|MadSci Network: Science History|
If you look on any U.S. electronics, you will see a label stating that the unit meets FCC specifications for radio emissions. European electronics have similar labeling. The reason that electronics need these labels is because all of them give off a little radio noise while operating. Without a metallic case your computer could give off enough radio energy to affect radios, cellular phones, other computers, etc. In a large, open office the emissions would give a whole new meaning to local area network.
I am not a tube expert, but I believe that the electron tubes used to receive the radio transmissions would give off radio waves at the same frequency as the incoming transmission. These emissions were much stronger than today's transistors and integrated circuits since the tubes operated between 120 V and as much as 900 V versus 2 V to 12 V for a transistor or IC. The radios themselves were not as well shielded either. So if the Germans knew the transmission frequency of the English, they could use radio tracking devices over short ranges (probably your 50 meters or so) to locate these radio sets.
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