Active electronically scanned array (AESA) system noise temperature

In calculating the antenna system noise temperature the question always arises as to how to account for the antenna mismatch. In this paper it is shown that the antenna mismatch introduces another term into the antenna system temperature equation. This term increases the system temperature. It is due to the reflection of the noise coming out of the receiver in the direction of the antenna that is reflected back into the receiver towards its signal output port. For a single port reflector antenna system this term is equal to the temperature Tr of the noise coming out of the receiver towards the antenna times the antenna power reflection coefficient |Γ|². This is usually included in the radiometer field but not by those in the radar and communication fields. For an active phased array the question that arises is: What reflection coefficient to use? It is shown that for an active electronically scanned array (AESA) the reflection coefficient to use for this incoherent noise is the same one used for a coherent signal transmitted in the direction of the main beam. This result is new for those in the radiometer, radar and communication fields. Sometimes system engineers argue that because antenna mismatch is nonohmic it has no effect in calculating the system temperature of an AESA. Doing this can yield a large underestimate of the system temperature at large scan angles, 1.2 to 2.4 dB for the examples cases considered here. It is shown that if the mismatch loss is treated as fully ohmic the correct system noise temperature results are obtained. However to determine the temperature of the ohmic mismatch requires understanding that this noise originates from the noise coming out of the receiver in the direction of the antenna. The temperature of the ohmic mismatch loss will be equal to the temperature of this receiver noise at the point of the mismatch. We will first analyze single port dish and lens antennas which use a single Low Noise Amplifier (LNA)- for the sum channel and then cover active electronically scanned arrays (transmit/receive or receive only).