Jim Choate wrote on 1998-02-09 17:35 UTC:
This is the hardest since the actual modulation of the e-beam is done via a screen grid (who said tube theory was out of date?) and that signal is quite small and generaly has a cardoid emission pattern aligned axialy along the central axis of the CRT tube.
Things are somewhat more complicated and I am not convinced that the e-beam is the primary source of radiation. Your claim that the Tempest radiation is modulated by the screen grid does not agree with my practical experience: All signals I get are close to harmonics of the dot clock and not of the screen grid rate. In addition, the Tempest monitor cannot distinguish between an all-black and an all-white image, which it should in the case of a screen-grid caused modulation. If there is indeed a screen-grid modulation, then it is *much* weaker than any modulation that you get by software dithering. Monitors are pretty strange antennas: For instance, my monitor still radiates quite well (although noticeably weaker) if I switch its power supply off. Just the passive resonance of the chassis gives a clear signal in around a meter radius with a simple untuned dipole antenna. Switching off a monitor alone does not protect you from eavesdropping a VDU signal, especially if the signal is not just text but a pattern optimized for reception. After I unplug the VGA cable however, I can't pick up any signal with our Tempest receiver unless I bring the antenna almost in contact with the cable or connector. The closed PC chassis also appears to be no very big source of VDU emanations, certainly much below the levels that our receiver can detect. Markus -- Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK email: mkuhn at acm.org, home page: <http://www.cl.cam.ac.uk/~mgk25/>