How does a near field become a far field?

[nemesiswes]

Ok so I understand that an antenna whether it is a electromagnet or a piece of long wire has two effects associated with it. Those two effect are the Near-Field and the Far-Field. The Near-Field is the reactive component, it is responsible for things like inductors, transformers, Electromagnets causing a force on something etc, basically in like a transformer, any energy induced into the secondary causes a decrease in the magnetic flux because the secondary opposes the primarys flux thus decreasing the flux in the core which causes the primary to pull more power, it is reactive. The Far-field is the actual EMR (electromagnetic radiation) it is self reinforcing and any EMR absorbed by say an antenna does not cause a increase in power pull on the transmitter.

So just as the title says, How does the near field become the far-field? Like what actually is happening in the transition zone between the two?

 

[skeptic2]

I don't think the near field becomes the far field. I think they are two separate fields with the near field diminishing more rapidly. I once saw a paper that described how to measure the E field and the H field separately and the phase angle between them. There was a formula in which the phase angle between the two would tell you how far you are from the antenna. This of course is more useful at lower frequencies.

 

[Dale]

There is just Maxwells equations which govern any antenna. You can (in principle) solve Maxwells equations for a given antenna and get the total field. There will be some term with a 1/r^2 factor, this is arbitrarily called the far field. Then the near field is simply the difference between the total field and the far field.

 

[mfb]

To extend DaleSpams answer: You can evaluate the electromagnetic field in powers of 1/r.
If you calculate them, you will see that there is no 1/r-field, but there is a 1/r^2-field, a 1/r^3-term, 1/r^4 and so on. For distances much larger than the antenna size, just the 1/r^2-term is relevant, and this is called electromagnetic radiation.

You can get similar things in other systems, too: General Relativity tells us that the gravitational potential is not really 1/r. However, you can write is a combination of a 1/r-potential and additional corrections with 1/r^2, 1/r^3, ... which are extremely small for distances much larger than the Schwarzschild radius of an object.

 

[marcusl]

The near field region contains, as you mentioned, reactive fields. These don't have to act as transformers, but they have the characteristic that they store energy. Magnetic energy, for instance, is the volume integral of B^2. Electric and magnetic fields are intense only near the antenna conductors, so B^2 (and magnetic energy density) drop as you move away from the antenna. Where these become negligible, you have the far field.

 

[the_emi_guy]

I see the difference between near field vs. far field as one of causality.

Imagine two small spheres separated by 1cm and oriented vertically and connected with wires to a reversable voltage source.
With the top sphere positively charged we will have E field directed downward between the spheres. With the top sphere negative
we will have an upward E field. The E field between spheres will follow the change in charge polarity. In fact,
the E field is being *caused* by the orientation of charge. This E-field, of whatever its polarity, will expand
at the speed of light to fill all of space (decreasing in magnitude of course).
Now imagine that I am reversing the polarity every 10nanoseconds (100MHz) starting with no fields
and applying the first charge at t=10ns:

t=0: no fields
t=10ns: upward field between spheres, no field 10 feet away.
t=20ns: downward field between spheres, upward field 10 feet away, no field 20 feet away.
t=30ns: upward field between spheres, downward field 10 feet away, upward field 20 feet away, no field 30 feet away.
t=40ns: downward field between spheres, upward field 10 feet away, downward field 20 feet away, upward field 30 feet away, no field 40 feet away.
Etc.

At t=40ns the downward field between the spheres is being caused by the dipole charge.
However, the upward field that appears at 30 feet is not being caused by the dipole charge.
It is being caused by the propagating field that was 20 feet away at t=30ns arriving at the 30 foot mark 10ns later.

To further illustrate this break in causality consider that at t=20ns that that the charged spheres disappear:

t=0: no fields
t=10ns: upward field between spheres, no field 10 feet away.
t=20ns: *no* field between spheres, upward field 10 feet away, no field 20 feet away.
t=30ns: *no* field between spheres, *no* field 10 feet away, upward field 20 feet away, no field 30 feet away.
t=40ns: *no* field between spheres, *no* field 10 feet away, *no* field 20 feet away, upward field 30 feet away, no field 40 feet away.

The field that arrives at the 30 foot mark at t=40ns is not caused by the charged spheres, the spheres stopped existing at t=20ns.

The near field gives birth to the far fields, but in the far field the fields now have a life of their own.
Their existence and behavior no longer depends on the mother source.

By the way, this is true of all waves. Consider sloshing water around with the oars of a rowboat in the middle of a very calm lake.
The motion of the water in the immediate vicinity of the oars is being caused by the motion of the oars.
However, 5 minutes later, 1/4 mile away, water that was calm begins to move.
Why did it move? The cause of its motion was the influence of adjacent water just slightly closer to the center of the lake.

 

[Bobbywhy]

As the emi guy mentioned above, near-field and far-fields occur with all kinds of waves. For an excellent review of the RF kinds, with great diagrams, see this wiki article:
http://en.wikipedia.org/wiki/Near_and_far_field

For sound in air, see:
http://www.lmsintl.com/acoustic-beamforming

For sound in water, see:
http://www.npl.co.uk/mathematics-scientific-computing/modelling/sectors/underwater-acoustics

 

[marcusl]

emi_guy's description applies to traveling waves, as far as it goes, but contains an error: there is no "break in causality." Potentials and fields at a distance propagate at the speed of light from the source (the potentials are even called "retarded potentials".) The description also does not indicate the difference between near and far fields. The key point is that energy is stored in the near field but not in the far field.

 

[the_emi_guy]

emi_guy's description applies to traveling waves, as far as it goes, but contains an error: there is no "break in causality." Potentials and fields at a distance propagate at the speed of light from the source (the potentials are even called "retarded potentials".QUOTE]

Sorry, I should have known than using the term "causality" would invite philosophical objections. Perhaps I will be permitted to describe this in terms of "direct causality"?

The motion of my oar directly caused the motion of the water in front of it. The leaf bobbing on the water 1/4 mile away was directly caused by the propagating water wave arriving at its location.

Of course the propagating water wave was caused by me sloshing the water with my oars, but this is an indirect cause, once removed, right? And I can continue backward with this chain of indirect causes ad nauseam. The cause of me sloshing the oars was that I wanted to reorient my boat northward. The cause of my wanting to reorient my boat northward was that the ramp is located on the north end of the lake and wind had oriented my boat eastward. The cause of the ramp being located at the north end of the lake was that in 1962 that was the cheapest real estate. The cause of that piece of real estate being cheap in 1962 was...

Ultimately we would have to say that the cause of the leaf bobbing was the big-bang,
or get into discussions about determinism and free will etc right?

I think the OP just wanted an intuitive, practical, description of the difference between far and near fields.

The description also does not indicate the difference between near and far fields.

Let's hear your superior description.

The key point is that energy is stored in the near field but not in the far field.

Energy certainly is stored in the far field. Go outside on a sunny day and feel the heat of the sun's rays on your face.

 

[marcusl]

Sorry emi_guy, your view of E&M is somewhat simplistic in respects, and incorrect in others.

1) Look up retarded potentials if you'd like to learn about the causal propagation of fields.

2) Energy is not stored in the far field, it is carried away from the antenna by traveling waves. The sun is not an electrical antenna so the analogy is inexact, but in any case the warmth you feel is a measure of what is called power flux density (not energy). The Poynting vector allows one to calculate the outgoing power flux density in the far field of an antenna. By contrast, there is field energy that is stored in the near field zone that neither propagates nor carries away energy.

 

[the_emi_guy]

Sorry emi_guy, your view of E&M is somewhat simplistic in respects, and incorrect in others.

Of course it is simplistic, intentionally so.

Once again, near fields are directly caused by their source and, as such, interact directly with those sources. This interaction often involves transfer of energy back and forth between the near field and the source (this is what you are referring to as “storage”). In the far field, the field exists on its own, independent of the source. Its existence and behavior no longer depend on the source. It is no longer interacting with the source. Retarded potentials is a tool used to quantify the transition between the near field and the far field. Its details are beyond the scope of this thread wouldn’t you agree?

 

[Bobbywhy]

The OP, written by "nemesiswes" indicates a fairly studied knowledge of the differences between near- and far-fields. Also, his personal profile is available, along with his academic achievements. It is often useful to know this before attempting to respond to questions posed here.
Seems to me the emi guy is right on target.

Note: Optical emitters of electromagnetic radiation behave the same as radio frequency ones: their near fields extend only a few wavelengths (microns) from the source.