On 17 Mar 2004 12:02:15 -0800, (John Michael
Williams) wrote:

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.

There were few spark transmitters mounted in automobiles. They were
just too inefficient, big, and clumsy to be functional. Transmitters
in vehicles really didn't start until tube type transmitters became
popular. The problem was that the typical mobile radio used a
dynamotor (motor-generator) combination to generate the necessary high
voltages. With the radio and dynamotor mounted in the trunk of the
vehicle, there was a good chance that gasoline fumes would accumulate
in the trunk of the vehicle and be ignited by the spark from the
dynamotor commutator. See the photo of the 80D at:
http://www.telmore.com/ka1nvz/old_tw...ola/49-59.html
The dynamotor is the black cylinder near the handle. The 140D was
twice as big and heavy.

Back in the 1960's, my 1960 Ford Falcon had an assortment of Motorola
80D and 140D radios in the trunk. I experienced a small explosion in
the trunk ignited by the dynamotor. I had filled up the gas tank at
the local gas stop (for 19 cents per gallon). Warm weather caused it
to expand and leak vapour into the trunk. Key the transmitter, the
dynamotor starts, sparks, and boom.

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.

Lovely. A harmonic generator. Any reason you want lots of harmonics?
Shottky diodes or any other non-linear device, do not belong in
antenna matching circuits.

It's home made, but it's probably
as good as any other wire about that long.

Wrong. Optimum for CB is either a 1/4 wave monopole (102 inches) or
two of them to form a half wave dipole.

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.

Find a 4 watt flourescent light. Attach a 1/4 wave antenna to your 5
watt CB radio. Transmit. Hold lamp in hand and touch the end of the
102" antenna. It will light when you talk. (Note: With AM
modulation, you only get 5 watts when you yell into the microphone.
Without modulation, you only get about 2.5 watts of RF). Now, ask
yourself what voltage is required to light the flourescent lamp.

In order to get a spark, you need to generate enough voltage to ionize
the air between the contacts. That's about 20KV/inch. If we
eliminate the antenna, 5 watts of RF into 50 ohms will generate:
P = E^2 / R
E = 16v rms
E(peak) = 1.4 * 16 = 22 volts
The gap necessary to create an arc with 22 volts is:
22V / 20,000V/in = 0.001 inches
Kinda small, but given a microscope, a 1 mil spark gap will arc. Of
course the VSWR protection circuity in the transmitter will instantly
shut down the transmitter when it arcs, but that takes a few millisec.

Notice that this is a voltage phenomenon, and is not dependent upon
the power level. Therefore, an antenna that offers a voltage step-up
will generate a higher voltage. However as the antenna is in the air
and nowhere near a close enough ground to arc, it doesn't matter. If
there's gonna be any arcing, it will be between the xmitter output and
the base of the antenna.

The typical mobile FM transmitter of the day (1960's) cranked out
between 15 and 150 watts. Most were around 75 watts. Run the calcs
again for 75 watts and see if the gap is more reasonable (I'm lazy).

The next question is how much heat is necessary from the arc to ignite
the gasoline vapour. I'll leave that as an exercise for when I have
more time to burn. Gotta get back to lying and cheating on my taxes.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831.336.2558 voice http://www.LearnByDestroying.com
#
# 831.421.6491 digital_pager AE6KS