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?
  • $\begingroup$ 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. $\endgroup$ – tomnexus Oct 14 '15 at 6:12
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    $\begingroup$ 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. $\endgroup$ – tomnexus Oct 14 '15 at 6:15
  • $\begingroup$ Thanks Tomnexus. The transmitter is a 5 dbi antenna. It makes sense that the length can be longer than 6 cm. $\endgroup$ – Shiva Mudide Oct 14 '15 at 13:47
  • $\begingroup$ That's a lot of questions for one question. $\endgroup$ – Phil Frost - W8II Oct 14 '15 at 14:31

I would say the near field would be no more than about 28cm max.
You could use a dipole or a coil, but the dipole would be more efficient.
If you wanted to find the impedance of a dipole in the reactance region, go for EZNEC. No way you'll find an analyzer in tat high a frequency.

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  • $\begingroup$ Can you please share info on why dipole is more efficient than a coil? $\endgroup$ – Shiva Mudide Oct 17 '15 at 19:52
  • $\begingroup$ Coils have much more loss than plain wire. Otherwise, you could pack a 160M antenna into a coffee mug and it would work just as well. It just doesn't follow. See sayedsaad.com/fundmental/… $\endgroup$ – Daniel Oct 18 '15 at 3:09
  • $\begingroup$ Thanks Daniel. Can I still use a dipole antenna in reactive field or it has to be a coil becuase it's reactive field? $\endgroup$ – Shiva Mudide Oct 18 '15 at 16:35
  • $\begingroup$ You can use it in the reactive field, but just make sure it's impedance matches pretty close. The formula for impedance from complex impedance is this: For Z = RΩ ± jXΩ, Z = √(R^2 + X^2) where R is the radiation resistance, and X is the reactance (+ for inductance, - for capacitance), and Z is the final impedance. Keep in mind, the capacitive reactance can be cancelled out with inductive reactance of the same absolute value. $\endgroup$ – Daniel Oct 18 '15 at 19:22

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