The antenna and transmitter described in this report were designed for integration into the remote acoustic assessment system for detection of sockeye salmon in the Bristol Bay region of the Bering Sea. The assessment system configuration consists of an upward directed sonar buoy anchored 150 ft below the surface and attached by cable to a spar buoy tethered some 300 ft laterally. The spar buoy contains a telemetry transmitter, power supply, data processing electronics, an antenna and a beacon light

Baldwin, H. A.

Freyman, R. W.

English

NASA Technical Reports Server

TELEMETRY LINK FOR
AN AUTOMATIC SALMON MIGRATION MONITOR
by
Howard A. Baldwin and Robert W. Freyman
(NASA-CR-138152) TELEMERY LINK FOR AN 574-2179 9
AUTCMATIC SALMOA MIGRAIIOn NONITOR Final
Technical Report (Sensory Systems Lab.)
-4-p HC $4.25 CSCL 17B Unclas
R3 G3/07 37067
Final Technical Report
Prepared Under NASA Contract NGR-03-041-008
With
Sensory Systems Laboratory
P. O. Box 208
Los Alamos, N.M. 87544
For
AMES RESEARCH CENTER
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
October 1973
https://ntrs.nasa.gov/search.jsp?R=19740013686 2020-03-23T09:49:23+00:00Z
TELEMETRY LINK FOR AN
AUTOMATIC SALMON MIGRATION MONITOR
INTRODUCTION
The antenna and transmitter described in this
report were designed for integration into the remote
acoustic assessment system for detection of sockeye
salmon in the Bristol Bay region of the Bering Sea.
This system is described in a report prepared by the
Applied Physics Laboratory and the Fisheries Institute
of the University of Washington, November 1972.
A sketch of the assessment system configuration
is shown in Figure 1. It consists of an upward
directed sonar buoy anchored 150 feet below the
surface and attached by cable to a spar buoy tethered
some 300 feet laterally. The spar buoy contains a
telemetry transmitter, power supply, data processing
electronics, an antenna and a beacon light.
Previous experience with telemetry from small
floats and buoys, especially in tracking marine
turtles,( 1 ) has led to some understanding of the
-2-
practical problems of transmitting data under these
conditions. An antenna on a small platform in
moderate seas is very seldom tuned or matched because
of the variable spacing between the active region
of the antenna and the sea water surface. We have
given considerable thought to the buoy antenna design
in this report in the hope of arriving at a useful
design compromise for the small buoy problem.
GROUND WAVE TRANSMISSION
The presence of the sea, a good conductive
surface, modifies propagation at low and high fre-
quencies by refraction, We have selected a frequency
of 8 MHz based on prior experience. Figure 2 is a
prediction of signal strength at various ranges to
100 miles for radiated power levels of 1 and 10 watts.
These curves are based on calculations given by
Terman(2 ) and include spherical spreading, diffraction
and refraction effects. We would expect signal
strengths of 58 dB at 20 miles and 45 dB above
1 microvolt per meter at 70 miles using 10 watts of
radiated power. These ranges are those assumed for
the transmission path lengths of interest. These
calculations are based on the assumption that both
-3-
the receiving and transmitting antennas will be
located at or near the sea surface.
SKY WAVE PROPAGATION
Ranges less than several hundred miles are
not suitable for sky wave propagation via the ionosphere
at 8 MHz. The major effect of skywave propagation
is to increase the noise level at sunrise and sunset
as the ionospheric ionization levels enhance re-
ception from distant stations. Final frequency
selection around 8 MHz should be made after a field
survey of the reception characteristics at the
receiver site. A directive antenna for the receiver
should help in the reduction of noise.
NOISE LEVELS ANTICIPATED
The major noise sources other than man-made
interference at 8 MHz expected in the Bristol Bay
region are atmospheric and galactic. A directive
receiving antenna should be of some benefit in the
reduction of all three noise sources.
The equation
En = Fa + 20 log F - 6 5.5 (3)
gives the rms noise field for a 1 kHz bandwidth in
dB above 1 uv/m for values of Fa, the atmospheric
-4-
noise level in dB above KTB, and frequency in
megahertz. Fa values are provided for various
seasons, time of day and localities by CCIR Report
322. (4 ) Using a value of 20 dB for Fa and 6 kHz
for bandwidth,
En = 12 dB below 1 uv/m
The equation for En is derived from measurements
made with a short vertical antenna.
SIGNAL TO NOISE RATIO
With the following assumptions:
transmitter power 10 watts
transmitter antenna efficiency 50%
receiver antenna gain 0 dB
receiver bandwidth 6 kHz
distance 70 miles
fade margin 10 dB
the signal to noise ratio should be at least 44 dB.
Reduction of the transmitted power to 1 watt
would still provide more than adequate signal to
noise ratio under these assumptions and this has
in fact been our experience in temperate and tropic
zones. For this study, 1 watt of power to the
transmitter antenna would provide a field strength
of at least 35 uv/m at the receiving antenna.
-5-
BUOY ANTENNA
The following specifications were established
for the buoy antenna:
1. Transmitted signal to be vertically polarized.
2. Total antenna height should not exceed 3 feet,
to limit wind and wave loading in storm
conditions.
3. The antenna must accept 10 watts average
power at 8.0 MHz.
4 . The maximum diameter ground plane at the
top of the spar buoy is approximately
24 inches.
5. The antenna efficiency should be as high as
practical within the above physical con-
straints.
ANTENNA CONFIGURATIONS CONSIDERED
The following antennas were studied to
determine feasibility:
1. /4 whip
2. 3-foot vertical antenna, base loaded
3. 3-foot vertical antenna, center loaded
4. 3-foot vertical antenna, top loaded
5. DDRR(5) antenna with a 6-inch vertical section
and spiral top section
6. 3-foot THC (Top Horizontal Coil: 3-foot
vertical section with horizontal solenoid top)
7. 2-foot THC (2-foot vertical section with
identical solenoid to that of 6)
An 8-foot uniformly loaded vertical whip was also
constructed for use as the standard comparison antenna
-6-
in field measurements, but was not included in the
analysis because of the excessively large ground
plane required.
RADIATION RESISTANCE AND LOSS RESISTANCE
If the impedance of a vertical antenna is
measured at the base with a vector impedance meter,
the impedance of a resonant antenna will be found to
be purely resistive--a value we will call Rb. For the
sake of this discussion, we will assume quadurature
components due to dielectric losses to be negligible.
The base resistance Rb will consist of the radiation
loss, RT and the heat loss due to the finite con-
ductance of the antenna, RL.
For an infinitesmal antenna the power into the
base is
W = Io2 Rb Io 2 (RT + RL)
Where
Io = average rf current into base of antenna
RT = total radiation loss
RL = total ohmic heat loss
The above statement is correct only if all radiation
and ohmic losses are subject to current Io . In
reality, the current in an antenna varies as Io cos 0
-7-
where 0 = 00 at the base, and 900 at the top of the
antenna if the system is resonant.
Each antenna is calculated in piece-wise
fashion to determine contribution to the horizontal
radiation resistance (RH), the vertical radiation
resistance (Rv ) and loss resistance RL. Where coil
loading is used, only the loss resistance in the
loading coil is used for RL because in practical
cases this is usually 99% of the loss.
For comparison of antennas, the ground loss
is assumed to be zero. Obviously during agitated
sea states the quarter wave whip would not be usable
and the effect of sea spray would affect all antennas.
The effect of the sea spray is impossible to
rigorously calculate, but we assume all three models
would be enclosed in a fiberglass shell or radome
filled with dielectric low-loss foam.
-8-
TABLE 1.
Summary of Antenna Calculations
Efficiency %
Vertically Horizontally
Radiation R Loss Polarized Polarized
Antenna RV  RH RL NV NH
4/4 whip (30.76 ft) 38 0 0.1 99.7 0
3-ft base loaded .94 0 1.2 0.7 0
3-ft center loaded .94 0 1.5 42.7 0
3-ft top loaded .94 0 2.6 42.25 0
DDRR .028 0 2.8 2 0
3-ft Top H Coil .94 .94 .38 59 29
2-ft Top H Coil .43 .94 .38 39.5 43
ANTENNA CALCULATIONS
The efficiency of an antenna, for the purposes
of this study, is the ratio of total useful radiated
power to the total power delivered to the antenna.
Efficiency = WV + WH
WV + WH + WL
where
WV = watts radiated, vertically polarized
WH = watts radiated, horizontally polarized
WL = total loss resistance
In these calculations we regard all radiated power
as "useful" and antennas are assumed to be impedance
matched to the radio frequency source.
-9-
Radiation resistance for vertical antennas of
various electrical lengths are plotted in Figure 3.
This curve is derived from equation 22, page 793 of
Terman. (2 ) An approximation to the equation for the
linear porti6n of the curve is given by
Rv = 1578 (H)
For 8 MHz a quarter wave antenna would have a radiation
resistance, R= ' 38 ohms.
To compare various antennas we have assumed a
base current of Io = A cos 0 where A = 1. The average
current in a quarter wave section would be .707. In
this case the power radiated would be
WR = (R + R) ave = 18.99 watts
Assuming a loss resistance of 0.1 ohms
2
WL = RL I v e = 0.05 watts
The efficiency is 99.7%
The calculations of Table 1 are based on
similar assumptions for each type of antenna.
FIELD MEASUREMENTS
After calculating the vertical and horizontal
efficiencies, full scale electrical models were
constructed of the DDRR, the 2-foot top horizontal
coil, and the 3-foot top horizontal coil loaded antennas.
A vertical whip was used as a reference antenna.
-10-
All antennas were connected to a ground plane
consisting of eight each #12 radials 20.76 feet long.
All antennas were adjusted for match to 50 ohm coax
with a Hewlett-Packard vector impedance meter. A
battery operated signal generator was used to drive
the antennas to eliminate AC lines in the vicinity
of the test antennas.
For the receiving antennas an 8 MHz horizontal
dipole and a quarter wave vertical dipole were used
with a precision antennuator between the antenna and
a GPR-90 receiver. The GPR-90 was operated at
constant input. Calibration runs utilizing the
standard vertical dipole on the transmitting end
were made before and after each experimental measure-
ment.
COMPARISON OF TEST ANTENNAS
Vertical Horizontal
Vertical Cal Antenna (8') +14 dB 0 dB
3-foot THC +17.5 dB +23 to +26 dB
2-foot THC +15 dB +20 dB
6-inch DDRR 0 dB 0 dB
From the above data the 3-foot THC is 2.5 dB
better than the 2-foot THC for vertical polarization
3 to 6 dB superior for horizontal polarization.
-11-
For tilt angles of 450 the 3-foot THC signal
decreases 5 dB and for 600 tilt 7 dB.
Since the horizontal coil section is a
semicircle (but not over 1800), the horizontal
pattern on the 3-foot THC is within 3 dB of an omni-
directional radiator and contributes significantly
to vertical radiation under tilt conditions.
Foreshortening antennas results in limited
bandwidth. The antennas tested were measured on a
Hewlett-Packard Vector Impedance Meter to insure
adequate bandwidth for this application. The
Smith Chart for the 3-foot THC is shown in Figure 4.
The DDRR antenna mathematics collapse for
structures with a high vertical section in comparison
with the base diameter. The characteristic impedance
of the top section becomes excessive and the antenna
degenerates into a capacity loaded vertical.
TELEMETRY TRANSMITTER DESIGN
Summary of Specifications
Range: nominal 20 to 70 miles, 100 miles maximum
Data Transmission: up to 100 bites
Bit Rate: 1 kHz
(cont......)
-12-
Summary of Specifications (Cont.)
Data Transmitted: Fish count
Distribution data
Synchronization Code
Buoy identification
Frequency: 8 MHz
Transmitter power output: 10 watts
Modulation: FSK - 2375 Hz mark
3500 Hz space
MODULATION
In the FSK or AFSK mode of operation the carrier
is on continuously, but the carrier frequency or
sideband is shifted to represent a space or mark.
The FCC regulation will permit any frequency
shift up to 900 Hz. The circuitry shown on APL Univer-
sity of Washington drawing 29787 indicates frequencies
of 3500 and 2375 Hz for a shift of 1125 Hz and
audio-shift keying (A2 transmission). Nominally for
operation below 50 MHz FCC regulation allows only
Fl or FSK to be used.
The transmitter design conforms to the APL
University of Washington report; however, special
authorization may be required from the FCC for this
mode of operation.
-13-
TRANSMITTER DESIGN
A block diagram of the telemetry transmitter
is shown in Figure 4. The transmitter consists of
an all solid state design beginning with a fundamental
frequency crystal oscillator on 8 MHz driving three rf
amplifier stages. The final rf amplifier stage uses
a 2N5070 transistor capable of a conservative 10.watts
output with high level modulation.
The modulator consists of an LM380 integrated
circuit driving a Motorola MJE3055 transistor. APL
drawing 29 774 shows an 18 volt battery supply gated
by a 2N3767 transistor for data transmission. The
circuit is compatible with this arrangement.
CONCLUSION
This report outlines a telemetry data link
design for use with the tethered automatic fish
counter described by the Applied Physics Laboratory,
University of Washington. For a small buoy in various
sea conditions, we recommend a short vertical antenna
with a horizontal semicircular loading coil to
operate at a frequency of 8 MHz. A transmitter design
is provided which will produce a nominal 10 watts of
radio frequency power and be capable of frequency
shift keying.
-14-
COMMENT ON 3-FOOT TOP HORIZONTAL COIL ANTENNA
TAPPED AT 1 3 /4 TURNS:
This impedance plot indicates the tap point
is at too high an impedance point on the coil, resulting
in dual resonances at 7.96 and 7.996 MHz and a re-
stricted range over which the antenna will provide a
VSWR of 2 or less to the transmitter.
When proper tap point is used (as shown in the
impedance plot for L turn), the antenna is resonated
with the BFC-12 capacitor. The tap point is relatively
broad with respect to the frequency of resonance,
but the BFC-12 is very sharp.
For a given tap point the shape of the impedance
plot remains.identical, but tuning the capacitor
rotates the position of any given frequency on the
curve.
150'
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REFERENCES
1. Baldwin, H.A., "Long Range Radio Tracking of
Sea Turtles and Polar Bear--Instrumentation and
Preliminary Results," Proceedings, Symposium on
Animal Orientation and Navigation, Wallops Island,
Va., NASA Publication SP 292.
2. Terman, F. E., Radio Engineers' Handbook, McGraw-
Hill, 1943.
3. Reference Data for-Radio Engineers, 5th Edition,
Howard W. Sams and Co., 1968, pg. 27-3.
4. World Distribution and Characteristics of Atmospheric
Radio Noise, CCIR Report 322, 10th Plenary Assembly,
Geneva, 1963.
5. Dome, R. "A Study of the DDRR Antenna'" QST, pg. 27-31,
July 1972.


Telemetry link for an automatic salmon migration monitor

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