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Originally Posted by MarshalLaw
Radar detectors are a passive device.
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No. Their functionality is passive, but their operation is
far from passive.
Called "signal leak" or "signal egress", anything with an oscillator circuit will emit RF. Your toaster, your wristwatch, your LCD/CRT monitor, printer, keyboard, mouse. The amount of power it transmits is a function of design, and some designs have more leakage than others.
For example, your computer's BIOS may have a "spread spectrum" function. People sometimes refer to it as electromagnetic-interference, but it's just normal signal egress from your sound card and hard drive that cause problems. Signal egress is why ethernet uses "twisted pair" cabling.
At the time, using radio-tubes, the receiver sensitivity was not altered like it is today. The output power was increased or decreased, and the receiver was always operated at maximum sensitivity. This is why so many receivers were going out all the time, but point being that they emitted RF at their maximum design power...
all the time. Radiotubes cannot be made sensitive enough to pickup the signal egress, but transistors can.
It's was theoretically possible to pickup the Metox detector, but the British would have needed a transistorized receiver (with SMD's of 1mm size and smaller) to do it. Which is exactly why the Germans were so stunned by that pilot's interrogation.