aboulang@bbn.com (Albert Boulanger) (05/21/89)
Dannie Gregoire (dannie@coplex) writes:
I am currently doing some research and study of lightning
and have come upon the mention of a field mill (a device
which measures the intesity of the electromagnetic pulse
from lightning) The schematics I have found for one
are non-standard as well as archaic. If any one has any
schematics of such a device, please send them my way. I am
also in search of a way to use a set of radio-directional
finders to triangulate upon where lightning strikes are occuring.
Any relevant info or comments on this project would be appreciated
as well.
Thanx-a-head-a-time!
First of all, you should be aware that there is coverage of most of the
US nowdays by a national network of lightning cross-loop recievers
made by a company called LLP in Tuscon Ariz. You can get online data
of lightning events across the US by talking to:
Dick Orville
SUNY Albany
You can buy these recievers from LLP.
Now a field-mill really detects the slow-varying electrostatic field
changes of thunderstorms. The fair-weather field is something like
100V/meter. I trust you have some info of the details on the
mechanical design. They are a rotating air-capacitor affair. The
bottom of the capacitor is split up into quadrants and the top is two
opposing conjoined quadrants (like a butterfly wing) that rotates.
1 | 2
-- --
3 | 4
The signal is picked off of quad 1&4, 2&3 by an instrumentation
amplifier (the top rotating vane is grounded). This rotating vane acts
as a chopper so we get an ac signal out of this (thus best to use a DC
motor). The front-end of the instrumentation amplifier should be FET
op amps. (For the design of an instrumentation amp, pull out the
schematics from your fav Brain-wave biofeedback device. There was one
in Popular Electronics in the early-to-mid 70s.) Note that we dealing
with VERY small currents so mounting the front-end on *clean* ceramic
is wise. The tail end is an integrator. I belive there was a field
mill design in the Amateur Scientist column in Sci Am many years ago.
At any rate, I can send you some info my snail mail. One possible
source for field mills is surplus from 'Nam. They were used to detect
the electrostatic potential of helicopters in a feedback circuit with
an electrostatic generator that was used to null-out the potential.
One source that had them a while back was:
Microwave Equipment Company
4121 N.W. 27th Street
Miami Fl. 33142
305 871-2171
Potentially yours,
Albert Boulanger
BBN Systems & Tewchnologies Corp.
aboulanger@bbn.com