What type of radiating cable you have? Coupled mode or radiating mode? Coupled mode cables have wider bandwidth, when Radiating mode cables are optimized for particular bands.
Radiating cables are defined by their coupling loss, which is defined as the difference between the power transmitted into the cable and the power received by a λ/2 dipole antenna located at a distance of 2m from the cable.
Are you using a λ/2 dipole antenna for your tests? Check the coupling loss mentioned in its datasheet.