August 1967 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
NASA (and its predecessor
NACA), and private and public operators have been monitoring solar events
in the optical realm for many decades while attempting to correlate terrestrial
phenomena with it. Auroral light displays in the extreme polar regions have long
been known to be caused by solar flare and coronal mass ejections (CME). With the
advent of radio, the electrical nature of the upper atmosphere became evident when
static (AM) and long range propagation affected long range communications. Extreme
CME activity eventually was associated with behavior of the electrical power grid;
indeed, massive blackouts and brownouts are to blame for many. Last but not least
came concern for sun-sourced electrons regarding satellites. More than one 'bird'
has been smoked by the sun's ejecta.
This November, on Thanksgiving
Day, actually, comet
ISON, which could produce the most spectacular astronomical display in our lifetime
("the comet of the century"), might actually impact the sun. Depending on where
it strikes (if it strikes), some of the most violent solar electrons ever could
be hurdled toward the earth. The effects cannot be predicted; we'll all find out
real-time what our fate is. You might want to make sure all your electronics devices
have a full charge, just in case. Update: It crashed into the sun.
432-Mc. Solar Patrol
A Study of Solar Noise in Relation to Radio Propagation
By Paul M, Wilson, W4HHK/A4HHK
In the spring of 1966, the "big dish" at W4HHK was comparatively idle. Oscar
IV was silent, moonbounce activity on 432 Mc. was nil, and there were no troposcatter
schedules in prospect. Then an item in Sky and Telescope caught my eye. It told
how NASA planned to maintain a solar flare patrol as part of the space program.
Why not a solar patrol on 432 Mc.?
This seemed like a worthwhile effort. Though the flux density of radio noise
from the sun is measured daily by observatories, their records are not immediately
available to the amateur. Our project would be a means of keeping track of solar
activity on a day-to-day basis. In addition it would be a way to evaluate the antenna
and receiving. system, and periodically check it.
Until now only an occasional "look" at the sun had been made. Observation had
been inconvenient because each session meant trips up and down the tower to release
and stow the dish. By the middle of April, 1966, several events had made daily observations
feasible. A stowing device was built that permitted operation from the ground. A
commercial step attenuator was obtained for calibrating solar noise recordings accurately.
Last but not least, a 432-Mc. preamplifier built with low-noise kMc. transistors
made a noticeable improvement in 432-Mc. receiver performance.
At this point it is desirable to examine the source of the noise to be observed
and measured. The sun is some 92 million miles from earth, or 8.3 minutes of travel
at the speed of light and radio emissions. It is not a true point source, but for
most amateur purposes can be considered as one. According to Bray and Kirchner,1
it can be represented as a ring of about one degree angular diameter on the outside,
and about one-half degree on the inside. The sun is a source of noise on radio frequencies,
caused by plasma oscillations and gyro-oscillations in the solar atmosphere, as
well as noise originating in random collision of electrons. Noise from the quiet
sun is of the latter type.2 A solar noise recording made when the sun
was relatively quiet is shown in Fig.1.
Fig. 1 - Noise of the sun on a relatively-quiet day, Dec.
1, 1966. Noise was running between 3 and 3.5 db. American sunspot count was 32.
A 1-db. calibration dip is seen near the center of the recording. A burst of ignition
noise is seen at the left.
Noise from a quiet sun appears to have random polarization, but bursts of high
intensity at 432 Mc. are elliptically to linearly polarized.3 Sunspot
activity of the current 11-year cycle (Cycle 20) is expected to peak some time in
1968. The quiet sun looks like an approximate 500,000-degree Kelvin source, rising
to about 1,000,000 degrees K. when disturbed." The amplitude of solar emission may
remain relatively constant for long periods, and then will be greatly enhanced during
a "noise storm." Such storms are often associated with solar flares and certain
geophysical disturbances, and may last for hours or days.5
Solar flares are sometimes, but not always, followed by aurora some 20 to 40
hours later. The rotation period of the sun is about 27 days, and there is a tendency
for aurora to recur at this rate. Some sunspots may survive several rotations before
disappearing. In a 1951 QST article,6 Moore pointed out that correlation
between sunspot number and aurora is not as great as one might expect. But what
about solar noise and aurora, or sporadic E, F2 or transequatorial v.h.f.
propagation? It was hoped that regular solar noise observation might serve as an
indicator of propagation conditions, and possibly give advance warning of events
such as major auroras.
Besides helping you to keep up to date on what the sun is doing, regular solar
noise measurements are a means of evaluating system performance, from antenna to
receiver output. For given antenna size, feed line loss and receiver temperature
(or noise figure) a certain minimum amount of solar noise should always be obtainable.
WIFZJ described how a 17-foot parabola and a receiver-feed line combination with
a 751-degrees Kelvin temperature would observe a 3-db. increase in noise when aimed
at the quiet sun.7 Another reference gives the quiet-sun noise level
at 432 Mc. as 21 db. below the receiver noise level when using a perfect receiver
and a dipole antenna. An increase of 10 db. could be expected from an active sun.8
At the outset several questions were raised. Was the writer's dish performing
up to specifications? What short-term variations would be observed? Could solar
noise be used for reasonably accurate comparisons of equipment? Text books on hand
didn't have all the answers. Thus one goal of the project was to resolve some of
The Noise-Observing Setup
The equipment used for solar noise measurement at W4HHK is shown in block-diagram
form in Fig. 2. It doesn't have the simplicity of visual devices such as the
Aurorascope9 for checking sunspot activity, but it does work, rain or
shine. The antenna is an 18-foot parabola, with a focal length of 90 inches, and
a diameter-to-focal-length ratio of 2.45. The slide-rule specifications for 432
Mc. are 26 db. gain over isotropic, and a beam-width of 9 degrees at the half-power
points. It was made by the D. S. Kennedy Company, and obtained through the Army
MARS program. The center is 35 feet above ground, at a 380-foot elevation. Geographic
location is latitude 35 02 48 North and longitude 89 40 04 West. The dish is fully
steerable in azimuth and elevation by an SCR-584 pedestal.
Fig. 2 - Block diagram of the 432-Mc. solar-noise measuring
setup at W4HHK.
A modified FPS-3 radar platform supports the pedestal and dish. Selsyn indicators
at the control point show the azimuth and elevation in one-degree intervals. Steering
is done by applying d.c. power to the 1/2-h.p. pedestal motors.
The feed for the dish is a horizontal folded dipole, with a plane reflector 16
inches in diameter. An air-dielectric balun mounted at the feed matches 50-ohm coax
to the balanced 300-ohm dipole. A 38-foot length of foam RG-8 connects the balun
to a 60-foot run of 7/8-inch Heliax, from the base of the pedestal to the shack.
Overall line loss is about 2.25 db., according to published figures.
Since the antenna is also used for transmitting, a coaxial relay, not shown,
is employed for transmit-receive switching. It is worth mentioning that after several
months of operating a kilowatt transmitter alongside, no measurable change in receiver
noise figure has been observed. The feed line is disconnected manually when the
equipment is not in use, as a precaution against damage from lightning and heavy
Each r.f. stage is in a separate aluminum box, with power supplied by individual
9-volt batteries. The first r.f. amplifier uses a KMC n.p.n. experimental transistor,
with a factory-measured noise figure of 2.8 db. (for the transistor). The second
uses a TIXMO5 p.n.p. transistor. Gain per stage is about 9 db. Each has the common-emitter
configuration, and is unneutralized. Both are stable, even in the absence of input
load. Both have double-tuned input circuits, to minimize response to out-of-band
signals. In-band signals are not a problem, as the nearest 432-Mc. station is 70
miles distant! The second r.f. stage feeds a 1N21F mixer, which works into a 50-Mc.
converter at 49.5 Mc., converting again to 7 Mc., and followed by a 75A3 receiver
and chart recorder. The converter and 75A3 are always kept on standby when not in
use, to minimize trouble. The r.f. amplifiers and 432-Mc. converter were built by
A constant-voltage line transformer was found to be necessary to maintain reasonably
good gain stability. Without it a recording of a constant-amplitude signal would
vary considerably, especially during lengthy recordings. Apparently the main cause
of this was heater-voltage change, as the converter plate voltages are all regulated.
The converter seemed more sensitive to line-voltage change than did the 75A3. This
and other problems were ironed out before regular observations began on June 1,
A 20-db. fixed pad is used at the converter output and a 10-db. fixed pad at
the receiver input, with the step attenuator in the 50-ohm line between them. This
was done to insure operation of the attenuator at its design impedance. The 75A3
mode switch was modified to permit reception with the b.f.o. and a.v.c. off. The
receiver 500-ohm output is connected to a bridge rectifier, which drives the Esterline-Angus
1-ma. strip-chart recorder. Rectifier output is not linear, crowding at the low-signal
end of the scale. Esterline-Angus chart paper, type 132020, that closely matches
the rectifier response, was obtained surplus. A chart speed of 3 inches per minute
has been found a good compromise between easily-read recordings and paper conservation.
For long-duration recordings a speed of 3 inches per hour has been used.
Radio observatories measure the flux density of solar radio emissions by using
the unit of 10-22 watts per square meter per cycle per second. The method used by
the writer measures the ratio of solar noise to quiet-sky (background) noise in
decibels. The results of a typical measurement, made as described below, are shown
in Fig. 3. The feed line is connected to the first r.f. amplifier and power
is applied to both r.f. stages. Converter crystal mixer current is adjusted to read
0.5 ma. The 75A3 S-meter is observed to verify that two S-units of converter noise
is read with the receiver in the a.m. mode. The 75A3 a.v.c. and b.f.o. are turned
off, and the a.m. and c.w. limiters are disabled. Audio gain is advanced to 75 percent
of full volume, and the" r.f." gain is reduced until the chart recorder reads about
one-third scale, as seen at the right edge of the recording.
Fig. 3 - Solar noise level of 5 db. recorded March 3, 1967.
Read chart from right to left. A chart speed of 3 inches per hour was used at the
start, as the dish was centered on the sun for the recording in the upper right
corner of the chart, then changed to 3 inches per minute for the measurement. Next
come three levels of attenuation, inserted for calibration, and another 15 seconds
of solar noise. The dish is then turned away from the sun, and the quiet-sky noise
is recorded. A 50-ohm termination is then substituted, giving about 0.5 db. more
noise than the quiet sky.
The dish is then steered toward the sun, and the azimuth and elevation controls
are adjusted until maximum noise is indicated on the recorder. Audio gain is readjusted
for a meter reading of about 0.9 ma. Chart speed is shifted from the 3 inches per
hour, used while steering, to 3 inches per minute, and solar noise is recorded for
about 30 seconds. This is the first 5-db. peak at the right. At this point several
steps of attenuation are inserted to calibrate the chart, usually beginning with
the 3-db. step. Normally the 3-, 4-, and 5-db. steps are each recorded for 15 to
30 seconds, depending on the undesired responses that may be present, such as from
ignition or radar. In this instance steps of 4, 5 and 6 db. were used. The audio
is monitored on a speaker, and the chart trace is observed, to make certain that
interference is not spoiling calibration and reception in general.
Following the last level of calibration the attenuator is switched out of the
circuit, and full solar noise is again recorded on the chart. If nothing has changed
(dish heading by wind gust, solar noise level, etc.) the reading will be about the
same as at the start. Though the sun is not tracked during the measurement, the
noise amplitude should remain constant, because of the antenna beamwidth and the
shortness of the measurement period.
The dish is then steered away from the sun to a point in the northwest sky, the
quietest heading for this location. Quiet-sky noise level is recorded for 30 seconds
to a minute, again making certain that unwanted noise does not obscure the quiet-sky
noise level. As each measurement is being made the GMT date-time, calibration steps,
and other information are noted quickly on the moving chart. Three to six measurements
are made each day in this manner, the entire operation taking 30 minutes or so.
More measurements are made if conditions warrant. Recordings are examined at the
end of the session, or later, and solar noise readings determined. Periodically
the noise of a 50-ohm termination at the receiver input is recorded, in addition
to quiet-sky noise, to confirm receiver performance. Termination noise is usually
about 0.5 db. higher than quiet-sky noise, as seen at the left edge of Fig. 3.
All readings are entered in the solar noise log, and the section of chart with
the highest reading is mounted and filed. The highest reading is of particular interest
because it shows maximum solar activity during the observing period, and because
it is usually the most accurate. Any errors caused by steering, noise, and equipment
failure tend to degrade readings.
A solar noise recording made while the sun moved across the dish is shown in
Fig. 4. This was obtained by pointing the dish at a point in the sky where
the sun would be at a later time. The Greenwich hour angle and declination of the
sun were determined from the Nautical Almanac.10 This information was
converted to azimuth and elevation by tables found in a Hydrographic Office publication.
11 Receiver gain was adjusted so that background noise deflected the
recording pen about one-third scale, and the chart speed set for 3 inches per hour.
Recording was started about an hour in advance of the time for which the dish was
positioned. When the sun came into view the solar noise rose from the background
level, peaked, and then slowly fell. Insertion of suitable levels of attenuation,
in this instance 4 and 5 db., provide a check on the performance and establish the
accuracy of the recording. Only the main lobe is seen at present levels of solar
activity. Minor lobes of the antenna are too far down to show.
Fig. 4 - Movement of the sun across the dish is shown in
this 3-hour recording made with slow chart speed. The antenna was aimed at a point
where the sun would be later, and left there to record the rise in noise level as
the sun passes. Calibrations of 4 and 5 db. inserted at the noise peak show the
strength of the solar noise. Numerous spikes are ignition and other noise.
The sun's position can be used to "boresight" an amateur antenna. The dish is
peaked carefully on the sun at a time and date listed in the Nautical Almanac, and
the azimuth and elevation indicators are adjusted to read the azimuth and elevation
given in the tables for that moment. This requires knowing the latitude and longitude
of the antenna site precisely, and making the test at the correct time.
A Quiet Summer Ends Dramatically
Noise varied monotonously between 3 and 4 db. through the summer of1966. QST
reported an aurora on July 7-8, and Sky and Telescope mentioned a big flare on July
11, but solar noise recordings on or about these dates gave no hint of anything
out of the ordinary. Obviously measurements made during a brief period daily do
not tell the whole story, and short-term events can be missed entirely. By late
August the daily routine of shooting the sun had become just that: a routine. Interest
Then on August 29 a reading of 4.7 db. was obtained. Sunspot records indicate
that it was probably higher the previous day, when a Class 3 flare and associated
sudden ionospheric disturbance (SID) occurred, but the 28th had been missed by this
observer. Here was the big change sought all summer. Would there be an aurora? Indeed
there was! QST reported auroral v.h.f. communication on Aug. 29, 30 and Sept. 1
through 4. The evening display of Sept. 3 was the best aurora in years, according
to Sky and Telescope,12 and amateur results on 6 and 2 bear this out.
An inconvenient work schedule and too much reliance on "N7" transmissions by WWV
caused the writer to miss the big event!
This called for careful monitoring of the sun for the remainder of the month,
with results shown in Fig. 5. Solar noise is plotted by the solid line, and
American sunspot numbers13 by the broken line. Aurora dates are identified
by the symbol, A. Note the high level of solar noise and sunspot number on Aug.
29, followed by a rapid drop to the low point Sept. 3, the period of auroral activity.
Solar noise peaks were observed again on the 16th and 20th, and corresponding sunspot
maxima occurred on the 17th, 19th and 21st. Both started dropping, and reached a
low point on the 27th and 28th. No radio aurora was reported around the noise peaks
of the 16th and 20th, but visual sightings were reported by Sky and Telescope. QST
reported 50-Mc. DX Sept. 21.
Fig. 5 - Correlation between sunspot number and 432-Mc.
solar noise is shown clearly by this graph. Dates when aurora was observed are indicated
by the circled A.
The graph indicates that 432-Mc. solar noise and sunspot numbers are related,
and vary in the same general way. One can see readily that there is also a correlation
with aurora. It would also appear that during September the sunspot maxima and minima
tended to trail solar noise highs and lows by a day or so. This is not always the
case. Records for eight other months show noise peaks and sunspot maxima coinciding,
and occasionally the sunspot count has peaked a day in advance of the noise.
Fall and Winter
During October solar noise was measured every day but three. Auroral propagation
on 50 and 144 Mc. was reported by W1HDQ Oct. 3, but noise and sunspot figures for
the period didn't indicate it. Presumably this was a 27-day recurrence of the disturbance
of early September. There was a noise peak on the 20th that coincided with maximum
sunspot number (82) for the month. The Ottawa Algonquin Radio Observatory report
for October shows solar flux at 2800 Mc. also peaked this day, but there were no
radio aurora reports. Sky and Telescope confirms that there was a small aurora on
the 20th. Transequatorial propagation on 50 Mc. was observed by PY5GK on 14 days
in October, including the 21st, but not the 20th.
Shortened daylight hours reduced our opportunities for daily observations in
November and December. Solar noise high for November was only 4.1 db., but activity
picked up in December. High noise reading for the last month of the year was 4.75
db. at 1900 GMT, Dec. 13. There were no reports of aurora. (Statistically, aurora
is rare in this hemisphere in December - Editor). Sagamore Hill Observatory reported
an outstanding solar radio emission on 2695 Mc. at this time, and American sunspot
numbers were at their month high on the 13th. A week later noise readings reached
a low point, and then began an upward climb toward the end of the month. A good
indicator of general solar activity, for the writer, has been the number of days
per month that solar noise reached 4 db. or higher. December had 13 such days, compared
with only 5 in November, for almost the same number of observing days.
No observations were made the first few days of 1967, but when they were resumed
Jan. 5 a whopping 5.1 db. was recorded, and it was up to 5.6 db. the following day.
The alert was sounded, and participants in the V.h.f. Sweepstakes the weekend of
Jan. 7-8 don't need to be told what happened. Working schedules the next few days
prevented observations, but on Jan. 10 a high of 4.75 db. was recorded. Three days
later, Jan. 13-14, another auroral session was reported. Like that of Jan. 7-8,
it was violent and widespread, yet January is normally a relatively quiet month
for aurora. Solar activity continued high, and every day when observations could
be made found noise exceeding 4 db.
The first day of February a high noise reading of 4.6 db. was obtained, and on
the 4th an all-time high of 6 db. was logged! Three days later, on the 7th, another
aurora was enjoyed by the v.h.f. gang. Activity continued high, with readings of
4.7 db. on the 7th and 4.5 db. on the 8th. Readings then settled down to about 4
db. There was no indication of aurora on Feb. 16, about the time that a recurring
disturbance might have been expected. It would appear that some of the "repeaters"
(auroras recurring on the 27-day solar rotation cycle) are not associated with solar
noise readings, or the related noise peaks are of short duration and are easily
Readings continued at 4 db. until Feb. 23, when a moderate rise to 4.5 db. was
observed. Something new to the writer was recorded on this date: a noise burst some
3 db. above the average level, lasting about 90 seconds. See Fig. 6. On the
25th solar noise was peaking at 4.6 db. A minor aurora was reported on the 27th.
Fig. 6 - 90-second burst of solar noise recorded at
1613 GMT Feb. 23, 1967. Peaks Were some 3.5 db. above the normal solar noise level.
The Overall Picture
Highlights for the period June 1, 1966, to March 1,1967, are shown in Table I.
The monthly mean value for American sunspot numbers is included to show the relationship
of sunspot count to solar noise. No doubt results might have been different if noise
readings had been made every day, or for longer periods of time. Nevertheless, the
figures do show that solar radio noise and solar activity have been increasing since
June, 1966, and are still climbing. During the entire period there were no changes
made in equipment or measuring procedure. Any known causes of error have been noted.
Various r.f. amplifiers were tested and compared from time to time, but only the
units described were used for the recorded data. Periodic checks of tubes and regular
measurement of noise figure was done in an effort to obtain consistent results.
The unchanging performance of the transistor r.f. stages contributed much to the
Paul Wilson, W4HHK, at work on the 18-foot dish used to make
solar noise measurements on 432 Mc. The driven element and its circular plane reflector
are just above the author's head.
Our 432-Mc. solar patrol has provided some answers to questions raised at the
beginning, but more information is needed. The receiving system has performed about
as expected, except that the azimuth beamwidth of the antenna is almost double what
it should be, probably because an 18-foot dish is "small" in terms of wavelength
at 432 Mc., and thus the dipole does not simulate a point source, as it must for
optimum performance with a parabolic reflector. Vertical beamwidth is correct, about
A minimum of about 3 db. of solar noise was always obtained, even during periods
of little or no solar activity. When used with care, solar noise readings under
quiet conditions can be used to evaluate systems, and make comparisons between antennas,
r.f. amplifiers and so on. By comparing readings with other amateurs, a v.h.f. operator
can determine if his system is "in the ball park," which can be very helpful in
setting up for moonbounce work, for example. But beware! Solar noise measurements
are subject to error and variations, just as noise figure measurements are.
Regular observations do give an indication of solar activity, but the solar noise
level is not an exact indicator of sunspot number. For example, the AA VSO daily
sunspot count has varied from 1 to 89 for noise readings of 3.5 to 3.6 db. Sunspot
number is usually in the range of 1 to 50 for a 3.5-db. noise level. Auroras are
associated with high solar noise levels, the .( magic number" for the writer apparently
being a minimum level or about 4.7 db. This level and higher has been observed on
ten separate occasions, and seven individual auroras, apparently related, have been
reported. Aurora appears more likely when high noise levels are observed on two
or more successive days. Delay between noise peak and aurora has been one to three
days, though some occurred on the same date. More observations should give a clearer
picture, and improve forecasting. Thus far the relationship of F2 and
transequatorial 50-Mc. propagation to solar noise is not well defined, though this
could improve when the F2 layer rises into the 50-Mc. region more consistently.14
Sporadic-E seems hardly related, if at all.
The equipment and techniques described should not be considered the ultimate
or best way. This is simply an account of a 432-Mc. experiment, in the hope that
it will shed some light on the subject as it relates to amateur v.h.f. enthusiasts,
and perhaps stimulate others to do similar work. Possibly observations by a number
of stations could be coordinated to provide an auroral warning service for the v.h.f.
community, during the peak years of Cycle 20.
The writer owes special thanks to Third Army MARS for the dish and related equipment;
to WIHDQ for propagation reports; to Sky and Telescope for auroral reports; to the
American Association of Variable Star Observers for sunspot data; to K2TKN, W3GKP,
W3O11, WA0IQN and others for their assistance in making this project possible.
Late Report On the afternoon of May 23, W4HHK recorded solar noise on 432 Mc.
in excess of 15 db. Visual observation showed exceptionally large spots approaching
the center of the solar disk. On May 25 came the most widespread auroral display
in recent years. V.h.f. communication via the aurora was reported as far south as
Sarasota, Florida, and auroral contacts were made in Southern California for the
first time. The disturbance continued through May 30, with peaks on the 25th, 28th
and 30th. For more details. see The World Above 50-Mc., July QST.
Table 1 - 432 Mc. Solar Noise June, 1966-February, 1967
(*) From Sky & Telescope Magazine.
1. Bray and Kirchner, "Antenna Patterns from the Sun," July, 1960,
QST, p. 13.
2. NBS Monograph 80, "Ionospheric Radio Propagation," April 1, 1965,
p. 38. Price $2.75, from Supt. of Documents, Washington, D. C. 20402.
3. M. H. Cohen, Cornell University "Measurement of Solar Radiation
at 430 Mc ." Quarterly Status Report, Feb. 1-July 31, 1965.
4. "The World Above 50 Mc.," October, 1965, QST, p. 112
5. Reference 2, p. 43.
6. Moore, "Aurora and Magnetic Storms," June, 1951, QST, p. 16.
7. "The World Above 50 Mc., October, 1960, QST, p. 112.
8. "IT & T Reference Data for Radio Engineers, 4th Edition, p
9. Tomcik, "The Aurorascope;" July, 1964, QST, p 43.
10. "Nautical Almanac for 1967," Supt. of Documents, Washington, D. C.
20402. Price $3.50.
11. "U. S. Navy Hydrographic Office Tables of Computed Altitude and Azimuth,"
Latitudes 30 to 39 degrees, inclusive. Pub. No. 214, Vol. IV. Order volume applicable
to your latitude, from Supt. of Documents, Price $3.00.
12. "Auroral Activity Increases," Observer's Page, Sky and Telescope, December,
1966, p. 380.
13. American sunspot numbers derived by the American Association of Variable
Star Observers (AAVSO), solar division, reported monthly in Sky and Telescope.
14 The correlation between solar activity and extreme peaks of F2-layer
m.u.f. was first observed by G6DH in the 1930's. For his slant on it, relating to
50-Mc. DX, see "Any DX Today?" January, 1948. QST, p. 27. The correlation with transequatorial.50-Mc.
DX is indicated in a summary of 1950 work by South American 50-Mc. operators, in
"The World Above 50 Mc.," May, 1950, QST, p. 49.
Posted September 24, 2013