Probing the Magnetic Field Probe By Roy Ediss, Philips Semiconductors, UK. Introduction Commercial and handcrafted probes similar to those shown in Fiure ! are commonly used in E"C dian ostic wor#, but ha$ e you e$er conside red how they ope rate % &he man etic field probes are made in the form of a loop with an inherent electrostatic shield, enerally from '( )hm semi*riid coa+ial cable. &hey $ary slihtly in confiuration and in characteristics, but essentially they are electrically small shielded loop antennas deri$ed from the antennas used since the !-(s for radio communication and direction findin /!,-0. Figure 1. Various shielded loops. How they work Refe r to the diar ams of the $ari ous 1*fiel d loop probes show n in Fiur e -. &he foll owin e+planation can be applied in eneral to all the probes, but the common probe type -2a3 will be considere d. &he e4ui$a lent circuit diaram is shown as Fiure 5, which has numbered locati on points correspon din to Fiure -2a3 /5,60. 7n eleant arran ement e+ists where electric fields may impine on the outer she ath but are shielded from the inn er sinal line. 7 small ap in the outer sheath is howe$er always included, pre$entin a shorted*turn to manetic fields. 7 ma netic field passin throuh th e probe loop enerate s a $oltae accor din to Fara days law, which states that the induced $oltae is proportional to the rate of chane of manetic flu+ throuh a circuit loop. 7t $ery low fre4uencies a $oltae would be induced directly in the internal loop conductor, but the copper sheath is 4uite a ood shield to manetic fields at fre4uencies e+ceedin the low K18 rane. So at hih fre4uency , a $oltae is then induced preferen tially in the outer sheath loop, and this appears across the sheath ap 2points ! 9 :3 /'0. &he metal sheath thic#ness is se$eral s#in depths, so this pre$ents direct interaction between currents on the e+ternal surfa ce and internal surfa ces of the shield. &he '( )hm tra nsmission line, formed by the inner surface of the sheath and the inner conductor, is then dri$en by this $oltae and is terminated by the '( )hm measurement system 2points 6 9 '3. 7n analo y to the effect described abo$e may be made, by consideri n a circuit loop arran ement where a coa+ial shield termina tion is unconnected at one end 2see Fiure 63. )r an impe rfec t coa+ ial shie ld terminat ion , wher e some ro und curren t may cre ate a $olta e across an inducti$e path such as a ;Pitail< round connection.