FM broadcast antenna manufacturers publish free space models for good reason. It makes it easy separate differences in performance related to antenna design versus differences in performance related to height above ground. Without free space you can't tell if power being radiated at undesired angles is a result of antenna design or ground reflections.
The higher above ground, the more secondary lobes are developed at angles that can vary widely with height. At the same time, the primary lobe will continue to drop closer to the horizon until it levels off many wavelengths above ground. This effect is fairly predictable with wavelength and soil type in terms of the primary lobe for line of sight propagation. Modeling at various heights above real ground can be a useful tool if you desire to use secondary lobes to target specific angles for DX propagation.
Results in free space do transfer over to real world and represent antenna performance without the variables associated with different mounting heights. Once a model is designed to provide peak gain at zero degrees in free space, altering the design will not typically improve performance on the horizon when mounted over real ground. However, placing the antenna above real ground always provides more gain then the free space model.
If this sounds like hocus pocus and you think free space is useless because no one mounts their antenna in free space, remember the characteristics described apply to all antennas. For example a half wave horizontal dipole mounted one wavelength above ground has several db over a dipole as W8JI and many others have pointed out. Once you understand this, then it becomes clear why we need free space to determine a relative fixed gain reference point for the dipole and the same playing field to compare other antennas against it.
Will your results vary from one installation location to another? Of course they will. At the same time the free space model provides significant information in which to base your predictions as to how a particular antenna should perform in your specific installation. Even things like adding a mast in free space show accurate results. In the case of the Sigma IV, the mast actually tightens the beamwidth and focuses more gain at 0 degrees in free space or at the horizon over real ground.
