For the ones that were designed in 50s, yes. I am talking about the main phenomena here, for digital comm and modulation it's different as I stated. On Mon, Apr 1, 2013 at 4:47 AM, Isaac Marino Bavaresco < isaacbavaresco@yahoo.com.br> wrote: > So you are saying that the receiver must farther away than 300m for a > carrier frequency of 1MHz? > > > Em 31/3/2013 22:13, Yigit Turgut escreveu: > > Distance between the transmitter and receiver should be greater than at > > least 1 wavelength. > > > > > > On Mon, Apr 1, 2013 at 4:03 AM, David VanHorn > wrote: > > > >> Magnetic loop audio systems have been around since the 50s at least. I= t > >> doesn't get more near field than that. > >> On Mar 31, 2013 6:40 PM, "Yigit Turgut" wrote: > >> > >>> Hi Sean, > >>> > >>> > >>> > >>> I think you are referring to passive RFID tags. They are powered from > the > >>> reader side via the near-field and the response is read from the card > >> back. > >>> Considering todays bandwidth, lets take the good old 802.11,it's not > >>> possible to satisfy the modulation ratio required by higher bandwidth= s > >> with > >>> the near-field. It can be done but it's just not suitable for it. > >>> > >>> I think my statement was mistaken. Digital communication is possible > via > >>> the near-field with certain limitations such as the bandwidth. > >>> > >>> Please consider the following phenomena ; > >>> > >>> You basically transmit some electromagnetic wave that has a particula= r > >>> amplitude, wavelength and the phase. When you get very close to the > >>> receiver, say less than a wavelength, it is not possible to transfer > the > >>> complete waveform already. That's what I meant with my expression, in > >>> previous years I performed computational electromagnetics simulations > >> and I > >>> know that the near-field is very wild and unpredictable unlike the > >>> far-field. > >>> > >>> Joe, > >>> > >>> Your wireless speaker mic is probably using the far-field.Surely it > would > >>> work when it's very close to the receiver but what makes it tick is t= he > >>> far-field if it has an analog transmitter/receiver. > >>> > >>> > >>> On Sun, Mar 31, 2013 at 10:36 PM, Sean Breheny > wrote: > >>> > >>>> Hi Yigit, > >>>> > >>>> What do you mean by "you can not design a communication system > >>>> that is based on the near-field"? There are indeed many near-field > >>>> communication systems, such as RFID. > >>>> > >>>> Sean > >>>> > >>>> > >>>> > >>>> On Sun, Mar 31, 2013 at 12:23 PM, Yigit Turgut > >>> wrote: > >>>>> Geometrical radius/size of the near field is usually in orders of a > >> few > >>>>> wavelength. Thus it depends on the frequency of the time-varying > >>> current > >>>>> source that feeds the antenna/coil (assuming appropriate antenna/co= il > >>>>> design). For higher frequencies, distance between coils will be ver= y > >>>> small > >>>>> thus transfer will not be practically wireless. It is possible to > >>>> decrease > >>>>> the frequency, for the same distance the near-field effect will be > >> much > >>>>> higher. > >>>>> > >>>>> That's why projects such as WiTricity are made to work in the MHz > >>> region > >>>> of > >>>>> the spectrum. > >>>>> > >>>>> The term magnetic induction comes from the fact that oscillating > >>> magnetic > >>>>> field components are dominant in the near-field when compared the f= ar > >>>>> field. One thing to consider is that evaluating the near-field > >> behavior > >>>> is > >>>>> not that straightforward like the far-field, thus it's an > >> uncontrolled > >>>> raw > >>>>> power transmission - for example you can not design a communication > >>>> system > >>>>> that is based on the near-field. Please note that it's possible to > >>> power > >>>>> wirelessly via the far-field but efficiency will be much lower. > >>>>> > >>>>> A good example/application for wireless energy transfer would be > >>>> induction > >>>>> cooking, also using the near-field effects. > >>>>> > >>>>> On Sun, Mar 31, 2013 at 5:16 PM, Sean Breheny > >>> wrote: > >>>>>> Hi David, > >>>>>> > >>>>>> As your message hints that you suspect, any structure that > >> generates > >>>>>> changing electric and magnetic fields IS an antenna to some extent= .. > >>>>>> > >>>>>> However, you can design structures which work better as an antenna > >>> and > >>>>>> those which do not radiate as much power for a given input current= .. > >>>>>> > >>>>>> If you make a coil which is much smaller than 1 wavelength at the > >>>> driving > >>>>>> frequency, it will not be a very efficient antenna. In other words= , > >>> it > >>>>> will > >>>>>> not "launch" much of an EM wave. However, depending on the > >>> construction > >>>>> and > >>>>>> the driving current, there may still be a very strong magnetic > >> field > >>> at > >>>>>> distances which are very close to the coil. This is also known as > >> the > >>>>> "near > >>>>>> field" of an antenna. > >>>>>> > >>>>>> If you place another coil close to the first, you can couple > >>>> significant > >>>>>> power between them through this near field magnetic coupling > >> without > >>>>>> transmitting much of a "far field" EM wave. > >>>>>> > >>>>>> You can even electrostatically-shield the coil to enhance this > >>> effect - > >>>>>> making it launch even less of an EM wave by limiting the amount of > >> E > >>>>> field > >>>>>> produced by the coil. > >>>>>> > >>>>>> There is no magic here - any time-varying magnetic field will also > >>>>> produce > >>>>>> some E field and vice-versa, thereby creating a propagating EM > >> wave. > >>>>>> However, depending on the geometry of the fields near the antenna, > >>> the > >>>>>> ratio of the EM wave strength to the local near field intensity ca= n > >>> be > >>>>>> varied. > >>>>>> > >>>>>> This is like a transformer - you can have an RF transformer which > >>>> couples > >>>>>> effectively between the primary and secondary but which does not > >>> create > >>>>>> *much* of a propagating EM wave. > >>>>>> > >>>>>> Sean > >>>>>> > >>>>>> > >>>>>> > >>>>>> On Sun, Mar 31, 2013 at 10:10 AM, David wrote: > >>>>>> > >>>>>>> Could anybody explain the difference between magnetic induction > >> and > >>>> RF? > >>>>>>> Obviously there is inductive charging with two coupled coils, > >> used > >>> as > >>>>>>> for charging small devices. However my confusion comes from this > >>>> part > >>>>>>> of a manual for transponder based timing: > >>>>>>> > >>>>>>> "Since the AMB transponders operate on magnetic induction, they > >>> have > >>>> no > >>>>>>> antenna but a built-in coil instead. The transponders do not > >>> produce > >>>> an > >>>>>>> electromagnetic (radio) wave but only a magnetic wave. The > >>> difference > >>>>>>> between an electromagnetic (radio) wave and a magnetic wave is > >> that > >>>> the > >>>>>>> electromagnetic wave travels by itself over great distances and > >> the > >>>>>>> magnetic wave does not." [1] > >>>>>>> > >>>>>>> I don't understand this difference, the transponders use a fixed > >>>>>>> frequency (near 4Mhz) and digital modulation. They probably use > >> a > >>>>>>> ferrite rod, but I am unsure how this is anything other than very > >>> low > >>>>>>> power RF. > >>>>>>> > >>>>>>> My interest comes as I'm trying to improve a simple transponder > >>> based > >>>>>>> system that I designed last year, and want to know if there is > >> some > >>>>>>> magic that I am missing. > >>>>>>> > >>>>>>> Thanks, > >>>>>>> > >>>>>>> David > >>>>>>> > >>>>>>> 1 - http://tinyurl.com/dypszke (Appendix G) > >>>>>>> -- > >>>>>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >>>>>>> View/change your membership options at > >>>>>>> http://mailman.mit.edu/mailman/listinfo/piclist > >>>>>>> > >>>>>> -- > >>>>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >>>>>> View/change your membership options at > >>>>>> http://mailman.mit.edu/mailman/listinfo/piclist > >>>>>> > >>>>> -- > >>>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >>>>> View/change your membership options at > >>>>> http://mailman.mit.edu/mailman/listinfo/piclist > >>>>> > >>>> -- > >>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >>>> View/change your membership options at > >>>> http://mailman.mit.edu/mailman/listinfo/piclist > >>>> > >>> -- > >>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >>> View/change your membership options at > >>> http://mailman.mit.edu/mailman/listinfo/piclist > >>> > >> -- > >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >> View/change your membership options at > >> http://mailman.mit.edu/mailman/listinfo/piclist > >> > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .