RSE). From the measurement accuracy, it is usually required to use the test under far field conditions. But in fact, there is not a particularly precise and accepted statement to divide what is "near field"? What is the "far field"?

Yesterday, a friend of the testing industry asked me if I could measure the radiation spurs of 2.4G products under the EMC standard 3M darkroom conditions. From an empirical point of view, of course, it is perfectly possible to complete the spurs of 2.4G products in the 3M darkroom. But if we go one step further, the question is actually to confirm the definition of the far field state of the 2.4G product.

According to this vertical detection (SKYLABS) also carefully look at the common RF standards of 2.4G products, taking EN300328 as an example, in the final appendix of the standard, it does have its definition of far-field conditions, as follows:

First of all, it is clearly stated in EN300328 that we should choose the "far field" condition in the radiation measurement of the measured object. In this standard, it defines the far-field conditions of two 2.4G wireless products. The two far-field conditions that need to be explained are based on which of the two conditions, whichever is greater.

Definition of far field: Select the wavelength λ of the measured object as the far field condition.

Far field definition 2: The distance between the measured object and the measuring antenna should be much larger than D*D/λ.

Then we know the frequency of the 2.4Ghz product, we can calculate its wavelength λ is equal to 0.125m. If you only look at the far field definition in EN300328, this far field condition is met in the standard 3 meter EMC chamber.

Then we calculate D*D/λ to see if it meets the definition of the far field. Before nesting this formula, we must first confirm what is "D", and the D inside is actually the geometric diameter of the antenna of the object under test. The figure below shows relatively clear.

In the standard 3M method anechoic chamber, the distance from the object to the test antenna is 3 meters, and the wavelength of the 2.4G product is 0.125 meters, so we can calculate that D can reach 0.61m, that is, as long as the object is measured. The antenna size is much smaller than 0.61m, so the measurement of spurious or antenna in the 3M method darkroom is required to meet the test distance (far field) in EN300328.

Based on the calculation of the above two far-field conditions, we can see that for the 2.4GHz wireless product, as long as its antenna size is less than 0.61m, it is in accordance with the definition of far-field test conditions in EN300328 under the condition of 3m method microwave darkroom. .

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