Myth from the 60's?

Yes, it probably used to work. There was even a specific channel or couple of channels that worked best. Won't work today, unless you have a really old TV. The theory is that the low pressure in the center of the tornado allows a massive gas discharge, not a lightning bolt but more like a fluorescent or neon light, to develop, and it becomes a huge wideband radiator of energy in the low VHF band. There are some photos of night tornadoes that gave a continuous glow.

If I recall how NTSC worked, the screen would turn black, not white, when a lot of signal was coming in.

Jon

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

Yes. NTSC used inverted video, and sync was referred to as "Blacker than Black" since the transmitter was only at full power during the sync pulses. The TV had to be on channel 2 or 3, where the atmospheric noise could travel further.

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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?

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

Bob

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.

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Not really. You needed a heavier tower, and larger antenna for the lower channels. The advantage was that the signal went further and was prefered in open areas with low population. Lower channels had group delay problems, and other artifacts that the high VHF & UHF channels didn't. The basic VHF transmitter had to be customized for whatever channel it was assigned to. Generally, UHF transmitters were built for groups of three channels without changing a lot of parts.

The FCC had to reassign a lot of channel numbers at one time, to eliminate co-channel interference. When a lot of those stations went on the air, they barely covered 10 miles. As transmitters got bigger & towers went higher they had problems. That caused the FCC to implement their 150 mile rule for same channel usage, and to eliminate adjacent channel headaches.

There is a gap between TV channels 4 & 5 at 72 MHz-76 MHz and a another between 6 & 7 from 88 MHz to 174 MHz, so those channel number could be used adjacent without problems.

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 nearby.

Ch 6 is suggested for the tornado detection thing.

There was this lightning sensor mentioned in a recent Future Electronics e-circular

that looks likt it may be fun to play with. It's "real" (available at Digikey et al.).

ar Electronics that claimed you could detect tornados by tuning your TV to an unused 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 baseme nt. I always wondered if this were really true. Any opinions?

Here's the article about it from the March 1969 Popular Mechanics cover sto ry:

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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 whiter.

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.

On a sunny day (Sat, 09 Feb 2013 15:00:42 +0200) it happened snipped-for-privacy@downunder.com wrote in :

Except France, it had positive modulation.

There were also interestng anti-noise spike circuits. Philps once uses a varicap diode over an IF bandfilter, that detuned the IF circuit in the presence of strong RF pulses.

Those noise pulses were common from cars and motorbike ignition, before those got equiped with screend cables to the sparkplugs.

I have build a small lightning detector that uses a ferrite rod tuned so some LW frequency, and a PIC comparator to flash a LED:

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.

On a sunny day (Sat, 09 Feb 2013 16:14:55 +0200) it happened snipped-for-privacy@downunder.com wrote in :

In the old tube IF amps, pulses of a hundred or so volt were possible versus a signal of maybe a few volt at most.

The problem with my box is that if you make it very sensitive, by setting a low comparator threshold in the PC software, then it also triggers in the house from light switch sparks. Works much better with an outside antenna connected to it.

There are interesting designs on the web that also can see difference between negative and positive kind of lighting, and measure volts per meter IIRC. I build mine to get advance warning when I had to disconnect my 27MHz GPA... Sometimes in the middle of the night, go out in the rain...

The other thing you could implement in software is a 'decrease increase' detector, and if you put a photocell outside perhaps a distance detector, variations are endless.