What is component 4 in figure T3?

Component 4 in the figure referred to as T3 is identified as an antenna. The antenna is a crucial part of a radio system, as it converts electrical energy into radio waves for transmission and vice versa for receiving. It plays a significant role in directing the radio signal into the surrounding environment or capturing incoming signals from the air.

In radio systems, antennas can take many forms and their design affects range and performance. Understanding antennas includes recognizing their properties such as gain, polarization, and directivity, which all contribute to how efficiently they transmit and receive signals.

The other components listed may also be present in a typical radio setup, but in this context, the function of an antenna is distinct and vital, highlighting its importance in communication systems.