ib technology 1 data sheet antenna_125.pdf 4 pages last revised 21/09/09 micro rwd 125khz antenna specification the micro rwd has been designed to interface to a s imple low q (10-20) antenna coil of around 700uh (micro henry) inductance operating at a carrier frequency of 125 khz. for maximum range and performance the following fac tors should be considered: 1) maximum range and coupling between transponder a nd rwd is based on the ratio of their antenna diameters. very approximately the rwd antenna loop diameter should be 2-3 times the diameter of the transponder coil. the basic method of communication is via magnetic flux linkage (like an air-cored transformer) so the more lines of flux that intersect the transponder c oil, the better the overall performance. for iso card transponders there is lit tle benefit in using an rwd antenna larger than 10cm diameter. circular antenna coils generally give a more uniform flux distribution. 2) the micro rwd is designed to give up to 400ma pu lse current with a peak voltage of up to 200v. this can give a 20cm read/write rang e with ht2 iso card transponders. lower burst current can be achieved b y inserting an additional series resistor in the antenna loop circuit (22r in series would typically reduce pulse current/voltage by about half using example antenna s). this is recommended in order to give the optimum signal-to-noise ratio and q value. 3) sample antenna supplied is for demonstration onl y. the characteristics of an antenna for emc/wt approval will vary according to shape of coil, type of wire used, style of winding, bobbin material, compaction of windings etc. 4) if the antenna is to be positioned remote to the micro rwd a screened twisted pair cable can be used to connect them. the screen shoul d be connected to the antenna gnd pin on the micro rwd. cable lengths up to 1.5 m etres have been tested and perform well with no significant system degradation . 5) the capacitance and inductive effect of antenna cable and physical positioning of the antenna should be taken into account when desig ning the antenna coil and if necessary the inductance of the coil should be adju sted to compensate. the 125khz tuned circuit is basically an rlc network with the c element fixed on the micro rwd board. tuning the antenna inductance (l) for op timum performance is quite simple, the positioned antenna system should be con nected to the micro rwd and power applied. an oscilloscope should be connected between the an 2 pin and the gnd pin. without a transponder in the field a pulsed 125khz sine wa ve will be seen with a peak voltage of up to +/-180v, with sma ll switching steps visible near the peaks and troughs of the sine wave.
ib technology 2 optimum tuning and performance is achieved when the se steps are seen exactly at the maximum and minimum points of the sine wave, th e peak voltage will also be at its maximum value. the antenna series resistor (r) should be set to limit the peak voltage to 110v maximum in order to give the best q value and optimum signal-to- noise ratio (for practical purposes use +/-100v peak voltage as limit) . 6) for optimum performance the antenna q should not exceed 20 and to achieve reliable tuning at 125khz the antenna inductance sh ould be around 700uh. higher q and inductance values will still function but wit h a reduced range and performance. the formula for calculating q = 2* *fl / rant = 549 / rant where f = resonant frequency, 125 khz, l = antenna inductance, 700uh rant = overall antenna resistance = rdriver + ra + (rcu + rrf) = 3.14159 etc rdriver = 3.5 r (from ic spec) and ra = 22 r (series resistor in antenna loop) rcu = resistance of copper (coil and cable) a nd rrf = rf resistive component (eddy current lo sses etc) by measurement at 125khz, (rcu + rrf) = approx 6r therefore rant = 3.5 + 22 + 6 = 31.5 ohms, q = 549 / 31.5 = 17 max peak antenna current (with 22r series resistor) , iant max = 4vdd / *rant = 20 / *31.5 = 200ma max peak antenna voltage, uant max = iant ma x . (2* *fl) = 110v 7) the micro rw d has been designed to work at optimum performance with rant = 31.5 (ra = 22r), uant max = 110v and iant ma x = 200ma . when designing and winding antenna coils, the wire gauge and its dc resistance must be considered to keep close to the design opti mum. significant differences in ra will affect resilience to emc and overall perfor mance. 110v peak 125khz correctly tuned antenna waveform with ra = 22 r to limit peak voltage micro rwd driver switching steps low antenna voltage detuned antenna. l too low (<700uh) low antenna voltage detuned antenna. l too high (>700uh) 700uh antenna microrwd antenna connection pins an1 an2 22r
ib technology 3 8) ferrite shielding when the rwd antenna coil is positioned close to me tal objects such as the reader housing or even the pcb ground plane then the rf fi eld induces eddy currents in the metal. this absorbs the rf field energy and has the effect of detuning the antenna (reducing the inductance). both these facto rs can very significantly affect the performance of the rwd system. therefore for op eration in metallic environments it may be necessary to shield the ante nna with ferrite. this shielding has the effect of concentrating the magnetic field lines close to the ferrite material, which introduces a fixed field component that detun es the antenna system (which can be accounted for in antenna design). practicall y, there is a compromise between the shielding effect (reduction of eddy currents) a nd the concentration of the magnetic field (reduced range), so the ferrite plan e should only slightly overlap the antenna coil. the optimum size of the ferrite plane , the distance from the coil and the degree of overlap are very hard to calculate an d must be determined practically. tests have shown that best performance is achieved when the antenna coil and ferrite plane overlap by around 5mm. a useful formula is given below for calculating the approximate number of turns required to achieve a particular inductance. l n = (approx) or l = 2 . a . ln(a / d). n 2 . a . ln(a / d) l = required inductance (nh) i.e h x 10 a = antenna circumference (cm) ie. x diameter (cm) d = wire diameter (cm) e.g 0.0236 cm n = approx number of windings 1.9 -9 1.9 metal plane magnetic field around antenna coil eddy currents eddy currents absorb energy and cause detuning of antenna metal plane magnetic field around antenna coil optimum field distribution, fixed antenna detuning with minimum loss of energy ferrite plane 5mm overlap
ib technology 4 example antennas a) 0.236mm diameter (34 swg) enamelled cu wire. antenna 14.5cm internal diameter, 45 turns. approx 700uh maximum range: approx 20cm with ht2 card transpond ers b) 0.236mm diameter (34 swg) enamelled cu wire. antenna 10cm internal diameter, 55 turns. approx 700uh maximum range: approx 15cm with ht2 card transpond ers c) 0.236mm diameter (34 swg) enamelled cu wire. antenna 7cm internal diameter, 67 turns. approx 700uh maximum range: approx 10-15cm with ht2 card transp onders examples of antenna (c) and a very small antenna fo r applications with limited space no responsibility is taken for the method of integr ation or final use of micro rwd more information on the micro rwd and other product s can be found at the internet web site: http://www.ibtechnology.co.uk or alternatively contact ib technology by email at: sales@ibtechnology.co.uk
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