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From: aboulang@bbn.com (Albert Boulanger)
Newsgroups: sci.electronics,sci.physics
Subject: Field Mills and all that
Message-ID: <40240@bbn.COM>
Date: 20 May 89 19:16:53 GMT
Sender: news@bbn.COM
Reply-To: aboulanger@bbn.com
Lines: 62


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