Your RF Cafe
August 1967 QST
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.
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.
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 these questions.
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.
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 static.
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 the writer.
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, 1966.
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.
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.
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 was lagging.
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.
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 missed.
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.
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 system reliability.
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 9 degrees.
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.
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 764.
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