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

The letter representing the subsystem where the audio and radio waves are combined is A.

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

1. Attenuation of electromagnetic waves: As the EM waves travel through the transmission path, they lose energy due to distance and environmental factors.

2. Reflection and absorption: Some energy from the radiated waves may be lost due to reflection off ground or natural topography, as well as absorption in the atmosphere, which diminishes the signal strength before it reaches stage D.

The combined amplitude modulated (AM) signal graph should show the modulating signal being superimposed on the carrier wave, where the amplitude of the carrier varies according to the modulating signal. The peaks of the modulating wave should align with the peaks of the carrier wave to indicate modulation.

Given that the allocated frequency spectrum for broadcasting is between 540 kHz and 1600 kHz, a full audio frequency of 20 kHz can be supported. However, the actual ability to broadcast hi-fi music would depend on the bandwidth each station can use. Since each station requires bandwidth to operate without interference, it may not be feasible for all 55 stations to broadcast hi-fi music simultaneously without limiting audio quality or causing cross-talk. Overall, not all stations can broadcast hi-fi music using the full 20 kHz frequency.