ABSTRACT
Further, it is very interesting to observe from Figure 1 that at the higher frequencies especially f > 50 MHz, the values of PTEr significantly predominate over the values of PTEa. This fact can be explained from Figure 2. The loss resistance (Rac) and radiation resistance (Rrad) of the Tx coil have been calculated as functions of frequency and plots of Rac, Rrad versus frequency are shown in Figure 2. It is observed from Figure 2 that the values of Rrad are much smaller and almost negligible as compared to the values of Rac at the lower frequencies. But the rate of increase of Rrad with respect to frequency (i.e. dRac/df) is much larger than that of Rac (i.e. dRac/df). It is interesting to note that the values of Rrad become almost comparable to the values of Rac, when the frequency increases beyond 10 MHz. At f = 54.1 MHz, both of them
become equal, Rrad = Rac = 1.83 ohm and corresponding radiation efficiency becomes €rad = 50%. The values of Rrad over take those of Rac while the frequency increases above 54.1 MHz; which leads to sharp increase of radiation efficiency of the Tx coil. Thus the near-field electromagnetic coupling in R-coupling orientation improves sharply for the higher frequencies f > 10 MHz and due to the significant increase of €rad beyond 54.1 MHz, PTEr increase significantly. But at the same time, it may be noted that these frequencies (f > 10 MHz) are much larger than the self-resonance frequency of the coils. Thus at higher frequencies (10 MHz ≤ f ≤ 60 MHz), the magnitude the input impedance of the coils are much greater than that of its value at resonance. Thus the AC current flowing in both the coils is very small, causing very small magnetic coupling. As a result of small amount of magnetic coupling, the PTEa values are very small at the said frequency range (10 MHz ≤ f ≤ 60 MHz).
