What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal?

To properly view the RF envelope of a transmitted signal on an oscilloscope, you need to sample the final output signal that is heading to the antenna. This allows you to inspect the actual waveform being broadcast over the air. However, connecting a transmitter's raw, full-power RF output directly to an oscilloscope would severely damage the sensitive vertical input circuits. Therefore, the RF output must be appropriately attenuated (reduced in power) using a sampler or dummy load tap before it reaches the test equipment. The other options represent internal, lower-level stages of the radio. Measuring the local oscillator or the balanced mixer output would not show you the final amplified envelope that the transmitter is actually producing.