This paper introduces the differences among active radar, semi-active radar and passive radar. Thank you, and God bless you.

Active and passive have one thing in common, because the change of beam direction can be controlled by phase. For passive phased array radar, the radiating elements of the antenna are concentrated by a centralized transmitter and then radiated. As a radiating unit, it has no independence and cannot radiate power independently. Therefore, it is called passivity, because we have a metaphor that radar is the eye of war, but in fact, radar and eyes are still very different. The biggest difference is that my eyes are passive and people's eyes want to see things. The light source is not the eye itself, it is external, and the eye only receives it. The active feature is that the radiation source that the radar wants to see is radiated by the radar itself, so a very important feature of the radar is the active feature. Passive radar is like looking at the light outside. When the sun shines, I can see the light when I receive the radiated sunlight. Active radar is equivalent to the function of ears and mouth. When I said it, I said I could hear it myself. Let's put it in a popular way. If the passive phased array radar is equivalent to talking with one mouth and going out through a decibel meter, then the active radar is equivalent to talking with dozens, hundreds or even tens of thousands of mouths and then distributing them, which is more powerful. I have tens of thousands of mouths talking at the same time, and the signal coming out is stronger. Sound waves are different from electromagnetic waves, but the principle is the same. The radiation capacity of each antenna element is divided by a centralized high-power transmitter through a distributor. Active phased array radar is to receive a small transmitter behind each radiating element, which is actually a receiver and a radar. Behind the radiation unit of active phased array antenna is not an imager, behind the passive phased array antenna is an imager, and behind the imager is a transceiver module, which can both transmit and receive. In fact, it is a small radar.