As per my understanding you want to plot the radiation pattern of a H-plane Horn Antenna in MATLAB
I will be demonstrating it using an example. First, I will create a H-plane Horn antenna and plot the radiation pattern for it. You can use the below code to verify that.
%% Antenna Properties
antennaObject = horn;
antennaObject.FlareWidth = 0.25715;
antennaObject.FlareHeight = 0.023782;
% Show
figure;
show(antennaObject)
%%Antenna Analysis
% Define plot frequency
plotFrequency = 15*1e9;
% Define frequency range
freqRange = (13.5:0.15:16.5)*1e9;
% Reference Impedance
refImpedance = 50;
% pattern
figure;
pattern(antennaObject, plotFrequency)