What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal?
Why An oscilloscope displays the instantaneous voltage applied to its vertical input, so to see the modulation envelope of your transmitted signal you must feed it a sample of the actual transmitter output. That output is far too powerful for a scope's input, so it is taken through a directional coupler, RF sampling probe, or resistive divider to attenuate it to a few volts. The resulting trace shows the RF carrier filled in with the shape of your audio, letting you spot flat-topping or overmodulation.
Watch out Looking at the local oscillator or the balanced mixer output only shows a stage inside the rig, not the amplified, filtered signal actually going to the antenna, and an external oscillator carries no modulation at all.
Envelope check = watch what you actually transmit, turned way down. Attenuated RF output into the vertical input.