My guess is that they're talking about the radiation efficiency -- the strength of signal you get for a unit of power. I would interpret that sentence to mean

My guess is that they're talking about the radiation efficiency -- the strength of signal you get for a unit of power. I would interpret that sentence to mean that communicating with a satellite over a dish antenna will take more power than using this helical antenna in its short state. https://en.wikipedia.org/wiki/Radiation_efficiency https://en.wikipedia.org/wiki/Radiation_efficiency