EM and Lightning Detector

7/29/2019 EM and Lightning Detector

1/6CIRCUIT CELLAR ONLINE July 1999 1www.circuitcellar.com/online

ferics is anabbreviation for

atmospheric electro-magnetic radiation and

is the subject of a severe-weather-

monitoring project that Ive beeninvolved with for nearly 40 years.The monitoring techniques haveprogressed from an omnidirectionalpulse counter to detailed measure-ment of pulse shape and relativetiming with the appropriate data-analysis procedures. The resultsfrom each monitoring systemdictated the requirements for thenext step. This project has been atedious journey because each step

generally required one or moreyearly seasons to determine therelevant factors for the next step.

Currently, the monitor consists ofa crossed-loop antenna array,analog preamplifiers, microcontrol-ler, and PC. The microcontrollerperforms real-time analysis of theinput pulses to determine thedirection-of-arrival and several pulsecharacteristics. A standard PC isused to display, store, and manipu-late this data for pattern analysisand correlation with severe weather.

Electromagnetic Radiationand Severe Weather

Unfortunately, not allsevere storms turnout to be just a baddream. So, if yourerunning toward theroot cellar, you mightwant to bringRichards electromag-

netic radiation sensorso that you can di-rectly monitor theseverity and distanceof approachingstorms.

MONITORING PARAMETERSThree parameters (pulse shape,

pulse polarity, and relative timing) ofthe electromagnetic radiation aremeasured with reference to thedirection-of-arrival. These measure-ments are used to observe changesin the spectral distribution rather

than the exact magnitude of theparameter. In general, signalamplitude is disregarded.

A crude measurement of electro-static pulse polarity/shape (width)component is made by noting thepolarity of the first signal excursionand the time (width) to the nextexcursion of the opposite polarity.These factors are accumulated withand referenced to direction-of-arrival.

The relative timing between aseries of events has been the pri-mary monitoring parameter formany years. For this data, eachseries of three events that occurfrom the same direction in less than1 s is sorted into one of two catego-ries. Either the time between thefirst and second event is greater orless than the time between thesecond and third event. In a longseries of events, each event will

contribute to more than one com-parison. Each series is also countedas a burst.

These accumulations are refer-enced to the signal direction-of-arrival (120 segments or 3/seg-ment). Periodically, (usually each 5min.), the accumulations are ana-lyzed and separated into categoriesof generated total activity, burstactivity, percentage of positiveevents, percentages of long pulses

(width), and a timing factor that isthe ratio of the greater-than/less-than relative timing accumulators.

These summations are furtherreduced to peak activity descriptors(e.g., bands of activity concentrationwith reference to direction-of-arrival). Each peak descriptorincludes direction, activity level(events per time period), timingfactor, positive event percentage,and long pulse-width percentages.This data is the basis for the patternanalysis.

Building An Electrostatic and MagneticPulse Monitor

s

FEATUREARTICLERichard Fergus


2/62 July 1999 CIRCUIT CELLAR ONLINE www.circuitcellar.com/online

ANALYSIS PROCEDURESAll peak descriptor data is saved

on disk for correlation studies at alater time. Raw data is retained incomputer memory for up to 8 h andcan be saved on disk by keypress ormouse button click. Several formatsare available for data analysis. The

two most useful are graphical plotsof activity direction versus time andactivity peak (descriptor data timing,polarity, and shape data) versustime.

The activity direction versus timeis not useful for pattern detection butdoes help determine the location ofthe activity. The peak-width changeand directional movement is used toestimate the activity location. Thisprocedure is more subjective than

analytical but is useful to limit thesearch for reported severe weather.

The peak-tracking format tracesa selected peak (weather formation)over a period of time. The eventactivity is used as criteria for thetrace function, which finds matchingdirectional peaks from each periodicdata record and plots the activity,timing factor, polarity percentage,and pulse-shape percentages as afunction of record (time). This

format produces the patterns thatare the basis for severe-weatherdetection and prediction. During realtime, a peak can be selected bykeypress or mouse click and trackedfor up to 8 h. Archived data can alsobe analyzed in a similar manner.

PATTERN OBSERVATIONSThe timing factors have cyclic

variations with periods of nearly anhour. Large polarity variations with

reference to direction-of-arrival(across a front) have been ob-served. Reoccurring patterns of thetiming factor, polarity, and pulseshape are observed as a function oftime. These patterns occur over awide variation of timespans. Thepolarity distribution can changeslowly throughout the day or rapidlyin a few minutes. Generally, thepulse-width distributions are similarto the polarity patterns.

Distance effects (signal strengthsand activity rate) are minimized by

using graphic plots of distributionfactors (e.g., ratios or percentages).Although quantitative data is avail-able, it is seldom used for patternobservation.

The relative timing betweenevents occurring in a short period(less than 1 s) has been studied for

several years. This data is gener-ated from the time between succes-sive events that arrive from thesame direction.

Each series of three events isused to determine one bit of data.As each event triplet arrives, thetime between the first and second iscompared to the time between thesecond and third event. If the timebetween the first and second isgreater than the time between the

second and third, one accumulatoris advanced. The opposite conditionadvances a second accumulator.

There is an accumulator pairreferenced to each of 120 directionchannels. If the relative timingbetween events were truly random,you would expect the two accumula-tors to be nearly equal (there issome statistical error). When thetiming factor (ratio of the twoaccumulators) is plotted with respect

to time, it does not show a flat linebut varies over a wide range.

The timing factor patterns havebeen compared with severe weatherfor several years. The earlier timingfactor analysis sorted the time-between-events into two categoriesbased on the actual time rather thanthe current less-than or greater-thancomparison. This simpler compari-son was incorporated after consider-ing the Chaos Theory, which sug-

gests that one event would influencethe next event. The patterns fromthis comparison are similar to theearlier method of comparison.

The following interpretations aresomewhat subjective because thedata is from a single monitoring sitewith limited access to real-timeweather information. When two ormore weather formations arelocated in the same general direc-tion or the activity is too distant(greater than 500 mi.), the confi-dence of the monitored data is

reduced. In many cases, someevidence of the expected patternsare present. After many hours ofobservation, the following conclu-sions have been assembled.

In the early stages of the severeweather, the timing factor variesover a wide range in a periodic

manner. The period of some ofthese variations can be nearly anhour. Minutes before severe weather(large hail, funnel cloud, tornado,etc.), the variation becomes muchsmaller. During the severe weather,the large variation returns. Thequantity of multiple events alsodecreases at the time of severeweather.

Numerous severe weatheroccurrences have been analyzed

with respect to the timing data withvarying degrees of correlation. Thebest example is the Lemont, Illinois,tornado of March 27, 1991. In thiscase, the timing variation decreasedabout 35 min. before the tornadoand remained low until a fewminutes before the actual tornado.As you can see in Figure 1, theburst activity increased rapidly andthen decreased at the same timethat the timing factor variation

increased. From the many observa-tions of other severe occurrences,this seems to be a picture-bookrepresentation. Other observationsgenerally have one or more of theseunique patterns.

The Lemont tornado came froma rapid buildup. Some severesystems build up rapidly and then

Figure 1This was the range and burst-level variationrecorded for the Lemont Tornado.

15:00 15:30 16:00 16:30

Burst timing factor

Burst activity level

Lemonttornado



maintain a relatively constantactivity level for several hoursbefore the severe weather develops.During this constant activity level,

the timing factor can vary across awide range in a periodic manner. Itappears that the severe weather willeither occur during the rapid buildupor after several hours of relativelyconstant activity.

HARDWAREThe hardware consists of a

remote antenna-preamp assemblyand signal-analysis package (seeFigure 2). The antenna assembly

should be mounted clear of metalobjects for several loop diametersin most cases, several feet above abuilding roof will be satisfactory.Generally, the interconnect cablebetween the antenna assembly andanalysis package can be up to 100long. The system requires a powersource of 12 VDC at about 25 mA.The RS-232 data format is used totransfer sferics data to the PC serialport.

Two concentric loops are used tosense the magnetic signal compo-

nent. The remote preamps shouldbe mounted directly below theseloops. As an example, the loops canbe constructed with PVC pipe and

copper tubing by threading thetubing through two sets of perpen-dicular holes in the pipe about 18

H

L

R1 C1

L1

+

HI

H

L

R2 C2

L2

+

HI

NS

EW

North

East

To sense amplifier

Alternative

88 mH toroiddetail

Insideturn

Outsideturn

HILO

Splice

Two added turns

+

+

C1 C2 = 3300 pf (select to tune)R1 R2 = 56 K

Top view

LO

HI

Remove core wiringfrom positive terminal.

Add center terminal at

left center.

Add two turns betweencore winding and pot shell.

C1 C2 = 18 pfR1 R2 = 100 K

One turn loopssingle 1824 diameter

Figure 2The loop construction should be sturdy because the direction resolution is related to physical looparrangement and stability.

apart. At the top of the pipe, eachloop of tubing can be bent slightly toeliminate any contact between theloops. A PVC pipe splice (securedwith several screws rather thanglue) can be used below the loopsto provide access to the loopconnections and remote preamps

shown in Figure 3.The magnetic signal component

is inductively coupled through thetwo turn loops of the input trans-formers, which are tuned to 10 kHz.By connecting the two loops to-gether at one point (center tap ofcoupling loops), the electrostaticsignal component may also besensed.

Additionally, the sense channelincludes a low-pass amplifier stage

that rolls off at 250 kHz to reducelocal broadcast interference. Largesignal protection (local lightning,etc.) is provided by a series ofresistance and clamping diodes forall amplifier inputs.

Briefly, the U2c and U2d op-amps are connected as integratorswith the dual four-position CMOSswitch (U3) during a specific half-cycle of the-10 kHz component ofthe sense signal as detected by

U7a/U7b comparators (see Figure4). After this integration, the twointegrators and U5a inverter are

Figure 3The preamps are standard op-amp circuits that provide antenna-component isolation and cable-drivecapability.



connected in a quadrature oscillatorarrangement via U3 by themicrocontroller. The time it takes theoscillator to reach a predeterminedcondition (U4a switch) is proportionalto the signal direction-of-arrival. The

virtual signal ground (REF) is biasedat +5 V to allow for bipolar signalswings of all op-amp circuitry.

The output of a second low-passfilter is applied to a series resonatecircuit (L3-C24) to select both the 0-

kHz component and provide a 90phase shift to align the electrostaticand magnetic signal components(see Figure 5). The two comparatorcircuits U7a and U7b generate bothpolarity trigger pulses for themicrocontroller analysis. Thecomparator threshold level is

adjustable by the microcontrollerthrough the current source (U8) andresistor network (R4245).

The microcontroller (U9) circuitryshown in Figure 6 generates athree-character serial data string foreach input event. These characters(binary value) indicate the direction-of-arrival, time from the last event,and the pulse width. Also, severalfunctions are available to control thedirection calibration, timing factors,

and signal threshold level (sensitiv-ity). Several LEDs are used toindicate relative signal levels.

MONITOR PROGRAMSA PC program sorts serial data

from the loop antenna hardware/microcontroller into categories withreference to direction-of-arrival. Theraw data is accumulated in severalcategories: total and burst activity,time-between-events, pulse width,

and polarity. This data is maintainedin memory for 8 h. Periodically(every 5 min.), the data is reducedto several activity peaks withseveral descriptor parameters.

A number of options to view andsave selected data are available viakeypresses or mouse clicks. InPhoto 1, the upper left is a plot ofthe last 8 h of data in 10-min.increments. The plot is updatedevery 5 min. with burst (white) and

total activity (red). The horizontalaxis is direction (3 resolution).The upper-right display is a track

of an activity peak for up to the last8 h, which is selected by the opera-tor (cursor not shown in the demo)for a specific direction from thehistogram or a direction/time fromthe activity/time plot. This data is asummation of data within the peak(concentrated activity over one ormore direction channels), which isalso the source of descriptors fordisk file archiving. From top to

Figure 4Signals from the loops preamps are applied to the comparator and converter circuitry (which interface tothe microntroller). Direction-of-arrival, pulse polarity, and pulse width data are developed from analysis of theseantenna signals.

Figure 5The peak amplitude circuitry (U6) and comparators (U7c and U7d) generate signals for microcontrolleranalysis of pulse polarity and width factors.

..

.



bottom, the following peak descrip-tors are plotted:

timing factor change for the lastthree readouts (15 min.)0change at the top

percentage of the narrow pulsewidths

the time-between-events timing

factor logarithmic plot of burst and totalactivity

range of percentage positiveevents for total activity across thepeak

range of percentage positive

events for burst activity across thepeak

Below the track display, numeri-cal data (peak direction limits,timing factor, and burst polarityfactors) for a selected time span areshown. Peak descriptor data isperiodically saved to disk for laterreview. The running display can besaved to disk at any time by opera-tor command. Specific histogram ortrack plots can also be saved to diskin a spreadsheet-compatible file(space delimited). All programs arewritten in the Forth language,

specifically Pygmy Forth.

FORECASTThe timing of the electromagnetic

energy pulses from weather systemsmay provide a key to the predictionof their severity. This sferics projectgenerates data that indicates arelationship between severe weatherand the relative timing betweenbursts of electromagnetic energyemanating from the weather system.With the limited data collection(single monitoring location), it isdifficult to determine the potential ofthis technique.

Polarity and pulse shape factorshave not been fully evaluated, andthe repeating nature of the patternssuggest that there must be condi-tions that cause the effect on elec-tromagnetic energy pulses. Observa-

tions appear to support a relationshipbetween the energy pulses andweather severity.

Obviously, additional effort will benecessary to prove or disprove theusefulness of this technique forsevere-weather prediction. Thistechnique is presented at this time todescribe the current state of theproject. To fully evaluate this sys-tem, more monitoring stations will beneeded to provide better coverage

and triangulation information.Photo 1Heres what the normal running display looks like

Figure 6The LEDs are pulsed for several milliseconds with each input event. Theres even one LED (FAULT) to indicate a watchdog timeout or possible program failure, highsignal rates, and so on.



These monitoring sites should belocated on a grid 300500 milesapart. The site equipment wouldoperate unattended and communi-cate with a central analysis facility.The site equipment would consist ofthe 2-diameter loop antennasystem, amplifiers, and computer. In

most cases, this could be accommo-dated in the top floor of an existingbuilding.

The sites should not requireattention that could not be handledthrough the communication link.Local readout or analysis equipmentcould be included but would not benecessary for the network to func-tion. The communication linkrequirements are minimal. Ingeneral, less than 1000 bytes of

data would be transmitted each 5min. During extremely severeweather, the data rate from selectedsites would increase by severaltimes. A standard telephone line canfulfill this requirement.

With additional monitoringlocations, the personnel at theanalysis facility will be able toselectively analyze the data fromthe site with the best view (i.e.,only one weather system within the

usable range). Pattern analysis fromone or more sites can be used todetermine the severity of theweather system. When necessary,data would be available from othersites to locate an occurrence bytriangulation. Because more better-quality data would be available atthis analysis facility, you wouldexpect to find additional patterns orsignatures which relate to severeweather.

Richard Fergus is a retired elec-tronic engineer with a background in

instrumentation for safety systems.This severe weather project and

amateur radio are his primaryinterests. You may reach him at

[email protected].

Circuit Cellar, the Magazine for Computer Applica-tions. Reprinted by permission. For subscriptioninformation, call (860) 875-2199 [email protected]

RESOURCE

Project information and docu-mentation, www.theramp.net/sferics

EM and Lightning Detector

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