Explanation: In a RADAR system, the antenna is shared for both transmitting powerful pulses and receiving faint echo signals. Block 46 represents the TR (Transmit/Receive) box, which is a crucial duplexing device. Its primary function is to protect the sensitive receiver components from the extremely high power of the outgoing transmit pulse. During transmission, the TR box acts as a switch, effectively shorting or blocking the path to the receiver. Immediately after the transmit pulse, it rapidly switches to allow the weak incoming echo signals to pass to the receiver's mixer and IF stages. The ATR (Anti-Transmit/Receive) box, while also part of the duplexer, typically works in conjunction with the TR box to fine-tune the impedance match and further protect the receiver, but the TR box is the main protective switch. An RF attenuator simply reduces signal strength and doesn't provide the switching action needed for duplexing. The crystal detector is located much later in the receiver chain, after the IF amplifier, to convert the IF signal to a video signal.