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. It >> 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 t= he >>> 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 bandwidths >> 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 vi= a >>> the near-field with certain limitations such as the bandwidth. >>> >>> Please consider the following phenomena ; >>> >>> You basically transmit some electromagnetic wave that has a particular >>> amplitude, wavelength and the phase. When you get very close to the >>> receiver, say less than a wavelength, it is not possible to transfer th= e >>> 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 wou= ld >>> work when it's very close to the receiver but what makes it tick is the >>> 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/coil >>>>> design). For higher frequencies, distance between coils will be very >>>> 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 far >>>>> 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 can >>> 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 .