# How far the near-field reactive region extends from the transmitting antenna?

I am doing research on near-field harvesting. In my situation the transmitting antenna is a dipole antenna operating at 2.45 GHz and its length is 14 cm long. I am trying to find how far near-field reactive region extends from the antenna.

I noticed that there are two kinds of formulas – one for very short antennas (i.e. $\lambda/(2\pi)$) and other for longer antennas ($0.62 \sqrt{(D\cdot D\cdot D)/\lambda}$). Here are my questions:

1. In my situation, do I have to use the second formula to find near-field reactive region boundary?
2. If I have to harvest within the reactive field of the transmitting antenna, can I use a dipole antenna or do I have to use a coil?
3. If I can use a dipole antenna within the reactive region for harvesting, how would I find its impedance?
• Quick comment: There are equations for the near fields of an antenna, which will give you E and H for any position. These fields extend forever, but most of the components decay as 1/radius^2. What we call far fields are an approximation, valid not too close to the antenna, and as travelling waves the fields decay with 1/r. The two "regions" are where reactive or travelling fields are stronger. Google for the near fields of a dipole antenna. – tomnexus Oct 14 '15 at 6:12
• A dipole at 2.45 GHz is about 6 cm long. Is this really the length of the antenna, or is it the length of the plastic housing? The antenna will also have a coaxial cable, while it does also form part of the antenna, I'd suggest ignoring it for now and using the most basic dipole, for which there are manageable equations. – tomnexus Oct 14 '15 at 6:15
• Thanks Tomnexus. The transmitter is a 5 dbi antenna. It makes sense that the length can be longer than 6 cm. – Shiva Mudide Oct 14 '15 at 13:47
• That's a lot of questions for one question. – Phil Frost - W8II Oct 14 '15 at 14:31