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SU-SEl-68-077
Test of a Monostatic FM-CW
Vertical -Incidence Sounder
by
R.B. Fenwick
J.M. Lomasney
October 1968
This document is subject to special export controls and each
transmittal to foreign governments or foreign nationals may
be made only with prior approval of the Office of Naval
Research, Field Projects Programs, Washington, D.C. 20360.
Technical Report No. 144
Prepared under
Office of Naval Research Contract
Nonr- 225(64), NR 088 019, and
Advanced Research Projects Agency ARPA
RRDIGSIIERCE IRB0RRT0RV
5TM1F0RD ElEITROnitS MBORDTORIES
SinnFORD UHIUERSITV • STHnFORD, CRUFORdin
TEST OF A MONOSTATIC FM-CW
VERTICAL- INCIDENCE SOUNDER
by
R. B. Fenwick and J. M. Lomasney
October 1968
This document is subject to special export controls and each
transmittal to foreign governments or foreign nationals may
be made only with prior approval of the Office of Naval
Research, Field Projects Programs, Washington, D. C. 20360.
Technical Report No. 144
Prepared under
Office of Naval Research Contract
Nonr-225(64) , NR 0£8 019, and
Advanced Research Projects Agency
ARPA Order No. 196
Radioscience Laboratory
Stanford Electronics Laboratories
Stanford University Stanford, California
ABSTRACT
The report describes a system for ionospheric vertical- incidence
sounding using a low-power monostatic, EM-CW homodyne-detection technique.
Advantages of such a system, as compared with the usual high- peak-power
pulse-type ionosonde, are a substantial reduction in interference to
other users of the HF radio spectrum, both local and distant, and a
corresponding reduction of the effects of other-user interference on the
sounder records.
A test was conducted on 24 August 1967, using a second-generation
version of the Stanford IM-CW generator, together with a commercial
digital spectrum analyzer and a facsimile recorder. Real-time ionograms
were obtained over a period of 2-1/2 minutes, using a transmitted power
of 1 watt. The relatively small amount of interference produced by such
a sounder would allow it to be operated — if desired — in an HF radio re¬
ceiving station. Elirther improvement in performance could be secured
by increasing the isolation between the transmitting and receiving
\
antennas.
ii SEL-68-077
CONTENTS
Page
I. INTRODUCTION . 1
II. EXPERIMENTAL DESIGN AND EQUIPMENT . 3
III. EXPERIMENTAL RESULTS . 8
IV. CONCLUSION . 10
REFERENCES . 11
ILLUSTRATIONS
Figure
1. Block diagram of FM-CW vertical- incidence sounder . 4
2. Vertical delta antenna used for vertical-incidence sounding
test . 5
3. Vertical delta antenna . 6
4. Plan view: positions of delta antennas for vertical-
incidence sounding . 7
5. Isolation between delta antennas of Fig. 4 . 7
6. Example of ionogram obtained by FM-CW sounding method ... 9
iii SEL-68-077
I . INTRODUCTION
The high-peak-power characteristic of a conventional ionosonde
(ionospheric sounder) is often a liability. For example, the sounder
should logically be located with other HF communication equipment. Be¬
cause it incorporates a sensitive receiver, it cannot be located at a
transmitting site because of the interference it would receive. Because
it has a high-powered transmitter, it cannot be located at a receiving
site because of the interference it would create. As a result, sounders
have not been used as widely as they might, because of the cost and
inconvenience of establishing a separate site especially for the sounder.
This report describes a different approach to ionospheric sounding.
This new approach calls for radiating an amount of power not far different
from that which is likely to leak from the high-frequency oscillator of
an unshielded receiver.
The "monostatic CW ionosonde" (a monostatic, FM-CW, homodyne-detection
sounder) can provide the advantages of a substantial reduction in inter¬
ference caused to other users of the radio spectrum (obtained, as already
noted, by the use of extremely low transmitted power) and of some reduc¬
tion in complexity of sounder design (achieved by the elimination of high-
voltage, high- power components and of the need for a separate super¬
heterodyne receiver). The monostatic ionosonde is practical to the
extent that spurious sidebands and hum components of the frequency- sweep
waveform can be reduced to a level significantly lower than that of the
energy received after reflection from the ionosphere.
An FM-CW ionosonde employing a precise, directly synthesized frequency
sweep was introduced in 1964 by Fenwick and Barry [Ref. 1], The original
equipment, designated "Chirp I," was superseded by a second-generation
equipment, "Chirp II," in 1966 [Ref. 2], The principal improvements
incorporated in Chirp II were provisions for easily set sweep rates and
sweep limits, and an increase in the time-delay resolution capability
from 10 (isec in Chirp I to better than 1 psec in Chirp II.
At the time the Chirp II equipment was made operational, a Federal
Scientific Corp. Model UA7 spectrum analyzer and an ITT facsimile re¬
corder were obtained; these gave a capability for real-time analysis of
1
SEL-68-077
ionospheric reflections at much lower frequency-sweep rates than could
be used previously. This capability, in turn, made possible the use of
very low transmitted power for vertical- incidence sounding; thus it be¬
came reasonable to attempt transmitting and receiving at a single site,
using a single .requency- sweep generator both for transmitting and for
received-signal demodulation. These enhanced equipment capabilities
made feasible a monostatic vertical-incidence sounding test. Such a
test was conducted at Stanford on 24 August 1967. The results are re¬
ported herein.
2
SEL-68-077
II. EXPERIMENTAL DESIGN AND EQUIPMENT
The EM-CW ionospheric sounder differs in principle from pulse sounders
in that it transmits a continuous, relatively low-power signal whose fre¬
quency varies at a constant rate. For vertical- incidence sounding, the
frequency of the signal received by ionospheric reflection will differ
from the frequency of the signal received by the direct path between
transmitting and receiving antennas because the transmitted frequency
changes during the additional time required for the ionospherically re¬
flected signal to travel to the ionosphere and back to the receiver. For
a given frequency-sweep rate, the virtual height of reflection of the
transmitted signal is directly proportional to the frequency difference
between the direct and the reflected signal:
h’ = 4 fQ (1)
2f
where
h' = virtual height of reflection in km;
fQ = difference frequency in Hz;
f = frequency sweep rate in Hz/sec;
c = velocity of propagation in km/sec.
Figure 1 is a block diagram of the experimental set-up. A pair of
vertical delta antennas [Ref. 3; Figs. 2 and 3] situated as shown in
Fig. 4 were used for transmitting and receiving. The two antennas were
nearly orthogonal to each other in order to minimize coupling, and were
oriented at an angle with respect to magnetic north so that both the
ordinary and extraordinary transmission modes could be excited with com¬
parable signal levels. Figure 5 shows the measured isolation between the
antennas.
The swept- frequency output power of the Chirp II equipment was di¬
vided by a wideband power divider. Half the power was amplified and
transmitted through coaxial cable to a second broadband amplifier located
at the feed point of the transmitting antenna. The return signal picked
3
SEL-68-077
■S"tf=
* %<y.
O
z
4
SEL-68-077
Fig. 1. BLOCK DIAGRAM OF FM-CW VERTICAL- INCIDENCE SOUNDER
Fig. 2. VERTICAL DELTA ANTENNA USED FOR VERTICAL-
INCIDENCE SOUNDING TEST.
up by the receiving antenna was band-pass filtered in order to reduce
interference, and was mixed (in a balanced mixer) with the other half of
the original signal from the power divider. The difference-frequency
output of the mixer was analyzed by the spectrum analyzer and recorded
on the facsimile recorder. On the resulting records, the horizontal
(time) axis can be calibrated in transmitted frequency, while the vertical
(difference- frequency) axis can be calibrated in virtual height to the
reflection point, in accordance with Eq. (1).
5
SEL-68-077
Fig. 3. VERTICAL DELTA ANTENNA.
6
SEL-68-077
III. EXPERIMENTAL RESULTS
The equipment was operated for several hours on 24 August 1967.
With the arrangement shown in Fig. 1, the nominal power output was 1 watt.
Figure 6 is an example of the vertical ionograms obtained. F-region X-
and O-modes are clearly shown above 5 MHz; the O-mode is still visible
at the 3-MHz lower limit of the ionogram. Note the interesting traveling-
disturbance effect at 4.5 MHz. The 60- Hz , 120-Hz, and 180- Hz hum lines
are quite apparent on the record of Fi0-. 6.
Some records were made with higher power, i.e., in the 50- to 60-watt
range. The records thus obtained appeared identical with those obtained
at lower power (since the noise level on the record is primarily sounder
self-noise and not atmospheric noise or interference).
Thus a nominal power of 1 watt appears ample for the purpose of
vertical-sounding. In fact, results from oblique sounding experiments
(unpublished) suggest that ••cable records ought to be obtainable with
only a few milliwatts of transmitted power. Levels of 100 milliwatts to
1 watt are probably optimum; increasing power much beyond this level will
only increase interference caused by the sounder to other equipment. The
use of antennas separated by a greater distance than was the case in this
experiment (270 feet) is desirable to reduce "self interference" by noise
sidebands on the frequency sweep because of mutual coupling between the
antenna^. Careful attention to feeder balance and antenna orthogonality
would also reduce mutual coupling.
8
SEL-68-077
IV. CONCLUSION
This experiment showed that usable high-resolution vertical-incidence
ionograms can be obtained with a monostatic FM-CW sounder transmitting a
power of the order of 1 watt. Using so low a power level, transmitting
and receiving functions can be performed conveniently at a single site
and with a single frequency-sweep generator. No separate receiver (in
the usual sense) is required. Also, interference caused by the sounder
to other spectrum users is minimized.
10
SEL-68-077
REFERENCES
1. R. B. Fenwick and G. H. Barry, "HF Measurements Using Extended Chirp-
Radar Techniques," Report SU-SEL-65-058 (TR No. 103), Stanford Elec¬
tronics Laboratories, Stanford, Calif., June, 1965.
2. L. L. Peden and R. B. Fenwick, "Chirp II — A Flexible, HF Sweep Fre¬
quency Sounder with Submicrosecond Resolution," Report SU-SEL-67-055
(TR No. 140), to be published.
3. H. N. Cones, H. V. Cottony, and J. M. Watts, "A 600-ohm Multiple-
Wire Delta Antenna for Ionosphere Studies," Journal of Research of
the National Bureau of Standards, Research Paper RP2094, Volume 44,
May, 1950.
11
SEL-68-077
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3 REPORT TITLE
TEST OF A MONOSTATIC FM-CW VERTICAL- INCIDENCE SOUNDER
4- DESCRIPTIVE NOTES ( Type of report and inclusive dates'
Technical Report No. 144-October 1968
5 authoriS) (First name, middle initial, last nome)
R. B. Fenwick
J. M. Lomasney
6 REPOR T DA TE
October 1968
««. CONTRACT OR GRANT NO
Nonr-225(64), NR 088 019
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SEL-68-077
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this report)
10. DISTRIBUTION STATEMENT
This document is subject to special export controls and each transmittal to foreign
governments or foreign nationals may be made only with prior approval of the Office
of Naval Research, Field Projects Programs, Washington, D.C. 20360.
11- SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY
Office of Naval Research and
Advanced Research Projects Agency
13 abstract ' rep0r{- describes a system for ionospheric vertical-incidence sounding
using a low- power monostatic. FM-CW homodyne-detection technique. Advantages of such
a system, as compared with the usual high-peak-power pulse-type ionosonde, are a sub¬
stantial reduction in interference to other users of the HF radio spectrum, both local
and distant, and a corresponding reduction of the effects of other-user interference
on the sounder records.
A test was conducted on 24 August 1967, using a second-generation version of the
Stanford FM-CW generator, together with a commercial digital spectrum analyzer and a
facsimile recorder. Real-time ionograms were obtained over a period of 2-1/2 minutes,
using a transmitted power of 1 watt. The relatively small amount of interference
produced by such a sounder would allow it to be operated — if desired — in an HF radio
receiving station. Farther improvement in performance could be secured by increasing
the isolation between the transmitting and receiving antennas.
DD ,Fr..1473
S/N 0101-807.6801
(PAGE I)
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