Читать книгу Antenna and EM Modeling with MATLAB Antenna Toolbox - Sergey N. Makarov - Страница 41

Using MATLAB Antenna Toolbox, determine antenna impedance bandwidth for the blade dipole antenna with l_A = 15 cm, w = 8 mm.

Solution: The approximate resonant frequency of the half wave dipole with the length of 15 cm is about 1 GHz. We again choose the frequency band from 200 to 1200 MHz for testing. A simple MATLAB script given below initializes the dipole antenna, plots the antenna geometry, and computes the dipole reflection coefficient Γ = S₁₁. Note that the computations are performed exactly following Eq. (1.17), i.e. without an extra cable. Figure 1.8 shows the resulting dipole geometry and the reflection coefficient comparison with the analytical result obtained from Eq. (1.14). The agreement between two solutions is good while the numerical solution is again expected to be somewhat more accurate for this particular geometry. Note that the exact reflection coefficient values below −10 dB do not really matter, they are very sensitive to small variations of the antenna impedance around 50 Ω and are subject to noise.

%% Setup analysis parameters f = linspace(200e6, 1200e6, 1000); % Frequency, Hz lA = 0.15; % Dipole total length, m a = 0.002; % Dipole radius, m %% Antenna toolbox model and analysis w = cylinder2strip(a); % Eq. strip width model d = dipole('Length',lA,'Width',w); % Strip dipole model figure; show(d) % Visualize geometry S11 = rfparam(sparameters(d,f,Rg),1,1); % Calculate s-parameters S11dB = 20*log10(abs(S11));

Figure 1.8 Magnitude of the reflection coefficient in dB for the dipole antenna and the antenna impedance bandwidth. Numerical solution is shown by a dashed curve.

In Figure 1.8, the antenna impedance bandwidth follows Eq. (1.32) or (1.34) with the minus 10 dB threshold. The threshold is shown by two dashed lines in Figure 1.8. The dipole antenna has the impedance bandwidth from the lower frequency of the band, f_L = 870.7 MHz, to the upper frequency of the band, f_U = 985.8 MHz (analytical solution is considered as an example). The center frequency of the band is given by

(1.35)

The antenna impedance bandwidth BW (or fractional bandwidth) is determined in the form

(1.36)

which is a very typical value for a wire dipole or a thin‐blade dipole. Generally, we always want to increase the antenna bandwidth for a better throughput.