antenna-theory.com

[Abu Maria.]

hello
the space surrounding an antenna is usually subdivided into 3 regions
1/-reactive near-field
2/-radiating near field
3/-far field
my question
why we have a reactive near field?(physically)
there is the reflection ??

thank you

[bigSteve]

A capacitor or an inductor has fields associated with it, that are purely reactive. That is, they don't radiate at all in the ideal case.

Antennas are part capacitor, part inductor. For instance, you could view the two arms of the dipole antenna as a capacitor, with stored charge on either arm. Or you could view the length of dipole arm as an open-circuited inductance.

Either way you look at it, the fields near the antenna are dominated by the reactive fields that don't radiate but store energy. I don't think it has anything to do with reflection (from what?).

[Abu Maria.]

thank you bigsteve
why in near region we have domination for the reactive power , and in far we have domination for the real power ? if I put an antenna in near region , what will happen?


and reflexion ,when poynting is imaginary , then we have stationnary regime , not progressive , there is reflexion , or no
thnk you

[Schubert]

Mathematically, it is because the near fields die off quicker - that is, the E- and H-fields fall off as 1/R^2 or 1/R^3. The radiated fields die off as 1/R.

Consequently, as R->0, the near fields get bigger faster, hence, they "dominate".

If you put an antenna in the near field of another antenna, a couple things happen. The first is that the fields are disturbed, so that some of the reactive fields become power-carrying fields. The result is a coupling together, and energy is transferred from one antenna to the other (either from the receivers attached to the antennas or re-radiated).

Last statement you made isn't clear what you're talking about

[bigSteve]

And note that the mathematical models are just models. Nothing actually dies off as 1/R^3, or the fields would go to infinity at the antenna. It's a decent model for the non-extreme near field.

[Abu Maria.]

thank you for everyone
for reflection
eg:reflection on a conducting plane under normal incidence

when we calculate Poynting vector , this vector will be imaginary
there is a phase shift between the two vectors (E and H)

then

in our case , and in the region near , we have an imaginary vector poynting , there is an phase shift between H and E

What do you think? is it purlly mathematic , or there is signification physic ??

sorry and tank you

[shabby]

As per the answer provided :

If you put an antenna in the near field of another antenna, a couple things happen. The first is that the fields are disturbed, so that some of the reactive fields become power-carrying fields. The result is a coupling together, and energy is transferred from one antenna to the other

Transfer of energy from one antenna to another should involve radiation ! So how does this coupling in the near field region occur without any radiation and wat exactly is storage of energy by fields physically? All standard texts mention the above statement while describing near field behaviour

[bigSteve]

shabby wrote:
"Transfer of energy from one antenna to another should involve radiation!"

Why is that?

First, let's define radiation simply. The radiated fields from an antenna are the (E- and H-) fields that die off as 1/R from the antenna, where R is the distance from the antenna. The power then falls off like 1/R^2. Hence, for any sphere from the antenna, no matter how large R is, the power travelling out of the sphere is constant. This is because the fields for any distance R have power density of 1/R^2, and the sphere has radius 4*pi*R^2, so the R's cancel and the power is constant. Hence, radiation in the antenna sense means propagation: the power propagates infinitely far away.

The near fields don't possess this property. Near fields die off quicker; the power dies off as 1/R^3 or 1/R^4 or whatever. Hence, they don't deliver power away or propagate. In addition, the E- and H- fields don't act as plane waves; the E- and H- fields are much more independent than in the radiated wave case.

Now, antennas have both fields around them - near fields and far (radiated) fields. In addition, a big key point is that antennas are reciprocal. This means that if an antenna radiates a vertically-polarized wave in one direction, it will also receive a vertically-polarized wave from the same direction.

So, if an antenna produces near fields, then it can certainly capture near fields.

In radiation, the currents or voltages on an antenna (produced by the source or transmitter) add up in phase to transmit far-field radiation. In the same way, when the antenna receives energy, an electromagnetic wave induces currents and voltages on the antenna, which travel down to the receiver. Such is reciprocity.

Similarly, currents and voltages add up in odder ways to produce near fields. Hence, near fields (non-propagating waves) can induce current and voltage distributions on an antenna, which then induce current and voltages on the transmission line which are detected by the receiver.

The effect occurs in all of electromagnetics, not just antennas: if a current in a loop produces a magnetic field, then a magnetic field will induce a current in a loop. Reciprocity is the way nature likes it.

Finally, it takes energy to produce fields. Hence, if there are fields, it takes energy to produce them. If the fields don't radiate, the energy can oscillate between E- and H- fields and voltage and current on a structure. This is the stored energy. The antenna in this case is acting like an inductor or a capacitor, which rely on fields but don't radiate.

[shabby]

Oh thanks a lot

[Abu Maria.]

THANK YOU