Title: APPARATUS AND METHOD USING CONTINUOUS-WAVE
RADIATION FOR DETECTING AND LOCATING TARGETS HIDDEN BEHIND A SURFACE
SPECIFICATION
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of provisional
Application No. 60/413,757 filed September 27, 2002, and
provisional Application No. 60/474,962 filed June 3, 2003,
both incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention is concerned with the detection and
location of targets hidden behind a surface, such as a
surface of the earth, using continuous-wave radiation.
Although the detailed description of the invention
refers to measurement of reflected electromagnetic
radiation at microwave wavelengths, the principles of the
invention are applicable to other types of continuous-wave
systems, such as those using sound waves (e.g., sonar).
Various means and methods have been developed for
detection and location of buried metallic and non-metallic
objects which rely on the reflectivity of the objects at
radar (microwave) wavelengths . These means and methods
include devices which attempt to image the objects through
beam scanning and determine distance (range) by timing
differences between transmitted and reflected pulses (i.e.,
ground-penetrating radars) as well as devices which attempt
to utilize expected differences between background (earth)
reflectivity and the reflectivity of the buried object.
Both methods are subject to significant difficulties in
their ability to locate buried objects (especially non-
metallic objects) due to several factors. These include:
(a) presence of other buried materials in surrounding
soil (rocks, tree roots, etc.) whose reflectivities are
comparable to that of the target object;
(b) rough or uneven terrain surface which produces
widely-varying background reflected signals;
(c) for continuous-wave devices, constructive and
destructive interference between transmitted and reflected
waves;
(d) interference between multi-path reflected signals;
and
(e) interference between the fundamental frequency and
harmonics in the reflected wave.
Although pulsed devices which rely on timing are less
subject to interference problems than continuous-wave
devices, continuous-wave devices are inherently less
complex, require less power, and may be made more easily
portable.
BRIEF DESCRIPTION OF THE INVENTION
The following description relates to a continuous-wave
device comprising a transmitter and two or more receivers
designed to detect and locate buried metallic and non-
metallic objects by measurement of reflected microwave
radiation, and discloses the means and methods used to
overcome or diminish some of the difficulties described
above .
The invention will be described with reference to two
embodiments which are designed to detect targets beneath
the surface of the earth, but it will become apparent in
later portions of the description that the invention is
useful in detecting targets hidden behind wall surfaces,
for example.
Both embodiments of the invention use a transmitter
that transmits a beam of continuous-wave radiation and a
pair of receivers of such radiation. Predetermined spatial
relationships (geometry) of the transmitter and the
receivers are provided such that the transmitter is farther
from the surface than the receivers and such that a
quadrature phase relationship exists for reflected
radiation at the receivers. In one embodiment, a
quadrature relationship also exists for direct radiation
that reaches the receivers from the transmitter.
Although not so restricted, in both embodiments the
transmitter and the receivers are mounted on an elongated
hand-held rod, with the receivers adjacent to an end of the
rod and the transmitter farther from the end of the rod
than the receivers. For microwave applications of the
invention, directional antennas are used at the transmitter
and each of the receivers. In one embodiment, the axis of
each beam pattern is along the length of the rod. In
another embodiment, parallel axes of the receiver beam
patterns are inclined with respect to the length of the
rod, and the axis of the transmitter beam pattern is also
inclined with respect to the length of the rod, but at a
different angle of inclination than that for the receivers.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described in conjunction
with the accompanying drawings, which illustrate preferred
(best mode) embodiments, and wherein:
Fig. 1 is a view showing a first embodiment of the
invention in use;
Fig. 2 is a perspective view of the first embodiment;
Fig. 3 is a fragmentary perspective view showing a
portion of the first embodiment;
Fig. 4 is a fragmentary perspective view showing
another portion of the first embodiment;
Fig. 5 is a plan view of an antenna that can be used
in the invention;
Fig. 6 is a graphical view showing the results of an
actual test of the first embodiment;
Fig. 7 is another graphical view showing test results
of the first embodiment;
Fig. 8 is a diagram showing spatial relationships
employed in the first embodiment;
Fig. 9 is a view showing a second embodiment of the
invention in use;
Fig. 10 is a fragmentary perspective view of the
second embodiment;
Fig. 11 is a fragmentary elevation view showing a
portion of the second embodiment;
Fig. 12 is a perspective view of the second
embodiment;
Fig. 13 is diagram showing spatial relationships
employed in the second embodiment;
Fig. 14 is another diagram showing spatial
relationships employed in the second embodiment; and
Figs. 15A and 15B constitute a block diagram showing a
circuit that can be used in the second embodiment.
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment
Figs. 1-5 show apparatus 10 employed in the first
embodiment. In this apparatus, there are three antennas,
namely, a transmitter antenna XMTR and two receiver
antennas RCV1 and RCV2. In the later description, the
receiver antenna RCV2 referred to as the "top antenna" is
actually closer to the ground than the "bottom antenna"
RCV1 in the use of the apparatus. All three antennas are
mounted on a rod 12, such as a 1-inch square fiberglass
tube. Basswood blocks 14 are used to attach the antennas
to the tube.
The two receiver antennas are mounted on opposite
sides of the rod 12, and in use the top antenna RCV2 is 0.6
inch closer to the ground than the bottom antenna RCV1.
The transmitter antenna XMTR is mounted on the same side of
the rod as the top antenna RCV2 and is at equal distances
from the receiver antennas. For example, the transmitter
antenna may be ten inches from the receiver antennas. This
distance is not critical, but should be the same for both
receiver antennas .
In the apparatus shown, each antenna is a directional
log periodic antenna having the gain of a Yagi but in a
smaller space. See Fig. 5. The antenna elements 16 and 18
are mounted on opposite sides of a PC board 20. The
elements on one side of the board are tied to the elements
on the opposite side of the board, which are fed with the
center conductor of a coaxial cable 22.
As shown in Fig. 2, the rod is angulated and has a
handle 24 at one end that is held by an operator when the
apparatus is in use as shown in Fig. 1. Mounted on the rod
adjacent to the handle is an electronics unit 26 that
includes a source of continuous microwave energy, a battery
power supply, one or more readout devices (e.g., visual
and/or audible) and various controls (e.g., background null
and gain adjust) . Typical circuitry for use in the
invention will be described later in connection with the
second embodiment .
In the use of the apparatus, as shown in Fig. 1, a
beam of radiation is transmitted into the ground toward a
hidden target and radiation reflected from the target is
received by the receivers and produces an output.
Continuous-Wave Transmitter/Receiver Design
Consideration of factors (a) through (e) above leads
to a number of constraints in the design of a continuous-
wave detector. Factors (a) and (b) lead to a requirement
that the transmitter be situated high enough above the
terrain surface that, for a reasonably restricted
transmitter beamwidth, the transmitter beam will illuminate
an area of the ground that is large compared to the size of
the non-target irregularities (rocks, etc.) and terrain
roughness scale. The same considerations apply to the
height requirement for the receiving antenna (s), subject to
the two additional requirements that the receiving
antenna (s) must be as close as possible to the ground for
maximum sensitivity to the reflected signal from the
target, and as far as possible from the transmitter to
reduce the direct signal.
Factor (c) above suggests that a pair of receiving
antennas separated by some appropriate fraction of a
wavelength might be used to insure that both receivers can
never simultaneously be located at an interference null,
while (d) and (e) impact the receiver antenna's beamwidth
pattern and tuning characteristics, respectively.
Analysis of wave pathlengths and phases indicates that the
transmitter antenna must lie above the pair of receiver
antennas for the reduction of interference effects.
Fig. 8 shows spatial relationships of the transmitter
and the receivers of the first embodiment, and more
particularly shows path lengths and phase differences, as
follows:
DIRECT PATHS: RCU1 : S1 = a
RCU2: S2 = a + b
REFLECTED PATHS: RCU1 : S1'= Rx + R1
RCU2: S2'= Rx + R2 with: Rx = SQR( χA2 + (a + b + z)Λ2 )
R1 = SQR( xA2 + (b + z)A2 )
R2 = SQR( XA2 + z"2 )
DIRECT PHASES: RCU1 : Ph1 = 36Θ a/L + Phβ
RCU2: Ph2 = 36β (a + b)/L + Phθ with: L = wavelength and Phθ = constant
REFLECTED PHASES: RCU1 : Ph1'= 36Θ S1'/L + Phθ
RCU2: Ph2'= 366 S2'/L + Phø
PHASE DIFFERENCES: RCU1 : d(Ph1) = Ph1 ' - Ph1 d(Ph1) = 360 (ST - a)/L
RCU2: d(Ph2) = Ph2" - Ph2 d(Ph2) = 36B (S2" - a - b)/L
TOTAL PHASE DIFFERENCE (RCU1 AND RCU2 REFLECTED SIGNALS): d(Ph1)-d(Ph2) = 360 (S11- S2'+ b)/L
At x = θ: S1'= (a + b + z) + (b + z) S2"= (a + b + z) + z and: d(Phl)-d(Ph2) = 36Θ (2b)/L
The reflected signals at RCU1 and RCU2 are in quadrature (90 degrees out of phase) when d(Ph1)-d(Ph2) = oø, or when: b = L/8
In accordance with the invention, the receiving
antennas are separated by a distance b=L/8 to fix the phase
difference between the two receiver antennas at 90°
(quadrature) when the antennas are directly above the
target. This provides a means of determining the true
amplitude of the continuous-wave radiation reflected from
the target independent of interference effects. In keeping
with the constraints discussed previously, for b<<z, the
amplitudes of the reflected signals at RCVl and RCV2 will
he nearly identical, and with 90 degrees phase difference
between their addition to the direct signals at RCVl and at
RCV2 , which for b<<a are also nearly identical. Hence, if
the direct signals at RCVl and RCV2 are denoted by Al and
A2, with the reflected signals at RCVl and RCV2 by Al • and
A2 ' , the corresponding total signals Tl and T2 may be
written:
Tl - A1 + A1'*cos(Phi) and T2 = A2 * fl2'*cos(Fhi+98) - A2 *■ A2,*sifi(Ptιi)
If the direct signals Al and A2 are measured before a
target is present and subtracted from outputs Tl and T2,
then the average reflected signal amplitude A' (which is
nearly equal to Al ' and A2 " ) can be found from:
fl'= S ((T1-A1)A2 + (T2-A2.A2)'- M " ~ A2" since (sin(Phi) )A2 *(cos(Pi))Λ2)= 1
5
Thus the amplitude A" is independent of phase angle Phi (hence without interference effects) when the device is directly above the target (x=0) . In the following section,
it will be shown that A" should' be a monotonic function of
.0 target distance (z) below the lower receiver antenna RCV2 for horizontal distances small compared to vertical distance (x«z) .
Transmitted and Reflected Signal Strengths
L5 The amplitude A' described above is a function of the transmitter power and beam pattern, distance and angle from
transmitter to the target, target size, shape and microwave
reflectivity, and the distance and angle from the target to
the receiving antennas. The following analysis considers
0 the strength of the transmitted signal at the target, the
reflection by the target, and the reflected signal at the receiving antennas. For simplicity, a cylindrical target of
radius Rp is assumed with reflectivity Q. The transmitted
signal strength is assumed to fall off as 1/r, and the
antenna pattern for transmitter and receiver is that of a
log periodic Yagi with measured cosine 12 angular falloff .
Since the length of the cylindrical target illuminated
increases with distance in the antenna beam pattern, the
strength of the reflected signal will fall off more slowly
than 1/r, and an arbitrary (l/r)n is, assumed for the
reflected signal. In the following, the transmitter signal
at 1 meter along the vertical axis is denoted Ax.
Transmitted signal strength at target:
At = (.x*((cos{(.lp a))Λl2)/Rx where tan(ήlpha) = x/(a+b+z)
Direct signal strengths at receiver:
A1 - Ax/a and A2 = flx/(a+br
Reflected signal strength at receiver: fit' = *At*((cos(Beta1))Λl2)*(Rp/Rl)An where tan(Betai) - x/(b+z) ή2" = Q*At*((cos{Befca2))A12)*(Rp/R2)Λn where tan(8eta2) « x/2
A device based on the described design was fabricated
and tested. In the test device, the parameters were: wave
freq= 2.452897 Ghz (L= 12.22 cm.), a= 21.6 cm., b= 1.528
cm. (L/8) , and the exponent n was determined to be 0.5.
Test results shown in Fig. 6 were obtained for an unburied
PVC pipe in air with Rp= 2.54 cm. The dashed curve is a
normalized theoretical fit from equations set forth
earlier.
Discussion of Results
As shown in the data plot in Fig. 6, the combination
of the two receiver outputs to obtain total reflected
signal amplitude results in a signal which agrees very well
with the theoretical prediction. The average deviation of
the combined outputs in this plot is approximately 12.5
millivolts. This small deviation is due to combined
effects of reflections over the illuminated target length,
small differences in the amplitude of the direct and
reflected signals at the two receiving antennas,
reflections off of nearby objects, and possible
interference effects of wave harmonics. By contrast, the
interference effects at the two receiver antennas cause
deviations as large as +/- 160 millivolts. Thus the 1/8
wavelength offset between the two receiving antennas to
obtain quadrature in the two receiver phases provides a
means of determining reflected signal amplitude through
elimination of the large deviations caused by interference
between the direct signal from the transmitter and the
reflected signals. Fig. 7 illustrates that the quadrature
output greatly reduces the interference pattern variations
seen in either receiver output.
The substantial reduction or elimination of
interference effects also allows the possibility of using
two or more pairs of quadrature receiving antennas,
collinear and spaced some distance apart, to determine
target depth. By measuring the true reflected signal
amplitude at each pair, the theoretical curve of signal
strength vs. target distance may be used to determine
target distance independent of absolute signal strength.
Signal strength may then be used to determine combined
reflectivity and target area, which may be helpful in the
determination of target characteristics such as composition
(metallic or non-metallic, etc.), size and possibly shape.
Applications
The device described in the previous sections, a
combination of a continuous-wave transmitter and two or
more pairs of quadrature receiving antennas has a wide
range of possible application, including but not limited
to:
(a) detection and location of underground metallic and
non-metallic pipes, cables, conduits and utility lines
(b) detection and location of buried metallic and non-
metallic munitions, including mines
(c) detection and location of underground anomalies
such as tunnels, shafts or graves.
Tests of the device in an urban environment also
indicate that the transmitted and reflected beams are
capable of penetrating building walls constructed of wood,
sheetroσk, stone or brick, and that the reflections
produced by anomalies within or behind the walls may be
used to locate such anomalies. Related applications
include :
(a) detection and location of beams, studs, electrical
conduit and gas or water pipes within building walls
(b) detection and location of moving objects,
including human or animal bodies, behind building walls.
Second Embodiment
In the above-described first embodiment of the
invention, quadrature of the two receiver phases for
reflected radiation eliminates the large deviations caused
by interference between the direct signal from the
transmitter and the two reflected signals. Additional
improvement in performance may be obtained by choice of a
geometry in which the transmitter antenna is shifted to the
side of the receiver antennas, with the receiver antennas
placed so that both the direct wave from the transmitter
and the reflected wave from a target below the receiver
antennas are 90 degrees out of phase at the two receiver
antennas (direct and reflected wave quadrature) . In
addition, the re-positioning of the receiver antennas away
from the center of the transmitter antenna beam greatly
reduces the direct signal strength at both receiver
antennas . A further benefit of this design is that the
reflected signal from the ground surface is not directly
below the receiver antennas, and the transmitter antenna
beam may be tilted toward the receiver antennas so that the
center of the transmitter beam illuminates a point at a
desired depth below the ground surface.
A simple embodiment of the design discussed above
comprises the transmitter antenna and the upper of the pair
of receiver antennas mounted at opposite ends of a portion
of a rigid rod or pole which is inclined at an angle
(theta) from the vertical, with the second (lower) receiver
antenna suspended below the rod or pole by a rigid strut in
such a position that it is 1/4 wavelength closer to the
ground surface than the upper receiver antenna and is also
1/4 wavelength closer to the transmitter antenna than the
upper receiver antenna.
The second embodiment of apparatus 28 of the invention
using this design is shown in Figs. 9-12, in which the rod
30 is a 3/4-inch aluminum tube ("main beam") . 1-inch
square fiberglass tubing mounting posts 32 (struts) support
the transmitter antenna XMTR and receiver antennas RCVl and
RCV2. The transmitter circuit is designated XC. The
mounting posts are pinned to the aluminum with 1/4-inch
fiberglass tubing and glued in place. Each of the antennas
is a directional log periodic type, such as that shown in
Fig. 5 described earlier and is enclosed in an ABS plastic
box 34. Each receiver antenna has a true power detector at
the base of the antenna on the same circuit board. This
detector gives a DC voltage output proportional to the
detected RF power at the antenna. The transmitter antenna
has a short coax cable to the transmitter. The circuitry
will be described later.
As shown in Fig. 9, the rod 30 has a handle 36
intermediate its length which is held by an operator when
the apparatus is in use. An electronics unit 38 of the
type referred to earlier is mounted on the rod between the
transmitter and the handle. A battery power supply 40 is
mounted on the rod adjacent to the opposite end of the rod
and includes batteries that form a counter-weight for the apparatus .
The geometry (spatial relationships) of such a design
is illustrated in Figs. 13 and 14, with theta, the angle
from vertical, set to 45 degrees. This choice of angle was
made for comfort of use and balance of the device in the
user's hands, and is not otherwise prescribed. The
receiver antennas RCVl and RCV2 are disposed in parallel
(vertically in Fig. 14) . In general, the exact dimensions
of the device required for both direct and reflected wave
quadrature may be calculated for an arbitrary angle theta
and transmitter wavelength L according to the following
relationships :
For chosen angle theta and chosen overall distance SI
between transmitter antenna and upper receiver antenna, the
lower receiver antenna will be supported at a vertical
distance D2 below the inclined rod from a point at a
distance k along the rod from the transmitter antenna, with
k and D2 calculated from:
Ik = S1*SQR(1-.5*L*((1+C0S(t eta))/(1-(C0S(theta))"2))/S1) |D2 = L/ι_*(S1- )*C0S(theta)
The vertical displacement h of the transmitter antenna
above the lower receiver antenna is given by:
- si*COS(t eta)+L/iι
and the horizontal distance dx between the two receiver
antennas is given by: dx » (Sl-k)*SlNO_heta)
The angle of tilt of the transmitter antenna from vertical (phi) required for the center of the transmitter beam to
cross the vertical mid-line between the two receiver antennas at a distance d below the lower receiver antenna is : , phi= RTN((S1*SIN(theta)-dx/2)/(h*d)}
Fig. 14 shows path lengths and phase differences in the second embodiment, as follows:
DIRECT PATHS: RCU1 : S1
RC 2: S2=S1-L/4 REFLECTED PATHS: RCU1 : S1"*Rx+R1
RCU2: S2'=Rχ+R2 with: Rx=SQR((X2-x)~2+(h+z)A2)
R1=SQR((x+dx)Λ2+(z+L/* A2) R2=SQR(XA2+z*2)
DIRECT PHASES: RCU1: Ph1=3όβ*S1/L+Phβ
RCU2: PI_2=36θ*S2/L+Phθ with: L = wauelength and Phθ = constant
DIRECT PHASE DIFFERENCE:
(qUADRATURE)
REFLECTED PHASES: RCU1 : Ph1 ^όββSI L+Phβ
RCU2: Ph2'=360*S2"/L+Phβ
REFLECTED PHASE DIFFERENCE: d(Phi" )=Ph1 *-PI.2*=3όθ*(R1-R2>/L d{Phi*)=36θ*(SQR((X+dx)"2+(z+L/i|)A2)-SQR(χA2+zΛ2})/L
For z»dx, at x=-dx/2: d(Phi')=9β (QUADRATURE)
Thus in this preferred design, both the direct signals
and the reflected signals at RCVl and RCV2 are in
quadrature (90 degrees out of phase) when the target is
along a vertical line mid-way between the two receiver
antennas .
As shown, the transmitter antenna is tilted 15 degrees
from the vertical, corresponding to the center of the
transmitter beam crossing the vertical mid-line of the two
receiver antennas at a point 15 inches below the RCV2
antenna for Sl=12.00 inches.
Selection of the various parameters (θ, Sx, d.l f and Zf)
specifying the instrument geometry involves trade-off
between instrument size, balance and ease of handling, and
expected target burial depth. The choices of parameters and
factors involved in their selection are discussed below.
Transmitter-Receiver Distance (S^ and Angle from
Vertical (θ)
Since both receiver antennas RCVl and RCV2 are always
vertical, selection of Sx for any particular value of θ
determines both the horizontal displacement between the
transmitter antenna and the receiver antennas and the
height of the transmitter above the ground. As previously
discussed, the transmitter height above the ground should
be sufficient for the transmitted beam to illuminate a spot
on the ground whose area is much larger than that of
irregularities on the ground or at shallow depths below it.
This produces an average ground background which remains
relatively constant, since it averages over many
irregularities. However, the transmitter antenna should be
far enough away from the feet of the user that the user's
feet do not produce reflections in the process of walking
normally while carrying the instrument. Likewise, the
combination of Sx and θ should position the two receiver
antennas RCVl and RCV2 close enough to the buried target to
receive a strong reflected signal, but high enough above
the ground surface to average over surface irregularities
reflections. A natural design scheme places the balance
point at the user's hand, with the batteries providing a
rear counter-weight for the weight of the antennas and
electronics. A practical limit of 52 inches was chosen for
overall length, and experimenting with various combinations
of S-L and θ for the required battery and instrument weights
led to a selection of S.,=12 inches and θ=45° as the optimum
configuration for all requirements.
Distance of RCVl Antenna Below Main Support Beam (d-,)
This distance depends entirely on the physical
dimensions of the rectangular box enclosing the receiver
antenna RCVl and associated rf detector. At θ =45°, the
minimum possible distance between the main support beam and
the center of the receiver antenna RCVl was dx = 3.5 in.
with d2 calculated to be 8.42 in.
Focus Depth of Transmitter Beam (Zf)
The transmitter and receiver antennas beam patterns
are identical, sharply focused around the forward direction
with power (or sensitivity) dropping to 50% at +/- 19°
about the forward axis. Since the receiver antennas are
always vertical in operation, the maximum reflected wave
signal strength at the receiver antennas will be obtained
when the target is along a vertical line mid-way between
the two receiver antennas and is illuminated at the center
of the transmitter antenna beam. Thus, for an expected
average target burial depth, the transmitter antenna angle
from vertical θ may be adjusted to have the antenna beam
center cross the vertical line between RCVl and RCV2 at the
expected target distance.) In the case of mines, the
expected burial depth is shallow (3 to 12 inches) , whereas
in the case of utility lines and pipes, burial may be
expected to be deeper (12 to 36 inches) . For test purposes,
a value of Z£ = 15 inches was chosen, resulting in θ = 15°
and d-^ = 4.95 inches, with Lx= 13.82 inches and L2= 8.56
inches .
Instrument Set-up and Operation
As shown in Figs. 15A and 15B, the instrument output
consists of an analog voltage (0-10 v.d.c.) which drives
both a voltage controlled oscillator (VCO) and a voltmeter.
The output voltage Vout is developed from the two receiver
r.f. outputs from RCVl (VI) and RCV2 (V2) through a series
of amplifiers and summing circuits, as described below:
(A) Both receiver antenna (RCVl and RCV2) outputs are
measured in power detectors whose output voltages Ax and A2
are inputs to difference amplifiers with xlO gain. The
other inputs to the difference amplifiers are voltages from
trim pots Cx and C2. The outputs from the two difference
amplifiers are thus 10 (Α^-C^ and 10(A2-C2).
(B) These outputs go to two variable gain amplifiers with
adjustable gain f, whose outputs are lOf (Α^-C^) and lOf
(A2-C2) .
(C) These outputs are squared, then summed together and the
square root of the resultant sum obtained. This voltage
V0= SQR [(10ftA1-C )2+ (10f(A2-C2))2]
which is equivalent to No = 10fSQR[(A.- d)2 + (A2- C2)2]
(D) The output V0 and an offset voltage CQ from a trim pot
are inputs to a difference amplifier whose output is V0-C0.
(E) The output V0-Co is then input to a variable gain
amplifier whose gain g is set to: g=10/(10-Co) which sets
the output to the meter and VCO so that their full range is
scaled (0-lOv) as V0 varies from C0 to ZC0.
The instrument is set up for operation in the following
steps :
1. Calibration for Direct signals at RCVl and RCV2
With the antennas pointed upward at the sky,
(a) the meter input is switched to the output
developed in step (B) for RCVl, which is lOf (A-L-C and the
voltage Cx is adjusted in the variable pot until the meter
reading is 0 (i.e., C = Ax) . This pot setting is held for
RCVl.
(b) meter input is switched to output for RCV2, which
is lOf (A2-C2) Pot voltage C2 is adjusted until meter reading
is 0 (C2=A2) and pot setting is held for RCV2.
(c) Meter input is switched to the output from step
(D) , which is V0-C0.
2. Neutralization of Earth Background
With the device held in normal operating position,
with antennas pointed toward the ground in a location
assumed to have no nearby buried targets,
(a) set pot adjusting voltage C0 to mid-range
(b) adjust variable gain f until meter reading is zero
(c) start research for buried objects, adjusting pot
controlling C0 as squelch control to compensate for changes
in earth background with terrain changes (gravel, bare
earth, grass, etc.)
While preferred embodiments of the invention have been
shown and described, it will be apparent that changes can
be made without departing from the principles and spirit of
the invention, the scope of which is defined in the
accompanying claims.