MANPACK DF SYSTEM FINAL REPORT

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP76-00451R000200010007-8
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 38
• Document Creation Date: December 9, 2016
• Document Release Date: August 28, 2001
• Sequence Number: 7
• Publication Date: December 1, 1968
• Content Type: REPORT

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AD
TECHNICAL REPORT ECOM-0503-P001-G806
MANPACK DF SYSTEM
FINAL REPORT
BY
RICHARD R. ERICKSON
MARVIN WAHL
DECEMBER 1968
.........................
UNITED STATES ARMY ELECTRONICS COMMAND, FORT MONMOUTH, N.J.
CONTRACT DAAB07-68-A-0503-P001
SYLVANIA
ELECTRONIC SYSTEMS - WESTERN DIVISION
Mountain View, California-
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Technical Report ECOM-0503-P001-G806
MANPACK DF SYSTEM
FINAL REPORT
Contract No. DAAB07-68-A-0503-P001
Prepared by
Richard R. Erickson
Marvin Wahl
Sylvania Electronic Systems - Western Division
Sylvania Electric Products Inc.
Mountain View, California
U. S. ARMY ELECTRONICS COMMAND, FORT MONMOUTH, N. J.
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Section
Title
Page
1.0
SUMMARY AND RECOMMENDATIONS
1
2.0
MANPACK DF SYSTEM DESIGN PHILOSOPHY
4
2.
1
Adcock Antenna Configuration
4
2.
2
Electrically Short Antenna Elements
4
2.
3
Elimination of Horizontally Polarized Sky Wave Currents
6
3. 0
DESCRIPTION OF MANPACK DF SYSTEM
3.
1
General Description
7
3.
2
Detailed Description of the Present MANPACK DF System
8
3.
2.
1
Modifications to the Antenna Configuration and End-Box
Circuitry
8
3.
2.
2
Modifications to the Center-Box Circuitry
9
3.
2.
3
Modifications to System Package
9
4.
0
EXPERIMENTAL TESTS OF PRESENT MANPACK DF
SYSTEM
13
4.
1
Sensitivity Tests
13
4.
2
Sky-Wave Rejection Tests
16
4.
3
Bearing Accuracy Tests
20
5.
0
REFERENCES
Appendix A Theoretical Discussion of Portable DF Systems
1.0
GENERAL PORTABLE DF SYSTEMS
1. 1
Loop-Type DF Systems
22
1.2
Adcock DF Systems
24
1.3
Spaced Loops
26
1. 4
Rotating Systems
26
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Section
Title
Page
Appendix B
History of MANPACK DF System
29
1.0
OBJECTIVE OF THE MANPACK DF STUDY
29
2.0
INITIAL DESIGN
3.0
IMPROVED SYSTEM DESIGN
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This report summarizes the past two year R&D effort at
Sylvania Electronic Systems-Western Division to develop
a small transportable high frequency radio direction find-
ing system. This system, the "MANPACK DF System,''
features "electrically short" antenna elements in an
H:-Adcock configuration along with frequency isolation of
the antenna elements from the horizontal transmission
lines to eliminate bearing errors due to downcoming
horizontally polarized sky-waves. System description
and test results are included in this report.
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1.0 SUMMARY AND RECOMMENDATIONS.
There is a need today for a short range portable HF radio direction-
finding unit that will work well in the presence of horizontally-polarized sky-wave.
As yet, there are no operational portable direction-finding systems which have
reliable performance under these conditions. The problem has been studied for
some time and several types of portable DF systems proposed to overcome this
difficulty but none of the solutions have proven satisfactory. This report describes
work performed at Sylvania Electronic Systems - Western Division during the past
two years, pertinent to the solution of this problem. This work is the research
and development of a particular portable direction-finding system which gives all
indications of alleviating the problems associated with horizontally-polarized sky-
wave. This DF system has been coined "The MANPACK DF System. "
Appendix A describes the problem of horizontally-polarized sky-wave
on portable direction-finding systems in general.
Appendix B is a brief history of the MANPACK DF system from the
initial conception to the present system design.
The body of the report is a technical description of present MANPACK
direction-finding system, and a performance evaluation based on the results of a
testing program which was carried out on the latest prototype.
We can summarize the entire MANPACK DF study up to this point as
(1)
(2)
The objective of the MANPACK study was to reduce or eliminate the
errors in bearing estimates which result from horizontally-polarized
downcoming sky-waves, for small, short-range, portable direction-
finding systems.
The problem was studied and a system was proposed which would
alleviate the sky-wave problem. The MANPACK system would
incorporate the following design features:
(a) Small H-Adcock antenna configuration for simple construction
and easy transportability.
(b) Wideband, electrically short antenna elements.
(c) Frequency isolation of the dipole elements from the horizontal
transmission lines.
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(3) A working model was designed, constructed, and tested, modified,
and retested. (The history of this model is discussed in Appendix B. )
Three areas of system performance were measured; sensitivity, sky-wave rejec-
tion, and accuracy. The results of the tests were:
(1) Sensitivity. The MANPACK DF system signal sensitivity was
slightly better than the sensitivity of a shielded loop type of system
which is current operational equipment.
(2 )
Sky-Wave Rejection. The MANPACK configuration attenuated
currents caused by horizontally-polarized downcoming sky-waves by
at least 10 db over the shielded loop.
(3) Accuracy. In the absence of sky-wave, the MANPACK and shielded
loop both made good estimates, with approximately 3-degree rms
error. Under heavy sky-wave conditions the MANPACK system rms
error was 4. 4 degrees; the shielded loop rms error was 13. 5 degrees.
In some situations, bearing estimates with errors less than 5 to
6 degrees could be made with the MANPACK when estimates made
with the loop resulted in errors greater than 90 degrees.
The MANPACK DF system, therefore, does possess certain advan-
tages over conventional portable DF systems, and can compare favorably with
these other systems in most all areas of performance.
The present system is not yet ready to be put into the field as an
operational unit. There are several problem areas which must first be studied
and improvements made:
The two amplifiers of the Adcock configuration cannot be maintained
in a balanced condition. To eliminate horizontal power supply lines,
a separate battery supply for each side of the antenna was installed
in the end-boxes. The present amplifier design draws considerable
current and the supplies drain down rapidly. The operating points
of the two amplifiers therefore drift independently of one another,
and the different gain of each side of the antenna configuration causes
the system to become unbalanced and bearing errors result.
(2) The problem of spurious inter-mod signals has been greatly reduced,
but these signals are still occasionally noticed, especially in denser
signal environments.
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(3)
Although the system has been repackaged into a much more functional
unit, it still would not be acceptable as operational field equipment.
In order to put an operational unit in the field, it is recommended
that the following tasks be undertaken:
(1)
(Z)
(3)
A detailed system study should be undertaken. The expected
operational use of the manpack should be determined, and desired
system parameters should be specified. These should include
sensitivity, accuracy, dynamic range, frequency band, size and
weight.
A detailed analytical study concerning electrically short antennas,
should be performed to determine the minimum physical dimensions
for the antenna configuration which will meet system requirements.
The necessary input characteristics of the high impedance antenna
amplifiers must also be determined. Some prefiltering might be
considered to eliminate the intermod problem.
Hardware development work must be undertaken to improve the
antenna amplifiers and the necessary power supplies. To operate
effectively, the amplifiers of the two halves of the Adcock antenna
must be stable relative to each other. A common power supply and/
or some compensating control between the two sides should be
installed.
(4) The present physical packaging should be modified so that it may be
easily disassembled and collapsed into a package small enough and
light enough to be carried by a person, The fiberglass boom should
be made into collapsible sections or telescopic (perhaps adjustable
length). The heavy wooden surveyor's transit tripod should be
replaced by a lighter fiberglass or magnesium structure which could
be collapsed into a small package.
(5)
Finally, a detailed testing program should be carried out to evaluate
the performance of the final model.
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2. 1 Adcock Antenna Configuration.
As explained in Appendix A of this report, the Adcock antenna systems
have been shown to be superior to loop type of antenna systems for taking bearings
on signals in the presence of sky wave. The design philosophy behind the Adcock
antenna is the elimination of currents from horizontally-polarized down-coming sky
wave.
The cancellation of these sky wave currents requires that the two
antenna elements be electrically balanced with respect to ground. The H-Adcock
antenna system is usually elevated high enough above the ground to reduce the
capacitive coupling of the antenna elements with the earth.
The H-Adcock configuration was chosen for the portable MANPACK
system due to the inherent sky wave rejection properties and the simplicity of
construction, two vertical dipole elements mounted at the ends of a horizontal
boom. This configuration causes problems because the desired size of the system
necessitates the use of small elements and the system must be used close to the
surface of the earth. This makes it almost impossible to maintain electrical
balance with respect to ground. To solve these problems, the currents caused
by horizontally polarized down-coming sky waves are further suppressed by other
methods as described below, and electrically short antennas are used.
2. 2 Electrically Short Antenna Elements.
Recently, extremely short antenna techniques have received much
attention. 1 An electrically short antenna is usually considered to be one which
has a total physical length less than one-fourth the shortest wavelength for the
operating range of the antenna. For small transportable antenna systems, such
as MANPACK, these techniques would be extremely useful.
The problem with using electrically short antennas is the small
physical size causes the antenna impedance to be almost all reactive, since the
resistance is so low. Therefore, the Q of the antenna equivalent circuit is
extremely large and a conjugate coupling to the first stage of the receiver would
result in very narrowband operation. The technique employed in achieveing wide-
band operation is based on coupling the antenna directly into the first stage of a
preamplifier whose input impedance is high in comparison to the antenna imped-
ance. This type of coupling provides for an efficient transfer of antenna voltage
to the receiver. The preamplifiers usually incorporate the latest developments
in semiconductor devices, such as field effect transistors (FET's) which are
capable of achieving good noise figures with the high source impedances. Feed-
back techniques are employed to achieve the high input impedances.
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The highly reactive impedance of a short antenna is not the optimum
source impedance to achieve the minimum noise figure in the first preamplifier
stage. At low frequencies, however, when the antenna sizes tend to be quite
large, and a reduction in sizes is most advantageous, even a significant deterior-
ation of the receiver noise figure will not impair the overall performance of the
receiver. This is due to the fact that at low frequencies, atmospheric noise
usually determines the signal-to-noise ratio before the signal is ever received
by the antenna.
Thus for a high Q antenna, the ohmic and ground losses contribute
only a small additive term to an already high noise figure, i. e. , while antenna
losses generate noise, the lower Q resulting from the losses improves the
noise figure of the receiver itself almost enough to compensate. This is, of
course, only true for a highly reactive source such as an "electrically short"
antenna.
The optimum source impedance cannot be achieved for high reactance
source impedance over wide bandwidths. For electrically short antennas, the
impedance of the antenna cannot be changed so the noise figure is optimized by
adjusting the first stage of the preamplifier.
The MANPACK system will take advantage of electrically short
antenna elements by using high impedance preamplifiers following the dipole
elements. For operation up to 30 MHz or a minimum wavelength of 10 meters
the overall length of the dipole should be approximately 10/4 = 2. 5 meters or
about 7 feet. The present model has dipoles of overall length of 70 inches.
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2. 3 Elimination of Horizontally Polarized Sky Wave Currents.
The small physical size of-the antenna makes it difficult to keep the
system balanced, and therefore currents which result from horizontally polarized
downcorning sky waves have to be suppressed by some other means.
One method by which this could be accomplished would be by the
elimination of all the horizontal conductors in the antenna. The vertical dipole
elements could be mounted on a dielectric boom and the electrical information
at each element could be transmitted via light beams to a central collection box.
In HF radio direction finding, we are usually interested in only one
signal at a time and over a limited range of frequencies. Therefore, it is only
necessary that the vertical antenna elements be isolated from the horizontal
conductors at the frequencies of interest. This could be done by changing the
frequency of the signal at each antenna element and passing this signal through
a filter which attenuates the original frequency into the horizontal transmission
lines which carries the signal at the new frequency to the phase combiner. The
amount of attenuation must be very large in order that the vertical dipoles appear
isolated from the horizontal transmission lines at the original frequency.
This is the technique that the MANPACK DF system will utilize in
order to eliminate the bearing errors due to horizontally polarized sky wave.
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3. 1 General Description.
As explained in the previous section, the MANPACK DF system will
take advantage of the following design features:
(1) Small H-Adcock antenna configuration
(2) Wideband, electrically short antennas
(3) Frequency isolation of the dipole elements from the horizontal trans-
mission lines.
A block diagram of the MANPACK DF system is shown in Figure I.
HIGH IMP
SUMMING
AMP
VERTICAL
DIPOLE
ELEMENTS
FILTER
ELEMENT END BOX
LOCAL
OSCILLATOR
MIXER I MIXER
HYBRID
COMBINER
'VHF
RECEIVER
FILTER
HIGH IMP
SUMMING
AMP
VERTICAL
DIPOLE
ELEMENTS
ELEMENT END BOX
Figure 1. Block Diagram of MANPACK HF DF System.
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The operation of the system is as follows: the signal is picked up by
the "electrically short" vertical dipole antenna elements and passed into extremely
high input impedance amplifiers. The output of the amplifiers then enters a mix-
ing stage where the signal is up-converted in frequency. The higher frequency is
then passed through a high-pass filter and then down transmission lines to a
collection box at the center of the horizontal boom. Here the vector difference
of the signals from the two dipoles is obtained with a hybrid combiner, and the
difference signal is sent to the input of the DF receiver.
This system is operated just the same as a loop system. A signal
of interest is monitored on the DF receiver, and the entire antenna configuration
is rotated on a central mast until a minimum or null is obtained at the receiver.
In this position, assuming the two halves of the Adcock are balanced, the boom
of the antenna will be perpendicular to the direction of arrival of the signal.
3. 2 Detailed Description of the Present MANPACK DF System.
The MANPACK system demonstrated at Fort Monmouth in 1968 was
redesigned in several areas to improve the performance. The two major items
that were deficient in the previous system was the overall sensitivity and the
spurious intermod signals generated by the upconversion of signal frequency.
To illustrate all of the modifications in a logical order, let us group the changes
into three categories: (1) the changes to the antenna and end box circuitry,
(2) the changes associated with the center box circuitry, and (3) the physical
changes to the packaging of the system.
3. 2. 1 Modifications to the Antenna Configuration and End-Box Circuitry.
(a) The antenna configuration was changed so that the lower half of both
vertical dipole elements was grounded resulting in the elimination of
one amplifier at each end-box with negligible change in sensitivity.
(b) The amplifier circuitry at each end-box was modified to increase the
sensitivity further.
(c) The original isolation filters in the horizontal transmission lines
carrying the local oscillator signals into and the IF signals out of
the end-boxes were replaced with bandpass filters of improved
design. The insertion loss was approximately 3 db and the isolation
was increased from about 40 to 60 db.
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3. 2. 1 -- Continued.
(d) High-pass filters were installed in the amplifier circuits to eliminate
intermodulations from signals below 2 MHz.
A schematic diagram illustrating the end box circuitry of the present
MANPACK DF system is shown in Figure 2.
3. 2. 2 Modifications to the Center-Box Circuitry.
The center-box circuitry which includes the 32-MHz local oscillator
and the summing amplifier, had several improvements incorporated:
(a) The local oscillator circuitry was left essentially unchanged, but the
output voltage was doubled by increasing the power supply levels and
running the oscillator at a higher operating point. This reduced the
level of intermod signals considerably.
(b) The summing amplifier which originally consisted of a two-channel
transistor amplifier, a "magic T" (180-degree hybrid) circuit, and
an output isolation amplifier, was changed to a completely passive
phase combining circuit. The output of the hybrid combiner is fed
directly to the VHF receiver. This resulted in simplified circuitry
and improved overall performance.
A schematic diagram of the center-box circuitry is shown in
Figure 3.
3. 2. 3 Modifications to System Package.
The entire antenna configuration was repackaged into a more functional
unit. The aluminum boom and mast were replaced with ones made entirely of
fiberglass tubing. This not only made the system more lightweight and durable,
but the elimination of the horizontal conducting boom enhances the capability for
sky-wave rejection. The horizontal transmission lines between the end-boxes
and the center-box are contained inside the boom.
The mast is mounted on the tripod from a surveyors transit, and a
360-degree azimuthal direction indicator was added. The system is now easier
to transport and operate as well as being a much more durable piece of equipment.
A photograph of the present MANPACK DF set is shown in Figure 4.
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s1
UI
N
0
U
O
m L
M
LH(._
r; x o
X
0
m
Z
W I--
0
00
u
Q
V
N
Zo
QI
I
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Figure 4. Photograph of Present Manpack Model.
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4.0 EXPERIMENTAL TESTS OF PRESENT MANPACK DF SYSTEM.
After all of the modifications that were described in the previous
section were implemented, the system was subjected to further testing to deter-
mine the level of performance of the present design. There are three character-
istics of the DF system which were selected to be measured and compared with
other antenna types. These characteristics are: (1) some measure of the
sensitivity of the MANPACK system, (2) the amount of sky-wave rejection that
could be expected from using the MANPACK design, and (3) the accuracy of
actual bearing estimates taken by the MANPACK system. We shall discuss the
measurements made of the above three performance specifications.
4. 1 Sensitivity Tests.
To gain some insight as to how the MANPACK DF system compares
in sensitivity with another antenna system which is now operational equipment,
the following experiment was performed.
A ten-inch-diameter shielded loop was selected as a representative
portable DF antenna to compare with the MANPACK system. This loop type of
antenna was designed for the Army by Sylvania Electronic Systems and currently
is operational Army equipment.
The test site was chosen to be a valley just south of Livermore,
California. This location is far enough away from industrial areas, and is
shielded by high mountains on all sides so that the external noise level is very
low.
The MANPACK and shielded loop were set up at one location along
with a battery powered National HRO-500 Communications Receiver. The IF
output of the receiver was monitored with an HP-403-B rms voltmeter. A
TRC-77 battery-powered transmitter was set up one mile away from the receiver
site. It was connected to a dipole antenna with a coupling device which allowed
the radiated power to be controlled. Another receiver with a signal strength
meter was located just out of the near-field of the HF transmitter to measure
the changes in radiated RF power. The objective of the experiment is to
measure the output signal-to-noise ratio of both the shielded loop and the
MANPACK DF systems as the incident signal power or field strength is varied
by known amounts. The distance between the receiver and transmitter is such
that the sky-wave present at the receiver should be negligible with respect to the
ground-wave. Therefore, we shall be measuring the sensitivity of both types of
antenna to ground wave signals.
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4. 1 -- Continued.
The frequency of the CW transmissions was 7.455 MHz. The up-
converting process of the MANPACK system with the 32-MHz local oscillator
required the receiver to tune in the up-converted signal at 24. 545 MHz.
Measurements of the HRO-500 receiver sensitivity and the output noise levels
using a 2. 5-kHz bandwidth at these two frequencies showed:
Receiver IF
Receiver Output Noise
Frequency Sensitivity (1 v = 0 db)
It was concluded that the receiver was internally noise limited since
the receiver output noise remained unchanged with the antenna connected or dis-
connected. Therefore, 9-db attenuation was put in the MANPACK antenna line
to compensate for the receiver's differential in sensitivity as the frequency was
changed.
The transmitter was tuned for maximum radiated power and the
received signal and noise measured for both the loop and MANPACK antennas.
Then the radiated power was decreased in increments of 10 db, and measure-
ments again made. This was continued until the signal was well down in the
noise.
The results of this experiment are illustrated in Figure 5. This
indicates that there is no loss in sensitivity by using the MANPACK system design.
The instrumental accuracies of the experiment were such that one may conclude
that the sensitivities of both systems were comparable. The experimental results
at the smaller signal-to-noise. ratios may not be reliable, since errors can
become significant when estimating signal-to-noise ratio at low signal-to-noise
ratios.
The output signal-to-noise ratio varies linearly with the input signal-
to-noise ratio as would be expected. The noise capture, or small signal suppres-
sion effect, of the receiver detector is noticed for the loop antenna at the low
signal-to-noise ratios, but again it should be pointed out that experimental
accuracies are low in this region.
Since direction-finding applications require an output signal-to-noise
ratio of at least 10 db, the conclusion is that the MANPACK design is comparable
to operational loop type of antenna systems as far as antenna sensitivity is
concerned.
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RELATIVE ANTENNA INPUT POWER --db
Figure 5. Antenna Output Signal-to-Noise Ratio as a Function of
Relative Input Signal Power.
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4. 2 Sky-Wave Rejection Tests.
The MANPACK system evolved as a result of trying to eliminate the
errors in bearing estimates which result from horizontally-polarized sky-wave.
The solution was to eliminate currents in the antenna structure resulting from
these sky-waves. To measure how much these currents were attenuated by the
MANPACK design, the following experiment was performed.
The shielded loop antenna described in the preceding section was
selected as a comparison DF system to measure the effects of sky-wave. The
test site was the parking lot behind the Sylvania facilities at Mountain View,
California. An HF transmitter was located a distance of 6 miles away, and
broadcast a CW signal at various frequencies selected by the experimentors.
The transmitter antenna was a horizontal windom. Computer propagation
predictions were run off so that frequency and time of day could be selected
when the sky-wave and ground-wave components of the received signal would
have comparable amplitudes. Most transmissions were made at frequencies of
about 7 MHz. The MANPACK and shielded loop antennas were set up in the
same general area, and both were connected to identical R-390 communications
receivers. The receivers were tuned so that both DF systems were monitoring
the same controlled signal simultaneously. The IF output of both receivers was
monitored by HP-403-B rms voltmeters, and also, in order to have a permanent
record;, the output of the two voltmeters was recorded on a two-channel visi-
corder or strip-chart recorder. The test situation is illustrated in Figure 6.
As was pointed out in Appendix A of this treatise, sky-wave
signals tend to be time varying in both amplitude and phase due to ionospheric
perturbations. This would cause a fading signal to be incident on the DF
antennas. The way we propose to measure the amount of sky-wave present at
the output of the two DF antennas is to estimate the amount of fading of the
signal from the R-390 receivers. This could be obtained by measuring the
amount of swing (fading) of the IF being charted by the visicorder.
Recordings were made for both DF systems for several trials and
during different hours of the day. As expected during the mid-day, almost no
fading was observed from either the MANPACK or the shielded loop, indicating
the absence of sky-wave. During the late afternoon hours the fading became
noticeable, especially from the shielded loop, showing that the strength of the
sky-wave was increasing. From the many recordings made of the fading, it
was impossible to determine absolutely how much the MANPACK configuration
attenuated currents caused by horizontally-polarized downcoming sky-wave, but
from the experimental results, it was concluded that the attenuation was at
least 10 db. The fading was measured with both the antennas in the null configur-
ation, which is the orientation where the output of the antenna should be minimum.
This was done because the fading due to the sky-wave causes difficulty determin-
ing just when the antenna is nulled, and makes the null very indistinct. An
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2-CHANNEL
VISICORDER
D F SYSTEM
SHIELDED LOOP
1
-
R-390
RECEIVER
IF
RMS
VOLTMETER
HP-403-B
TRANSMITTER
RMS
VOLTMETER
HP-403-B
IF
R-390
RECEIVER
Figure 6. Experimental Setup For Sky-Wave Rejection and Bearing
Accuracy Tests.
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example of the experimental results is shown in Figure 7 which is a photograph
of the Visicorder output taken when the sky-wave signal was fairly strong. The
photograph shows that the output of the loop antenna is varying in amplitude by
amounts of 15 db. This is about the depth of the null of the shielded loop system
which was measured in the next series of tests on bearing accuracies. There-
fore, the loop antenna is varying completely out of a null configuration under
heavy sky-wave conditions. The MANPACK system, on the other hand, remained
sharply nulled on the signal with the output variations on the order of only 5 db.
This is about 25 percent of the total null depth (20 db).
The gain of the MANPACK amplifiers required different electronics
and ranges of operation to be used on the two channels of the Visicorder which is
why the scaling is slightly different on the two recordings. Some quantitative
measurements may still be made, and the general conclusion reached that the
MANPACK DF system attenuates currents from horizontally-polarized sky-wave
by at least 10 db and can maintain a null configuration on a signal under heavy
sky-wave conditions when the shielded loop cannot.
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G806
C- Cr o 0
+ +
qp -- 30f11IldWV qp -- 3 If11lldWV
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4. 3 Bearing Accuracy Tests.
To compare the accuracy of bearing estimates taken with the MAN-
PACK DF system with those taken by the shielded loop system, the same experi-
mental situation described previously which was utilized for testing sky-wave
rejection was used. Bearing estimates were taken with both DF systems under
varying sky-wave conditions. In the absence of sky-wave, both systems gave
comparable bearing estimates, the superiority of the MANPACK was clearly
demonstrated for heavy sky-wave condition. The results are summarized below:
Null Depth
RMS Bearing Error
Shielded loop
25-30 db
3 deg.
No sky-wave
MANPACK
16-28 db
3 deg.
Shielded loop
- 15 db
13.5 deg.
Heavy sky-wave
MANPACK
16-22 db
4. 4 deg.
It should be mentioned that the width of the null (in degrees) was
narrower for the MANPACK than the loop. Under heavy sky-wave conditions
the null of the loop became very smeared and indistinct, and sometimes com-
pletely absent. The null of the MANPACK system remained fairly sharp,
but drifted because the amplifiers in each side of the Adcock antenna could not
be maintained in a balanced condition. This is due to the large current drain
on the battery power supplies. Since each amplifier has its separate power
supply, the operating points of the two amplifiers drift independently, causing
the Adcock to become unbalanced. This causes the position of the null to drift.
To compensate for this, the amplifiers of the two dipoles were tuned to a balanced
condition before each bearing attempt on the target transmitter. This condition
was also present during each of the previous tests and the system had to be
constantly retuned. This should be kept in mind when considering the results of
the tests.
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1. S. T. Fisher and S. Czorpita, "Extremely Short Antenna Techniques",
Technical Report ECOM-0221(E)-1, Philco Corporation, August 1966.
2. F. E. Terman, Electronic and Radio Engineering, McGraw-Hill Book Co.
New York, 1955, pp 1046-1051.
3. D. S. Bond, Radio Direction Finders, McGraw-Hill Book Co. , New York,
1944.
4. J., D. Moore and M. P. Castles, "HF Spaced Loop Antenna", Technical
Report ECOM-01960-F, Southwest Research Institute, San Antonio, Texas,
July, 1967.
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Appendix A
Theoretical Discussion of Portable DF Systems
The fact that to a first approximation high-frequency radio waves
propagate outward from the transmitting antenna along a great-circle route,
enables a person who is picking up the signal with an HF radio receiver to
estimate the line-of-bearing from his geographical location to that of the trans-
mitting antenna by measuring the direction of arrival of the HF wave.
Direction-finding (DF) systems are radio receivers which have the
capability to make a measurement of the direction of arrival of a radio wave that
is being monitored. These systems are useful for determining the actual location
of the radio transmitter or for navigation purposes.
This project was concerned with small portable direction-finding
systems. We do not refer to the ones which are installed in trucks or mounted
on jeeps, but the smaller types which can be transported by one or two men.
They must have the capability to be disassembled, transported, and reassembled
in a relatively short time for tactical operation. Their primary function is to
make line-of-bearing estimates on high-frequency radio emitters in the near
vicinity. The range of operation is usually required to be 10 miles or less. The
high mobility is the chief requirement.
There are several general types of portable DF systems being used
today. We will mention and briefly discuss a few of the principle categories of
these DF systems. 2, 3
1. 1 Loop-Type DF Systems.
The oldest and most familiar type of direction finder is the loop
antenna. The loop antenna is arranged with the plane of the loop vertical and is
rotated about a vertical axis. The output of the antenna is connected to an HF
radio receiver. The loop is rotated about its vertical axis until a "null" or
minimum of the receiver output is obtained (Figure A-1). In this position, the
plane of the loop will be perpendicular to the direction of arrival of the HF wave.
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HF RECEIVER
Figure A-1. Drawing of Loop Type DF System.
The propagated wave sets up currents in antenna conductors with
amplitude and phase dependent on the amplitude and phase of the electromagnetic
wave. If the received signal is ground wave, that is, if the radio wave is propa-
gating horizontally across the surface of the earth, the currents set up in the
two horizontal elements of the-loop antenna have the same amplitude and phase,
since the radio wave incident on each of these elements is the same. These
currents tend to travel around the loop in opposite directions, and thus cancel
each other out no matter how the loop is oriented. But the currents set up in the
spaced vertical elements of the loop may differ in phase due to the relative time
delay between incidence of the EM wave. This phase difference causes a result-
ant current to flow in the loop antenna dependent on the orientation of the loop.
Complete cancellation can occur only when the incidence time difference is zero
which happens when the plane of the loop is perpendicular to the direction of
arrival of the electromagnetic plane wave. The loop antenna, besides being
subject to errors due to unknown polarization, will give spurious readings unless
it is balanced with respect to ground due to unbalanced reactive currents flowing.
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These errors can be reduced by using circuit arrangements which are symmetric
with respect to ground and by using shielded loops. The operator of this type of
system tunes his receiver to the desired signal and rotates the loop antenna until
a null is obtained. The direction of arrival is then indicated by the orientation
of the loop.
1. 2 Adcock DF Systems.
A discussion of the motivations that led to the development of the
Adcock systems will be postponed until the problems associated with loop-type
antenna systems are reviewed. There are basically two types of Adcock antenna
configurations, the U and the H Adcocks. The U-Adcock is illustrated in
Figure A^2. The antenna system is basically two vertical antenna elements erected
at ground level, and fed with horizontal feeders usually laid under the surface of
the ground. This antenna is usually balanced with respect to ground. It would
be difficult to physically rotate this type of antenna system, so usually two such
antennas are employed at right angles to each other, and the output of the antennas
are combined in a goniometer. Rotating the goniometer is equivalent electrically
to physically rotating a single Adcock (or loop). Eight-element Adcocks employ
four of the single Adcock antenna systems at 45-degree angles to each other, and
combine all their outputs in a goniometer.
ANTENNA
ELEMENT
TRANSMISSION LINES
TO HF RECEIVER
EARTH
SURFACE
ANTENNA
ELEMENT
Figure A-2. U-Adcock Antenna Configuration.
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1.2 -- Continued.
The H-Adcock antenna is illustrated in Figure A-3. It consists of
two vertical dipole elements connected by horizontal feed lines and a supporting
boom. This type of configuration can either be rotated physically or several
fixed antennas combined with a goniometer. This type of antenna is usually
elevated a quarter wavelength or more above the ground so that it can be balanced
with respect to ground. The dipole antenna arrangement is difficult to balance
when located close to the ground since it is assymetric with respect to ground.
DIPOLE ANTENNA
ELEMENTS
J
I r
TRANSMISSION LINES
TO HF RECEIVER
DIPOLE ANTENNA
ELEMENTS
EARTH SURFACE
poll 11 1 pop 11 F lip IF1
Figure A-3. H-Adcock Antenna Configuration.
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1. 3 Spaced Loops.
There is currently considerable development work being done on
systems of spaced loops. 2 The spaced loop antenna is essentially two standard
loop antennas one at each end of a horizontal boom. The output of the two loop
antenna. elements are usually combined in parallel opposition. There are three
common configurations for mounting the two loops: coaxial, vertical coplanar,
and horizontal coplanar. Some favorable results from experimental investiga-
tions of this type of direction-finding system have been reported, especially
concerning sky-wave performance.
1.4 Rotating Systems.
All of the previously described antenna systems, loops, Adcocks,
or spaced loops can be continuously rotating, either physically or by means of a
goniometer. The output signals of these rotating systems are displayed on a
cathode ray tube. Depending upon the particular system, some sort of pattern
indicating direction of arrival of incoming signals is obtained. The advantages
of rotating systems are that they look in all directions (360 degrees) in a short
period of time, depending on the rotation speed of the antenna. Most portable
systems have variable speed controls. For example, the AN/PRD-8 DF system
has a 30-rpm continuous rotation speed and slow-speed variable operation
between 1 and 15 rpm. The display, which is an averaged look in all directions
due to the persistence of the scope and the human eye, averages out some of the
noise and is convenient to obtain bearing estimates from. Most of the small
portable rotating systems have the antenna elements physically whirling. It has
been experimentally noticed the rotating systems seem to be more effective when
sky-wave is present.
The first type of antennas used for direction-finding purposes were
the previously described loop antennas. The early experimenters noted a
phenomenon that occurred with loop-type direction finders which they termed
the "Night Effect" for obvious reasons. It was noticed that during the daylight
hours the direction-finding equipment performed very well when taking bearings
on HF emitters located several miles away. The bearing measurements made
were very accurate and consistent, but during the evening and night the bearing
estimates became very erratic and unreliable. Since these irregularities were
predominant at night the phenomenon was termed the "night" effect.
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2. 0 -- Continued.
The reason for the irregularities is the presence of sky wave at the
direction-finding receiver site (Figure A-4). During the daylight hours the
conditions for HF sky wave are not good since the D-layer of the ionosphere
absorbes the electromagnetic energy which is propagated skyward from the
antenna of the HF transmitter. Under these conditions the signal received at
the direction-finding site is just the ground wave which propagates horizontally
across the surface of the earth. This received signal is almost always steady
and nonfading and travels directly from the transmitter antenna to the receiver
antenna. With this strong steady signal the loop type of direction finders were
able to make reliable estimates of the line of bearing.
During the evening hours the D-layer begins to disappear, and the
conditions for the sky-wave propagation improves. The energy propagated sky-
ward from the transmitter antenna is reflected back to earth by the higher layers
of the ionosphere and received at the DF receiver along with the ground wave.
A radio signal propagated via the ionosphere undergoes changes in amplitude
and phase due to perturbations of the ionosphere. Also multipath conditions
usually exist during ionospheric propagation causing interference at the receiver
site. The result is that the received signal has random variations in polarization,
amplitude and phase; and the direction of arrival of the reflected wave has random
IONOSPHERE
D-LAYER
ANT
DF
RECEIVER
SKY-WAVE
GROUND-WAVE
TRANSMITTER
Figure A-4. Illustration of Direction Finding Signal Propagation.
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variations and errors due to ionospheric tilts and other propagation phenomenon.
The results of all this is that as the strength of the sky wave received at the
direction-finding site becomes comparable with that of the ground wave, the
bearing estimates obtained by trying to null the currents in the loop antenna
become very erroneous. There are two major reasons for this. The first reason
is that the currents set up in the horizontal elements of the loop antenna due to the
downcoming horizontally-polarized sky waves do not cancel each other out as with
ground wave. This is because there isa phase differential between the currents
caused by the time difference of arrival of the downcoming wavefront at the top
and bottom horizontal antenna elements. This causes a resultant current to flow
around the loop which cannot be nulled out by orienting the plane of the loop
perpendicular to the azimuthal direction of arrival, and the result then is either
a false bearing, an indistinct null, or both. The second reason is the random
variations in amplitude, phase, and angle of arrival of propagated sky wave;
these cause the null to become very smeared and indistinct, and it may be impos-
sible to obtain a null by any orientation of the loop.
As the frequency gets higher and the distance between the HF trans-
mitter and receiver becomes greater, the "night" effect becomes more pronounced,
because under these conditions the ratio of sky-wave strength to ground-wave
strength is higher.
The errors in bearing caused by downcoming horizontally-polarized
sky waves can be eliminated with an Adcock antenna. The action of the previously
described Adcock antenna system is the same as the loop for vertically-polarized
signals. This is because the output current is proportional to the vector difference
of the voltages induced in the vertical elements of the Adcock antenna the same as
the loop. Horizontally-polarized downcoming waves induce currents inthe
horizontal members (feed lines) which are the same in amplitude and phase, and
since the feeders are connected in an arrangement to take the vector difference of
two sides of the antenna, the horizontal currents tend to cancel each other out.
The Adcock antenna system is effective, however, only if symmetry
is maintained with respect to ground. If the antenna elements are not balanced
to ground, reactive currents will flow in the antenna when in the null configuration,
thus again causing bearing errors.
The U-Adcock antenna system is balanced by the manner of its
construction, and the H-Adcock may also be balanced if the entire antenna is
elevated at least a quarter wavelength above the earth to reduce capacitive coupling.
Small H-Adcock systems which are not sufficiently elevated are usually unbalanced
with respect to ground due to the asymmetric arrangement. The problem we are
concerned with is the elimination of currents caused by horizontally-polarized
sky waves in small unbalanced H-Adcock antenna systems.
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Appendix B
History Of MANPACK DF System
1.0 OBJECTIVE OF THE MANPACK DF STUDY.
The objective of the MANPACK DF study was to demonstrate the
feasibility of a new type of portable direction-finding antenna system. The moti-
vation for the new design was to eliminate the effects of sky wave which have
plagued all small transportable direction finders. The proposed system had
several innovations incorporated in order to achieve the objective. Some of
these features are:
(1)
Small H-Adcock antenna configuration for simple construction and
easy transportability
(2) Electrically short, integrated antennas utilized to allow small dipole
elements
(3)
Frequency conversion at the antenna elements to provide isolation
from all horizontal conductors and eliminate sky-wave effects.
2. 0 INITIAL DESIGN.
In 1966 the MANPACK DF system was initially conceived and the
first working model was designed and built. A technical report, File No. EE-
RD-351-4123, was published in January, 1967, which describes in detail the
construction, circuit arrangements, and test results of this first model. All of
the technical details will not be repeated here, but some of the more pertinent
facts are listed.
The unit was designed to operate over the frequency range 2 to 8 MHz.
The preamplifier and up-converter circuitry was contained in two
aluminum boxes which also served as a mount for the two halves of the dipole
antenna. These boxes were mounted one at each end of a 10-foot aluminum boom.
The hybrid combiner and local oscillator were contained in an aluminum box
mounted at the center of the boom.
The antenna elements were connected directly to a differential FET
amplifier without any preselection or matching networks.
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A post-selector followed the amplifier with the purpose of narrowing
the bandwidth to reduce the number of spurious signals produced in the nonlinear
mixing stage. The post-selector had frequency bands of 2 to 3 MHz, 3 to 4 MHz,
4 to 6 MHz, and 6 to 8 MHz.
The local oscillator was crystal controlled and operated at a frequency
The hybrid combiner was used in conjunction with a two-channel
summing amplifier to obtain the difference signal.
A battery power supply for the entire system was contained in the
The testing program was carried out and the conclusions reached
were the following:
(1)
The sensitivity of the system was poor and was attributed to matching
loss and poor amplifier noise figure
(2) The system suffered from intermodulation problems due to spurious
signals being introduced from the nonlinear mixing operation
(3)
The amount of isolation of the vertical dipole elements from the
horizontal conductors was not sufficient.
3. 0 IMPROVED SYSTEM DESIGN.
Late in 1967 some improvements were made on the first model to
reduce the previously mentioned deficiencies:
(a) The first stage amplifier was redesigned to increase the sensitivity
and reduce the intermodulation problem
(b) The post-selector was removed and replaced with a filter which
increased the sensitivity
(c) Finally, dielectric spacers were installed between the aluminum end
boxes and the horizontal boom to reduce the coupling between the
boom and the vertical dipole elements and thus increase the amount
of isolation between the antenna elements and horizontal conductors.
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(d) The battery power supplies were moved to each end-box eliminating
the need for horizontal power supply lines.
In early 1968, this model was demonstrated at Fort Monmouth for the
Army. Although the sensitivity of the system was acceptable, the dense signal
environment at Fort Monmouth caused the system to suffer badly from spurious
intermod signals being introduced into the passband of the receiver.
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ccrim i*rv riAce irIPATInMM (This page is Unclassified)
14
LINK A
LINK B
LINK C
KEY WORDS
ROLE
WT
ROLE
WT
ROLE
WT
Manpa.ck
4
DF
4
.Antenna
4
Portable
4
HF
4
Adcock
4
Sky-wave
4
Isolation
1
Bearing
1
Electrically-Short
1
Polarization
1
Short Range
1
(This page is Unclassified)
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DOCUMENT CONTROL DATA - R & D
(Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified
I. ORIGINATING ACTIVITY (Corporate author)
2a. REPORT SECURITY CLASSIFICATION
Sylvania Electronic Systems - Western Division
UNCLASSIFIED
P. O. Box 205, Mt. View, Calif. 94040
21). GROUP
N/A
3. REPORT TITLE
MANPACK DF SYSTEM - FINAL REPORT
4. DESCRIPTIVE NOTES (Type of report and inclusive dates)
Technical Report
5. AUTHORISI (First name, middle initial, last name)
Richard R. Erickson
Marvin Wahl
6 REPORT DATE
AL NO. OF PAGES
7a. TO
7b. NO. OF REFS
December 1968
36
4
Be CONTRACT OR GRANT NO.
fa. ORIGINATOR'S REPORT NUMBER(S)
DAAB07-68-A-0503-P001
b PROJECT NO.
C
!b. OTHER REPORT NO(S) (Any othernumbere that may be &..J ,.d
this report)
d
ECOM-0503-P001-G806
10 DISTRIOUTION STATEMENT
I t SUPPLEMENTARY NOTES
12. SPONSORING MILITARY ACTIVITY
U. S. Army Electronics Command
Fort Monmouth, New Jersey
13 ABSTRACT
This report summarizes the past two year R&D effort at Sylvania Electronic
Systems-Western Division to develop a small transportable high frequency
radio direction finding system. This system, the "MANPACK DF System,"
features "electrically short" antenna elements in an H-Adcock configuration
along with frequency isolation of the antenna elements from the horizontal
transmission lines to eliminate bearing errors due to downcoming horizon-
tally polarized sky-waves. System description and test results are included
in this report.
DD FORM
I NOV 65
4