Radio Science,Volume 9, Number 3, pages387-401, March 1974
A new wideband, fully steerable, decametric
array at Clark Lake
William
C. Erickson
and J. Richard
Fisher •
Astronomy Program,
University o[ Maryland, College Park, Maryland 20742
(Received August 20, 1973.)
A new, fully steerable,decametricarray for radio astronomyis under constructionat the
Clark Lake Radio Observatorynear Borrego Springs,California. This array will be a "T"
of 720 conicalspiral antennas(teepee-shaped
antennas,hencethe array is calledthe TPT),
3.0 by 1.8 km capableof operatingbetween15 and 125 MHz. Both its operatingfrequency
and beam positionwill be adjustablein less than one msec, and the TPT will provide a
49-element picture around the central beam position for extended-sourceobservations.
Considerableexperiencehas been gained in the operationof completedportions of the
array, and successfuloperationof the final array is assured.This paper describesthe results
of the testswhich have been conductedwith the conicalspiralsand outlinesthe planned
electronics.
INTRODUCTION
nearlyinstantaneous
frequency
andbeampositioning
Most of the radio.astronomicalobservationsbelow
100 MHz have been the resultof considerableeffort
capability.
The instrumenthas been designedwith both solar
on the part of a relativelysmallnumberof astron- and siderealprogramsin mind.The spacingof the
omersand engineers.
The size of the instruments phasecentersof the banksof elementshas been
requiredhas precludedthe construction
of more arrangedto providea "fieldof view"clearof gratthan a few in the world, and, until three or four ing responsesseveralsolar diametersacross.One
yearsago, technologyhad not allowedthe design of the firstusesof the instrument
will be as a multiof a large decametricarray whichwouldoperate frequencyradioheliograph,
similarto the Culgoora
over more than a limitedfrequencyrangeand be instrument.
The systemhas mostof the flexibility
steerable
in twocoordinates
withreasonable
speed. of a hugeparaboloidal
antenna,
andhasa number
Consequently,
• this part of the radiospectrum
has of advantages
and disadvantages.
First of all, its
attractedvery few astronomers
eventhoughmuch
informationabout the physicsof celestialobjects
may be foundfrom the studyof radiationat these
wavelengths.
Recent advanceshave been made in the tech-
resolvingpower is approximately
.thatof a 3-km
paraboloid.
The abilityto slewrapidlyandto change
frequency
rapidlypermitsmanymodesof operation
that are impossible
with paraboloids.
On the other
hand, the collectingarea of the untilled apertureis
nologyof decadebandwidthantennas[Rumsey, lessthan 1% of that of a 3 km paraboloid,
so the
1966], and reliable, wideband,solid state devices systemis most useful for programswhere high
havereacheda price whereit is practicalto. use
hundreds
or thousands
of unitsin a largesystem.
Aroundthesedevelopments,
a designfor a fully
steerabledecametricarray has evolved.It is operable anywherebetwen 15 and 125 MHz ,with
x Present address' National Radio Astronomy Observa-
tory,
Green
Bank,
West
Virginia
24944.
angularresolutionand only modestsensitivity
are
required.
The system
is adaptable
to the studyof dynamic
sources
suchas the sun,Jupiter,or pulsars,for it
canbe usedin an adaptivesensewith prioritygiven
to dynamic sourceswhenever they are active. The
beamcan be steppedaroundthe sky to monitorthe
sun,flarestars,
planets,
orionospheric
scintillations.
Spectral observationscan be made on many thou-
Copyright
(•) 1974by theAmerican
Geophysical
Union.
sands of sidereal radio sources. Sources can be fol387
388
ERICKSON
AND
FISHER
lowed acrossthe sky and observedsuccessively
at Clark's dry lake which is not exactly tangential to
differentfrequenciesfor spectralmeasurements.Ob- the geoid. The south arm, which is perpendicularto
servationsof calibration sourcescan be interspersed the E-W arms, has been laid out 18 arc sec from the
with observations of unknown sources on a secondplane containingthe earth's axis and the center of
to-secondbasis. At our sensitivitylevel, lunar oc- the array. The array's effective coordinatesbecome
cultations of radio sourceswill occur every few lat. 33ø20'29", long. 116017'25". The signalsfrom
each arm are combined
hours.
The instrument, called the TPT because of its
to form
7 fan beams
and
the E-W fan beam signals are multiplied by the
N-S fan beam signalsto producepencil beams.The
beam shape is equivalent to that obtained with a
full crossbut the collecting area of the fourth arm
geometryand antennadesign,is a "T" of 720
conical spiral antennasunder constructionat the
Universityof Maryland'sClark Lake Radio ObservatorynearBorregoSprings,California.Elements is lost with the T. Phase tolerances between the
of the array have been in operationfor about 3 orthogonal arms are more critical in the T array
yearsfor testpurposes
andfor somelimitedobserva- than in a full cross[Christiansenand H6gbom, 1969].
tions. Experiencegainedwith portionsof the array
Each log spiral elementhas a collectingarea of
assuresthe successof the design and allows some aboutx2/3, andis designed
to operatebetween20 and
definitivestatementsto be made aboutthe properties 125 MHz. The low frequencylimit hasbeenextended
of the individual elementsand the overall phasing to 15 MHz at reducedefficiencyby terminatingthe
scheme.It is the purposeof this paper to describe base of the spiral with resistors.The teepeesare at
the system,to outlinethe measuredparametersof 6.25-m intervals in the E-W arms and 7.5-m interthe conicalspirals,and to comparethe calculated vals in the southarm. This spacinggivesrise to gratand observed array patterns with particular em- ing responses
above50 MHz whichwill be discussed
phasis on an incremental phasing scheme not in a later section. Since the elements are all fixed
normally employed in radio astronomicalinstru- in the vertical direction, beam positioning is acments.The sidelobestructureof the systemis com- complishedpurely throughadjustmentof the phase
plex and affectsthe confusionlimit of the systemas gradientacrosseach arm. The gain of the systemis
estimatedby Fisher [1972]. A brief descriptionof modulatedby the response
patternof the individual
the proposedelectronicsis includedto give a co- elements.For goodzenith distancecoverage,the reherent picture of the final instrument.
sponsepatternmustbe wide, and the gain of each
OUTLINE
OF ARRAY
AND
ELECTRONICS
The new array is a 3.0 x 1.8 km T with the
direction of its legs being approximatelyeast, west,
and south. The array is laid out in the plane of the
E-W
Arm
::5OOOm
Banks of Elements
i
i
i
i
••
I
i
0 bse•a
elementis correspondingly
low.
As shownin Figures1 and 2, phasingof the array
is accomplished
in two stages.The elementsare divided into 48 banks of 15, and the signalsfrom the
15 antennasin each bank are combined,then preamplifiedandsentto the centralbuildingon separate
coaxial feed lines. Phasingwithin a bank is accom-
plishedby electronically
"rotating"eachconicalspiral
antenna with a diode switch controlled from the cen-
tral building.
The signalsfrom the 48 banks are processedsep-
arately, in carefullymatchedamplifiersand digital
networks.Figures3 and 4 are block diagramsof the
proposedelectronics.The slope filter aheadof the
postamplifier
is a highpassfilter with a cutoffof 100
MHz and is designedto compensateapproximately
for increaseof sky noise and decreaseof cable attenuationwith decreasingfrequency.It also reduces
the dynamic range requirementson the following
electronics,particularly with respect to the strong
N-S
ARM
18OO m
16 Banks
of
Elements
man-made
Fig. 1.
Array layout.
interference
below 30 MHz.
The remain-
ing RF and IF componentsare relativelystraightfor-
CLARK
1
2
5
14
LAKE
ARRAY
389
15
• VAR,ABLE
j PHASE
SEVEN-WA
S'GNAL
LINE1
I •l
GRADIENT
I FROM
qOLJPPER
J DIGITALDELAY
I 10MHzIF
I
DIVIDERWITH LINEAR
ACROSS
OUTPUTS
I
BEAM
COMBINER
i 8 MHzI
RO,,/VII1' ' Phase
and
Amplitude
J
CLOCKI
JCalibration
Signal
bower
FromObservatory
Combinet
CHANNEL
OUTPUT
Dummy
Løad
'• Jc!axial,
SwitchJ-•Auxiliary
Antenna
IFAN
BEAM
COMBINER
-W
• FAN
49
CORR
49
BEAMS
FROM
EACH E
BEAM
INPUTS
I•
iOUTPUTS
16(SOUTHARM) OR
32 (EAST- WESTARMS)
7 EAST- WEST FAN
FAN BEAM
INPUTS
TOFORM
FAN OUTPUT
ARM
OF
ENCIL
BEAM
INPUTS
ANDJlBEAM
7 NORTH
- SOUTH
ARRAY
'"""'"'
ttttttt
N-S
10-140
MHz
Preamp
Fig. 4.
Block diagram of digital delay and correlation sys-
I 1600m-7/8"
Styroflex
Coaxial
tem.
Cable
tiers are digitizedin 2-bit levelsand delayedin random accessmemories.All of the real-timedelaysare
controlledby the memory combinations.Phase adjustmentbetweenthe channelsis accomplished
by
appropriatebit permutations.
Finally, the digitalsignals are convertedto a.nalog levelsand combinedin
Observatory
Building
Fig. 2.
One bank of 15 conical spiral elements.
ward. Each RF signal goes through two frequency
conversions.It is first convertedup,to an intermediate frequency(IF) of 170 MHz to providegoodimage rejection. Then the signal is convertedto 10
MHz, where bandwidths between 0.1 and 3 MHz
can be selected. The first local oscillator (LO)
is
variable from 185 to 295 MHz to selectthe appropriate operatingfrequency.The secondLO is fixed
in frequencyat 160 MHz.
The high level outputsof the 10 MHz IF ampliI Observatory
15Element
I Building
Bank Pro
W•de-Band
Hybrid
Cable I
Post
T Amp
Loss
I
10-140 MHz
-8 db at 10MHzl
10-140 MHz
36db Gain -25 at 110MHzl
N,F,= 4 db
I
J
36dbGmn
N,F,=4 db
,•
1st
I F Amp
2nd
M•xer
Bandpass
Filter
•
Other
Signal
Systems
Input
Adjustable
1st
Mixer
•
O
OSystem
OutputTo
Test
2nd
I F Amp
+ldbm
Nmse
resistor networks to form 7 E-W and 7 N-S fan
beams. All fan beams are cross-correlated to form
49 pictureelementsaroundthe primarydirectionfor
whichthe arrayis phased.The instrument's
specificationsand capabilitiesare listedin Table 1.
For comparison,the total collectingarea of the
elementsin this array is equivalentto the geometrical area of a 50-cm (165-ft) dish at 100 MHz and a
250-m (800-ft) dishat 20. MHz. The conicalspiral
has a constantgain with respectto an isotropicradiator, and its collectingareavariesas the wavelength
squared.At 80 MHz the total collectingarea is half
that of the Culgoora array. The resolution values
given in Table 1 are approximateand dependon
TABLE 1. TPT specifications.
Frequency range
15 to 125 MHz
Instantaneous
0.10 to 3 MHz
Total collectingarea
250 X•-
Resolution
20 arc min
20 MHz
100 MHz
Level
o
bandwidth
time
t fc:I?OMHz
T r.•.OOMH,
fc:
10MHz
BW:6
MHz
1stLocal Gmn=26db
Osc•l lator
Programmable
185-:>95 MHz
msl-J 1,50 MHz
"'"-10,75
MHz
Gm
n
L0,15MHz Adiustable
Local
Oscillator
160 MHz
Fig. 3. Block diagram of one of 48 channels in which
signalsfrom the 15-element banks are processed.
4 arc min
Steeringand frequencychanging
<< 1 msec
Sky coverage
Sensitivity (r --- 10s; BW --2 MHz)
Confusion limit
<45 ø zenith distance
'•1 JyXat all frequencies
'•1 jyt at all frequencies
Polarization
Left circular
t 1 Jy (Jansky, flux unit) = 10-•-6 w m-'- Hz-L
390
ERICKSON
AND
FISHER
the load, and the same impedanceat the antenna
terminalsis thenmaintainedto verylow frequencies.
At frequencies
where X •, ,•im someof the power
#1
CONDUCTOR
#2
on the antenna is lost in the resistive lo.ads before
it can be launchedinto space.Thereforethe limit on
the operatingfrequencyis setby the lossof efficiency
one can tolerate. In principle, at least, the radiation
pattern and the circularityof the polarizationshould
not changeat lower frequencies.
Second, the antenna is unidirectional toward the
apex and the polarizationis in the oppositesense
from the openingdirectionof the spiral, i.e., a fighthand (clockwiseopening) spiral as viewedfrom the
top radiatespredominatelya left circularwave. The
far field radiationpattern is determinedby the apex
angleof the cone and the pitch angleof the conductors. Considerablymore detail may be obtainedfrom
other sources[Rumsey, 1966; Dyson, 1965; Yeh and
Mei, 1967, 1968].
In actual practice the use of conductingsheetsis
very difficult because of cost and wind resistance
for large antennas.A good approximationto a conductingsheetcan be made by usingthree wires, one
at the location of each edge of the conductor and
one in the center.Thus the elementsin this array use
six coaxiallywound spiral wires, three connectedto
each side of the transmission line.
Fig. 5.
Basic form of conical log spiral antenna.
Each elementin the array is phasedby electrically
rotating it in 45 ø increments.Antenna rotation is a
practicalphasingschemein this array only if the
taper and digitalprocessing
of the correlatedoutput polarizationremains very nearly circular in all diand on the zenith distance of the observed sources.
rectionsobserved.The conicalspiral antennasmeet
this requirementquite well between half-power
SINGLE
ELEMENT
CONSTRUCTION
AND
pointsof their radiation pattern. Antenna rotationis
OPERATION
accomplishedby winding the spiralswith eight inThe basicbuildingblockof the arrayis the conical stead of six wires and a diode switch has been delog spiral antenna.Ideally this antennawould con- vised to selectsix of the wires at any given time.
sist of two conductingsheetswound on the surface With the simplicityof this phasingschemecomes
of a coneasshownin Figure5. It is a self-conjugate the disadvantageof not having continuousrotation.
antennawith a characteristic
impedance
of 189 fl and The phaseerror of any element can be as much as
isfedby a balanced
transmission
lineat itsapex.
22.5ø dueto incrementalphasing.
The electricalandradiativepropertiesof this anFigure 6 showsthe appearanceof the antennas
tenna are as follows.First, the low-frequencylimit after the supportsand switch circuitry are added.
of the antennais determined
by the sizeof the base The diodeswitchis insidethe top of the centralpipe;
of thespiral(circumference
•,•nm) andthehighfre- the signalpassesthrough a balanced-to-unbalanced
quencylimit is setby the pointat whichthe top of transformer and is brought down the center of the
the spiralis truncatedand the accuracy
with which pipe with 75 a coaxial cable.
it is wound.The low frequencylimit of the antennais
ELEMENT
GROUPING
AND
PHASING
SCHEME
extended
by terminating
the baseof the spiralwith
a resistive
load.Powerthat wouldordinarilybe reThe phasingwithin eachbank is accomplished
by
flectedfromthe baseof the antennais dissipated
in rotatingeachelement,so there is no real time delay
CLARK
Fig. 6.
LAKE
ARRAY
391
The working version of the TP antenna in the array.
added to the signals from individual elements. All
array the line lengthshave beencut pseudo-randomly
time delaysare added to the 48 signalpaths in the to reduce the array's responseto unwanted rightcentralbuilding.
circularly polarized radiation [Swenson and Lo,
The. use of simple phasingas opposedto delays 1961]. The basicidea is the following.Each element
in the. 15-element banks limits the. size of banks due
has a small responseto. right circular polarization
to coherence loss with wide receiver ,bandwidths.
(crosspolarizatio.n).When an elementis rotated,the
One bank is approximately100 rn in length. This phaseof the fight-handpolarizedsignalis changed
results in a coherence loss of 9% with a bandwidth
in the oppo.sitedirectionfrom that of the left-circuof 3 MHz at a zenith distanceof 45 ø. It is normally lady polarized signal, i.e., when the left-polarized
much less with smaller bandwidths and zenith disphase is advancedthe. right-polarizedphase is retances.
tarded. The result for an array with equal feeder
In actual operation a computer-controlled
set of lengthsis that a "ghostbeam" is formed with crosspowertransistorssuppliesthe phasingsignals.Every polarizedradiationon the oppositesideof the zenith
bank in each arm is identicallyphased,and elements from the main beam. This ghost beam can be subwith the same positionin each bank have their con- stantially reduced by staggeringthe lengths of the
trol wires connectedin parallel. Also., the east and cablesfeeding the elementsin a bank of 15. Thus,
west arms are normally phased alike, so only 120 if a feeder cable is shorter than normal, the antenna
independentcontrol wires are needed for the full is rotated counterclockwiseto retard the left-polararray (4 wires per element times 15 elements per ized wave to bring it back into phase.In the process
bank times 2 independentarms).
the cross-polafized
wavegetsadvancedby the shorter
Normally,
equaliengths
oftransmission
linewould cable and againby rotation and, in general,is out of
be run to. the 15 elements in each bank, but in this phasewith respectto similarly polarizedwavesfrom
392
ERICKSON
AND
FISHER
other elements.Also, after the signalsfrom the 48
banks are delayedand combined,the chanceof having a pencil beam fall in one of the cross-polarized
ghostbeamsis quite small.
As shownin Figure 2, the signalsfrom the 15 elementsin eachbank are combined,and the resulting
signal is amplified approximately36 db in a wideband amplifier.This preamplifierhas a noisefigure
less than 4 db (400 K) and a bandpassextending
from 10 to 140.MHz. It is particularlydesignedto
handle large interfering signalswithout producing
spuriousintermodulationproducts.
After amplificationthe combinedsignalentersan
air-filled coaxial cable of 1600 m length for transmissionto the centralbuilding.The lossin this cable
been treated in detail. A considerable
number
of
fundamentalpatterncalculations
hadto.be performed
to assurethe feasibilityof the scheme.We had to
determinethe magnitudeof the sidelobesgenerated
by phaseerrors up to 22.5 ø and calculatethe confusion limits due to these sidelobes. The confusion
calculationwascarriedout by Fisher[1972].
For pattern calculationsthe array is most conveniently divided into two levels. First, the electric
field pattern of a bank of 15 elementsis calculated
usingincrementalelementphases.Then the pattern
of one bank is multipliedby the array factor generatedby isotropicradiatorsat the phasecenterof
eachbankto obtainthetotalresponse
pattern.
To illustrate incrementalphasing effects, assume
is proportionalto the squareroot of the frequency that the elementsare fed with equal lengthsof transand is about 25 db at 110 MHz. Even at the highest missionline. Figure 7 showstwo examplesof calcufrequencythe preamplifierhas adequategain so that lated patterns resulting from incremental phasing
noiseproducedby succeeding
componentshas little (solid lines). Superimposedon each is a plot of a
effecton the systemnoisefigure.After accounting
for pattern with perfect phasing (dashed lines). The
the loss in all the cable and transformers ahead of
most important differencebetweenperfect and increthe preamplifier, the system noise temperature is mental phasingin Figure 7 is that the zero crossings
1800 K at 110 MHz. At this frequencythe sky noise are not identical. Therefore, when the 32-element
is about 1000 K so Tsy• + T•, _• 2800 K. The ga- gratingresponseis multipliedby the 15-elementpatlactic backgroundradiation (Ts•y) is approximately tern, the gratinglobeswhich ordinarily would fall on
proportionalto X2'ø,and the lossesaheadof the pre- zeroes of the 15-elementpattern will no longer be
amplifier decreasein proportion to the squareroot
completelycancelled.Anotherway of lookingat this
of the frequency,so below about 70 MHz the sysis that there will be a periodic phaseerror distributem noiseis insignificantcomparedto sky noise.
tion in the array which repeatsevery 15 elements.
ARRAY
PATTERN
CALCULATIONS
This will createsmallgratingresponses
with angular
To the authors'knowledge,the use of incremental spacingsof (X/15d) radians,where d is the distance
phasingin linear arraysfor radio astronomyhas not
between individual
elements.
The slightdegradationof the mainbeamis a quasirandom function of direction so there will be about a
3% peak-to-peakmodulationof the response
as the
array tracks a source.This is also a result of the imperfectphasing.
The randomized feed cable lengths mentioned
abovedo not introducelarger phaseerrors,but they
do changethe patternobtainedwith a 15-element
0--•-
Fig. 7. 15-element array patterns with incremental phasing (solid lines) compared to perfectly phased array pattern (dashed lines).
bank. Someof the patternscalculatedusingthe feeder
systemactually incorporatedin the array are shown
in Figures8 and 9. Equal and unequalfeedersystems give the same average sidelobelevel, so the
sidelobeand confusioncalculationsgiven by Fisher
[1972] are valid for the actualsystemalthoughthey
were made for equal feeder lengths. Incremental
phasingis not the onlysourceOf phaseerrors,but
it is by far the mostimportant.
CLARK
LAKE
ARRAY
393
sidelobemaxima (see Figures 8 and 9). This, and
the phasecentermotion, are resultsof the incomplete
cancellationof the imaginarypart of the vectorsum.
In other words, the phase of the resultant vector
varies continuouslyfrom 0 ø to 180ø but, after the
signalsfrom the E-W and southarms are multiplied,
the combinedpattern in the cosine receiver output
will have zero amplitudewhen the E-W and south
arm signalsare in phase-quadrature.
Note that the 15-elementpatternsare voltagepatterns and that the pattern resultingfrom the correlaion of the E-W and south arm signalsis a product
of two voltages.At a given direction in the sky the
array responseis a vectorproductof the voltageresp.onse
of the E-W arm, the voltageresponseof the
southarm, andthe individualelementpowerresponse
0ø
in that direction.
Fig. 8. Representative voltage pattern of incrementally
phased 15-elementbanks at 28 MHz using staggeredfeeder
cable lengths (solid lines) compared to perfect phasingpatterns (sinc x, dashed lines). Only the amplitude of the resultant voltage vector is plotted (see text).
TOTAL
ARRAY
PATTERN
After compensatingfor phase center motion the
relative phasesof the signalsfrom the 48 banks will
be within a few degreesof the perfect phasingcase
so the 32-element grating responseis a sinc x --(sin •rx)/ ('•rx) pattern.
SIDELOBES
In principle,to get a completepictureof the sidelobes,the array pattern shouldbe evaluatedfor all
directionsin the sky for all beam positionsat many
wavelengths.This is a formidable task even for a
With unequal feeder lengths in the 15-element
banks the element phases are not antisymmetric
around the center element, #8; e.g., the incremental
phasingerror of element#7 is not necessarily
equal
in amplitudeand oppositein signto the phaseerror
of element #9. As a result, the phasecenter of the
15-element banks can be displacedby as much as
0.02 wavelength(7 ø) from the center element.Becauseall of the banks within an arm are identically
phased,the phasecenter displacement
producesno
additionalphaseerror betweenthe banksin one arm.
The south arm is phaseddifferently from the E-W
arms, so.there will be a variation in relative phase
of the combinedsignalfrom the southarm with r•spect to the combinedsignal from the E-W arms.
This phaseerror can be calculatedin the controlling
computer'sphasingprogram and compensatedby
adding a small phase shift in the signalpaths from
the south arm.
Another consequenceof the asymmetricphasing
in the 15-elementbanks is that the amplitudeof the
voltage vector which is the resultantof the addition
of the 15 signalsdoesnot go throughzero between Fig. 9. Sameas Figure8 but at a frequencyof 110 MHz.
394
ERICKSON
AND
FISHER
Full
Array
Pattern
////•\•,•15-Element
Bank
Pattern
/111•\\
B
Fig. 10. Total one-dimensional
array voltagepattern (solid line). The dashedenvelopeis
the 15-element pattern.
high speedcomputer,so.some simplifications
are
andsoutharmsarecombined,sidelo,be
responses
oc-
made.
cur wherever a main beam in one direction crosses a
Figure 10 is a representative
plot of a portionof
the patternof the full array (solidline). The dashed
line is a plot of the 15-elementbank pattern.This
figureillustratestwo of the threebasictypesof sidelobescharacteristic
of this array.First, very closeto
significant
response
from the orthogonalarm. Figure
11 is a projectionof the celestialsphereonto the
planeof the horizonand showsthe positionsof the
varioustypesof sidelobes
at 70 MHz. Figure12 is an
expansionof the area aroundthe main beam.
the mainbeamare the sidelobes
typicalof any uniTP IMPEDANCE
CHARACTERISTICS
form linear array.The firstnegativesidelobe(A) is
21% of the main beam,and the close-insidelobes Becausethe conical spiral elementsin this array
fall off to lessthana fewpercentabout8 beamwidths incorporate several features which have not been
away.Thistypeof sidelobecanbe reducedeitherby tried with these antennasbefore, it was imperative
taperingthe illuminationof the array or by adding that impedanceand radiation characteristicsbe ina portionof the responses
of 2 adjacentbeamsat the vestigatedbefore building 720 units. For practical
zero crossingsof the centralpattern so.as to cancel
the sinc x sidelobes[Perini, 1964]. Both methods
broadenthe mainbeamslightly.
The secondtype of sidelobe (B) is due to. the
imperfectphasingof the array. The zeroesof the
15-elementpattern do.not fall on the maxima of the
32-elementgrating response,resultingin sidelobes
averagingabout 5 % of the main beam which occur
every 32 beamwidths on either side of the main
beam.Below 50 MHz theseare the mostimportant
sidelobes as far as confusion is concerned.
Above 50 MHz the spacingof the individualelementsis greaterthan one wavelength,givingrise to
primarygratingresponses
equalin amplitudeto the
mainbeam.Thesewill be treatedas a third typeof
sidelobe(C). The onlymethodfor reducingtypesB
and C is through the use of a receiver bandwidth
large enoughto reducethe coherenceof the radiation
at the position of these sidelobes.Bandwidths of a
few percent are needed to lower these sidelobesto
manageablelevelsfor weak sourceobservations.
It must be remembered that the above discussion
is only for the one-dimensional
case.When the E-W
reasonsthe antenna impedancecould not be measured directly. There was a length of cable, a transformer, and the phase switch between the impedancebridge and the antennaterminals.Since we
are interestedin the operation of the total system,
the standing wave ratio (VSWR) and impedance
measuredthrough these componentsare perfectly
valid providedthe power lossin the individualcomponentsis not more than 20% or so.
Significantstrayreactancesin the feedsystemarise
due to the physicallayout of the diode switchinside
the central support pipe. These reactanceswere
measuredand compensatedwith small inductorsincorporatedin the switch. The combinationof stray
and lumped reactancesforms a nearly symmetrical
low passfilter with a cutoff frequencyof about 250
MHz
in series with each antenna wire.
Figure 13 is a Smith chart plot of the antennaimpedanceas a function of frequency.The measurement was made at the base of the TP and corrected
for delay in the 20-ft feeder cable. The maximum
VSWR encounteredis 1.4'1, whichcorresponds
to a
reflectedpower loss of 4%. Ohmic lossesin the
CLARK
LAKE
ARRAY
395
Horizon
Fig. 11. Projection of the 70 MHz TPT array pattern
Fig. 12. Expansionof Figure 11 around the main beam.
onto the plane of the horizon. Beam sizes are not to scale.
Beam sizesare approximately to scale.
transformerand switchare lessthan 1 db (20%) and randomlypolarized,the difference
in polarization
of
the lossin the 20-ft coaxial cable is 0.4 db (8%)
thetwoantennas
should
beof littleconsequence.
Half-wave folded dipoles were constructedfor
at 110 MHz. When onetakesthe 75:200 impedance
transformer into account it is evident that the char110 and 33 MHz. The testprocedureat 110 MHz
acteristicimpedanceof the antennais closeto 189 •,
was to recordthe total power from the TP for 24 hr
whichis the theoreticalvaluefor a self-conjugate
an- andthento repeattheobservation
usingthedipole.
tenna.
ANTENNA
EFFICIENCY
Determinationof the efficiencyof a singleelement
is very difficultbecauseit requiresan absolutemeasurement. There are no isolated radio sources with
accurately
knownintensities
whicharestrongenough
to be measuredwith a singleantenna.The next best
alternativeis to comparethe noiseoutputof the TP
dueto the galacticbackground
with that of dipoles
at two frequencies.This type of measurementdoes
not give the gain of the antennain any particular
direction, but it should account for ohmic losses
in the antennaand absorption
of radiationby the
ground.Sincethe galacticbackgroundis concentrated
in the galacticplane, this measurementhas to assume
that the radiationpatternsof the TP and dipoleare
approximatelythe same and that the measurements
are taken at the same sidereal time. A half-wave di-
pole, one quarterwavelengthabovea conducting
plane, producesa power pattern not too different Fig. 13. Impedancecharacteristics
of the productionverfrom that o• the conicalspiralantennawith which sion of the TP antennacorrectedfor delay in the feeder
it is compared.
Because
the galacticbackground
is
cable.
396
ERICKSON
AND
FISHER
The antennatemperaturewas calibratedby substituting a noisesourcefor the antennaat the preamplifier
input and recordinga seriesof noiselevels.The noise
output from both antennaspeakedat the samesidereal time and the ratio of their outputswas appro.ximately the same throughoutthe siderealday. This
confirmsthe assumptionthat the two antennashave
nearly the sameradiationpatterns.
The dipoleantennatemperaturesat similarsidereal
times, correctedfor cablelosses,are plottedon Figure 14 as x's. The error bars representoutsidelimits
on the uncertaintyin reflectionlossesand calibration
errors.Other pointson the graphhave approximately
the same error estimate.The dipole measurements
fit the expectedbackgroundspectrumas shownby
the straightline. The solid circlesare the TP measurementswhich were directlycomparedwith the dipole values.The 110 MHz TP antennatemperature
is 20% -- 20% below the dipolevalue and the 33
MHz TP antenna temperaturewas 50% -+ 20%
below the dipole standard.
ionosphereand ground reflectionsinto account.The
constructionof a lower-frequencydipole at Clark
Lake was impractical.
It can be seen from Figure 14 that the antenna
begins to lose efficiencyrapidly below 25 MHz,
where a considerablefraction of the power is absorbedby the terminations.As a result the conidal
spiral efficiencybelow 25 MHz is not well known.
The systemtemperatureis still dominatedby sky
noisedownto at least 12 MHz asis easilyshownby
replacingthe antennawith a 300 K terminationand
notingthe output noiselevel variation.
ELEMENT
POWER
PATTERNS
Many componentsof the elementswere too small
to be scaleddown in size by any significantfactor,
so all pattern measurements
were made on full size
antenna elements. The most convenient far-field
sources for the measurements were natural ones and
theywereusedfor all of the patternmeasurements.
In order to discriminateagainstgalacticback-
To •et an estimate
of theefficiency
of theTP at ground emission and to allow the observation of
other frequencies,its noiseoutput was measuredat
discretefrequenciesdown to 14 MHz. After correctionfor lossesbetweenthe antennaandthepreamplitier, the antennatemperaturesare plotted as open
circleson Figure 14. Thesevalueswere normalizedto
agreewith the TP/dipole ratios.Below 30 MHz the
sky noiseincreaseslessrapidlywith decreasing
frequency,but the exact value of the integratedsky
temperature in the antenna beam is not known because it is very difficult to take the effects of the
discrete radio sources, an interferometer was con-
structed of two. elements. The responseof the
antenna elements as a function of direction was de-
terminedin two stages.First, the azimuthaldependenceof the power patternwas determinedat several
frequencies
by observinga radio sourceat a given
elevationon a seriesof nightswith one elementfixed
andthe otherone rotatedto a differentpositioneach
night with the phaseswitch.The zenith angle dependencewas studiedby comparingthe apparent
strength of two known radio sourcesat different
elevations
on thesameinterferometer
fringe.
The two radio sourcesused in nearly all of the
MHz
5
14 16 20 25 33
I i\
i
I
I
\
antennatestswere CassiopeiaA [, - 23h 22m, 8 58041' (1970)] and CygnusA [, = 19h 58m, 8 =
40036 ' (1970)]. Where the flux ratio of the two
\
sourceswas needed,the spectracompiledby Viner
[1973] were used. Cas A is about 20 to 50%
strongerthanCygA depending
on frequency.
The azimuthal pattern was measuredat four frequenciesand three zenith distances,and the results
Frequencies
o TP ot Other
I
I
1.2
1,4
I
1.6
I
1.8
2.0
LOG F
Fig. 14. Spectrum of noise output
from a single TP (circles and curved
line). The crossesare dipole measurements, and the straight line represents
a background spectrum of T
are shownin Table 2. With four independentsamples at a given frequencyand zenith distance,the
azimuthalvoltagepattern can be approximatedby
an ellipse.Table2 givesthe axialratioof eachellipse
(minor/major). The horizontal extent of the radiating regionof this antennais aboutX/.,r,so it should
not producestructurein the azimuthalpatternwith a
scalesizeof lessthana radian.Thusthe45ø sampling
CLARK
intervalshouldbe adequate.Sincethe valuesin Table
2 refer to the voltagepattern (not power), they are
a direct indication of how the illumination
TABLE
2.
LAKE
ARRAY
Axial ratios of azimuthal voltage patterns.
Zenith
distances
of one ele-
ment will vary as it is rotated.
One drawbackto this methodfor measuringthe
electricfield pattern of the TP's is that the unwanted
right-hand polarization responsecontributesto the
fringe amplitude.This will causevery little error if
the right-handinterferometerresponseremainsconstantin amplitudeand phaseas the antennasare rotated. The electric field pattern in the right-hand
polarizationprobablydoesvary in amplitudein different azimuthaldirections,however.This may cause
397
Estimated
Frequency
28
45
70
110
MHz
MHz
MHz
MHz
20 ø
0.86
0.97
0.94
0.91
32 ø
0.95
0.93
0.95
0.83
42 ø
0.87
0.86
0.77
0.79
errors
4-0.07
4-0.05
4-0.04
4-0.07
The azimuthaldependenceshouldcontributean error of only about ---5% to an individualratio in the
worst case. Also, care was taken to be sure that a
cross-polarization
(right-hand) fan beamdid not fall
an errorin addition
to thosequotedin Table2 of ,as on one of the sourceswhen a left-handpolarizedfan
much as ---0.07 at 110 MHz
and ---0.04 at other fre-
beammeasurement
wastakingplace.Thus,the cross-
quencies.
polarizedresponsecausedno significanterror in this
Unless otherwisestated,the errors quotedfor the part of the antennapatternsudy.
measurementsare outer limits on the possiblerange
Adoptingthe flux ratiosof CasA and Cyg A given
of each value. These limits, which take the signal-to- by Viner [1973] at the severalobservedfrequencies,
noise ratio (snr) into account,are mainly derived the procedurewas to use a bootstraptechniqueto
from the estimatedreadingerror on the strip.charts constructthe zenith distancedependenceof the anusedto recordthe measurementsand from experience tennapattern.Ratiosof responseto Cyg A and Cas A
with the calibrationaccuraciesand reproducibilityof on the same fan beam were measured for five to seven
the measurements.
fan beamsat each of four differentfrequencies.The
As one mightexpect,the azimuthalpatternis more responseto Cyg A nearestthe zenithwas assumedto
nearly circular at small zenith distances.Also, the be unity and the relative value for Cas A, corrected
patternhas a largervariationat 110 MHz, where the for its higherflux, was plottedat its larger zenith disantennais operatingnear its designlimit, than it does tance as in Figure 15. By interpolatingbetweenthe
at lower frequencies.Naturally, we would like to first two points,the next largestzenith-distance
value
samplethe patternbelow25 MHz, wherethe antenna for Cyg A could be normalizedand the power patoperatesbelow its natural frequencylimit, but man- tern extendedto largerZ with the corresponding
Cas
made interferenceprohibits the use of bandwidths A value, and so forth.
necessaryto get a sufficientsnr using only two anAs a point of interest,the abovemethodfor meastennas.
uring antennapatternswill work for any linear array
Measurement of the zenith-distancedependence of elementswhich have a pattern that is a surfaceof
of the electricfield pattern was lessstraightforward. rotationwhoseaxisis perpendicularto the array axis,
A methodhad to be devisedto separatethe observed e.g., an E-W array of N-S dipoles.
intensityvariation due to the antennapattern from
Figure 15 showsthe zenith distancedependence
that due to. coherencelosswhen usinga wide band- of the TP power pattern at four frequencies.In each
width and antennaseparation.
graph similar symbolsrefer to measurements
of Cas
For this measurement, the two central E-W banks A and Cyg A on the samefan beam.Cyg A is always
of elementswere used as the two componentsof a at the lesser zenith distance. The assumedflux ratio,
phase-switchedinterferometer. This configuration a, of Cas A to Cyg A is given on each graph.
produced
several
N-S fanbeams
whichcouldbejuThe errorsin the curvesmeasuredin Figure 15 are
diciouslyplaced so that only o.ne of the two test rather complicatedin that they tend to accumulateat
sources was in a beam at one time. The coherence
higherzenith distances.The initial responseratio has
lossfactorwaseliminatedby comparingthe response an estimatederror limit of -+5% and the point at
to Cas A and Cyg A in the samefan beam.Phasing largestzenith distancemay be in error by -+10%.
combinations
were chosenso that the eight phase The small scatterat large zenith distanceindicatesa
positionswere fairly evenly representedand the fairly small error.
azimuthal antennapattern tended to averageout.
In interpretingthe graphsin Figure 15, it should
398
ERICKSON
AND
I
1,0
--
FISHER
i
I
ß
0
sinusaidcould be explainedby a ground reflection
20 db below the direct wave (10% in electric field)
from a groundplane.7 m below the phasecenter of
the antenna.This would mean the effectivegroundis
I
- 'xo.l•o
•+
0,5
110 MHz
• 0,0
o..,1,0
70MHz
a=1.21
a:1,27
I
I
I
I
I
...
eX o + o
-- eXø+
o ,!•I¾0
0,5
0.0
29 MHz
40 MHz a =1.35
I
I
:3ø
20ø
a= 1,48
I
40 ø
60 ø 0 o
Zenith
I
20ø
40 ø
60 ø
Distance
Fig. 15. Zenith distancedependence
of the TP power pattern at four frequencies
measuredwih the centralbanksof
elements. a is the ratio of the flux of Cas A to Cyg A at
the given frequency.
about 1 rn belowthe surfaceof the dry lake. There
may be a weak ground-reflectedwave at the other
frequencies,but it is not possibleto distinguishits
effect from the direct-wavepower pattern features.
In summary,the half-power beamwidthof the
conicalspiral antennausedin the TPT is about 100ø
exceptnear 110 MHz wherethe patternis apparently
distortedby a ground-reflected
wave. This groundreflectioncausesa secondary
peakin the zenithdistance
dependenceof the power pattern at Z --• 30ø. The
variationof the antennapattern in the azimuthaldirection at a constantzenith distancerangesfrom -+1
db at 110 MHz and Z = 45 o to practicallyno variatio.n at 45 MHz
and Z = 20 ø. In no case is the modu-
lation of the patternin azimuthlarge enoughto cause
unacceptableirregularitiesin the array illumination.
CROSS-POLARIZATION
RESPONSE
be remembered
thattheyshowan averagezenithdisThe cross-polarization
response(responseof the
tancedependence,
and a plot of the powerpatternat
antennas to right-hand circular polarization) was
a fixed azimuthwill differ slightlyfrom the average measured with the same observations which were
curve.The mostimportantresultof thesecurvesis
used to determinethe zenith-distancedependenceof
that theygivethe zenithdistanceto whichthe array
the antennapower pattern. As was explainedin an
is useful.It appearsthat the half-powerpointfalls
earlier section, the cross-polarizedwave forms a
ghostbeam on the oppositeside of the zenith from
distancedependence
is fairly smooth.At 110 MHz
the properlypolarizedbeam.
thereis a roughlysinusoidal
component
with a 25%
The amplitude of the right-hand polarized fan
peak-to-peak
amplitudesuperimposed
on a smooth
beam was measuredfor at least four differentposicurvewith a half-powerwidth of about55ø. This
tions in the sky for each frequency.Its amplitude,
relative to the left-hand polarizedbeam, was some15
15
what dependenton the beam position. All of the
121.5 MHz
109 MHz
measurementswere within 25 ø of the meridian, so
1.0 ...""""".
1.0 :'.
coherenceand antenna pattern effects were unimportant. Table 3 givesthe range or upper limit on
• 0.5
.
0.5
ß
the relative amplitudesof the cross-polarizedresponseswith respectto the left-hand polarizedfan
w
'''i
410
ø
i
i
'
'
•
• Ooo •oo ,,oo •oo ida" Ooo ida
•'oo
•: 1.5
1.5
beam. The detectionlimit of about 5% was set by
confusionwith the minor sidelobesof the properly
w
63.4 MHz
.
30.9 MHz
>- 1.0 ' ...'
1.0 ..
polarizedresponse.
As would be expected,the cross-polarizationre0.5
ß.
0.5
sponseis highestat 110 MHz where the antennais
at about Z = 50'ø. At all but 110 MHz the zenith
ß
:•,
o
0
w
.
0
ß
n•
,
ß
ß
%'
i
i
•o'
i
J
ß
,
I
,;oo' •'oo ,•o' %o •oo ,;o'-•oo
ZENITH
•o'
DISTANCE
Fig. 16. Zenith distance dependenceof the TP power pattern at various frequencies measured using the full E-W
arm.
TABLE 3. Right-hand polarized response relative to the
left-hand polarized response.
Frequency
Response
110 MHz
70 MHz
10to20% <5to10%
40 MHz
<5to8%
20 MHz
<6%
CLARK
LAKE
ARRAY
399
TABLE 4. TP phaseerror upper limits.
workingnear its designlimit. The wires from the
phasingswitchto the spiralare no doubtradiating Frequency
110 MHz
70 MHz
40 MHz
20 MHz
<20 ø
<10 ø
<5 ø
<7 ø
a significant
amountof powerwhichis linearlypolar- Error
ized.Possibly,
at theotherdesignlimitbelow20 MHz
thecross-polarization
response
will alsobe fairlyhigh, ture. A 20% cross-polarizedresponsewill produce
but it couldnot be measuredbecauseof strongbroad-
an apparent phase error of up to 20 ø. The values
given in Table 3, then, set upper limits on the acIn a previoussectionthe methodfor reducingthe
curacyof the phasemeasurements.
cross-polarization
response
of a bank of elementsusTable 4 lists the limits on the phase errors in the
ing staggeredlengthsof feed cableswas outlined. conical spiral antennasas derived from these measWith unequalfeederlengthsin both banks,all of the urements.The snr and ionosphericseeingset a decross-polarized
fanbeams
werebelowa 4% dete,
ction tectionlimit of about 5 ø on the phaseerrors, and the
limit at all four frequencies.On the average, the
higher upper limits were due to the cross-polarizastaggering
of phasesin the right-handwavesshould tion response.It can be stated that no phase errors
reducethe unwantedfan beamsby a factor of ( 15)
were measured which would not be consistent with
provided the phasedifferencebetweenthe shortest
the errors expectedfrom the cross-polarizationreand lo.ngestcablesis more than 2 radians.This apsponsegivenin Table 3.
pearsto be confirmedby the measurements.
Thus far, all of the discussionof phase errors has
The difference in electrical length between the
been concernedwith those produced by the indishortestand longestcablesis about 11.5 m, so there
vidual elements.There are further phase errors inis not quite enoughphasescramblingbelow 20 MHz
troduced by the transmissionlines and amplifiers
to realize the full potential of the cross-polarization
which are summarizedin Table 5. Becausethe sigreduction.If the differencewere made larger, the loss
nals are combinedin two stages,the phase errors
differential in the cables at 110 MHz would cause
are divided into two groups: those associatedwith
excessiveillumination irregularitiesacrossthe banks
the 15-elementbanks and those occurringin the 48
of elements,creatingsidelobesmore difficultto elimisignal paths from the 48 banks.
nate than the cross-polarization
responses.It should
Two values are given in Table 5, the larger of
also be noted that a further reduction in the unwhich is the outsidelimit on the phasedeviationdue
wantedpolarizationis realizedin the phasingof the to each component.In practicethe phasedeviations
full array becausethe chanceof havingthe overall are considerablysmallerthan the specificationlimits,
phasingcorrectfor both the mainbeamdirectionand
as is indicated by the rms values. Also, the phase
the cross-polarizedbeam directionis very small. The
errors on items 2 through6 are given for an operatright-hand polarized wave should cause very few
ing frequencyof 110 MHz with theseerrorsdecreasconfusionproblems.
ing with decreasingfrequency.It can be seen that
the largestphaseerror is due to incrementalphasing,
PHASE
MEASUREMENTS
cast interference.
To. confirmthat the 45 ø phase-stepping
systemon
the conical spiral antennasdoes indeed provide the
proper phase increments,a pair of TP's were connectedin a phase-switched
interferometeras in the
azimuthal pattern measurements.The fringe phase
as was assumed in the sidelobe calculations.
TABLE 5.
15-element
Phase error budget. Items 2 through 6 assume a
frequency of 110 MHz.
bank
limit
rms
nights with one of the antennasrotated in stepsof
1. Incremental phasing
2. Cable, switch, and transformer
4-22.5 ø
4-5 ø
4-13.9 ø
4-2 ø
4/5 ø.
3.
4-5 ø
4-2 ø
of this interferometer
was measured
on successive
Power combiner
rms total
This techniquewas not completelydefinitive,however. There was no practical way to eliminate the 48 signalpaths
4. Preamps
responseof the antennato right-handpolarizedradi5. High passfilters
ation. The cross-polarizedinterferometer pattern
6. Cables
7. RF and IF electronics
will cause an apparent phase shift in the fringes
8. Digital delays
measuredwith the above techniquewhen the oprms
positely polarized patterns are near phase-quadra-
total
4- t 4.1 o
4-5 ø
4-5 ø
4-2 ø
4-2 ø
4-2 ø
4-1 o
4-5 ø
4-2 ø
4-5 ø
4-2 ø
4-5.8 ø
400
ERICKSON
AND
PROTOTYPE
FISHER
T OPERATION
As an initial check on the performance of the
full T, the central three banks of elements were
combined as a prototype T. This formed a small
telescopeof about 200 m effectiveaperture which
includesthe central regionwhere the arms may interact. Most of the difficultiesin the operationof a T
usuallyoccurin this centralregion.A few beampositionsweretried at eachof the four testfrequencies,
affected
by theionosphere
whilepeak-to-peak
scintillationsof up to 20% are seenon othernights.
Theagreement
between
calculated
andobserved
patterns is excellent.
OPERATIONAL
TESTS OF THE FULL APERTURE
The 48 coaxialfeedlines,preamplifiers,
slopefilters, and postamplifierswere next installed. The
cableswereburiedat a depthof about1 m, but were
and the resultswere as predicted.The pointingac- broughtabovegroundat everysplice.All cablesare
curacywaswithinlimits setby the incrementalphas- pressurizedwith dry air. The relative attenuations
ing and the sidelobelevel was no higher than ex- and electricallengthsof each of the 1600 m cables
werefoundto remainidenticalat all frequencies
and
pected.
A more detailed analysiswas done at 70 MHz,
where
the best snr was obtained
with
moderate
to
low ionosphericscintillationlevels. Figure 17 shows
7 drift scansof Cyg A at differentbeam positions.
No attemptwas made to.maintainreceivergain calibrations over the 7 nights, so the intensitiesshould
not be compareddirectly. The expectedbeam positions as indicatedby the tick marks in Figure 17 coincided with the measuredvalueswithin the timing
accuracythat could be obtained.Some examplesof
ionosphericintensity scintillationsare seen in the
drift scans.Observations4 and 5 are virtually un-
LST
:>040 2020
:>O
h
1940
1920
19h
Cyg A
outdoortemperatures
within the accuracyof our
measurements,
about-+0.2db and-+0.3cm,respectively. The absoluteattenuationof the cablesvaries
with underground
temperature,
as expected,and in
the calibration of observed intensities it will be
necessaryto correct for this effect. The cables and
othercomponents
appearto operatewithintheerror
budgetgivenin Table5, soweshouldexpectthefull
apertureto operatein accordance
with theory.
The center element of each bank was constructed
and eachwas connected
to a preamplifierand feed
line. E-W and N-S gratingarrayswere thusformed.
They yield the full angular resolutionof the final
system,but have largegratingresponses
and only
1/15 of the final array'scollectingarea. As accurately as we can measure,thesearraysproducethe
expectedresponse
of a uniformlyilluminatedaperture.Below50 MHz, theseresponse
patternmeasurementsare accurateto a few percent;above50 MHz,
• BEAM'S
GREAT
CIRCLE
DISTANCE
MERIDIAN
FROM
all of the stronger
radiosources
are partlyresolved
by the arraysand the interpretationof the observed
profilesis complicated.
In fact,the arraysare being
usedfor somemeasurements
of the angularstructure
$
_1
4
0ø
5
Fig. ]7.
_
Iø
_
Drift scans of Cygnus A through the
prototype T beam at severalbeam positions.Points
on 4 and 6 are calculated
values.
of strong sources.
The E-W grating array has also been correlated
with the completedcentral, E-W banks of elements:
This processeliminatesall gratingresponses
except
the one that falls within the pattern of the banksof
elements.The elementbanksare pointedundercomputer control and their responsepattern can be
placedon any of the gratinglobesto form singleN-S
fan beams.By taking successive
drift scansacrossa
given source at many zenith distances,the data
shownin Figure 15 have been checked.Thesenewer
pattern measurementsare shownin Figure 16. In all
cases,thesepatternsrepresentreasonableinterpolationsbetweenthoseshownin Figure 15.
The E-W arm of the instrumentis nowcomplete
CLARK
LAKE
ARRAY
401
and in full operation.The N-S arm is nearly finished astronomicalcommunity. We will welcome visitor
and the electronicssystemwill next be completed. usage of the system.
The grating arrays mentionedabove are being used
.4cknowledgments. The authors wish to acknowledge the
for studies of solar bursts, while the E-W arm is
being employed in multifrequencyobservationsof
the quiet sun, X-ray sources,supernovaremnants,
and other objects.An example of solar burst data,
and the output spectrumof the system,is shown in
Figure 18.
All data indicate that the completedsystemwill
operateas planned,so its constructionis proceeding
as rapidly as fundswill permit. During the next few
months, as the system comes into operation, a
powerful new instrument will be available to the
work of John Hubbard, Stig Johansson, and Kenneth Barbier. They have all devoted a significantfraction of their professional careers to the design and construction of this instrument. Many other temporary staff and students have
devoted themselvestirelessly to this effort. The work is supported by the National Science Foundation under grant
GP-19401 and by the National Aeronautics and Space Administration under grant NGR 21-002-367.
REFERENCES
Christiansen, W. N., and J. A. H/3gbom, (1969), Radiotelescopes, pp. 155-156, Cambridge University Press,
London.
Dyson,J. D. (19i55), The characteristics
and designof the
AUGUST2,1972
:...•,:.•,".'...:..i
....' -.'I10
•
:4:'"'::4•.?..
9o
' •.-i':"';:;':.:':?.i):';:,'i'."•:'-:.-.....-':..'-.•-,:•:•-.'.,.•',
........-'";.• '
•
=
o
•
-'.......
zo •
:::-;:•;:-::?:.!::.:•::::::?
"•':;;
..:..:.
-.-.:/....- ---•-•---..-;•:......
'--•:.;
.........
½,
......
;'2• ...,:
.....
..
Z "
•'
.•,•
- •..............
.. ::.:'•.
*. ;•;.
,•,., ,:,..;
50
': • •::"•:-'• ..... '............
::'":•::'::•%":;)-•Z•"'
'"½•::-•:•.'
.......'....
conical log-spiral antenna, IEEE
Propagat., .4P-13(4), 488-499.
Trans.
.4ntennas
Fisher, J. R. (1972), Design tests of the fully steerable,
wideband, decametric array at the Clark Lake Radio Observatory, Ph.D. thesis, pp. 31-42, University of Maryland, College Park, Maryland.
Perini, J. (1964), Sidelobe reduction by beam shifting, IEEE
Trans..4ntennas Propagat., .4P-12(6), 791-792.
Rumsey, ¾. H. (1966), Frequency Independent .4ntennas,
.......
'•:•
.......
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.....................................
"
':::;:;:•:•":'•"-:':•z:•::?:::';:$•'•
....•:•::':'
"•o
pp. 39-54,
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...
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=•
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if, •
19:30
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IiO
...
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...............
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•
Academic, New York.
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Fig. 18. Swept frequency records of solar radio emission
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