In RF circuits, the signal strengths are generally referred in terms of power, rather than voltage or current. The ultimate objective of a radio transmitter is to deliver maximum power to antenna meeting the other constraints. Maximum power transfer theorem states that the condition for maximum power transfer is a source and it’s load impedances must be complex conjugate. In practice the load and source impedance never be complex conjugate, hence a matching circuit is necessary to meet this requirement. In Rx chain, noise is more important than signal power. So the matching network between antenna and LNA is not simple complex conjugate, instead it is designed from noise constrains.
In summary, input/ouput impedance matching in radio circuits is necessary either to achieve
- Maximize power transfer
- Noise performance
- minimize reflections from source or load
All of these trade with each other