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I have my Hack RF plugged in my TV antenna to have a cheap setup and I have observed on the spectrum analyser a signal (Bandwidth: ~400Khz) that is emitted by my old analog TV from 80ies. These are my observations:

  • By switching the TV on and off, the signal did the same
  • The spectrum is not at the frequency tuned to, but some Mhz above
  • when I switch a PAL Signal source on and off, the TV emission are changing from a constellation diagram with two concentric sources (like a donut, perhaps QAM) to a filled circle (probably noise)

what is that signal for? What kind of information can an old analog TV emit?

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The TV is likely not emitting any information, but rather just noise. A TV from the 80s would have been a CRT set. These work by sweeping a beam of electrons through a raster scan. The aiming of that electron beam is accomplished by subjecting it to a controlled electric field, and as we know, radios are machines for detecting electric fields.

Since the raster is scanned at a regular rhythm, you are most likely receiving signals at the same frequency as the horizontal scan, or some multiple thereof. It looks like PAL scans 625 lines, 50 times per second. So the horizontal sync frequency is 625*50 = 31.25 kHz. It is probably approximately a triangle wave, so you will also find harmonics at every odd multiple: 93.75 kHz, 156.25 kHz, 218.75 kHz, and so on.

The intensity of the electron beam is further modulated as it scans the display, and so multiplied by the 31.25 kHz line frequency can generate frequency components into several MHz, depending on what's being displayed on the TV.

Lastly, if your HackRF antenna is very close to the TV, you may be overloading the receiver, causing clipping and associated harmonic distortion, which will in effect create copies of signals at regular intervals throughout the spectrum. A CRT produces a very strong electric field with very little current, which is why they don't radiate noise very well far away. But up close the electric fields can be very strong (that's why they attract your hair).

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    $\begingroup$ I love this answer because a) it's informative and b) it gives me an excuse to link to my favorite thing ever: en.wikipedia.org/wiki/Van_Eck_phreaking (which also features prominently in the plot of a book about cryptography that I love). $\endgroup$ – Rafael KR7MJ Jan 25 '17 at 5:23

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