VLF EARTH-CURRENT COMMUNICATION INTERIM REPORT

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03424A000400090001-9
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 81
• Document Creation Date: December 23, 2016
• Document Release Date: March 31, 2014
• Sequence Number: 1
• Publication Date: October 27, 1960
• Content Type: REPORT

Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
C NFIDENTIAU
Hi
VLF EARTH- CURRENT COMMUNICATION
INTERIM REPORT
27 October 19b0
NF1DENTBAL
V Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
50X1
,-- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
LJ
TABLE OF CONTENTS
Page
SECTION I - SUNNARY
1.1
Summary
2
1.1.1
Surface Wave Propagation
2
1.1.2
Conducting Layer Propagation
2
SECTION II - SURFACE WAVE PROPAGATION
2.1
Introduction . . . .
4
2.2
Surface Wave Propagation at Low Frequencies
6
2.3
Surface Layer Propagation at Ultra Low Frequencies .
8
2.4
Noise Characteristics
13
2.5
Communication Range . . . . .
22
2.6
Equipment Size and Weight
32
3.1
3.2
SECTION III - CONDUCTING LAYER PROPAGATION
Introduction
Noise Characteristics for Vertical Electrode Configuration .
38
38
3.3
Propagation of Earth Currents from Vertically Spaced
Electrodes
41
3.3.1
Homogeneous Earth ? ? ? .
41
3.3.2
Layered Earth . . .
46
3.3.3
Range, Frequency, and Data Rate Considerations
48
3.3.4
Discussion . . ? .
48
APPENDICES
Appendix A - Ultra Low Frequency Earth Current Propagation
Appendix B - Ultra Low Frequency Field Tests at 1/2 Cycle per Second
-
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
nx?I
50X1
I
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
LJ
-1
Figure No.
Portable
Plot of
LIST OF ILLUSTRATIONS '
Page
Earth Current Communication
do
1
1
2
5
7
- (2r-v-)2 + j .2)1_2.1
?1
3
Signal Strength
vs. Range (1 to 30 kc)
9
It
Signal Strength vs. Range (f = 100 kc) .
lo
5
Two Layer Earth
12
6
Ep vs. Range (d = 10 Meters)
14
7
E, Vs. Range (d = 100 Meters)
15
8
E/0 vs. Range = 1000 Meters)
16
9
Skin Depth 6 and Air Wavelength A vs. Frequency .
17
10
Ratio Ev/Eh of Vertical Field in Air to Horizontal
Field in Earth
19
11
Summary of Measurements of Horizontal Noise Field at
Surface of Earth
20
12
Comparison of Theoretical and Measured Signal
at 3 kc
21
13
Standard Noise Spectra Used in Computation
23
14
Communication Range vs. Frequency and Data Rate
(Low Noise, 5000 Watts Power) . .
25
15
Communication Range vs. Frequency and Data Rate
- (High Noise, 5000 Watts Power)
27
16
Communication Range vs. Frequency and Data Rate
(Low Noise, 1 Watt Power)
fl
17
Communication Range vs. Frequency and Data Rate
(High Noise, 1 Watt Power) . .
31
18
Equipment Weight vs. Transmitter Power
34
19
Range vs. Power for 5 wpm Data Rate .
35
20
Range vs. Power for 80 wpm Data Rate
36
-
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
fl
,LJ
?
LIST OF ILLUSTRATIONS (Continued)
Figure No.
21 Vertical Electrode Configuration . .
22 Ratio of Vertical Received Noise to Horizontal
Noise at Surface
23 Received Noise vs. Frequency (One Cycle Bandwidth,
Low Noise Condition)
24 Received Noise vs. Frequency (One Cycle Bandwidth,
High Noise Condition)
Homogeneous Earth
25
26
27
28
29
30
Layered Earth
Received Signal vs
Range vs. Frequency
Range
Range
vs. Frequency
vs. Power for
Condition) .
31 Range vs. Power for
Condition) .
32 Range vs. Power for
Condition) .
33 Range vs. Power for
Condition) .
Table No.
Range
(Law Noise Condition)
(High Noise Condition)
5 wpm Data Rate (Low Noise
5 wpm Data Rate (High Noise
? ?
80 wpm Data Rate (Law Noise
8o wpm Data Rate (High Noise
LIST OF TABLES
Communication Range vs. Frequency and Data Rate
(Low Noise, 5000 Watts Power) . .
-
Page
39
42
44
45
47
49
50
51
52
53
54
55
Page
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
-1
fl
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
LIST OF TABLES (Continued)
Table No.
Page
II Communication Range vs. Frequency and Data Rate
(High Noise, 5000 Watts Power) . 26
III Communication Range vs. Frequency and Data Rate
(Low Noise, 1 Watt Power). ? .
IV Communication Range vs. Frequency and Data Rate
(High Noise, 1 Watt Power) . .. 30
- iv -
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Page 1
SECTION I
SUMMARY
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1.1 SUMMARY
This report covers the application of earth current communication
techniques to a portable coiinnunications equipment for operation over ranges from
2 to 300 miles. Propagation and noise characteristics are considered for fre-
quencies up to 100 kc.
1.1.1 Surface Wave Propagation (Horizontal Electrode Spacing)
Using data rates of about 5 words per minute, a pocket-sized set can
communicate at ranges from 3 to 30 miles, depending upon noise conditions. A
larger battery-powered set weighting 150 lbs. total (in two or three portable
cases) has a range from about 15 to 700 miles, depending upon frequency and noise
conditions. Even under high noise ,conditions, the 150 lb. set can communicate at
80 words per minute to a range of 55 miles. A frequency of 100 kc results in the
most advantageous propagation characteristics, but such equipment could readily
be detected and jammed, even at relatively long ranges. For ranges less than 10
miles, frequencies on the order of 100 cps and lower are advantageous. Jamming
at long range would be virtually impossible with such a system. For longer ranges,
frequencies of 3 kc or 30 kc are applicable, With 3 kc having shorter range cap-
ability but greater resistance to jamming.
1.1.2 Conducting Layer Propagation (Vertical Electrode Spacing)
Using a data rate of about 5 words per minute, a pocket-sized set can
communicate at ranges from about 4 to 8 miles, depending upon noise. The larger
150 lb. set can similarly communicate from about 10 to 18 miles at 5 wpm. At
80 wpm the corresponding figures are about 2 to 3 miles and 4 to 8 miles, respecti-
vely. Frequencies of around 1 kc and less are advantageous, and the system is
extremely resistant to jamming.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1
1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
rage 5
SECTION II
SURFACE WAVE PROPAGATION
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Li
,J
rage L.
2.1 INTRODUCTION
This section is concerned with the application of surface. earth current
communication techniques for two-way communication over ranges from 2 to 300
miles, between a central fixed station and portable field station.
A typical earth current transmitter feeds current into two electrodes
buried in the earth. The resulting electromagnetic field propagates along the
surface of the earth where it can be received by appropriate detection equip-
ment. Receiving equipment can use either horizontal antennas (electrodes buried
in the earth) or vertical antennas. Horizontal electrode antennas pick up a
smaller signal but result in the same signal-to-noise ratio. A wave similar
to that generated by earth current equipment could be launched by a vertical
antenna; but, such an antenna would be very tall and conspicuous and extremely
vulnerable to attack. For these reasons it seems appropriate that both.trans-
mitter and receiver use earth electrode type antennas.
The signal strength is proportional to both electrode current (I) and
electrode current (L). For the purpose of this report it is assumed that a
maximum electrode separation of 100 meters can be used with portable equipment
For a given transmitter power, current can be maximized by minimizing the im-
pedance (using large electrodes). Where a permanent deep electrode installation
can be made, it is possible to achieve an electrode earth resistance of an ohm or
less. For a portable station, however, it has been .assumed that no buried con-
ductors (such as water pipes) are available, and that earth contact is made by
means of two iron pipes about 5 feet long driven into the earth about 100 meters
apart. When the stakes are set in typical soil and the area around them is wet,
an impedance of about 50 ohms results - this value being assumed throughout this
section. However, when a buried conductor such as a water well casing or water
main is available to serve as one electrode, the total resistance may become
much less than 50 ohms, and the same transmitter power would then result in a
signal improvement of about 10 db per decade reduction of electrode impedance
over that assumed in this analysis.
An artist's conception of a portable earth current field station for trans-
mitting and receiving is shown in Figure 1. The central station would be similar,
except that it would be permanently housed, and thus could use permanent electrodes
having a lower impedance.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
LJ
fl
50X1
IRON STAKE
FIGURE 1. Portable Earth Current Communication Equipment
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
rage
BATTERY POWERED RECEIVER-
TRANSMITTER
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page 6
2.2 SURFACE WAVE PROPAGATION AT LOW FREQUENCIES
When both transmitting and receiving electrodes are located at the surface
of a homogeneous earth, the horizontal electric field at the receiving electrode
1
is given by.
2
Ep =
IL
1
- (27)
27c 6p3
where I is the current in amperes
L is the electrode separation in meters
a is the conductivity
p is the range in meters
X is the wavelength in air
(1)
It can be seen that for small values of p/X, the field drops off as inverse
range cubed.
At longer ranges, however, the term on the right begins to be important,
2
until at long range this term becomes equal to0216)-) , and the field becomes
X
of
2
Ep IL [ 27cp X
21TOp3
p/X > 1 (2)
Thus, at long ranges, the field is proportional to inverse range. A plot
1 l_() + j (112)
X
is shown in Figure 2. The break point occurs at p/X 0.16. Using the first
equation, the field can be plotted as a function of range for a given transmitter
power. Thus for 5000 watts, I = 10 amperes into 50 ohms. Then, using L = 100
meters and o = 10-2 mho/meter, the field can be computed from the equation.
1A. Banos and J. P. Wesley: The Horizontal Electric Dipole In A Conducting
Half-Space", University of California Report S10 - Reference 53-33, to Bureau
of Ships, September, 1953.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
70
6o
10
-10
-20
? -30
140
f-1 f-1 f-1 r-1 r-1 I1 r-1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
0.16 =
.02 0.05 0.1
1 10
( pn
2 2
FIGURE 2. Plot of 1 + j .)1In Decibels
X X
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
.Lc1,6e
This procedure is good for frequencies up to about 30 kc. At 100 kc, however,
an earth current antenna of 100 meter length (consisting of a wire lain along
the ground and two electrodes) reaches resonance, resulting in a high resistive
component of impedance, thus severely limiting the current for a given power.
Also, the current distribution along the wire becomes non-uniform. At 100 kc
it would be more advantageous to use a 50 meter length and thus avoid resonance.
The antenna impedance would then be the same as at lower frequencies (i.e.,
50 ohms). Thus only 6 db (due to shortening L) is lost. Also, at 100 kc the
curvature of the earth becomes important at long ranges and causes the field
to decay somewhat faster than inverse range. This effect2 has been, taken into
account in the 100 kc case Effects of the ionosphere begin to be important
also at very long range. However, at the 300-mile maximum range requirement in
this study, such effects can be neglected. Figure 3 gives a plot of equation
(1) for frequencies of 1 kc, 3 kc, 10 kc, and 30 kc for a power of 5000 watts
(current of 10 amperes) with a 100 meter antenna. Figure 4 gives a plot for
the 100 kc case, including a 6 db loss (using a 50 meter antenna) and greater
attenuation at long range due to the curvature of the earth.
2.3 SURFACE LAYER PROPAGATION AT ULTRA LOW FREQUENCIES
The conductivity of the outer earth's crust varies widely among the many
materials of which it is composed. Sea water has a conductivity of about 4
mhos/meter, while gneiss and granite of pre-Cambrian age have conductivities as
-5
low as 10 mho/meter. The surface of the earth typically has a conductivity of
about 10-2 mho/meter. The geologically older material at greater depths usually
has considerably smaller conductivity. In some areas, particularly where sedimentary
formations exist, alternating layers of low and highly conducting material may occur.
A typical condition, however, is that in which a weathered surface layer of rela-
tively shallow depth, and having a high conductivity, overlays a geologically
older, denser, and less conductive material. At high frequencies, the skin depth
may be so shallow that appreciable current does not penetrate to the lower layer,
in which case the earth current propagation characteristics may be determined from
the usual analysis for a homogeneous earth. When the surface layer depth is ap-
preciably less than a skin depth in the upper medium, then the effect of the lower
2Terman: Radio Engineer's Handbook.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
r---1
Dri eclassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
RANGE IN KILOMETERS
FIGURE 3. Signal Strength vs Range, 1 to 30 kc
For I = 10 Amperes, L = 100 Meters
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
(:=3 r--, r--, r--, r--n r--n r--n E:=3 E:=3 E=23
u? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
20
10
0
-10
-8o
2
10 RANGE KILOMELLERS
100
FIGURE 4. Signal Strength vs Range, f = 100 kc
For I = 10 Amperes, L = 50 Meters
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1000
at
CD
0
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Lai
Page 11
medium must be taken into account in the analysis. A:useful approximation is
-0
obtained from an analysis of the static case (i.e., from the d-c current case).
The condition of interest is diagrammed in Figure 5, where the surface layer has
conductivity al, and the basement material has conductivity 62. The transmitter
electrodes are spaced a distance L, and carry current I. The depth of the layer
-1
is d, and p is the distance from transmitter to receiver. The radial component
of the electric field at the surface of the earth is then given by3:
[1d3
IL
,0
where E is a dimensionless quantity given by
7 Po
in volts per meter
o
-1 E = 1
Y3
Li where y = IL
a
LAIL
2 -
I A-
1/2
(2 X)21
Y
(2 x)21 5/2
Y
(7 6
1 - 2
a =
When 6 = 6 2' then a = and the solution reduces to
1
IL
E -
P 3
g6
1P
cr cr
1 2
dx
which is the solution for a homogeneous earth in the d-c case. A plot of EPo in
decibels is given in Appendix A as a function of the quantity
fl
3cf_ Appendix A.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
cr1
Page 12
SURFACE OF EARTH
RECEIVER
cr2
FIGURE 5. Two Layer Earth
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
eage 13
When C" < a (i.e., when the lower material is less conducting), then E is
2 1
greater than in the case of the homogeneous earth, as can be seen from the
figure. Three layer depths d which are chosen for study are 10, 100, and 1000
meters. These cover the range of typical conducting layer depths. Also it
was assumed that
= 10-2 mho/meter
1
I = 10 amperes (5000 watts into 50 ohms)
L = 100 meters.
The resulting curves of E vs. range in kilometers were plotted in Figures
6, 7 and 8. The curves are plotted for three values of C2/0) (i.e., or2/C1 =
1, 0.1, and 0.01). The curve for C21 = 1 represents the C.-ccase for a homo-
geneous earth. Although the graphs are plotted for a 5000 watt input, they are
readily transformable to any power input at the rate of 10 db loss in signal per
decade reduction in power.
The curves shown, while derived on a a
mation to the propagation of earth current energy out to ranges not exceeding
about o.16 wavelength in air, provided the skin depth E is greater than the layer
depth d. Skin depths and wavelength X are plotted in Figure 9 as a function of
cbasis, should form a good approxi-
frequency, for dl = 10-2 mho/meter. At a frequency of 10 cps,
at 100 cps, 6 = 500 meters; and at 1000 cps, g= 160 meters,
8 = 1600 meters;
for c - 10-2
mho/meter. Thus when the layer depth is 1000 meters, the presence of the lower
material has little or no effect at frequencies much above 10 cps since then the
skin depth is less than the layer depth. When the layer depth is 100 meters, the
layer has effect out to a little above 100 cps,and thus the curves sho.m are
applicable.
2.4 liCaSE CHARACTERISTICS
The chief source of noise in the usual earth current communication system
is atmospheric in origin. Atmospheric noise arises chiefly from lightning dis-
charge, where the predominantly vertical current stroke results in a large
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
30
20
10
-10
-20
L r--n r-Tn r--, r--n r--n r--n r--n r n --
6 Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
r
62la1 0.1
012la= 0.01
-70
-8o
1
10
RANGE IN KILOMETERS
100
FIGURE 6. E vs. Range, d = 10 Meters
I = 10 Amperes
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
500
_D E
50X1
30
20
10
0
10
20
r r
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 . CIA-RDP78-03424A000400090001-9
= 1
2 1
Pq
A
-50
-6o
-10
-8o
a2ic1 = 0.1
2la1= 0.01
1
I = 10 Amperes
L = 100 Meters
cr = 10-2 Mho/meter
1
10
RANGE IN KILOMETERS
100
FIGURE 7. E vs Range, a = 100 Meters
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Li L
500
50X1
30
20
10
0
-60
-70
-8o
1-2 r-7:aI g ua r----4r u u U I?A I?A
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
I = 10 Amps.
L = 1000 Meters
a = 0.01 mho/meter
1
d = 1000 Meters
21 = 0.01
=1
2
0.
1N.
1
10
Range in Kilometers
100
Declassified in Part - Sanitized Copy Approved for-Release-2014/0-3-/ii :.CIA-RDP78-03424A000400090001-9
0
011
50X1
CD
E:=1 (7=1 [7-' r-7, r-111 1-1111 C-111 1-111 1-1111 1-41
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Skin Depth 5 in Meters
10,000
1,000
100
10
1
10 160
Frequency in CPS
FIGURE 9, Skin Depth 5 and Air Wavelength X vs
- /
a = 102 ' mhometer
1,000
Frequency
10,000
-00,000 KM
10,000 KM
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
100 KM
50X1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page 18
radiated field. Such pulses have appreciable energy in the spectrum from a
few cycles up to above 100 kc. The'electric field in air is predominantly
vertical, but the much slower phase velocity in the earth results in a pre-
dominantly horizontal field in the ground at the surface. The ratio of the
vertical field in air to the horizontal field in the earth is large at low
frequencies and is given by'/ . This is plotted in Figure 10 for
a = 10-2 mho/meter. This ratio was used to convert noise measurements made
in air to the applicable values for earth current receivers using electrode
antennas. Measurements have also been made by
and others of the noise voltage between earth electrodes. A summary of such
measurements is given in Figure 11. This figure Shove the equivalent mean
value of the noise envelope in a 1 cps bandwidth. The noise level can vary
widely during a single day, and from season to season. There is a pronounced
minimum of noise in the neighborhood of 3 kc This minimum is predicted by
the mode theory of VLF propagation which shows greater attenuation, at ranges
beyond about 500 miles, at these frequencies. At shorter ranges, the propaga-
tion at 3 kc follows the usual inverse range dependence. Thus the ionosphere,
in the 3 kc range, operates to reduce the level of noise originating over 500
miles away more than that at other frequencies. Applying the mode theory to
the close-in measured spectra of lightning discharges gives long-range spectra
whose shape agrees well with the measured values5. The fact that noise is low
at 3 kc and that this frequency discriminates against jamming sources located
500 miles or more away makes 3 kc an attractive frequency for this application.
Recent tests by have confirmed the theo-
retical propagation characteristics at 3 kc. High power was. used in the tests,
and when the measured data are scaled to the assumptions (i.e., I = 10 amps,
L = 100 meters) used in this report, the measured values can be compared, as
shown in Figure 12. The attenuation over the inverse range line is negligible
for the ranges considered here for communication purposes (2 to 300 miles), and
so is neglected in the analysis.
4cf. Proceedings of the IRE, June 1957.
5ibid.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
50X1
50X1
r=71 EDI (7-m r-711 r--0 r--m r--m r--"M r--111 r--111 r--11 r--111 r-711 r-72 r-71
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
10 100 1000
Frequency in CPS
FIGURE 10. Ratio EV/EH of Vertical Field in
Air to Horizontal Field in Earth
10,000
100,000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
CD
C
50X1
1.)
0
0
-20
-40
cz -6o
Pq
?rt. ?70
a)
?ri
-8o
7---1 r---) r
L----Eleclassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
, 1
_
?
1
Summer
4,8 PM
3 PM
Measurementslat
Winter_
Night
?
?
6 PM
NOON
g AM 0J
1/2 CPS
4:4o FM Aprip
4:oo PM m.
40
e
?
4,,,00
IAl
74
0
?
inter
Day ?
,
?
.
4
a).* 6;
Winter
4.8 PM
'
MST
CPS 1 CPS
10 CPS
100 CPS 1 KC
Frequency
10 KC
100 NC
FIGURE 11. SprrimRry of Measurements of Horizontal Noise Field at Surface of Earth
(Mean Value of Noise Envelope for 1 CPS Bandwidth)
,
g = mho/meter
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
J 1 1 L
1 MC
50X1
Eli EDI r-u i
F-7, F--s r--u r--u r--u r--U r--U r--U r--u r--u -
Declassified n Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9 r71 11 r-71
-30
0
^ -40d
r-1
(1/
P:1
P -6o
? -70
tr)
-8o
-90
-100
Theory,
=
0
Flat Earth,
3 kc
Measured Values
-,,,...
w,N
NN
\
10 ?
100
Range in Kilometers
FIGURE 12. Comparison of Theoretical and Measured Signal at 3 kc
1000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
3000
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
U
50X1
lage 22
For the purposes of this report, two standard noise spectra will be con-
sidered, as shown in Figure 13. The upper curve in the figure represents the
"high" noise condition, or the value of noise exceeded roughly 10 percent of the
time. The lower curve represents the "low" noise condition, or that value ex-
ceeded roughly 90 percent of the time.
The curves were drawn so as to represent reasonable noise conditions in
the Western U.S. The "high" noise condition can be taken to represent a rough
diurnal maximum noise condition. The "low" noise condition can be taken to re-
present the diurnal minimum. Thus if the communication system must work at any
-1 time on short notice, the "high" noise condition would be assumed. If, on the
other hand, the operator can choose the time of day to send his message, then the
"low" noise curve can be used. In the subsequent analysis, both noise conditions
are considered and included in the determination of maximum range.
2.5 COMMUNICATION RANGE
The range to which a transmitter-receiver system can communicate depends
upon the data rate and upon the signal/noise ratio at the receiver. If a S/N ratio
of about 10 db is realized in a one cps bandwidth, then an information rate of two
bits/second can be transmitted. If a 10 db S/N ratio is realized in a 4 cps band-
width, then about 8 bits/second can be transmitted. Similarly, in a 16 cps band-
fl 32 bits/second can be transmitted. Using optimum coding techniques, two
_J bits/second carries a word rate of about 5 wpm.
Thus, if a system provides a 10 db S/N at 1 cps bandwidth, a 5 wpm data
rate can be handled. If the bandwidth is widened to 4 cps, noise will be increased
by 6 db. Thus, the signal must be increased by 6 db to provide a 20 wpm capability.
Similarly, a 12 db increase would be necessary to realize 80 wpm capability with
16 cps bandwidth. The noise curves presented earlier are drawn for a 1 cps band-
width. Then using these curves, the range at which a 10 db S/N ratio just occurs
represents a maximum range for a 5 wpm capability. The range at which a 22 db S/N
occurs represents 80 wpm, and 28 db S/N represents 320 wpm capability. These
ranges were determined for both the high and the low noise condition, for powers of
1 watt and 5000 watts. At the lower frequencies (f 100 cps) surface layers of
J 10, 100, and 1000 meters depth were used, corresponding to the propagation curves
presented earlier. The results are given in Tables I, II, III and IV. The results
are plotted in Figures 14, 15, 16 and 17, where a layer depth of 100 meters, and a
conductivity ratio a2/cs = 0.1 were assumed.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
E-13 F-13 U 1 1 1 1LA iA L_A
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
.1 CPS
1 CPS
10 CPS
100 CPS
Frequency
FIGURE 13. Standard Noise Spectra Used in Computation
1 KC
10 KC
100 ICC
, Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
f"--1 f-1 r i L j 1, 1L ],
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
TABLE I
COMMUNICATION RANGE VS. FREQUENCY AND DATA RATE
LOW NOISE, 5000 WATTS POWER
Frequency
a 2'1 a
d
(Meters)
5 WPM
20 WPM
80 WPM
320 WPM
KM
Miles
KM
Miles
KM
Mlles
KM
Mlles
100 cps
1 kc
3 kc
10 kc
30 kc
100 kc
462/01 = 1
a / = 0.1
2 1
a /a. = 0.01
2 1
10
100
1000
10
100
l000
18
38
38
35
83
80
50
17
63
22
360
1100
11.2
23.6
23.6
21.8
51.5
49.7
31
10.5
39
14
220
683
14
31
30
27
66
64
37
13.5
31.5
11
180
780
8.7
19.2
18.6
16.8
41
39.7
23
8.5
19.5
6.8
112
484
11
24
24
20
52
48
26
11
16.5
.5.6
90
520
6.8
15
15
12.4
32
30
16
6.8
10.3
3.5
56
323
8.6
12.6
4
45
330
5.3
7.8
2.5
28
205
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
J
1000
100
10
1
3 CPS
J
L r-771 r--n r-77 r--n r--n r--n r--n r--n r--n
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
C.
(a) Bandwidth = 1 CPS (5 WPM)
(b) Bandwidth = 4 CPS (20 WPM)
(c) Bandwidth = 16 CPS (80 WPM)
(d) Bandwidth = 64 CPS (320 )PM)
Z
a
b
.... ...
. d
10CPS
100 CPS
1 KC
FREQUENCY
10 KC
FIGURE 14. Communication Range vs Frequency And Data Rate
(Low Noise, 5000 Watts, 01-2/01-1 = 0.1, d = 100 Meters)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
100 KC
L L.:
50X1
 lr ir F
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
TABLE II
COMMUNICATION RANGE VS. FREQUENCY AND DATA RATE
HIGH NOISE, 5000 WATTS POWER
Frequency
02/a1
a
(Meters)
5 WPM
20 WPM
,
80 WPM
320 WPM
KM
Miles
Mil
Miles
KM
Miles
KM
Miles
100 cps
0 / = 1
8.2
5.1
6.6
4.1
5.2
3.2
02101 = 0.1
10
18
11.2
14
8.7
11
6.8
loo
18
11.2
14
8.7
11
6.8
1000
14
8.7
10.4
6.5
7.6
4.7
02/a1 = 0.01
10
38
23.6
30
18-6
24
15
100
35
21.7
27
16.8
20
12.4
1000
17
10.5
22
7.5
8.3
5.1
1 kc
7.9
4.9
6.4
4.0
5.0
3.1
4.0
2.5
3 kc
12
7.5
9.2
5.7
7.4
4.6
5.8
3.6
10 kc
3.7
2.3
2.9
1.8
2.3
1.4
1.86
1.15
30 kc
35
21.7
18
11.2
9.0
5.6
4.4
2.7
100 kc
280
174
160
99
90
56
46
29
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1000
100
10
7-1 7-1 f---1 r 1 f-1 L
J L_
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
(a)
(b)
(c)
(d)
Bandwidth =
Bandwidth =
Bandwidth =
Bandwidth =
1 CPS (5 WPM)
4- CPS (20 WPM)
16 as (80 wpm)
64 CPS (320 WPM)
3 CPS 10 CPS
100 CPS?
1 KC
FREQUENCY
10 KC
FIGURE 15. Communication Range vs Frequency And. Data Rate
(High Noise, 5000 Watts, a2/1 = o.1, d = 100 Meters)
/ 1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
100 KC
L
50X1
L
r r
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 . CIA-RDP78-03424A000400090001-9
TABLE III
COMMUNICATION RANGE VS. FREQUENCY AND DATA RATE
LOW NOISE, 1 WATT POWER
Frequency
a2la1
5 WPM
20 WPM
80 WPM
320 WPM
a
(Meters)
KM
Miles
KM
Miles
KM
Miles
KM
Miles
100 cps
.02As1 = 1
4.3
2.7
' 3.45
2.1
2.72
1.7
02/(51 = 0.1
lo
9.3
5.8
7.4
4.6
5.85
3.6
loo
9
5.6
7.2
4.5
5.6
3.5
l000
5.9
3.7
4.4
2.7
3.3
2.1
c2/ d1 = 0.01
10
20
12.4
15.6
9.7
12.4
7.7
loo
16
lo
12
7.5
9
5.6
l000
6.2
3.9
4.5
2.8
3.3
2.1
1 kc
4.1
2.6
3.3
2.1
2.6
1.6
2.1
1.3
3 kc
6
3.7
4.8
3.0
3.8
2.4
3
1.9
10 kc
1.9
1.2
1.53
0.95
1.2
0.75
0.98
0.61
30 kc
5.0
3.1
2.5
1.6
1.47
0.91
1.16
0.72
100 kc
50
31
25
16
12.6
7.8
6.4
4.o
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
C_]
50X1
Range in Miles
1000
10
1
1
r--, r--n r-- r--7 f r--m
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
(a) Bandwidth = 1 CPS (5 WPM)
CO Bandwidth = 4 CPS (20 WPM)
(c) Bandwidth = 16 CPS (80 WPM)
(d) Bandwidth = 64 CPS (320 WPM)
lo
c
,
10 CPS
100 CPS
1 KC
Frequency
FIGURE 16. Communication Range vs. Frequency and Data Rate
(Low Noisel 1 Watt, a2/al = 0.1, d = 100 Meters)
10 KC
100 KC
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
5 OX 1
L_]
r r 1 r-----1
"- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
TABLE IV
COMMUNICATION RANGE VS. FREQUENCY AND DATA RATE
HIGH NOISE, 1 WATT POWER
L )
Frequency
a2/o'1
d
(Meters)
5 WPM
e
20 WPM
80 WPM
320 WPM
KM
Miles
KM
Miles
KM
Miles
KM
Miles
p
100 cps
cr2/ci1 = 1
1.97
1.22
1.56
0.97
1.25
0.78
c2io1 = 0.1
10
4.3
2.7
3.44
2.14
2.72
1.7
100
4
2.5
3.13
1.95
2.4
1.5
l000
2.28
1.42
1.70
1.05
1.3
0.8
a2ic1 = 0.0
10
9.1
5.65
7.3
4.5
5.6
3.5
loo
6
3.7
4.4
2.7
3.1
1.92
l000
2.28
1.42
1.70
1.05
1.3
0.8
1 kc
1.9
1.2
1.54
0.95
1.2
0.75
0.98
0.61
3 kc
2.8
1.7
2.2
1.4
1.8
1.1
1.42
0.88
10 kc
0.90
0.56
30 kc
1.1
0.68
0.90
0.56
0.70
0.44
100 kc
5.0
3.1
2.6
1.6
1.26
0.78
t
.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
t7-7 L7-1 r--1 I r, 1 E
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 . CIA-RDP78-03424A000400090001-9
1000
100
(a)
(b)
(c)
(d)
Bandwidth = 1 CPS
Bandwidth = 4 cps
Bandwidth = 16 cps
Bandwidth = 64 CPS
1 ,
(5 WPM)
(20 Wpm)
(80 wpm)
(320 WPM)
a
_ --------6---N.
b
10 CPS
100 CPS
1 KC
10 KC
FIGURE 17. Communication Range vs Frequency and Data Rate
(High Noise, 1 Watt, c/2/1 = 0.1, d = 100 Meters)
/ 1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
0
CD
100 KC
50X1
Declassified in
Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
kage 32
50X1
2.6 EQUIPMENT ST71 AND WEIGHT
The necessary equipment for a portable station in general consists of:
(1) electronic equipment and its carrying cases (including batteries, if any);
(2) wire to connect to electrodes (100 meters of wire required); and (3) elec-
trodes (stakes) and a hammer or other means of driving and extracting them. The
electrodes may consist of ordinary iron pipe driven about 3 to 5 feet into the
ground, or they may consist of existing buried conductors (water pipes, iron sur-
vey stakes, etc). The wire need only be large enough so that its resistance be
not more than a few ohms. For example, wire as small as AWG No. 18 copper wire
can be used. A 100 meter length of this wire weighs a little over one pound and
has a resistance of about two ohms. The heat dissipation in such a wire is about
200 watts for a 5000.0watt transmitter, or less than one watt per lineal foot of
wire. Larger wire can be used at a slightly increased weight penalty. However,
in this report, further size and weight considerations will be limited to that of
the electronic equipment and its carrying cases.
A transmitter power of 5000 watts can be supplied by batteries. Power can
be supplied by silver-zinc cells, for a total battery weight of about 100 lbs. The
cells can be recharged from the power line by a rectifier weighing about 2 lbs. The
coder/decoder is estimated to weigh one pound with a transistorized receiver weight
of half a pound. The transmitter itself would be transistorized and would weigh in
the neighborhood of 20 lbs. Weight of carrying cases (2 or 3) is estimated at 26 1/2
lbs., giving a total weight of 150 lbs. The weight can be summarized as follows:
Batteries 100 lbs.
Transmitter 20 lbs.
Coder/decoder 1 lb.
Receiver 0.5 lb.
Rectifier 2 lbs.
Case 26.5 lbs.
TOTAL 150 lbs.
Batteries which will meet this minimal weight requirement have a relatively
short life. They will provide operation for a 15-minute period, and require about
3 hours to reach a full charge. They can be recharged 25 times. Greater battery
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
ri Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
^
1(1.t, 33
life and shorter recharge time can be obtained at an extra penalty in battery
weight. This 5000-watt transmitter provides 10 amperes into a (50 ohm) 100
meter antenna terminated in driven pipe electrodes.
At the other end of the power spectrum, a one-watt transmitter can be
considered. The battery for such a transmitter would be a rechargeable type
capable of about 100 recharge cycles and would weigh 8 ounces. The total weight
breakdown is as follows:
Battery 8 oz.
Transmitter 8 oz.
Coder/decoder 1 lb.
Receiver 8 oz.
Rectifier and case 8 oz.
TOTAL 3 lbs.
50X1
For an a-cline-powered transmitter, the weight of batteries is eliminated,
but allowance must be made for power conversion equipment.
Figure 18 shows a summary of battery weight and total weight estimates for
both battery-powered and a-cline-powered sets as a function, of transmitter power.
Figures 19 and 20 show range in miles vs. transmitter power for 5 wpm and 80 wpm
data rates. Indicated on the power scale are several representative set sizes.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1000
500
100
50
5
1
u-s f---% f-11 EDI ic=3
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
...---
L
-----
0-
.0
.0
...0
...--""
0
0
.0
...---
0
......-- ...---
-:--?
-k.-1....,..--
-40-
c 1\,?
----
...----
r?
...----
'0)
.
10
100
TRANSMITTER POWER IN WATTS
moo
10,000
FIGURE 18. Equipment Weight vs. Transmitter Power
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
FC
Ott
CD
CAJ
50X1
r-----? r--a 11-11 1-11
1-111 1171 reclassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
to
BATTERY POWERED
"POCKET t' SIZE
3 LBS .
1000
100
10
1
BATTERY POWERED
BATTERY POWERED
AC POWERED
BATTERY POWERED!
10 LBS.
25 LBS
25 LBS.
150 LBS
1
, 100300
----------
'
S
0
,
,
--= 100
? I
*
0
.
...--1.-----...
f --: 100 *0 . 11.3:GE. .3101.SZ
= 3 KC IIIGli 'ROVE,
C)
-
-
10 100
POWER IN WATTS
1000
10,000
FIGURE 19. Range vs. Power For 5 WPM Data Rate
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
a?,
50X1
" ir-21 11"---71 1-111 r--m s---at r--14 r-i
DU E3 eclassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
BATTERY POWERED
"POCKET" STZE
3 LBS.
BATTERY POWERED
10 LBS.
AC POWERED
25 LBS.
100
10
1
10 100
POWER IN WATTS
1000
FIGURE 20, Range Vs Power For 80 WPM Data Rate
10,000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
r,1 Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Page 37
SECTION III
CONDUCTING LAYER PROPAGATION
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Eage 3d,
3.1 INTRODUCTION
In the surface wave type of earth current propagation (cf. Section II), the
transmitting and receiving antennas use electrodes at the surface of the earth, but
the greater part of the actual propagation of usable energy takes place at the earth
air interface in the form of a surface wave. It is also possible to conceive of a
system in which the chief propagation path is through the earth itself. Such a
system can ordinarily be expected to suffer much greater signal attenuation. - By
use of appropriate receiving electrode configurations, however, the noise may be
greatly reduced in detecting such signals, and thus communication can be established
at appreciable ranges.
In this study, a vertically spaced electrode configuration will be assumed for
both transmitter and receiver. This configuration is less efficient for launching a
surface wave, but more efficient for launching an earth-propagated wave. The lower
electrode may be placed at the bottom of a water well or other vertical shaft in the
earth. An upper electrode (e.g., a driven iron rod) is required at the surface. An
artist's conception of such a communication system is shown in Figure 21.
3.2 NOISE CHARACTERISTICS FOR VERTICAL ELECTRODE CONFIGURATION
The noise voltage appearing between the electrodes arises from atmospheric
distrubances, telluric currents and magnetic disturbances. Above approximately
one cycle per second, the chief source of noise is atmospheric in origin- Tel-
luric and magnetic micropulsations have some effect at these frequencies, but some
preliminary measurements of the vertical component of potential at extremely low
frequencies in drilled wells have given values on the order of 40 db less than the
horizontal. Field measurements of vertical underground noise have, however, not
been as extensive as those at and above the surface of the earth. Thus the charac-
teristics of the noise must be reduced from the characteristics of the atmospheric
noise at the surface. Since the surface noise arises chiefly from vertically polar-
ized surface waves in the atmosphere, this source will be assumed in the subsequent
analysis.
In the propagation of a surface wave over an imperfect conductor such as the
earth, a set of inhomogeneous plane waves is generated within the conductor. In
the case of the earth, and at the frequencies of interest here, the electric field
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
EARTH
STRATA
rage j9
FIGURE 21. Vertical
Declassified in Part - Sanitized Copy Approved for Release
Electrode Configuration
2014/03/31: CIA-RDP78-03424A000400090001-9
50X1
- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
L_r.1
rage 4(,)
in the earth is much smaller than that in the air, and the earth waves propagate
almost vertically downward. The planes of constant phase are not exactly parallel
to the planes of constant amplitude. The vertical component of electric field
E(Z) varies with depth z according to the relation
E(z) = Ev(o)E-a(1-1)z
where
a
The ratio of the horizontal field to the vertical field within the earth at
the surface is given by
Thus
EH(o)
a
E(o) - (DE
0
E(z)
?
EH(0)
SinceI/
a
where Eli(o) is the horizontal field at the surface. is
much less than unity, the vertical field is much smaller than the horizontal.
To find the potential difference between vertically spaced electrodes, it is
only necessary to integrate Ev(z)? Thus
^ Ev(z)d
from which
VD
10E
a EH(o)
1
12-7a
EH(o)
E-a(1-i
ESD EiaD
-1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
L
V
A plot ofD is given in Figure 22 for lower electrode depths of 10, 30, 100,
mill 0 -2
and 300 meters for a = 10 mho/meter. Using the low and high noise values for
EH (o) as shown in the section on surface wave propagation gives the final noise
voltages as plotted in Figures 23 and 24.
3.3 PROPAGATION OF EARTH CURRENTS FROM VERTICALLY SPACED FLECTRODES
It is. assumed that one electrode is located at the surface, and the Other at
depth d in the earth.
50X1
3.3.1 Bbmegenous Earth
The analysis is relatively simple in the static case for a homogeneous earth.
The situation may be diagrammed as shown in Figure 25. It is assumed that the upper
electrode (No. 1) carries current +I and the lower electrode (No. 2) carries -I.
Then the voltage Vv measured by a receiver at range r is the difference in potential
between electrodes Nos. 3 and 4. Using the method of images, this potential difference
is found to be
3/
V LOtar
which reduces to
Aa 1 +(-47--) 47r C-
II.
v d4
V = neglecting higher order terms.
8A a275
If a horizontal electrode spacing d were used at the surface Of the earth,
the potential difference between electrodes would be
_31d3
VH
4A ar
This is greater than V at all ranges of interests and thus it might appear
better to use a horizontal antenna for receiving. However, the noise is much less
for the vertical electrode spacing. For example, if a 100-meter electrode spacing
is used, the noise for a vertical configuration at a frequency of 100 cps is -23.5
db relative to 114v for a horizontal field of 14v/m, as can be seen from Figure 22.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
E-773 1 r
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
-100 L
1
10
100
FREQUENCY IN CPS
1000 10,000
FIGURE 22. Ratio of Vertical Received Noise to Horizontal
Noise At Surface
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
OX1
f"--1
fl1 TJ "- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
A
0
C 3
1 10 100
FREQUENCY IN CPS
FIGURE 23. Received Noise vs. Frequency (One Cycle Bandwidth)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1000
10,000
Li
NOISE DB
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
100
FREQUENCY IN CPS
FIGURE 24. Received Noise vs. Frequency (One Cycle Bandwidth)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
E_] E
E Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
TRANSMITTER
ELECTRODE NO. 1
RECEIVER
NO. 3
d.
ELECTRODE NO. 2
0
NO. 4
FIGURE. 25. Homogeneous Earth
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Li
P3
Oz1
50X1
CD
E_J
al
The corresponding noise in a 100 meter horizontal antenna would be +40 db
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Page 46
????
111
Ell
roe
T-41
relative to 111v. Thus the ratio of horizontal to Vertical noise is 63.5 db
at 100 cycles/second. This result applies to spacings up to about 1000 meters.
At greater spacings, the ratio is still larger. At 10 kc? the ratio is about
50 db for spacings of 100 meters or less? and greater for greater spacings.
Taking the applicable ratio to be 6o db means that it is advantageous to use
the vertical electrode configuration provided that
Y.11 2r
V 3d
-1000
This implies that, for d = 100 meters,
r 150 kilometers, or
r 93 miles
Thus, at ranges less than about 93 milesy the vertical electrode spacing is preferable.
It is worth noting that in the case where the transmitter uses horizontally
spaced electrodes at the surface, the ratio is
VH 2r
Vv d
which would imply that vertical receiving electrodes are advantageous at ranges
less than about 30miles. However, horizontal transmitting electrodes are more
efficient at launching a surface wave in the air, which carries most of the
received energy, In such a case the ratio of VH is the same as for the noise,
since both arrive as surface waves. Vv
343.2 Layered Earth
When certain layering conditions occur in the surface region of the earth's
crusty the received signal may be enhanced over that for a homogeneous earth.
For example) suppose that there is a surface layer of high conductivity, under-
lain by a layer of low conductivity, followed by another layer of high conducti-
vity, and all underlain by a basement of low conductivity. This situation is
diagrammed in Figure 26. If electrodes are placed as shown in the figure. then
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
cr2
ci
02
r 1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
SURFACE OF EARTH
HIGH CONDUCTIVITY
LOW CONDUCTIVITY d2
FIGURE 26. Layered Earth
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
r- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page 48
the two conducting layers, separated by the relatively non-conducting layer, act
to increase the signal received between similarly spaced receiving electrodes.
The structure acts crudely like a lossy flat-plate waveguide, concentrating the
propagation of energy between the layers to some extent. The degree of signal
enhancement will depend on the conductivities and upon the relative thicknesses
of the layers. For the purposes of this analysis, it will be assumed that a
signal enchancement of 20 db is realized from a ratio of conductivities, ar2/al =
0.1. The resulting curves of received potential difference vs. range are shown
in Figure 27 for several values of electrode separation distance d.
3.3.3 Range, Frequency, and Data Rate Considerations
Range and data rate will be determined in a manner similar to that used in
the previous section on surface waves. Thus a 5 wpm data rate requires a one
cycle/second bandwidth and about 10 db signal-to-noise ratio.
An electrode impedance of 50 ohms will be assumed, as before, so that 5000
watts corresponds to 10 amperes electrode current. Figures 28 and 29 are plots
-1 of range vs. frequency under high and low noise for a 5000 watt transmitter for
data rates of 5, 20, 80, and 320 wpm. Figures 30 and 31 are plots of range vs.
-7 power for a?5 wpm data rate, under the low and high noise conditions. Figures
32 and 33 are similar plots for an 80 wpm data rate.
50X1
3-3.4 Discussion
The vertically-spaced electrode method is in general well suited for short
range communications, and has several unique advantages.
The electric field falls off very rapidly with distance (i.e., as range to
the fifth power). This makes such a communication link virtually impossible for
an enemy to jam from a distance, using similar equipment. An enemy could attempt
to launch a jamming wave using horizontally spaced electrodes, but could do so
with reasonable power only if his distance from the receiver were comparable to
that of the communicating transmitter. Detection of the signal by an enemy will
be extremely difficult because of the narrow bandwidths, low S/N ratio, the low
frequencies used, and the rapid attenuation with distance. Conventional receivers
and direction finders would be relatively ineffective.
This method is most applicable at the extremely low frequencies (below about
1 kc). The analysis given here covered only the static case. At frequencies
npriacsified in Part- Sanitized CopyApprovedforRelease2014/03/31 CIA-RDP78-03424A000400090001-9
17721 u?n 1-2 r---2 r"--4
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
0
Pq
0
-10
-20
-30
-50
-60
-70
-80
-90
-100
-110
-120
1
10
RANGE IN KILOMETERS
FIGURE 27. Received Signal vs Range
100
I = 10 Amps
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
100
10
f r 7--1LJ L i[
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
5 WPM
10 100
1000 10,000
FIGURE 28. Range vs. Frequency Low Noise Condition
I = 10 AMPS
d = 100 METERS
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
otl
CD
J
50X1
1---1 r--, ; V-- k
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
10
10
320 WP1v1?__
1
10 100
FREQUENCY IN CPS
1000
FIGURE 29. Range Vs. Frequency High Noise Condition
I = 10 AMPS
d = 100 METERS
10,000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
rJ r-3 fr-t r--1 f"--1 /-1 .1-1 L-1 7-1 fr-1 r"---1 17-1 r--1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
100
10
10
100
POWER IN WATTS
1000
FIGURE 30 . Range vs. Power For 5 WPM Data Rate
Low .Noise Condition
10,000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1 CPS
10 CPS
3 KC
100 CPS
1 KC
10 KC
50X1
100
10
r--t (7-1 1.--1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1 CPS
10 CPS
3 KC
100 CPS
'l KC
10 KC
10
100
POWER 11T WATTS
1000
FIGITRE 31. Range Vs. Power For 5 WPM Data Rate
High Noise Condition
10.000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
(D
50X1
r--1 f--1 L
- [7Declassifiedin Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Range in M_tles
10
100
Power in Watts
1000
FIGURE 32. Range vs. Power for 80 WPM Data Rate(Low Noise Condition)
10)000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
L C-J
50X1
r
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
10
5
10
100 1000
Power in Watts
FIGURE 33. Range vs. Power for 80 WPM Data Rate (High Noise Condition)
10,000
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
L_ 1
3 K.C.
1000 CPS
1 KC
10 KC
E
50X1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page 56
much above 1 kc, the attenuation of the earth begins to reduce the received
signal strength still further. While some surface wave may be generated, the
amplitude of such a wave would be far less than if horizontal electrodes were
[] used.
While it is necessary to drill a "well" for the installation of the lower
L-1
r"
electrode, the installation when complete is quite inconspicuous (much less so
than in the horizontal electrode case where a 100-meter wire must be strung
along the surface of the earth). Also, all exposed parts of the installation
are in a single location, so that accidental damage to the electrode cable is
not a problem.
Portable equipment can be used for field stations, provided only that
lower electrode installations be available wherever the stations are required
to operate*
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-1
APPENDIX A
ULTRA LOW FREQUENCY EARTH CURRENT PROPAGATION
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
7
ULTRA-LOW FREQUENCY EARTH CURRENT PROPAGATION
Page A-2
I. Introduction
For propagation at short ranges, less than twice the height of the iono-
sphere, the effect of the curvature of the earth or the ionosphere should be
negligible. Theoretical assumptions and subsequent empirical testing has de-
monstrated that propagation may he analyzed on the basis of a flat earth and
no ionosphere. In the sub-paragraphs below, a theoretical treatment of the
direct current case, which is a useful approximation to propagation in the
ultra-low frequency range, will be given. In this treatment, a single layer
earth is assumed in which a surface layer overlies a base material of rela-
tively low conductivity. This is a useful approximation of the usual geo-
physical conditions of the earth in which the conductivity decreases with age
of the geological formation.
In higher frequency earth current propagation, it is unnecessary to
consider the conductivities at depths much greater than that at which the elec-
trodes are buried. This is true because the earth attenuation is higher at the
higher frequencies and the chief path of importance is upward from the electrodes
to the surface of the earth. For this case the radial component of the field,
in a direction along the axis of the earth current dipole, is:
where
- IL
21top3
27cp2 , , 231p
X ) X
? length of the horizontal element in meters
a= ground conductivity in mhos per meter
X = free space wavelength in meters
radial distance in meters
dt ? burial depth of transmitter in meters
dr burial depth of receiver in meters
= permeability of free space in MKS units
current in amperes
t + dr
wpo/2
(Eq. 1)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
. PaiT. A-3
This equation has been verified at very low frequencies in many field tests
conducted by For ultra-low frequencies, where
distances are small compared to a wavelength, this equation indicates an
inverse range-cubed relationship.
The presence of a deeper layer of low conductivity appreciably alters the
range dependence factor when the frequency is low enough so that the lower layer
is on the order of a skin depth or less beneath the earth current electrodes.
As shown here for the DC current case, the range dependence factor is inverse
cubed at extremely short ranges and at extremely long ranges, but there is an
intermediate area in which the dependence is inverse range squared (smaller
attenuation versus distance). The presence of the lower layer results in a
long range signal advantage approximately equal to the ratio of conductivities.
The propagation characteristics result in integral equations which are not
expressable in closed form, but have been numerically integrated to Obtain the
normalized curves shown here. It should be noted that although the analysis
considers the electrodes to be located at the surface, the surface potential for
ranges greater than one layer of depth would be very close to that shown even
for relatively deep burial of the electrodes within the upper layer. Similarly,
burial of the receiving antenna would cause negligible change.
II. General Considerations
Consider a current (I) flawing between' two surface electrodes whose
spacing is denoted as 'a'. The resulting surface electric vector can be con-
sidered. to be composed of radial and angular components as Shown in the sketCh
below.
E/D
-
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
7
-1
fl
The potential at the point P is given by:
p
V(r,
_ 0) r.-; V(r - 5- cos 0) - V + a
? cos 0)
2 2
dV -
-a cos 0 a
dr
Pace A-4
(Eq. 2)
where v'( r) is the potential at range r from a single current source of +I.
Then':
and
,.vp(r, 0)
d r
-a cos 0
-- a sin
d2V
dr2
dV . 1
dr r
III. The Surface Potential from a Monopole
The earth model considered is diagrammed below:
Air (a = o)
Surface of Earth
///////277///i IWO/ ///,7/177, '777777777///7/71/7/1117
1 Depth of layer = d
a2
(Eq. 3)
(Eq. ).i)
The surface potential due to a current monopole of strength +I (i. e,
an earth current electrode near the surface carrying current +I) can be found
using the method of images to be:
co
ai
1 (Eq. 5)
I 1
24al + 2 r
V-r2 + 4i2 d
1 - a2/aI
where: a
1 + a2/a1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
At short ranges the potential approaches the 'asymptote
V
2Trcrir
rid?w
Page A-5
(Eq. 6)
This is the potential which would exist for all ranges on a homogeneous earth
of conductivity 01.
At long ranges, the potential approaches the asymptote
1:
7fa2r (a2/a1) ? 0157
V 2
(Eq. 7)
In the singular case when o2 = 0, the radial rate of change of potential is
asymptotic to:
27(r
(Eq. 8)
The discrete image current distribution can be approximated by a continuous
distribution and written, in one form, as:
V
2
CO
1
This approximation satisfies the asymptotic relations given above.
(Eq. 9)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-6
IV* The Electric Vector from a Dipole
(A) Radial Component
The radial component of the electric vector WAS given by Equation 3
as:
2
d V
-a cos 0
By differentiating,Equation 9, one obtains
and
oo
-3/2
+ 2r ax
1 r2 +2x2)
dx (Eq. 10)
where y = r/d
Thus, from Equation 3:
1/2
1
00
1/2
rxr
{ 112 5/2
A normalized (dimensionless) parameter EO can be written:
2
Eto -
--I a cos 0-
77. ald3
1
1+
00
ax
+fLo12 1572
12
(Eq. 13)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-7 50X1
er---.
-1
ro
1
3
F-737) ?
4....1?111,10?11.1.1MOMMK.700?1941 1 / 2
r/d?> co
2 =0
(Eq. 1)4.)
(Eq. 15)
(Eq. 16)
E/490 was obtained from Equation 13 by numerical integration, using
Simpson's Rule. Values were computed for y = r/d = 1, 2, 5, 10, 20, 50, 100,
200, 500 for conductivity ratios (02/01) = 0, .01, .1, and 1. The result is
plotted in Figure Al where smooth curves have been passed through the plotted
'points. The asymptotes are shown by dashed lines on the graph.
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
II
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
1?-age c.? 50X1
Transmitter Receiver
I
al
Nit?
a2
,-......---,
/
/
/
/
#
V) y
447,
/
/
/
/
#
\ th,
clif
0
>
\ O '?,,
1
aq,
/
/
6 q,
&
#
#'?
/
/
/
/
/
/
/
/
1.-
/
-
---
0
co
0
0
FIGURE L:1 . Normalized Radial Component of Electric
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
0
0
F-t
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
b__11
r-
as:
Page A-9
(B) Angular Component
The angular component of the electric vector is given by Equation 4
a sin dV
dr
Ia sln# 1
3 3
_I 2Y
A normalized parameter E0o can be written:
E0
1
E00 - r -Ia sin
2y
1+ 2
CO
1/2
OD
1/2
Eq. 17)
ax
x. 2 372
[1 4- rd
(Eq. 18)
The asymptotes can be found from the equations previously supplied. Thus, at
short ranges, E00 is asymptotic to:
EOo
At long ranges,
00
7
(Eq. 19)
(Eq. 20)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
When a2.. 0'
Page A-10
(Eq. 21)
E can be computed from Equation 18 by numerical integration. This has
00
been done, and a plot is given in Figure Ag, on which the.asymptotes are shown
by dashed lines.
(C) An Approximate "Break Point" Analysis
Equation 9 for the potential due to a monopole was obtained by approx-
imating the discrete image current source distribution by a single discrete
current source at the surface, and a continuous distribution extending from a
depth d downward. Alternately, a continuous current distribution extending over
the complete range from depths o to oo may be considered. In this case, the
continuous linear current source density q (D) at depth D is given by:
I ? D/2d I eD/2d lnm
q(D), a a' a
A "depth of penetration" can be defined by
2d
1
which is the depth at which q(D) =7 q(o)0
(Eq. 22)
(Eq. 23)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
? Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-11
0
o
/
/
*
\ 6y
a (-'
/
*
/
\6',
/
/
6
0,
?
',
It
/
\6"
/
/
/
/
///
'
//
/ ''
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
?
/
/
/
/
0
1
0
0
0
0
rd
FIGURE A2, Normalized Anular Component of Electric Vector E/
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
50X1
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-12
Then it is reasonable to assume that beyond some range (Ro 8 the potential
will be approximately proportional to inverse range as if there were a single
discrete current source. Beyond this range the electric vector from a current
dipole would be proportional to inverse range cubed.
Thus,
F.
R 2do
ln
and
2d?,
Ro
2)4.)
(Eq.. 25)
The normalized range (Ro/d) corresponds, in Figures 6 and 7 to the
break point where the asymptote for Eo crosses the asymptote for Eo
50X1
r/d--co r/d co
a2 00
2
From Figure Al, or from Equations (15) and (16), it can be seen that the
break point for Et? occurs at normalized range
Ro 2
o d (G2/c11)
From Equations (24) and (26),
a2 ,
Since -ma 2 ? (for a2/a1 1), then 2.
a1
Similarly, the break point for E occurs at normalized range
and
1
Yo
7-- 1.
(Eq. 26)
(Eq. 27)
(Eq. 28)
Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
r- Declassified in Part - Sanitized Copy Approved for Release 2014/03/31 : CIA-RDP78-03424A000400090001-9
Page A-13
The break points for E/9 and Es, given by Equations (26) and (28), are
.plotted as a function of a2/a1 in Figure A3,
V. Discussion
At very short range's (i.e. y = r/d