If you want to change the frequency response of the antenna, you would need to use a different type of antenna object that allows for this kind of customization. For example, you could use the 'phased.CustomAntennaElement' object, which allows you to specify a custom frequency response.
% Define the frequency range
fc = 1e9*(1:0.1:2);
% Define a custom frequency response function
myresp = @(f) (f - 1e9) / 1e9; % Linear response from 0 at 1GHz to 1 at 2GHz
% Create a custom antenna element with the custom frequency response
antenna = phased.CustomAntennaElement('FrequencyVector', fc, 'FrequencyResponse', myresp(fc));
% Compute the response at the frequencies of interest
resp = antenna(fc,[0;0]);
