how can I do a 3D plot for S-parameter based on S21, Frequency and Distance?

Hello all,

I have 30 S21 measurments between two seperate anttennas. the disrance between two antenna is D. Now I wannt do a 3D plot for this expriment. Z axis equal to s21, x axis equals to distance and y axis equals to frequency. here is my code but it returns errors aboud dimension.

numfiles = 42;

filename = "D_"+(12:numfiles)+".s2p";

S = sparameters(filename(1));

D = [12:1:42];

freq = S.Frequencies;

numfreq = numel(freq);

s21_data = zeros(numfreq,numfiles);

% Read Touchstone files

for n = 1: (numfiles - 12);

S = sparameters(filename(n));

s21 = rfparam(S,2,1);

s21_data(:,n) = s21;

txt{n}= sprintf('D %i',n+12);

end

s21_db = 20*log10(abs(s21_data));

[D,freq]=meshgrid (D,freq);

mesh(D,freq,s21_db);

axis on;

grid on

Answer 1

[Abhinaya Kennedy]

Hi Peyman,

"meshgrid" creates two-dimensional grids from one-dimensional inputs. However, your "s21_db" data has three dimensions (frequency, distance, and amplitude). Here's how you can fix your code for a 3D plot:

% Read Touchstone files
for n = 1: (numfiles - 12);
    S = sparameters(filename(n));
    s21 = rfparam(S,2,1);
    s21_data(:,n) = s21;
    txt{n}= sprintf('D %i',n+12);
end
s21_db = 20*log10(abs(s21_data));

% No need for meshgrid, use surf for 3D surface plot
surf(D, freq, s21_db);

axis on;
grid on;
xlabel('Distance (D)');
ylabel('Frequency (Hz)');
zlabel('S21 (dB)');
title('S21 vs. Distance and Frequency');

We don't need "meshgrid" as "surf" can handle 3D data directly. This code will create a 3D surface plot with distance (D) on the x-axis, frequency on the y-axis, and S21 in dB on the z-axis.

You can look into this link for more information on "surf": https://www.mathworks.com/help/matlab/ref/surf.html