Figure 4 shows a block (subsystem) diagram for a radio communication system - AQA - A-Level Physics - Question 3 - 2018 - Paper 8

The subsystem where the audio and radio waves are combined is represented by the letter A (the modulator).

The signal strength at stage D is weak due to factors such as:

• Attenuation of the electromagnetic wave across the transmission path, meaning some energy is lost as the signal travels.
• Directivity of the receiver: Only a small fraction of the radiated energy reaches the receiver.
• Obstructions or reflections from the ground, natural topography, and absorption in the atmosphere may also contribute to weakening the signal.

The combined amplitude-modulated (AM) signal should depict the modulating signal modifying the amplitude of the carrier wave, resulting in a waveform that oscillates with the envelope of the modulating signal.

The stations cannot broadcast hi-fi music using the full audio frequency of 20 kHz. Given that the frequency range allocated for AM broadcasts is between 540 kHz to 1600 kHz, the bandwidth available per station is limited. Specifically, a typical AM signal bandwidth would be around $10 ext{kHz}$ to $15 ext{kHz}$, which is insufficient to transmit the full audio frequency of 20 kHz.