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Re: EMP =- PULSED PHONE DESTRUCTION!
Tesla List wrote:
>
> Original Poster: "Sam Barros" <sambarros-at-hotmail-dot-com>
>
>
> REALLY? Could you tell that to my next door neighbour next time he
> calls the police? :-).
We had a similar problem at my previous work when we were testing rapid
fire on a acetylene/oxygen cannon about a week after a spectacular bank
robbery with much automatic weapons fire.
> The spark gap consisted of 2 bolts. Therefore the energy would be
> concentrated between the bolt heads and therefore it would not radiate
> in all directions, but only in front of the bolts.
The light from the arc may radiate only from the gap, but most of the
electromagnetic energy is being radiated from the current in the wires.
Antenna current is what radiates energy, not voltage, which is why tesla
coils, with high voltage and low current, make terrible radio
transmitters.
> The phone was closer than 1m. It was more like 20cm distance. The phone
> shuts down by itself at 1m distance…
That would, of course, make a difference. The fields close up would be
highly nonuniform, so you couldn't even begin to use inverse square.
Actually, straight inverse (1/r) might be a better model for the field
falloff at that distance. I assume the dimensions of your circuit (cap,
wires, and gap) are on the order of 30 cm?
>
> >I am surprised that it damaged the phone, though. Perhaps your LC
> >circuit was resonant at just the right frequency to match that the
> phone
> >receiver's front end and blew up the first stage. I'd be interested to
> >know the make and model of the phone and the handset receiver frequency
> >(1.6 MHz, 49 MHz, or 900 MHz). A schematic of the receiver would be
> even
> >more interesting.
>
> I don’t have the manual for the phone and it doesn’t say anywhere the
> frequency. I would send you a diagram but I don’t have one either.
> The phone is a standard PTT TELECOM (the Dutch Phone Company) model. I
> have no more info on it… The antenna is about 1m long… (Telescopic)
Often, cordless phones use a different frequency for the base to handset
than for the handset to base. At one time, in the US, 1.7 MHz from the
base and 49 MHz from the handset was popular. This made the receiving
antenna in the handset easy (a ferrite loopstick, just like in a
portable AM radio), and allowed the use of the AC line wiring as the
transmitting antenna for the base to handset link.
These days, both up and down link are in the 49 MHz band, typically. A
quarter wave antenna would be 1.5 m long, but they're willing to use a
short antenna (perhaps 30 cm long) and give up a bit on gain, especially
because the licensing laws allow a certain field strength, so you just
boost the transmitter power to make up for the reduced antenna
efficiency (shorter battery life, of course,..)
THis little digression is slightly relevant, because I certainly
wouldn't expect your spark gap to radiate any appreciable amount of
power in the 50 MHz band, so it is my guess that your cooked handset
uses the 1.6-1.7 MHz receiver technique. Open it up (if you still have
it) and see if there is a ferrite loopstick in there. Typically, it
would be about 4 cm long, 0.5 cm in diameter, and wound with many many
turns of very fine (40 AWG) wire.
>
>
> >The EMP power is radiated by the wires in the circuit, not the gap
> >itself, so shielding the gap would make no difference.
>
> What makes you so sure???
To be a bit flip, Maxwell's equations make me sure. EM energy is
radiated by accelerated charges. The more charges accelerated, the more
radiation. A common way to accelerate charges is to have a varying
current. More current means more charges means more radiation. For a
circuit of some conductor (including the spark), the radiation from each
little chunk of the circuit is basically proportional to the change in
current per unit time. If you have a spark which is 1 cm long in a
circuit consisting of a piece of wire 100 cm long, you'll radiate 100
times as much energy from the wire as the spark. Of course, if you have
the wires twisted together (or even directly adjacent), the radiated
fields will cancel each other out once you get some distance away.
A transmission line works because it is arranged so that it doesn't
radiate the energy but transmits it, i.e. the fields in the conductors
are exactly balanced.
Try an experiment... Take a neon bulb (like a NE-2) and hook wires to
each lead a few tens of cm long, separate the wires like wings. This
will be a crude em field detector:
-------wire----------- neon -------------- wire--------------
Now, set up your sparking apparatus. Fire the sparks and move the neon
bulb detector closer and farther. Note the distance at which the neon
bulb just fails to light.
Rearrange your sparking apparatus by using longer wires (say a couple of
meters long) and separating them out, making a large loop with the cap
and gap in series. Do the neon distance measurement experiment again.
Now move the two wires close together (twist them, if the insulation can
stand it). Do the neon distance measurement.
You should notice that with the wires spread out, the neon lights up a
lot farther away. The spark, though, is almost the same.
You can also use a storage oscilloscope to do this, instead of the neon
bulb.
Another experiment. Put the spark gap inside a grounded metal can.
You'll probably still get the same radiated power.
> From my experience, whenever there are sparks, there is interference.
True, because sparks imply large changes in current in a short time.
However that spark gap typically is part of a large circuit. The TV
cascade, particularly if it arcs from the second anode connector to
ground, forms a loop a few tens of cm across. Ignition coils have spark
plug wires a meter long, although modern ignitions use resistive wires
to limit the rise time, reducing the radiated EMI. When you discharge a
Leyden jar, the discharge wire forms a loop.
A
> TV cascade will interfere with a phone when it sparks. Ignition coil
> circuits, leiden jars… You name it!
> Most of them produce very little interference and are hard to notice.
In a large part because radiated EMI is heavily regulated by various
governmental agencies, so a lot of work goes into reducing it.
> That’s why I didn’t expect the results to be so dramatic with my phone…
I am surprised at the dramatic results too! That's why I think you must
have a 1.7 MHz phone, because there is a reasonable mechanism by which
the damage can occur.
>
> Thanks for replying so fast!
Just satisfying my pedagogical urges...