Indian Journal of Radio & Space PhysicsVol. 6, December 1977, pp. 288-292

Modulation Structure of Sweepers in hf & vhf Bands

A K SEN, S K TREHAN & S SHEKHAR DEY

Centre of Advanced Study in Radio Physics & Electronics, University of Calcutta, Calcutta 700 009

Received 8 December 1977

Studies are made of a peculiar radio noise usually appearing in the form of a modulated radio carriersweeping from high to low frequencies. The noise known as "sweepers" has been observed in Calcutta in thehf & vhf bands. An examination of the modulation structure of sweepers revealed few characteristic modu-lation waveforms and a remarkable correlation of the waveforms occurri ng at harmonically related frequencieswith a harmonic spacing of about 29 MHz. The correlation is found to be close even for the cases of repetitivesweepers. The results of our studies are discussed in relation to the existing knowledge of magnetosphericphenomena.

1. IntroductionWhile carrying on investigations in hf propagation

characteristics in Calcutta; we obtained some evidenceof an unusual type of interference appearing in theform of a radio signal sweeping usually from high tolow frequency or sometimes in the reverse order andmodulated in amplitude by a repetitive pulse wave-form. A similar disturbance called the "sweepingatmospheric" or "sweeper" was reported fromCanada in 1958 by Gerson and Gossard,' who, con-cluded from their observations that the disturbancemay be of geophysical origin. They, however, couldnot observe any modulation structure of the sweepersand the origin of the sweepers, apparently remainedunresolved for the last eighteen years. It appears thatthe radio interference we observed and the sweeperreported by Gerson and Gossard are really the samephenomena and that they originate from the exo-sphere. A search for the sweepers over a wide fre-quency range, made in Calcutta, revealed that theycan occur at frequencies as high as 150 MHz; whileon the lower side the occurrence exhibits a sort ofcut-off at about 20-25 MHz. Some of the importantcharacteristics of the sweepers observed in Calcuttain the range of 20-30 MHz during the winter months(Dec. 1974-Feb. 1975) are presented in this paper.

2. Experimental ProcedureTo determine the frequency drift rate of the

sweepers we also made simultaneous spaced frequencyobservat ions by employing two tunable narrow bandreceivers, each of which has an effective bandwidthof 3 kHz and covers the range 15-30 MHz. The set-up is shown in Fig. I. The antenna is a folded dipoleat 20 MHz. The variation of signal pick-up withfrequency for the folded dipole was taken into con-s ideration to estimate the intensity. The tuning of288

the receiver 1 was changed in steps of 0'5 MHz from20 to 30 MHz. For each of these steps, the frequencyof receiver 2 was tuned to a higher frequency in afew selected steps ranging from I to 4 MHz. Thetuning and duration of reception at each setting ofthe two receivers were arranged so regularly that weeffectively get samples of the disturbance on a time-sharing basis. As a result, although, the same distur-bance may not exist for the two consecutive steps inthe reception schedule, the statistical disturbance ofthe picture is revealed. This is shown in Fig. 2. Itmay be mentioned here that the initial detection of

rlNHR~~r-~ .

ICAT~g~~RIfOLLOWER~_y N HRO ~\c."."

Fig, 1- Experimental set-up of receiving system

~:h:'-l!~'~:~'~-~ Fallii:g SWQ2~ I 2;5i,'g S'S';;~p I t·/tlJltlr.lc sweep

c; 0 1 2 '0 0 2 " '3 c-' ~ . J ' :0 ' ..JTI~t::,S~( T:MF..s:;z,: ·,-H·'a::.S·;(

'"'"«u25

O'---2~-4.L· --=-6--=-G-~1O::---:':~2::--~1"'''--~'6-=-....:'1'e2c;-:'224:IME .se c

Fig.2-Variations of carrier frequencies as functions oftime for sweepers (estimated from spaced frequency

measurements)

SEN et al. : SWEEPERS IN hf & vhf BANDS

the sweeping nature of the disturbance was achievedwith only one receiver by changing the receivertuning quickly within a fraction of a second, themoment a disturbance arrives. It was found that thedisturbance appears again with the same waveformof modulation at the new setting of the receivers,depending on, whether the new setting was up ordown in frequency in a particular case. The noisefigure of the receiver is 6 dB over the entire frequencyrange of the reception.

3. Observation and AnalysisFig. 3 shows the modulation waveform of sweepers

as can be observed at the output of a sensitive radioreceiver tuned to a spot frequency of 30 MHz. Thewaveforms (a)-(e) exhibit a repetitive pulse with apulse repetition frequency of the order of 200 Hz,while the last waveform (f) contains a single pulsenot repeated at all. The types (a) and (b) occur veryfrequently whereas types (c)-(f) are of a relativelyrarer OCcurrence.

The observations were made for different fre-quency spacings of the two receivers extending from

0::W>WUW0::

(f)

5mSflC~

TIME

Fig. 3- Modulation waveforms of sweeper on 30 MHz

1 to 4 MHz, with their tuning frequencies in 20-30MHz range. To explore the range of frequency sweepduring a sweeper, observations were made with largeras well as smaller frequency spacings. Some typicalestimates of the spectral form of the sweepers obtain-ed from the spaced frequency observations are shownin Fig. 2. It shows the nature of the variation of thecarrier frequency with time for different types ofsweepers. The width of a trace indicates the intensityof the sweepers. Figs. 2 (a) and 2 (b) show the driftsfor falling and rising sweeps, respectively, whileFig. 2 (c) shows that of the multiple sweep. InFig. 2 (c) the frequency spacing between consecutivesweeps is found to be 1 MHz. Fig. 2 (d) shows thedrift curve for a periodic sweep, in which the intervalbetween successive sweeps is 5 sec. During such anoccurrence if the receivers be tuned to 27 and 26MHz, respectively, the sweeps will appear alternatelyat 27,26,27,26 MHz and so on.

From our observations it appears that theoccurrence rate of sweepers, their amplitudes, rangesof frequency sweeps as well as the structure andrepetition rate of the pulse modulation exhibit signi-ficant variability as would be expected if a sweeperbe of a natural origin. A statistics of the occurrenceof the sweepers at a spot frequency of 30 MHzsupported by monitoring at a spaced frequency of29'5 MHz made during the period under considera-tion can be summarized as follows.(i) The sweepers occur more frequently in daytimethan at night tending to follow the variation of F2-region ionization at the exospheric heights. Typicalrates of occurrence vary from 1 to 10 per min in themorning, 5 to 20 per min at midday, I to 5 per minin the evening, while the rate at night is less than1 per min. The rate increases rapidly with receiversensitivity.(ii) The sweepers showing a repetitive pulse in themodulation structure occur more frequently than thosehaving a simple structure in the form of a singlepulse modulation. On a few rare occasions, thesweeper appears to be swishy exhibiting a narrowband noise structure.(iii) The occurrence is often periodic, the successivesweepers occurring at intervals of the order of2-5 sec. During a particular periodic occurrence, theinterval tends to be constant.

4. ResultsThe results of the analysis of the spaced fre-

quency observations can be summarized as folIows.(i) The frequency sweep may occur from high to lowfrequencies as well as from low to high frequencies.The drift rate in the steeper portion is typically of

289

INDIAN J. RADIO SPACE PHYS., VOL. 6, DECEMBER 1977

the order of 1-0'3 Mlfzjsec, In latter portion, the driftrate becomes progressively lower or higher dependingon whether the sweeps are of falling or rising form,respectively. In some cases the sweeps are of multipletype being repeated 2-4 times closely at intervals ofabout I see, but successive amplitudes diminishrapidly.(ii) The amplitude of the sweeping signal during asweeper tends to increase steadily with time. Thepeak amplitude of a sweeper field intensity may varytypically from 10 to 100 p.V 1m during a sweep.(iii) The range of frequency sweep during a sweeperis typically about 1-4 MHz, although in some casesmuch smaller sweep range was noticeable.(iv) The rate of occurrence of the sweeper decreasesrapidly at frequencies below the range 20-25 MHz,exhibiting a sort of cut-off starting around thisfrequency.

5. DiscussionA search for the occurrence of sweepers in the vhf

band revealed an interesting harmonic band structureof occurrence. It has been observed that the hfsweeper occurring within a narrow band around29 MHz, for instance, is most intense and isaccompanied by its harmonics around 58, 87,

~ 10~

.- 0:::>0..

Z 30

0::W 20>w 10UW0:: 0

'-

- SWEEPER29 MHz-

f-

-- SWEEPER

116 MHz- I 4 f

-SWEEPER

- 145 MHz

I 5fI I I

40

30

20

20

10

o2 3 4 5 6

T IME,s(2cFig. 4 - Association of hf sweeper amplitude and vhf

harmonics290

UNSOL D, 1969

\~ Direction of Vibration,..____, of electric vector

IeDirection of radiation

Fig. 5 -Synchrotron radiation due to relativistic electronsspiralling around geomagnetic field line

0·0 0'1 0-5 os 1·1 1'5 1·9 2.3Y

Fig. 6-Spectrum of synchrotron radiation

116 and 145 MHz. The close association between thefundamental at 29 MHz and vhfharmonics is remark-able even in the case of repetitive sweepers as shownin Fig. 4. It may be mentioned here that no sweeperis detectable in the wide bands occupying the spacebetween the narrow harmonically related bands wheresweepers occur. The fact that the lower cut-off of theoccurrence of sweepers is around 20-25 MHz, suggest-ed an origin of the source of emission located abovethe peak of the F2-region ionization in the exosphere.A plausible explanation for its generation might bethe synchrotron radiation from relativistic electronsspiralling about a geomagnetic field line at exosphericheights, undergoing an electron beam cyclotronresonance interaction with a Doppler shifted whistlermode wave. Fig. 5 shows the radiation pattern dueto the synchrotron process." The peak intensity of thesweeper activity around 29 MHz, may, infact beassociated with the peak in the spectrum of synchro-tron radiation" as shown in Fig. 6. The harmonicspacing of a repetitive sweeper is about 1-0'3 MHzas calculated from their drift rate and repetition time.Assuming a synchrotron process to be operative, thegeneration region will be located where the gyro-frequency corresponds to this harmonic spacing. Suchgyrofrequency occurs in the range 2 <L<3. For asynchrotron radiation the frequency of maximumradiation is given by

SEN et af. : SWEEPERS IN hf & vhf BANDS

IIIII

Reg io n of tro pped II outer zone electrons 3 (40 < E> 20 ke~) It--- where cyclotron rosononce interactions I

couse vlf phose onomoly4 I

I Region of high

I latitude whistlerqenerntlon 2

III

u,0·40

>-0·35

ili0.2~

o0·' g:

41. MAEDA .52. CARPENTER

63 HESS 74. ROSENBERG

85. I"~HOF ct 01

Fig. 7-Region of generation of sweepers, whistlers and vlf emissions (Chorous, Hiss)

Vm = 4·6 X l O:" Hl £2

In our case Vm = 29 X 106 (in Hz)

and Hl. = 0·04 gauss at L = 2

( )

112

:. E= 29 xl07:::d·26x107eV:::d·26xI04keV18·4

It is interesting to note that the same order of electronenergy with about the same geocentric distance isrequired to explain the intensity of vlf emissionsthrough an electron beam cyclotron resonance inter-action with a whistler mode wave. Fig. 7 shows theregions of occurrence of whistlers, vlf emissions andrelated phenomena along with a probable location ofthe generation region of sweepers. The correspon-dence of the generation region of vlf emissions andsweepers suggest that the two phenomena may berelated to each other. It may well be, that the sameelectrons which are involved in the generation of vlfemissions radiate synchrotron radiation detected ashf and vhf sweepers, through the ionosphere at lowlatitudes. The synchrotron radiation in fact, would bereceived on the earth's surface only at latitudes lyingwithin _. 25~<L< +25~ for which the tangent to thespiralling electron orbit may pass through the obser-ver. The harmonic band structure of sweeper occur-rence suggests that the amplitude grows sufficiently toinvolve non-linear action at the source. The receptionof sweeper at another low latitude station, Berham-

pore (lat. 42~6' N; long. 88" 19' E) has, in fact, beenconfirmed through the observation by a radio amateur(VU2KX) working in the area; while a joint obser-vation by the authors and another radio amateur(VU2SA) at New Delhi (lat. 28° 63' N; long. 77 °24'E)indicates that the sweepers are detectable even at thatlatitude.

6. ConclusionWhatever might be the exact source mechanism,

the rather large intensities to which a sweeper builds upmay cause interference to radio communication linksand TV broadcasts in the vhf band, and often toradio communication in the higher end of the hfband. In fact, the extension of the sweepers in thevhf band was first noticed by us through its inter-fering effect in TV reception in Calcutta around65 MHz, during an experimental transmission of I kWpower on 28 Dec. 1974. It appears that the occurrenceof sweepers would necessitate a revision of radio noisedata for hf and vhf radio communication systems forlow latitudes. Investigations employing dynamicspectrum analyzers, covering the range 20-300 MHzfed from a wide band antenna, are in progress.

AcknowledgementThe authors are grateful to Prof. M K Das Gupta,

Director, Centre of Advanced Study in Radio Physics& Electronics, for his valuable suggestions.

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INDIAN J. RADIO SPACE PHYS .• VOL. 6, DECEMBER 1971

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292

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