A calibrated, unidirectional antenna used to make precise antenna gain measurements
An omnidirectional, horizontally polarized, precisely calibrated antenna used to make field measurements of antenna gain
A hypothetical, lossless antenna having equal radiation intensity in all directions used as a reference for antenna gain✓Правильный ответ
A spacecraft antenna used to direct signals toward Earth
Объяснение
An isotropic radiator is a mathematical reference rather than a real antenna: it is lossless and radiates with equal intensity in every direction, so the field strength is identical everywhere on a sphere around it. Gain quoted in dBi is measured against this ideal, which is why the answer cannot be a calibrated measurement antenna or a spacecraft antenna.