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
reactive power....
Started by ●July 13, 2010
Reply by ●July 13, 20102010-07-13
On 7/13/2010 4:06 PM, fisico32 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...How solid is your grasp of AC circuits? Do you know what aero power factor means? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 13, 20102010-07-13
On 07/13/2010 01:06 PM, fisico32 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......Says who? This is probably one way of looking at the near field of an antenna, but not the only way. If it is, then the author is separating the effects of the reactive and radiated fields, and the reactive field is just a _component_ of the total field that does not participate in radiation. The reactive field wouldn't define an area in space, rather it would be just one part of the total field.> 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?It means that energy goes into the reactive field then comes back out, once each half-cycle of the RF.> To what is it given?To the antenna.> And why does it give it back?Because of the electrical behavior of the antenna.> 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?Do you mean temporally or spatially?> Further away from an antenna the power becomes radiated and E and H are not > in phase...Further away from the antenna the radiated field dominates, but for power to flow from the antenna the radiated field must go all the way to it. Close to the antenna the reactive field dominates, but that doesn't mean that you can't interact with the antenna from miles away -- that is, after all, how radar works. This whole thing sounds like an interesting way of looking at antenna fields, but it also sounds like it could confuse a student (e.g. you), and I'm just not sure that it isn't either all BS or a way to add useless complication. So -- cite your sources. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●July 13, 20102010-07-13
On 7/13/2010 4:26 PM, Tim Wescott wrote: ...> This whole thing sounds like an interesting way of looking at antenna > fields, but it also sounds like it could confuse a student (e.g. you), > and I'm just not sure that it isn't either all BS or a way to add > useless complication.... When a wave undergoes total internal reflection in a dielectric, there is what is called an evanescent wave just outside the dielectric. The evanescent wave decays exponentially with distance from the dielectric surface and normally bleeds no power from the wave being reflected. However, if a receiving element is placed close enough, some of the power will enter it, diminishing the power being reflected. The geometry of antennas makes the fields more difficult to visualize than the corresponding case with prisms that I just described, but the math is not much different. (The prism case is further simplified by total internal reflection suppressing the radiated wave completely. There is also -- in theory -- an evanescent wave with ordinary refraction, but I know no practical way to demonstrate its presence.) Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 13, 20102010-07-13
>On 07/13/2010 01:06 PM, fisico32 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. Thereactive>> field is region where there is no radiated power...... > >Says who? > >This is probably one way of looking at the near field of an antenna, but >not the only way. If it is, then the author is separating the effects >of the reactive and radiated fields, and the reactive field is just a >_component_ of the total field that does not participate in radiation. >The reactive field wouldn't define an area in space, rather it would be >just one part of the total field. > >> 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? > >It means that energy goes into the reactive field then comes back out, >once each half-cycle of the RF. > >> To what is it given? > >To the antenna. > >> And why does it give it back? > >Because of the electrical behavior of the antenna. > >> It is said that if we are in the reactive field we can absorb energyand>> also affect the behavior of the source. In the radiation field we canonly>> absorb energy. >> >> what if the angle between E and H is not zero but either 90? What doesthat>> mean? > >Do you mean temporally or spatially? > >> Further away from an antenna the power becomes radiated and E and H arenot>> in phase... > >Further away from the antenna the radiated field dominates, but for >power to flow from the antenna the radiated field must go all the way to >it. Close to the antenna the reactive field dominates, but that doesn't >mean that you can't interact with the antenna from miles away -- that >is, after all, how radar works. > >This whole thing sounds like an interesting way of looking at antenna >fields, but it also sounds like it could confuse a student (e.g. you), >and I'm just not sure that it isn't either all BS or a way to add >useless complication. > >So -- cite your sources. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >Do you need to implement control loops in software? >"Applied Control Theory for Embedded Systems" was written for you. >See details at http://www.wescottdesign.com/actfes/actfes.html >As far as the E and H fields: in the radiation part of the total field they are in phase in time, and always in quadrature spatially,in any region of space, close or far from the antenna. If we are in the region of space dominated by the radiation field anything we do there will not influence the functioning of the source antenna... In the regions where the reactive fields dominate it is said that the interaction with the reactive field will affect the antenna...(the book I read is Electromagnetics Explained by Ron Schmitt). "....The reactive field is called the storage field...... For a dipole antenna the storage field dominates in an region within a distance lambda/2pi from the antenna...." "....When an observer absorbs energy in the near field things are different: any time we try to measure or receive a signal in the near field, you will cause an effect on the source circuit. That is why it is called a "reactive field"...." "....A reactive field can store or transfer energy, depending on what other electrical or magnetic device are in the field...." "...so the reactive field interacts with devices that are present within it..." "...we can couple to the reactive field and remove energy stored in the field..." "...the wave impedance of the near field is a function of the source circuit, whereas in the far field the wave impedance is dependent only on the medium..." No, I don't know what the aero power factor is...
Reply by ●July 13, 20102010-07-13
On 7/13/2010 4:55 PM, fisico32 wrote:>> On 07/13/2010 01:06 PM, fisico32 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...... >> >> Says who? >> >> This is probably one way of looking at the near field of an antenna, but >> not the only way. If it is, then the author is separating the effects >> of the reactive and radiated fields, and the reactive field is just a >> _component_ of the total field that does not participate in radiation. >> The reactive field wouldn't define an area in space, rather it would be >> just one part of the total field. >> >>> 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? >> >> It means that energy goes into the reactive field then comes back out, >> once each half-cycle of the RF. >> >>> To what is it given? >> >> To the antenna. >> >>> And why does it give it back? >> >> Because of the electrical behavior of the antenna. >> >>> 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? >> >> Do you mean temporally or spatially? >> >>> Further away from an antenna the power becomes radiated and E and H are > not >>> in phase... >> >> Further away from the antenna the radiated field dominates, but for >> power to flow from the antenna the radiated field must go all the way to >> it. Close to the antenna the reactive field dominates, but that doesn't >> mean that you can't interact with the antenna from miles away -- that >> is, after all, how radar works. >> >> This whole thing sounds like an interesting way of looking at antenna >> fields, but it also sounds like it could confuse a student (e.g. you), >> and I'm just not sure that it isn't either all BS or a way to add >> useless complication. >> >> So -- cite your sources. >> >> -- >> >> Tim Wescott >> Wescott Design Services >> http://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" was written for you. >> See details at http://www.wescottdesign.com/actfes/actfes.html >> > > > As far as the E and H fields: in the radiation part of the total field they > are in phase in time, and always in quadrature spatially,in any region of > space, close or far from the antenna.If the E and H fields were in phase in time, there would be two instants in every cycle when both were zero. That can't work. The total energy remains constant as it shuttles back and forth between the two fields.> If we are in the region of space dominated by the radiation field anything > we do there will not influence the functioning of the source antenna... > In the regions where the reactive fields dominate it is said that the > interaction with the reactive field will affect the antenna...(the book I > read is Electromagnetics Explained by Ron Schmitt)."Reactive" refers here to to reactive power, just as in AC power circuits.> "....The reactive field is called the storage field...... For a dipole > antenna the storage field dominates in an region within a distance > lambda/2pi from the antenna...." > "....When an observer absorbs energy in the near field things are > different: any time we try to measure or receive a signal in the near > field, you will cause an effect on the source circuit. That is why it is > called a "reactive field"...." > > "....A reactive field can store or transfer energy, depending on what other > electrical or magnetic device are in the field...." > > "...so the reactive field interacts with devices that are present within > it..." > > "...we can couple to the reactive field and remove energy stored in the > field..." > > "...the wave impedance of the near field is a function of the source > circuit, whereas in the far field the wave impedance is dependent only on > the medium..." > > > No, I don't know what the aero power factor is...Please excuse the typo. I meant to write "zero power factor". Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 14, 20102010-07-14
Hi the same problem happen when there is an inductive load. In this case percentage of energy come back. For resolve this issue, you must apply a Condenser that compensate the shift between I and V. So for understand this phenomenon start to study Power Factor. Search on Google "Power Factor.pdf" Giuseppe>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. Thereactive>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 canonly>absorb energy. > >what if the angle between E and H is not zero but either 90? What doesthat>mean? > >Further away from an antenna the power becomes radiated and E and H arenot>in phase... > >thanks >fisico32 >
Reply by ●July 14, 20102010-07-14
On Jul 13, 4:06�pm, "fisico32" <marcoscipioni1@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 > fisico32Find the book "Antennas" by John D Kraus - he covers all of this in great detail. Clay
Reply by ●July 14, 20102010-07-14
> > 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
Reply by ●July 14, 20102010-07-14
On Jul 13, 4:06�pm, "fisico32" <marcoscipioni1@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 > fisico32Acually 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
