# Double Trapezoidal Wave Transmitting System with Controllable Turn-Off Edge
## 4 Experiment with Wire Loop
### Experiment and Analyses  
Figure 9 shows the turning-off edge waveform of emission current observed by a clip-on ammeter. After data analysis and curve fitting, the emission current decreased from 10 A to 0 using 150 us with a linearity of 94% at fast turn-off and using 600 us with a linearity of 90% at slow turn-off. The tail of the current descended gently without oscillation and the waveform quality was good. Here, we performed two TEM measurements with and without this polarized wire loop. Figure 10 shows the response curve at a 150 us turn-off time of the emission current. Curve I, with the wire loop, presents the sum of the background and the wire loop response. Curve II, without the wire loop, is only the background response. The first intersection of the two curves occurs at P1. The induction field of the wire loop is dominant in the early stage of the response and superimposed on the background response, so curve I is stronger before P1. With the passage of time, the polarization characteristics of the wire loop begin to appear, a negative response is superimposed on the background, and curve I decreases obviously from P1 to P2. After P2, the response gradually weakens into the receiving system noise area and the two curves tend to coincide. The two curves are consistent with the theoretical analysis, which proves that the polarized wire loop can simulate some characteristics of polarized earth. Figure 10: The response with and without the polarized wire loop at a 150 μs turn-off time. Figure 9: The current waveform at 150 μs turn-off time and 600 μs turn-off time. Similarly, we performed two TEM measurements with and without the polarized wire loop at a 600 \(\upmu\)s turn-off time. Curve III is the response with the wire loop and curve IV is the pure background response, as shown in Figure 11.  
The polarized wire loop response at the 150 \(\upmu\)s turn-off was obtained by subtracting curve II from I. The response at 600 \(\upmu\)s turn-off time can be obtained by subtracting curve IV from III. Figure 12 shows the comparison of the wire loop responses at different turn-off times. Both curves are positive at the beginning where the induction effect is dominant. The curves then descend to zero as the polarization effect begins to appear. Finally, the curves slowly return to the positive sign and enter the system noise area. There are three obvious phenomena: (1) Before the sign becomes negative, the response intensity of curve (III - IV) is weaker than that of curve (I - II); (2) curve (III - IV) becomes negative at 2 ms, earlier than curve (I - II) by 2.3 ms; (3) the maximum negative response of the two curves is almost the same, which is \(10^{-4}\) nT. Because the polarized wire loop was designed to be used as a multi-turn coil equivalent to inductance L, resistance will inevitably be introduced into the circuit and its value cannot be ignored  
Figure 11: The response with and without the “polarized wire loop” at 600 \(\upmu\)s turn-off time. Figure 12: The response of the wire loop at 150 \(\upmu\)s turn-off time and 600 \(\upmu\)s turn-off time. compared with R1 and R0. Therefore, it is difficult to accurately extract the polarization parameters according to the filament model. We can only qualitatively analyze the influence of the transmitting system on the induced polarization effect measurement. Combining phenomena (1) and (3), extending the turn-off time can reduce the response of the induction field, while the response of polarization field is hardly affected under the balance of the turn-off rate and duration. For the same TEM receiving system, the ratio of positive maximum to negative maximum of the response determines the measurement accuracy of polarization effect. The positive response can be amplified to the maximum range close to the receiving system, and the negative signal is amplified at the same time. Hence the response of the polarized target excited by long turn-off times can be amplified and observed more easily.