From your description, this seems to work:
angl = 0:10:360;
pow = [-18.2 -18.5 -19.7 -21 -22.3 -23 -24.1 -28.1 -33 -37.8 -34 -32.4 -33.1 -32.8 -34.2 -36 -37.9 -39.2 -40 -40 -39.2 -36 -34.2 -32.8 -33.1 -32.4 -34 -37.8 -33 -28.1 -24.1 -23 -22.3 -21 -19.7 -18.5 -18.2];
R = deg2rad(angl);
powc = pow - max(pow);
powc(powc<-40) = -40;
hpp = -21.2;
db3(1) = interp1(pow(1:5), R(1:5), hpp); % Half-Power Angle 1
db3(2) = interp1(pow(end-4:end), R(end-4:end), hpp); % Half-Power Angle 2
figure
h = polarplot(R, pow + 40, 'Linewidth', 1, 'color', [.21 .81 .94]);
hold on
polarplot([0 0; db3(1) db3(2)], [-40 -40; hpp hpp]+40, '-r') % -3dB Opening’ Lines
hold off
haxes = get(h, 'Parent');
haxes.RTickLabel = {'-40 dB', '-30 dB', '-20 dB'};
title('Diagramme de rayonnement antenne TP-LINK TL-AN2409A');
producing:
If you want the lines to go to the edge of the plot, replace ‘hpp’ here:
polarplot([0 0; db3(1) db3(2)], [-40 -40; hpp hpp]+40, '-r') % -3dB Opening’ Lines
with the radius of the edge of the plot, that being ‘max(rlim)’.
You could smoothe the plot with a cubic or similar interpolation, using the interp1 function, first using the linspace function to define ‘R’ with a larger number of grid points.
Experiment to get the result you want.
