My wireless speaker mic is MI, not RF, and it's used in my communication system. Joe M. Sean Breheny wrote: > Hi Yigit, >=20 > 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. >=20 > Sean >=20 >=20 >=20 > On Sun, Mar 31, 2013 at 12:23 PM, Yigit Turgut wrote= : >=20 >> 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 sma= ll >> thus transfer will not be practically wireless. It is possible to decrea= se >> 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 magneti= c >> 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 r= aw >> power transmission - for example you can not design a communication syst= em >> 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 inducti= on >> 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 drivi= ng >>> 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 significan= t >>> 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 coupl= es >>> 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 n= o >>>> antenna but a built-in coil instead. The transponders do not produce a= n >>>> electromagnetic (radio) wave but only a magnetic wave. The difference >>>> between an electromagnetic (radio) wave and a magnetic wave is that th= e >>>> 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 >> >> >> ------------------------------------------------------------------------ >> >> >> No virus found in this incoming message. >> Checked by AVG - www.avg.com=20 >> Version: 9.0.932 / Virus Database: 2641.1.1/5717 - Release Date: 03/31/1= 3 13:34:00 >> --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .