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Myth from the 60's?

Discussion in 'Electronic Design' started by [email protected], Feb 8, 2013.

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  1. Guest

    When I was a nerdy kid growing up in the 60's, I read an article in PopularElectronics that claimed you could detect tornados by tuning your TV to anunused channel; if you see a normal snow pattern, no tornado, but if the screen suddenly turns all-white then you should high-tail it to the basement.. I always wondered if this were really true. Any opinions?

  2. Guest

    The idea was apparently to use the TV as a lightning detector, on the
    theory that a storm producing a tornado would also be producing a lot
    of lightning. There is a version of it here which mentions US TV
    channel 2 and also mentions using the low end of the US AM broadcast
    band on a radio.

    NOAA and the University of Oklahoma say it doesn't work - it may detect
    lightning, but not all tornadic storms produce a lot of lightning. (scroll down to the
    last question under "Detecting Tornadoes")

    Matt Roberds
  3. Robert Macy

    Robert Macy Guest

    From memory it is likely that channel 2, 3 or 4, 5, etc are very
    llikely to be occupied. Since low 'high frequency' equipment cost a
    bit less, stations throughout the midwest grabbed channel 2 first. I
    know in the Bay Area we received over 66 channels just using rabbit
    ears with Channel 4 being used, but not channel 3; and in Boston Area
    Channel 3 is used, but not channel 4. Again, from memory.recall
    something about those two adjacent channels are very prone to
    interfering, so the FCC did not grant licenses to both in the same
    metro service area.

    Anybody out there confirm? Deny?
  4. Guest

    Thanks, looks like it just detects strong electromagnetic emissions but isn't very reliable. I'll ignore that advice from Popular Electronics and instead stick with tbe numerous articles about how to make an audible turn-signal click sound using a 555 timer :)

  5. miso

    miso Guest

    A similar scheme could be use to detect meteors. The meteor leaves an
    ionization trail.

    TO be clear here you need an unused channel AND a TV station just beyond
    the radio horizon on that channel.

    Using AM radio to detect lightning storms isn't particularly useful
    since the noise propagates so well that the warning isn't particular
    very geographically significant.
  6. It is easy to use a TV. Not for individual lightning strikes as that
    provide little info other than timeline.

    They can be used quite well as tornado detectors, however.

    Same thing. It catches the lightning strikes. The difference being
    that when they are very high in number, it is very likely a funnel is

  7. Ch 6 is suggested for the tornado detection thing.
  8. Bob Boblaw

    Bob Boblaw Guest

    Here's the article about it from the March 1969 Popular Mechanics cover story: mechanics march 1969 tornadoes&f=false
  9. Guest

    Negative modulation was used in most TV systems in the world, so the
    screen would indeed go black with a stronger signal.

    However, you have to consider AGC in RF and IF stages. If the AGC
    drops the front end gain, less band noise and less front end thermal
    noise will enter the video detector, the snow pattern would become

    In normal operation, the signal strength is sampled only during the
    synch pulse and the AGC level is hold constant during the entire line
    time. However, if there are no synch pulses, how is the AGC voltage
    generated ? Apparently this depends on the circuit design for a
    particular receiver type.
  10. Guest

    The French always want to be original, running metro trains with
    rubber wheels and using positive modulation on both 625 as well as
    their own 819 line B&W system. The pre-war UK 405 line System-A also
    used positive modulation.

    The nice thing about negative modulation is that the carrier is always
    present, making it possible to use intercarrier sound. On positive
    modulation systems, audio recovery had to be done separately,
    requiring a higher frequency stability.
    Noise blankers should be positioned as close to the antenna as
    possible, before any sharp band pass filters. A short but high
    amplitude pulse is easy to kill, but if it goes through some band pass
    filter, it will be widened and reduced in amplitude and it is
    impossible to distinguish from the real signal.
    The majority of the LEMP energy is at frequencies well below 1 MHz, so
    the LF band is a good place to start.
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