The way an antenna manufacturer would test a certain antenna model is by using a tuned dipole as a benchmark for comparison. They would measure the gain (usually in decibels or dB) of their antenna over the tuned dipole and work out how much extra signal their antenna is picking up.
When we say tuned dipole, what is meant is that the diameter and length of the element is designed for or ‘tuned’ for a specific frequency. So manufacturers would have a set of ‘tuned dipoles’, one for each frequency the antenna is being tested for. For instance, in many Australian capital cities, this would be one for each of the channels 7, 9, 10, SBS and the ABC. The manufacturer would measure the amount of signal strength their antenna achieves under the same conditions as the tuned dipole in a particular frequency. They may then plot these readings on a graph and take an average. This may be something like ‘8dB signal gain over the tuned dipole’ for example. A second method of testing is to give the signal gain when compared to a theoretical omni-directional single point antenna called an Isotropic antenna.