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From: Mark on 14 Jul 2010 10:48

>
> Find the book "Antennas" by John D Kraus - he covers all of this in
> great detail.
>
> Clay- Hide quoted text -
>
> - Show quoted text -

also look at the electrodynamic applets here:

http://www.falstad.com/mathphysics.html

Mark
From: Clay on 14 Jul 2010 15:04
On Jul 13, 4:06 pm, "fisico32" <marcoscipioni1(a)n_o_s_p_a_m.gmail.com>
wrote:
> Hello Forum,
>
> near any radiating source like an antenna  there is a region called the
> reactive field: the E and H field are 90 degrees out of phase. The reactive
> field is region where there is no radiated power......
>
> It acts as a storage power. During one cycle power is given and taken
> back...
> I am having a hard time understanding what that means.
> What does it means that energy is given and taken back? To what is it
> given?
> And why does it give it back?
>
> It is said that if we are in the reactive field we can absorb energy and
> also affect the behavior of the source. In the radiation field we can only
> absorb energy.
>
> what if the angle between E and H is not zero but either 90? What does that
> mean?
>
> Further away from an antenna the power becomes radiated and E and H are not
> in phase...
>
> thanks
> fisico32

Acually far away from the antenna, E and H are in phase. Plane waves
(the extreme far field case) have both E and H in phase! In the near
field the energy is stored in space and it transistions back and forth
between being electric and magnetic. Think about a resonant tank
circuit where the energy moves back and forth between the inductor and
the capacitor.

IHTH,
Clay

From: Jerry Avins on 14 Jul 2010 15:17
On 7/14/2010 3:04 PM, Clay wrote:

> ... Think about a resonant tank
> circuit where the energy moves back and forth between the inductor and
> the capacitor.

I suspect that if he knew what a tank circuit is, he wouldn't have
needed to ask the question. He certainly wouldn't have asked it the way
he did. "What is this modern generation coming to?"*

Jerry
_________________________________
* I heard that a lot 70 years ago.
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
From: fisico32 on 14 Jul 2010 15:46
>On 7/14/2010 3:04 PM, Clay wrote:
>
>> ... Think about a resonant tank
>> circuit where the energy moves back and forth between the inductor and
>> the capacitor.
>
>I suspect that if he knew what a tank circuit is, he wouldn't have
>needed to ask the question. He certainly wouldn't have asked it the way
>he did. "What is this modern generation coming to?"*
>
>Jerry
>_________________________________
>* I heard that a lot 70 years ago.
>--
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������
>

Hi, I know what a tank circuit is: the L and C of the circuit cyclically
exchange energy...but it seems strange that this energy exchange happens in
free space and as we move further away from the antenna the energy exchange
dissapears and both E and B fields have an equal amount of energy at any
instant of time....The way and reason that happens is still not clear to
me...
From: Jerry Avins on 14 Jul 2010 16:25
On 7/14/2010 3:46 PM, fisico32 wrote:
>> On 7/14/2010 3:04 PM, Clay wrote:
>>
>>> ... Think about a resonant tank
>>> circuit where the energy moves back and forth between the inductor and
>>> the capacitor.
>>
>> I suspect that if he knew what a tank circuit is, he wouldn't have
>> needed to ask the question. He certainly wouldn't have asked it the way
>> he did. "What is this modern generation coming to?"*
>>
>> Jerry
>> _________________________________
>> * I heard that a lot 70 years ago.
>> --
>> Engineering is the art of making what you want from things you can get.
>> �����������������������������������������������������������������������
>>
>
> Hi, I know what a tank circuit is: the L and C of the circuit cyclically
> exchange energy...but it seems strange that this energy exchange happens in
> free space and as we move further away from the antenna the energy exchange
> dissapears and both E and B fields have an equal amount of energy at any
> instant of time....The way and reason that happens is still not clear to
> me...

That's the wrong interpretation, as I wrote before. If the E and H
fields were to become zero at the same time, what would regenerate them?
Constants aside, H = dE/dt and E = dH/dt, where the differentials are
partials with respect to time.

Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
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