% (first, make the attached .txt file an .ffs file)
copyfile('SpiralAntennaBox_v02_FarField_4p5GHz.txt','SpiralAntennaBox_v02_FarField_4p5GHz.ffs');
It looks like readmatrix will work; just specify 'FileType','text' due to the unrecognized extension:
% Option 1: use readmatrix:
data = readmatrix('SpiralAntennaBox_v02_FarField_4p5GHz.ffs','FileType','text')
Or, here's one way to complete the approach you were working on, using fileread:
% Option 2: use fileread:
radiationfile = fileread('SpiralAntennaBox_v02_FarField_4p5GHz.ffs');
idx = strfind(radiationfile,':');
data = str2num(radiationfile(idx(end)+1:end))
