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What is the significance of the 3 lambda AUT/probe separation distance?

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What is the significance of the 3 lambda AUT/probe separation distance?

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A NF measurement needs to be taken at a point in space where the NF probe is sampling the AUT radiating field. If the probe comes too close to the AUT it can start to detect the AUT reactive NF (fields that are coupled to the AUT and that do NOT contribute to radiation – just AUT reactance). This is typically within 1 wavelength (lambda) of the antenna. In this region the probe is not able to detect radiating fields since the reactive fields are dominant. We therefore have to ensure that the probe is outside of this region and NSI have found that 3 lambda is a safe distance. There is no maximum distance and the NF theory works even if the probe is at infinity. However, maximizing the distance leads to power loss due to space attentuation. It is accetable to reduce the distance from 3 to 2 lambda if needed, but one does run the risk of the NF probe detecing the reactive near-field, so this should be avoided unless some experimental data is available to mitigate the risk.

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