Application of NEC-2 Code to the Analysis of Tower Surge Response
2.8 Surge Simulation by NEC.-2
2.8.4 Influence of the Method of Current Injection
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0.5
Case) Case Case(ä' 11
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flowing into the conductor are shown in Fig. 2.17. The broken line, solid line and chain lines results correspond to case (i), case (ii) and case (iii) respectively. The voltage waveform for case (iii) reaches its peak value at I = 2h / c = 4 ns which shows that the propagation velocity is same as light. On the other hand for other two cases there is a time delay of 3.4 ns in case (ii) and 5 ns in case (i), which also show that the electromagnetic wave is propagating through the conductor with the velocity of light. For the current waveforms it is observed that only in case (iii) the reflection from the perfectly conducting ground occurs after 4 ns. But in other two cases there are time delays for reflection due to time required to propagate electromagnetic wave through the vertical conductor.
2.8.5 Effect of Changing Segment Length
The main electrical consideration is segment length AL relative to the wavelength A.
Generally, AL should be less than about 0.1 A. at the desired frequency. Somewhat longer segments may be acceptable on long wires with no abrupt changes while shorter segments, 0.05 A.or less, may be needed in modeling critical regions of an antenna. The size of the segments determines the resolution in solving for the current on the model since the current is computed at the center of each segment. Extremely short segments, less than about 10- A., should also be avoided since the similarity of the constant and cosine components of the current expansion leads to numerical inaccuracy. So the vertical conductor model of height 90 cm was taken into account for analysis by changing segments length according to the modeling guidelines. Here for simulation with NEC-2 the segment size was taken as 5 cm, 10
1.25 nsldry
Figure 2.18: Voltage at the tower top in case of changing segment length.
cm and 20 cm in length. The output waveforms of tower top voltage were obtained are almost similar to the Fig. 2.10 (b) means it is acceptable within the accuracy maintain in the analysis.
So , within the modeling limit the segment length should be as small as possible to obtain the better result, also to maintain the accuracy although if the segment length decreases the computation time increases.
2.8.6 Effect of Changing Radius and Height
A
550
SOD
450
N 430
350
300
05 1.0 15 20 2.5 3.0 3.5 40 4.5 5,0
Radious [mm]
Figure 2.19: Waveforms by varying radius and height of the conductor.
For the numerical analysis with NEC-2, the entire structure needs to be modeled by combination of cylindrical segments that must be short enough compared to the wavelength of interest. The accuracy of result of NEC-2 by varying the radius of the considered reduced scale vertical conductor model over perfectly conducting ground is investigated here by simulation approach. In the current analysis the height of the vertical conductor is set to 60 cm and being considered it as a cylinder the radius of the conductor is varied from 0.05 mm to 5 mm for observation. A step current pulse generator having pulse voltage of 5 V in magnitude, risetime of 1 ns and pulse width of 40 ns was taken for the analysis. Here the segment length AL = 10 cm was taken which must satis1' the condition that radius should be smaller than AL / 8 . Computation is carried out in time range of 0.25 ns with stepping frequency of 7.8125 MHz. The voltage at the top of the vertical conductor was taken and current was computed. Then from the computed value of voltage and current the surge impedance as defined by the ratio of the instantaneous values of the voltage at the tower top of vertical conductor to the current flowing through it at the moment of voltage peak is
determined. The value of surge impedance can also be determined by the theoretical formula which is very close to the well-known empirical formula given as in case of with ground plane. Similarly, 90 cm of height and 120 cm of height vertical conductors were considered and the data was taken. By plotting the simulation result, the effects of varying radius and height on the surge impedance are shown in Fig. 2.19.
From the graph it is seen that if the radius of the vertical conductor is increased then the value of surge impedance decreases. It was also noticeable that with the increase of the length of the vertical conductor the value of surge impedance increases. Thus it can be shown that the accuracy is satisfactory when a model comprises segments with a uniform radius smaller than about AL / S
Although, the simulation was carried out for the arrangement with ground plane, however the other model also gives the same results.
2.8.7 Computation Time
For the case of NEC-2, computation is carried out in the frequency range of 7.813 MI-Iz to 4 GHz with the increment step of 7.813 MI-Iz. This corresponds to the time range of 0 to 128 ns with 0.25 ns increments. The computation time for the output of NEC-2 block of the flow chart of Fig. 2.6, with Intel Celeron 700 MHz processor with 128 MB RAM is about 56 seconds with the ground plane case and 30 seconds without ground plane case for 60 cm model. For 90 cm model, 60 seconds with the ground plane case and 35 seconds without ground plane case and for 120 cm model, 70 seconds with the ground plane case and 40 seconds without ground plane case are observed.