Читать книгу Antenna and EM Modeling with MATLAB Antenna Toolbox - Sergey N. Makarov - Страница 44

Plot the reflection coefficient in dB and VSWR for the dipole with l_A = 15 cm, a = 2 mm over the band 200–1200 MHz using Eq. (1.14) and MATLAB, and determine the antenna impedance bandwidth.

Solution: We repeat the task of Example 1.5, but instead of the impedance plot, the reflection coefficient and the VSWR will be evaluated and plotted. Extra lines of the MATLAB code may be added such as

c = figure; Rg = 50; RC =(Za-Rg)./(Za+Rg); temp = abs(RC); VSWR = (1 + temp)./(1 - temp); semilogy(f/1e6, VSWR, 'b', 'LineWidth', 2); grid on; xlabel ('frequency, MHz'); ylabel ('VSWR, a.u.'); title('VSWR');

The result is shown in Figure 1.9. The same antenna impedance bandwidth is marked on every plot – for the reflection coefficient and for the VSWR, respectively. In fact, the VSWR ≤ 2 bandwidth on the right plot is slightly greater than the |Γ|_dB ≤ − 10 dB bandwidth since, strictly speaking, the condition VSWR = 2 corresponds to |Γ|_dB = − 9.5 dB. This difference is usually ignored.

Figure 1.9 Reflection coefficient in dB (left) versus VSWR (right) for the same dipole antenna.