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Caveman RadioW i t h underg round induc t i ve transmission, 300 feet is almost DX.

Frank S . Reid W Y M K VPO Box 5283Bloornington IN 47402 M gne t i c - i nduc t i one q u i p m e n t w h i c htransmits signals through

the ground is a valuable aidto cave-mapping and under-

ear Park City, Kentucky, penetrated cave wi th in tw o feet o f radioby42 73 Magazine February, 1984

ground rescue. Even moreuseful than its communica-tion ability is its ability to ac-curately find a spot on thesurface above an under-ground transmitter. It canalso determine depth withina few percent, using field-geometry measurements.

It's legal! Magnetic induc-tion is not real radio-it'ssimply very-loosely-coupledtransformer act ion. The FCCdoes not define equipmentoperating below 10 kHz as"radio frequency devices."How It Works

Inductive communicationis a very old technique (see"Who Real ly InventedRadio? The Twisted Taleof Nathan B. Stubblefield,"73, December, 1980). Whenamateur radio was bannedduring World War II, manyhams communicated by"ground wave," i.e., magnet-ic induction and earth-cur-rent. ("Earth-current" istransmission of audio-fre-quency signals through theground between pairs ofwidely-spaced ground rodsconnected to amplifiers.)Ranges greater than onemile were claimed.

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Skin effect, which causes

n" increases as frequen-

Audio-frequency magnet-penetrate most geo-

structures easily. Theremethods for locating oreion equipment as a sortJune,1928).

Inductive communica-i s inherently short-range

which obey an in-

radio (electromag-

netic) waves at audio fre-quencies would requireenormous antennas.

E . R. Roeschlein suggest-ed using the directionalproperties of magneticfields t o map caves in an ar-ticle in Electronics, Septem-ber 23,1960. Cavers, notablyWilliam Mixon and RichardBlenz, refined the equip-ment and developed depth-measuring techniques whichare independent of signalstrength (several articles ap-pear in Speleo Digest, 1964).Equipment

It's easy to get 300-footrange with very simpleequipment. Longer rangesare more challenging.

A transmitter is just an au-dio oscillator driving an am-plifier which i s driving a coil.Impedance matching is im-portant for maximum coilcurrent. Perhaps the mostimportant part of the trans-mitter is the keyer-a circuitto make it go "beep.. .beep. . . beep." In additionto the advantage of savingbattery power, a pulsed sig-nal is much easier for the re-ceiver operator to distin-guish against a backgroundof interference than is asteady tone.

A simple resonant coilconnected to an audio am-

t---)ctE L E C T T O R E S O N A T E

RE GE NE RA TI ON-

1. One-chip transceiver uses Q-multiplier effect for highd selectivity. Antenna needs no electrostatic

U? is any 741-type op amp.

WB9TLH operates underground transmitter on 3500-Hz CW,using microswitch for telegraph key.

plifier wi ll work for a receiv-er. Use crystal earphones,because magnetic phoneswil l cause feedback.

The circuit of Fig. 1 is aQ-multiplier. The resonantcircuit is in negative feed-back instead of being simplyconnected to the amplifier'sinput. The Q (regeneration)control taps some of theoutput and feeds it back tothe noninverting (+ ) input.

The amplifier forms a nega-tive resistance which cancelsthe resistance of the coil. Asthe Q control is advanced,sensitivity and selectivityget higher and higher untilthe circuit goes into oscilla-tion (infinite Q). Since i t willoscillate, the circu it can alsobe used as a very-low-pow-ered transmitter.A 60-Hz notch filter willnot get rid of power-line in-

A N T E N N AQOS CI LLA TOR1 z::.~ I

T U N I N G - F O R KFI LTE R

4 M O S D j ' l D E R S 11 2 8 1

3 5 0 0 ~ ~*

Fig. 2. Receiver with frequency conversion allows very highgain without feedback problems.

B A L A N C E DMI X E Ri M C 1 5 9 6 1

r - - - i

A4 5 0 0H i L - - - _ J~

0 - M U L T I P L I E R M U R A T A E F M - R1 1 0 0 0 H z 1

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3 TU R N S t

15551 M A N U A L K E Y

C R Y S T A LO S C I L L A T O R1 26 K H z

3. A cave-radio transmitter. Precise frequency control is n

Hz bu t many harmonics.use active filters indis-

Even with crystal ear-

of shielding and de-

DI'JIDERS1 - 3 6 ]

but then it's not portable. Abalanced mixer and local os-cillator can convert the in-pu t frequency to some otherfrequency, which can thenbe filtered and greatly am-piified without feedbackproblems. Fig. 2 is a blockdiagram of one such receiver.

3 5 0 0 ~ ~

InterferencePower lines are the majorsource of interference, evenin isolated areas. Harmonicsof 60 Hz extend well into theultrasonic frequencies. Pow-

rface locati on and depth of transmitter are found by null-g wi th direct ional antenna and by measuring shape of

T U R N :

OU TPU T IN D IC ATOR

ecessary if receiver uses very-

er-line interference is usual-ly directional and can bepartially nulled out by thereceiving antenna. To mini-mize interference, choosean operating frequency inbetween a pair of power-lineharmonics and use a receiv-ing filter narrow enough toreject the adjacent signals.Resonant-reed or tuning-fork filters of the type usedin radio pagers can providethe necessary selectivity.Such extremely narrowbandwidths require precisefrequency control and veryslow CW speeds.Atmospheric noise fromdistant thunderstorms canbe a problem in summer.Day time atmospheric noiseis minimal around 3.5 kHz(National Speleological So-ciety Bulletin, vol. 32, no. l ,January, 1970). The noiselevel increases appreciablyafter dark. Atmosphericnoise is polarized such thatit nulls when the receive coilis horizontal.What's the best frequen-cy to use? Mid-range audiofrequencies work well, andthe equipment i s easy to

build. I use 3500 Hz. 3276.8Hz would be a good fre-quency because i t is easy togenerate from a 32.768-kHzwristwatch crystal. 3276.8Hz falls in between harmon-ics of both 50- and 60-Hzpower lines, and so cou ld beused in any country. Athigher frequencies, groundabsorption increases andaudio amplifiers becomeless efficient. Some experi-menters have tried SSB onultrasonic frequencies, buthave found no advantagesto justify the complexity ofthe equipment. Below 2kHz, atmospheric noise andpower-line harmonics arevery strong. Subaudible fre-quencies bel ow 60 Hz havebeen used, with very com-plex receiving equipment.The OMECA navigationsystem transmits very strongsignals on several frequen-cies between 1 0 and 14 kHz.OMECA s ta t ions makegood beacons for testing re-ceivers. Each station trans-mits for one second in a se-quence that repeats everyten secondsAntennas

For best performance,maximize the magnetic mo-ment of the coils. Magneticmoment is Ampere-turnsmul tip lied by the coil's area.Doubling the range of aninductive system requires ane~ gh tfo ld ncrease in mag-netic moment, other factorsbeing constant. Self-reso-nance limits the number ofturns a coil may have Aneightfold increase in currentimplies either much largerwire or a 64-fold power in-crease. It's easy to see that

L A R G E S I N G L E O RM U L T I T U R N L OO P

ew2" 0 0 A T T 4 0 0 H zS I R E N l P A A M P L IF I E R v2R2C. . -, - .-

mR E C E I V E R

Fig.4. Surface-to-cave transmitter uses large antenna and highpower, so that underground equipment can be small. Surplus400-Hz transformers are very cheap or free because there isli tt le demand for them. (Caution- ossible shock hazardbetween chassis and earth grounds i f amplifier has no internaloutput transformer.)

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N U L Lt

P E A K I

N U L L

5. (a] Received signal disappears when coi l is paral lel tob) Note that magnetic-induction receivers nul l directions in the plane of the, unl ike radio loop antennas. (c) Nul ls are much sharperaks, bu t wi th very weak signals you may have to seekaks instead of nulls.

II

R E C E I V E R C O I L I NV E R T I CA L P L A N E

Fig. 6. Finding the approximate surface point above thetransmitter (plan view).

For a given length o f wire,

OK forns, bu t coils fo r

tb

t to the coil, and con-on a good receiver.Ferr i te-core antennas

entice-Hall, is an excel-Nathan 6. Stubblefield

receiver will detect the sig-nal. Likewise, a pair ofground probes can detectvoltage induced by a distantcurrent-carrying coil. Somecave-radio experimentershave buil t equipment whichoperates in either mode, al-lowing greater flexibility inv a r y i n g c o n d i t i o n s o fground conductivity.

Voice O perat ionM y own equipment was

designed primarily for direc-tion-finding and minimumweight. It can transmit fromcave to surface by CW, butit does not transceive. T w eway communication is notessential for surveying oper-ations, but i t can be veryuseful. (People who don'tknow Morse code can usual-

ly send it intelligibly if prc-vided wit h a code list and afew minutes of instructionon lengths of dots, dashes,spaces, letter and wordspacing, and abbreviations.)

For a "downlink" I use a12-volt-operated, 100-Wattpolice siren/PA amplifierdriving either a large loop ofwire lying on the surface or apair of ground rods. A surLlnlus 400-Hz variable autc-transformer matches theamplifier to different loads.The underground voicereceiver has a ferrite-corecoil connected to an audioamplifier through a high-pass LC filter which cuts offat 600 Hz, with 70 dB of re-jection below 300 Hz. Thefilter rejects the strongestpower-line harmonics. Aband of voice energy calledthe first formant is lost, re-sulting in loss of the quali-ties that distinguish individ-uals' voices, but intelligibil-ity remains. The femalevoice works best here.Ma gne tic Direction-Finding

Someone must take thetransmitter int o the cave tothe point of interest and turnit on at an appointed time.The transmit coil must behorizontal and very accu-rately level.Received signal strengthdepends on how much mag-netic flux Dasses through theReceiving antenna has inclinometer made from vernier radio coil. with ' the plane gf thedial and spirit-level for measuring vertical angles. coil parallel to the field, no

73 Magazine * February,1984

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7. Cround zero is pinpoin ted by finding the spot whereeld is vertical. Then, distances [L ) and vertical angles[el

f transmitter.

A N G L E OF F I E L D F R O M V ER T I C A L I D E G )

8. Cave Radio Depth Chartlafter W. Mixon). Each curve isvs. fie ld angle for a different horizontal distance from

ound zero. Use any distance units: feet, meters, etc. Exam-50'and angle 40 depth is 102'. Thanks to

. Blakely for providing this HP-85 computer p lot.

ten-and the signal disap-

Viewed from above, the

om the center (Fig.6).

Once the approximate

inches, using vertical

view o f the curved shape ofthe field. Point the coiltoward maximum signal,then t ilt it back and forth tofind a null which indicatesthe direction of the fieldcoming up out o f theground. Move in the direc-tion of decreasing verticalangle to find a place wherethe null direction is straightdown. Turn 90 horizontallyand repeat the procedure,getting closer to the centerof the field each time."Cround zero" is the pointwhere the vertical null isstraight down, no matterwhat horizontal directionyou point the coil's axis. Anexperienced operator canusually find ground zero inabout ten minutes and de-

CALCULATOR METHODFinding depth by calculator is fast, easy, eliminates plott ing

errors, and provides wider range than the graph. (The graph st illhas the advantages of low cost and easier error detection.) Aprogrammable pocket calculator with nonvolatile memory,such as the Hewlett-Packard HP-29C, is ideal for calculatingdepth while on location.

HP-29C Program for Depth of Cave RadioEquation solved for depth:~ = ~ ( 3 + d 9 + 8an20 )4 tan O

g LBL 0g DEGf TANENTERg x28

X9+fn3+x2y4X-

Xg RTNTo use: Key in L.

ENTERKey in O (in degrees).GSB 0

Example: L= 50', O = 45 ": Depth= 89.04'

termine depth in anotherten.

Finding DepthThe receiver antenna

should be mounted on a rig-id, flat board or frameworkand must be equipped withsome type of inclinometer,such as a carpenter's pro-tractor. Estimate vertical an-gles to the nearest 1/10 de-gree when taking data fordepth.Mark ground zero with astake or rock. Stretch a mea-suring tape horizontally awayfrom ground zero and mea-sure the vertical angle ofthe field at several differentdistances away. Use the dis-tance-and-angle data in thecalculator formula aboveor p lo t the data on the fami-

ly of curves in Fig. 8. Aver-age the results of severalpairs of data. The depthsshould be consistent, fall ingnear the average value andrandomly either side of theaverage. An increasing ordecreasing trend indicatesan error in ground zero loca-tion or an unlevel transmitcoil. Most of the error canbe recovered by taking an-other set of data in the op-posite direction away fromground zero and averagingthe results of bo th sets.

Note that the slope of thedepth function (Fig.8) is verysteep for small angles, i.e., asmall error in measuring theangle will produce a largedepth error. For best results,use only angles between 1 2 Oand 75O. (At vertical angles

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ear and greater than 90course, the signal is weak-

The depth chart (Fig. 8)an e = 3LD/(2D2- LZ),

e = angle of field=0), L = horizontal dis-

om ground zero, and= depth. The formula is

very small re lative t oNote that the c losedves of the magnetic field

t circles. Sim-ed to determine depth ( DL when e = 71.57O, not). An 8 112" X 11 " work-copy o f the depth chart

available from the author

e FutureExtending the range of

makes a fine project for

hams, especially VLF enthu-siasts. Experiments on 1750meters should be especiallyinteresting.

Correlation, signal-aver-aging, and other sophisticat-ed techniques for weak-sig-nal recovery are becomingincreasingly attractive toamateurs wi th new develop-ments in integrated circuits.Very-long-range cave radiocould, of course, be accom-plished by interfacing short-range cave-to-surface linkswith conventional amateurradio equipment. Futuretechnology may allow corn-munication through the en-tire Earth on modulatedbeams of neutrinos!

The National Speleo-logical Society is an or-ganization promotingsafety and conservationin the sport and scienceof cave exploring. Theiraddress is Cave Avenue,Huntsv i l le , A labama35810.

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73 Magazine * February, 1984 49