# 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. 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. 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. Oct 14 '15 at 13:47
• That's a lot of questions for one question. Oct 14 '15 at 14:31
• If it's much longer than 6cm then it's probably a colinear, not a dipole. Colinear is typically λ/4*N, N>3, where λ for 2.4 is ~6.5cm Sep 5 '20 at 12:11

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.

• Can you please share info on why dipole is more efficient than a coil? Oct 17 '15 at 19:52
• 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/… Oct 18 '15 at 3:09
• Thanks Daniel. Can I still use a dipole antenna in reactive field or it has to be a coil becuase it's reactive field? Oct 18 '15 at 16:35
• 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. Oct 18 '15 at 19:22
• NanoVNA goes to 3GHz, and commercial analyzers are available well beyond 40GHz, so don't say you can't get an analyzer that hgh... but I suppose there wasn't one in 2015. Sep 5 '20 at 12:13

This problem is very easy to model with any MOM tool, like EZNEC. E and H fields can be calculated using Near Field grid. Even better, exact power transfer from Tx to Rx antenna can be modeled. Modeling should include both Tx and Rx antenna, including Rx antenna termination resistance. Ratio of transmitted power and dissipation on Rx termination resistor becomes your coupling.

• Hello Tino, and welcome to ham.stackexchange.com! Sep 8 '20 at 17:08

In my situation, do I have to use the second formula to find near-field reactive region boundary?

The choice between the two equations you provide is determined by if the antenna is "electromagnetically short", so it isn't about if it is "very short" from a human's perspective of short, but rather how its length compares to the wavelength it emits. More specifically the equation $$n_n = \lambda$$, where $$n_n$$ is the radius of the near field, is used anytime the length of the antenna is less than half of the wavelength of the frequency it is transmitting. Note that the far-field is considered to be $$n_f = 2\lambda$$ where $$n_f$$ is the distance from the antenna where the far-field beings, so there is some area between these two where you are neither in the near-field or the far-field and instead expiernce effects of both.

2.45 GHz has a wavelength of 12cm. Since your antenna is 14 cm in length it is not "electromagnetically short" and as such would not use the above equation and instead would use what is called the Fraunhofer Distance which is governed by the following equation $$n_n = \frac{2D^2}{\lambda}$$.

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?

Coils are basically space-efficient for interacting with the near-field, specifically the magnetic field. They wont be very good at transmitting or receiving far-field, but they will be optimal at interacting with near-field. A capacitor design would be similarly true for the electric field component of the near-field. So a dipole would be less efficient for the space it took up for either meanwhile a inductor or capacitor configuration or a combination of the two would be your best bet if your only interest is harvesting in the near-field. the more the better. You can actually think of a faraday cage as the capacitor version of this if it were to encase the antenna completely. then the usual grounding cable you would attach to it would instead be your power line where you siphon off the power. Since it would completely envelop the antenna it would be pretty effective too. But a series of coils would also be effective.

If I can use a dipole antenna within the reactive region for harvesting, how would I find its impedance?

You can use it, and it will harvest energy, but you'll just waste a lot of space and it wont be nearly as efficient. You wont even be able to harvest all the flux that passes through it as it will re-radiate any energy it harvests to a certain extent. So a dipole really isnt your best way to harvest at all. But if you do use it just make sure its an antenna tuned to the frequency of the transmission for best results. The exact impedance isnt really the important part in the dipole case. The impedance will matter, however, if your using a coil. In that case just calculate the cut off frequency of the coil and make sure your coil is ideally about 10x bigger than that. If you want to harvest even more energy use additional coils rather than a single bigger coil.

All the above equations and a bit of detail and references on them can be found on wikipedia here: https://en.wikipedia.org/wiki/Near_and_far_field#Regions_according_to_electromagnetic_length

• Hello Jeffrey and welcome to ham.stackexchange.com! Nice answer! Sep 8 '20 at 17:14
• @rclocher3 Thanks, good to be here. Sep 8 '20 at 17:53