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?