demo_antennas.m

%% Demo:  Computing and Displaying Antenna Patterns
%
% In this demo, we will illustrate some basic MATLAB tools for
% computing and displaying antenna patterns.  Specifically, you will learn
% to:
% 
% * Perform basic manipulations in spherical coordinates
% * Define simple antennas using MATLAB's antenna toolbox
% * Plot antenna patterns in 2D and 3D
% * Use the antenna patterns and free-space path loss functions to 
%   compute the path loss along a trajectory

%% Spherical coordiantes.
% We first demonstrate how to perform basic manipulations
% in spherical coordinates.  
%
% For example, the code below generates four random points in 3D 
% and converts them to spherical coordinates

% Generate random data
X = randn(3,4);

% Compute spherical coordinates of a matrix of points
% Note these are in radians!
[az, el, rad] = cart2sph(X(1,:), X(2,:), X(3,:));

% We can then convert back
[x,y,z] = sph2cart(az,el,rad);
Xhat = [x; y; z];

%% Simulation constants
% For the remainder of the demo, we will use the following 
% simulation constants
%
% Note:  In MATLAB, all values are in metric units m, s, Hz, etc.
% Not GHz or MHz.
fc = 2.3e9;     % Carrier frequency
vp = physconst('lightspeed');  % speed of light
lambda = vp/fc;   % wavelength

%% Dipole antenna
% For a first antenna, we construct a simple dipole.

% Construct the antenna object
ant = dipole(...
    'Length', lambda/2,...
    'Width', 0.01*lambda );

% Display the antenna
ant.show();

%% Displaying the pattern
% We can display the antenna pattern with the following command.
ant.pattern(fc)


%% Patch Element
% We now consider a more complex antenna.  The antenna toolbox can analyze 
% a number of antennas in use.  However, once the antenna is more complex,
% you will start to notice that the analysis becomes very slow.
len = 0.49*lambda;
groundPlaneLen = lambda;
ant2 = patchMicrostrip(...
    'Length', len, 'Width', 1.5*len, ...
    'GroundPlaneLength', groundPlaneLen, ...
    'GroundPlaneWidth', groundPlaneLen, ...
    'Height', 0.01*lambda, ...
    'FeedOffset', [0.25*len 0]);

% Tilt the element so that the maximum energy is in the x-axis
ant2.Tilt = 90;
ant2.TiltAxis = [0 1 0];

% Display the antenna pattern after rotation.
% This may take a few minutes.  So be patient
ant2.pattern(fc, 'Type', 'Directivity');

% You can also save the pattern 
[dir,az,el] = ant2.pattern(fc, 'Type', 'Directivity');

%% Plotting a cross-section
% Once the antenna pattern is stored in an array, you can
% plot cross sections as follows.  Suppose we want to plot
% the cross-section at an elevation angle of 0

% Elevation angle to plot
elPlot = 0;

% Find the index closest to the desired angle
[~, iel] = min(abs(el - elPlot));

% Plot using the polar plot.
% Note the conversion to radians.  You also have to use the |rlim|
% command to set the limits.
polarplot(deg2rad(az), dir(iel,:),'LineWidth', 3);
rlim([-30, 15]);
title('Directivity (dBi)');


%% Creating a custom antenna pattern
% While MATLAB has many common antennas, you will often need
% to load antenna data from a manufacturer or other source.
% Also, even when using MATLAB's antenna elements, it is often
% useful to compute the pattern once and store it. For this purpose,
% you can create a custom antenna element.  Here, we will create
% a custom antenna element with directivity pattern we just 
% computed from the microstrip element.
phasePattern = zeros(size(dir));
ant3 = phased.CustomAntennaElement(...
    'AzimuthAngles', az, 'ElevationAngles', el, ...
    'MagnitudePattern', dir, ...
    'PhasePattern', phasePattern);

% Plot the antenna pattern.
% Note the format is slightly different since we are using
% the pattern routine from the phased array toolbox
ant3.pattern(fc);

%% Interpolating the directivity in the custom pattern
% Once we have the antenna pattern, we can interpolate the 
% values of the gain at other directions.   To illustrate we will
% plot the total path loss between a TX at the origin and an object
% traveling in a linear path along a 3D path.  First,
% we create and plot the path

% Define the linear path
npts = 100;
xstart = [50 -50 0]'; 
xend = [-50 50 50]';
t = linspace(0,1,npts);
X = xstart*(1-t) + xend*t;

% Plot the path in 3D along with the location of the TX at the origin
plot3(X(1,:), X(2,:), X(3,:), 'Linewidth', 3);
hold on;
plot3(0, 0, 0, 'o', 'Linewidth', 3);
grid();
hold off;

%%
% We compute the angle from the transmitter to the target.
% Remember to convert to degrees.
[azpath, elpath, dist] = cart2sph(X(1,:), X(2,:), X(3,:));
azpath = rad2deg(azpath);
elpath = rad2deg(elpath);

% Compute the free space path loss along the path without 
% the antenna gain.  We can use MATLAB's built-in function
plOmni = fspl(dist, lambda);

% Compute the directivity using interpolation of the pattern.
% We can use the ant3.resp method for this purpose, but the
% interpolation is not smooth.  So, we will do this using
% MATLAB's interpolation objects.   First, we create the 
% interpolation object.
F = griddedInterpolant({el,az},dir);

% Then, we compute the directivity using the object
dirPath = F(elpath,azpath);

% Compute the total path loss including the directivity
plDir = plOmni - dirPath;

% Plot the path loss over time.  Can you explain the 
plot(t, [plOmni; plDir]', 'Linewidth', 3);
grid();
set(gca, 'Fontsize', 16);
legend('Omni', 'With directivity', 'Location', 'SouthEast');
xlabel('Time');
ylabel('Path loss (dB)');