"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