Electromagnetic radiation (EMR) threshold

[Homestar1]

What is the threshold energy (or frequency) required for an electromagnetic field to transition from a near field to become self propagating (EMR), far field? (If I'm using the right definitions to ask the question correctly). Is this constant or are there other details needed to calculate this?

 

[Drakkith]

There is no threshold. Given some distance from the source, some portion of the field will be near-field and some will be far-field, with the near-field falling off extremely rapidly as distance increases. Once the distance is more than a few wavelengths, there is virtually no near-field left.

 

[Homestar1]

Ah, that makes sense! Thanks so much!