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Abu Maria.
Antenna Theory Regular

Joined: 22 Jan 2011
Posts: 39

 Posted: Thu Mar 17, 2011 9:48 am    Post subject: reactive near field 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
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Thu Mar 17, 2011 9:31 pm    Post subject: 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.
Antenna Theory Regular

Joined: 22 Jan 2011
Posts: 39

 Posted: Mon Mar 21, 2011 2:29 pm    Post subject: 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
Antenna Wizard

Joined: 08 Apr 2009
Posts: 161

 Posted: Tue Mar 22, 2011 5:20 pm    Post subject: 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
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Sun Mar 27, 2011 2:40 am    Post subject: 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.
Antenna Theory Regular

Joined: 22 Jan 2011
Posts: 39

 Posted: Thu Apr 07, 2011 10:10 am    Post subject: 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
Antenna-Theory.com Newbie

Joined: 31 May 2011
Posts: 4

 Posted: Tue May 31, 2011 11:35 am    Post subject: 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
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

shabby
Antenna-Theory.com Newbie

Joined: 31 May 2011
Posts: 4

 Posted: Wed Jun 01, 2011 6:12 am    Post subject: Oh thanks a lot
Abu Maria.
Antenna Theory Regular

Joined: 22 Jan 2011
Posts: 39

 Posted: Thu Jun 30, 2011 2:43 pm    Post subject: THANK YOU
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