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This is question a3a1-1 from the set of practice questions in the "amateur radio Exam Secrets" book:

An antenna is connected by a short feeder to the input to a radio receiver. Everything is correctly matched to 50Ω and a sensitive, high impedance RF voltmeter placed across the feeder records an input to the receiver of 10μV. The receiver is disconnected and the level of the RF signal noted. The reading will be about:

and the correct answer in 20μV. Why is that?

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The situations described, schematically:

schematic

simulate this circuit – Schematic created using CircuitLab

When the source and load are matched (Rantenna = Rreceiver), you will be measuring the voltage in the first case through a voltage divider such that the measured voltage will always be half the open-circuit voltage.

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The receiver is effectively acting as a shunt across the meter. Once the receiver is removed from the circuit the "shunt" is removed so the meter shows the true reading.

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