Home |
Search |
Today's Posts |
#10
|
|||
|
|||
"John Michael Williams" wrote in message
m... Claims that people have started fires by using their cell phone while refueling a car apparently are false: See http://www.snopes.com/autos/hazards/gasvapor.asp and other sites. However, the first radios transmitted sparks, so in principle it should be possible to transmit near a long wire separated by a small gap from ground or another wire and get a small spark. So, I decided to try an experiment. I don't use a cell phone, so I assumed a CB radio transceiver would be a reasonable substitute: The power output of a nominal 5 W CB also is consistent and nonadaptive, so a possible unknown (actual output power) is avoided. Cell phones are adaptive and not very consistent in power output, so power should be monitored during a cell phone experiment. 5 W is considerably more than the 0.2 to 2 W typically possible from a cell phone; the power should be the important factor, although maybe someone should repeat this experiment with a cell phone, which would operate at a much higher frequency. I used a Radio Shack TRC-231 handheld (stock #21-1675) with xmit power on high and set on Channel 40. The antenna was the one that came with it (about 25 cm long). I set the volume to max and the squelch at min to be able to detect anyone else trying to use the channel; this was just to be sure that my brief, silent transmissions would not interfere with anyone. I used the CB indoors, in a mostly metal-shielded room. Because CB wavelength is around 10 m, everything I did was in the near field; however, the inverse square law for power still holds, allowing that the CB antenna is more of a line than a point source under my conditions below. The first thing I noticed was that every time I keyed the transmit button, the CB would switch the light level of a nearby touch-dimmed lamp, and it made a Microalert microwave detector scream. I unplugged the lamp and turned off the Microalert. Then, I tried to light a 120VAC indicator neon lamp attached to two solid copper switchback wires totalling about 1 m long, so the lamp was in the middle effectively of a dipole antenna. I tested the lamp and found it would light with 10 microamps current. The CB had no effect, even if held parallel to, and almost touching, the wires. Thus, the near field of a 5 W CB radio can not supply about 90 V at even 10 uA, under these conditions. I then attached a 1.2 m monopole antenna to an oscilloscope. This antenna has a Schottky hot carrier diode and impedance matching resistors builtin. It's home made, but it's probably as good as any other wire about that long. I hooked the antenna coax to an oscilloscope: With the CB transmitting, and its antenna parallel and 1 m away from the monopole, the amplitude was about 100 mV p-p, at 27 MHz or so. I could not get more amplitude no matter how close I held the CB, or at what angle. Touching the bare monopole wire increased the amplitude by no more than 10%. So, first conclusion: To get even a 1 V spark would take a wire at least 9 m long, all somehow kept within 1 m of the transmitter. Thus, it appears it is not feasible to create a hazardous spark with a CB at a gas station. Just to be sure, I taped a 1 m wire to a table top in the dark and slowly brought it closer and closer to another wire plugged into a wall socket 3rd wire ground (yes, I verified that the socket was wired to ground first!). At each distance, I briefly keyed the CB. I could not see any spark, even after dark-adapting my eyes for 10 min and letting the wires touch. I might have dark-adapted longer, but I don't know whether I should have been able to see a 50 mV spark or not. So, I think sliding over on a car seat, and thus generating a possible static charge, would be more likely to ignite gasoline vapor than talking on a cell phone while refueling. However, it would be useful for someone to repeat this kind of test with an actual cell phone, as opposed to a CB radio. The wires should be shorter, for one thing . . .. I'm cross posting to an antenna group, looking for criticism. John John Michael Williams It's a bit of a stretch to think that cell-phones are a problem, whereas the car driving off next to you, with a set of spark plugs going for their lives, is not. Hmmmm. Ken |