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IEEE TRANSACTIONS ON ANTENATAS AND PROPAGATIOM
contributions
The Characteristicsand Design of the Conical Log-Spiral Antenna
JOHN D. DYSON,
Abstract-Thebalancedconicallogarithmic-spiral
antennais
considered as a slow-wave locally periodic structure with a slowly
varying period. A study of the near fields and their relationship to
identification of the active region or
the f a r fields has led to the
effective radiating aperture on the antenna and to a clearer understanding of its operating characteristics.
Information of the near- and far-field characteristics and on the
input impedance for a wide range of parameters is presented in a
form suitable for use in the design of practical antennas.
INTRODUCTION
HE FREQUEKCY-1NDEPEN:DEST antennas
havefoundwideapplicationandtheirgeneral
characteristicshavebeenoutlinedinnumerous
publications [l], [2]. The purpose of the present paper
is t o presentthecharacteristicsandpracticaldesign
information for one member
of this general class, the
balancedtwo-armconicallogarithmic-spiralantenna,
Dyson [3].
T
Manuscript receix-ed March 23, 1964; revised February 25, 1965.
This research was sponsored by the Avionics Laboratory, rVrightPatterson AFB, DaJ-ton, Ohio, under Contract AF33(657)-10471.
The authoris with the Dept. of Electrical Engineering, University
qf Illinois, Urbana, Ill.
SENIOR M E D E R , B E E
This study has been guided b~7 the concept, introduced by Mayes, Deschamps, and Patton [4], t h a t t h e
logarithmic-periodic antennas could be consideredt o be
locall>- periodicstructureswhoseperiodvariesslowly
with distance from the point
of excitation. It extends
this basic concept to the conical log-spiral antenna, by
initially comparing the propagation constant measured
along the surface
of the conical antenna toknown propagation constants for the cylindrical bifilar helix.
Thestudy of thepropagationconstantandother
characteristics of the near and far fields of these antennas has led to the identification of the ‘(active region”
or effective radiating apertureon the antenna. Successful design of the antenna depends upon knowledge
of
the position and size of this active region as a function
of antenna parameters.
The basic parameters of the conical antenna are defined in Fig. l. The parameter 0 0 determines the cone
angle, and a the rate of wrap of the arms. The angular
width of the exponentially expanding armsis defined b y
theangle 6 which is theprojection of 6’ on a plane
perpendicular to the axis of the antenna.-These angles
Uyson: ConacaC Log-apzraL Antenna
981u
are constantfor any given antenna and the radius vectorsurface of the cylinder. If we consider the difference
between these k / P ratios for thehelix and conical spiral,
to any point on the arms is given by [3]
and express this as a function of the helix ratio, we see
P = PO exp b ( 4 - 6)
(1) in Fig. 2 that thedifference issimply 1-cos Bo. For cones
with an included angle of 20°, for example, this differwhere
ence is approximately 1+ percent. For allconicallogspiral antennas which are good unidirectional radiators,
this difference is only a few percent.
This small difference in k / / 3 ratios for the smaller inThe edges of the first arm are defined by letting 6 = 0,
cluded
cone angles ~ o u l dlead one to expect that if we
and a fixed value between 0 and T. T h e second arm is
limit
our
discussion at any one time toa limited portion
obtained by multiplying the defining equations for the
of
the
conical
antenna there should be,a t least for these
first arm by e-bz. The orientation of the antenna in the
smaller
cones,
a close relationship between this portion
associated spherical coordinate system used for radiaof
the
conical
Structure
and some corresponding cylintion pattern measurements is also indicated.
drical structure.
The variationof the propagation constant on periodic
antenna structures can
beconvenientlydisplayed
on
the Brillouin or k / p diagram [S]-[S]. One such diagram
for the balanced bifilarhelix is shown in Fig. 3. T h e
vertical coordinate is given in units of k a l t a n a which,
since k = 27r/X, is simply the pitch distancein free-space
wavelengths. The horizontal scale is the pitch distance
expressedinguidewavelengths
on the surface of the
antenna. For any one helix, since a and a are constant,
the only variable involved is the wavelength of operation.
the frequency of operation is increased the propagation constant increases. If we assume a nondispersive
wave along the conductors, we find that there is first a
region of S ~ O W , closelyboundsurfacewaves.
As the
propagation constant increases still further, there
will
be strong coupling to
a space wave traveling in the
opposite direction. The propagation constant becomes
complex [ 9 ] as the structure radiates with
aphasing
that provides an
end-fire beam directed back towardthe
Fig. 1. Conical antenna with associated parameters.
point of excitation,i.e., a backfirewave. As the frequency of operation is increasedstillmore,energyis
THEAYTENNA
AS a LOCALLY
PERIODIC
STRUCTUREradiated a t a nangle from the axisof the structure, and
The parameters involved in a comparison of the coni- then in an end-fire beam in the opposite direction [lo].
calspiralandcylindricalhelicalgeometriesareindiFor the conical antenna there are two variables, the
cated in Fig. 2, in which is shown one turn or cell of a wavelength and the radius a , which vary linearly with
conical antenna with infinitesimally narrow arms, withI distance from the pointof excitation. For a fixed wavethe length of the turn, and a thc radius at any pointin length, wc SCC t h a t as a currcnt wavc progresses from
question. Superimposed upon this is one cell of a cylin- the pointof excitation along a structure with an increasdrical helix with the same pitch angle, and with a radius ingradius a , thepropagationconstantmightbe
exequal to the geometric mean radius of the conical cell. pected to behavein the manner thatwe attribute to the
Forobservationsparalleltotheaxis,theturn-tocylindrical structure with increasing frequency.
turn phasing of the helix is determined to a first approxi- Thesolutioninvolvingthepropagationconstant
mation by the ratio of the pitch distance p to the turn along the surface of the cylindrical helix applies t o t h e
length 2. This assumes a current wave progressing down infinite structure; however,suchsolutionshavebeen
the arm at the intrinsic phase velocityof the surround- shown to be useful for interpretation of the characterising medium. On the helix this ratio p / l is equal to the tics of the finite monofilar endfire helix
and the finite
sine of the pitch angle or cos a. The ratio of the pitch backfire bifilar helix with thin arms. I t is also recognized
distance (parallel to the axis) to the turn length on the
that extending these concepts to the tapered structure
conical structure is equal tocos a cos Bo.
is movingevenfartherfromthebasicpremiseupon
The ratio ( S ) of pitch to turn length is also the ratio which this diagram is based.I t is, however, a useful tool
of the propagation constant
k along the arms to the
in the interpretation of the characteristics of these anpropagation constant p of a wave propagating along the tennas [11], [12].
SPIRAL
s, = shCOS e,
FACTOR= s,_sh(IOO%)
sh
= (I-cos8,)
100%
I
Fig. 2.
0
Corresponding conical spiral and helicalcells.
Y
Fig. 4.
Fig. 3.
1
2
Brillouindiagram for bifilar helix.
8 -
Amplitude of near fields measured with a small shielded loop probe along the surface of one cone and electric far-field
radiation patterns corresponding to a truncationa t indicated points (280 = 20", 01 = SO", and 6 = 90").
The curve indicated bya solid line in Fig. 4 is a typical plot of the amplitudeof the fields measured b y moving a smallshieldedloopprobeparallelto,and
0.03
wavelength from, the surface
of oneconicalantenna.
The vertical axis is the relative magnitude
of the probed
field in decibels, and the horizontal axis is the distance
from the apex along the surface
of the cone in wavelengths (p/X) andtheradius
of the cone (a,'X). WTe
observe that there is a region of closely bound waves
near the apex of the cone. &4sthe probe is moved along
thesurfaceawayfromtheapex,thiswave
becomes
more loosely bound, and more energy is coupled to the
probe,untilthewavebecomes
so loosely bound that
energy is rapidly lost through radiation, and the amplitude of the near field decays to a negligible value.
As the frequency is changed, this region of rapid decay moves on the antenna so that its location and size
in wavelengths remains constant,i.e., the antenna aperture scales with frequency. An arm-to-arm variation in
amplitude isvisiblein
the region to the right of the
peak amplitude. The standing waveon the apex side of
the region of rapid decap appears to be due to an interaction between the wave progressing from thetip down
theantennaandthespacewaves
progressing in the
opposite direction. )Then the fields are measured with
the probe very close to the antenna (0.004X) the amplitude of thisfirstregion
is onlya few d B below the
maximum and the amplitude of the standing wave has
decreased, e.g., from 10 t o 1 2 d B t o less t h a n 1 d B a t a
radius of 0 . 0 3 . \JThen the probe is moved out to only
0.03 wavelength from the surface of the antenna, the
relative amplitude in this region drops by approximately
10 t o 15 dB, indicating how tightly the fields are bound
to the structure
when the conediameterissmallin
wavelengths.
THEACTIVEREGIOK
Since there appears to be a waveguide region and a
radiating region on the antenna,we define the radiating
or "active region" to be that region which controls t h e
primary characteristics of the radiated field.
To indicate the extent of the radiating or active regionon thisantenna,the
near-fielddistributionand
radiation patterns are shown in Fig. 4 as the base diameter becomes smaller in wavelengths. The near-field
amplitude indicated by the
solid curve and the radiation
patterns numbered (1) are representativeof the antenna
withoutend effect. As successive turnsareremoved
from the base end, thereis a negligible change until the
antenna is truncated at a radius such that the original
near-field amplitude is approximately 15 d B below the envelope along the peaks of t h e field in the region of
rapiddecaywouldbeunchanged.
Theactive region
recorded maximum. By the time the base is reduced to
this size, a definite change in the half-power beamwidth limits were obtained from such a smoothed curve.
Figure5
is aBrillouindiagramconstructedfrom
and the axial ratio is noted. These data and the nearfield and far-field data for other antennas,as they were amplitude and phase data measured on one narrow arm
maintained with a fixed size but with a changing fre- antenna at one frequency of operation. There is indeed
of the
quency of operation, led to the conclusion that the near a strikingresemblancebetweenthevariation
fields more than 15 dB down from the maximum con- propagation constant with position along the structure
tributed little to the radiation patterns. The radius
of and the asymptotic valuewhich we would expect on the
the cone a t this point, which we identify as a15+, could bifilar helix with variationof frequency of operation [9].
be considered t o be the lower edge of the active region. At the distancefrom the antenna surfaceat which these
measurements were made, a propagation constant beAs the antenna becomes still smallerinwavelengths,
comes evident by the tenth turn that has
a leading
the radiation in the direction of the base rapidly increases. Thepatternsnumbered
phase with increasing distance from the apex. This plot
( 3 ) , whichwererecorded for the antenna with a base radius (ala+), such of the variation of the real part of the propagation conthat the original near-field amplitude was 10 d B below stant along the surface lies just below the asymptotic
line cos a cos 0 0 . Near turn 18 this propagation constant
the recordedmaximum,aretypical
of those obtained
for a fixed cone size, at a frequency of operation such becomes complex, the real partincreases, and the atten18 t o 23
thattheantenna
is toosmall.The
effect of these uation per cell rapidly increases. From arms
there
is
more
than
25
d
B
of
attenuation.
This
is the
changes in base size, in wavelengths, on the measured
“active
region”
and
the
phase
center
of
the
radiated
field distribution on the small end of the antenna is not
great,indicatingthat
we shouldexpectonlyminor
field is consistently located in this region, in this case
variations in the input impedance.
near arm 20. Comparing this figure t o Figs. 3 and 4, we
T h e effect of a similar truncation of the apex of the note that the majorityof this activeregion and the porcone is shown by the curve labeled a3 and pattern (4). tion with greatest amplitude near
fields is phased f o r .
backfire radiation.
The vertical solid line indicates the original truncated
When data is recorded at a higher frequency, and the
apex a t a radius of approximately 0.03X. Whenthe
antenna is truncated ata radius such that the smoothed probe separation maintained fixed in wavelengths, the
level of original field distributionis 3 d B below the phasevariationobservedherefromturns
11 t o 22 is
maximum,the
near-field distributiononthelarger
closely repeated at turns 3 through 14. Thus, in conturns is perturbed enough to make majormodifications trast to the uniformly periodic structure, the radius at
in the radiation pattern. This radiuswe identify as as-. the region of rapid decay (i.e., the active region),exFurther truncation produced large changes in the fields pressed in wavelengths, remains essentially fixed on the
and hence in the aperture distribution in the radiating antenna. If the coupling to the space wave is strong,
region. In general agreement with these results, calcuessentially all of the energy is radiatedintheregion
which is phased for backfire wave radiation and there
lated radiation patterns based upon the measured near
fields of the unperturbed antenna, i.e., without end ef- is a negligible amount left to radiate at any angle from
fect, as the smaller turns were eliminated, showed little the axis of the structure.
change until the antennawas truncated at a point such
Having defined the active region of the conical anthat the originalnear-field
distributionwasapproxitenna in terms of radii of the cone we note how this
mately 3 d B below the maximum.
active region depends upon the antenna parameters. In
Although changes, and in particular a small decrease
Fig. 6, the boundaries of this region are plotted as a
in beamwidth, may be noted
before this limit is reached, function of the included cone angle 280 and the spiral
these results correlated with the other information led angle a. Since both axes of this graph are normalized t o
to the adoption of the region from a point 3 d B below the same wavelength these curves give the active region
is considered t o be
the maximum on the apex side (radius a3-) to a point bounds. If, however, the vertical axis
15 d B below themaximum on thebaseside(radius
normalized to the shortest wavelengthof operation and
al5+) as these fields were measured, as the effective radithe horizontal axis to the longest wavelength of operaating aperture of the antenna in what wecouldcall
tion, these curves give the required radii
of the trunnormal
frequency-independent
operation.
Near-field
cated apex and base of the conical antenna.
data recorded under different conditions would necesA study of the Brillouin diagram in Fig. 5 indicates
sarily be subject toa different set of limiting values on that the active region of the or=60° antenna, for exthe active region.
ample, should be closer to the apexof the cone than for
I t should be noted that if the line along which the
the a = 80’ antenna. Figure 6 shows that the onset of
fields are measured were to be rotated around the anradiation does occur at a smaller radius, but the loose
tenna by 90°, the arms would all be shifted down onespirals are less efficient radiators; the net effect is t h a t
half period and hence the arm-to-arm variations in the these antennas radiate over a considerably wider region
field structure would be likewise shifted.Theoverall
on the antenna. The result
is an activeregion that starts
492
TRANSACTIONS
IEEE
O N ANTENNAS AND PROPAGATION
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ACTIVE REGION EANDYIIDTH (6-1
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a t a smaller radius and, particularly for
200>10", ex- Fig. 6. Bounds of the active regionin terms of the radius of the
cone. Data for the self-complementary antenna (6 = 90").
tends to a larger radius than the equivalent region on
the antennas with tighter spirals.
ACTIVE REGION BMlWYlDTH [E,
These limits on the active region were based upon
near-field measurements. In terms of the far-field patterns, they are most accurate for cones with total included angles of approximately 20". They tend to become more conservative as the cone angle decreases and
slightly optimistic for greater cone angles.
If the activeregion had no width, the operating bandwidth of the antennawould be given by the ratio of the
radius of the base to the radius of the truncated apes.
Following the convention proposed by Carrel
[13]for the
log-periodic dipole arrays, this ratiois defined to be the
bandwidth of the structureB,.
I t is apparent that the operating bandwidth of the antenna is always less than the bandwidth
of the structure Fig. 7. Bounds of the active region that may be used for design if
appreciable pattern distortion is permissible at lowest operating
by a factor defined to be the bandwidth of the active
frequency (6 =go").
region B,,
CHARACTERISTICS
OF THE RADIATION
FIELDS
In Fig. 8 typicalelectric
field radiationpatterns
are shown as a functionof t h e included cone angle, and
the spiral angle a. T h e well-formed relatively narrow
B = BJB,,.
beam for small cone angles witha = 80" is indicative of
essentially all turns of the active region being phased
Figure 6 includes a gridindicatingtheactiveregion
for backfire radiation.
As the active region broadens,
bandwidth B,, as a function of antenna parameters.
the radiation pattern broadens and exhibits a tendency
For many applications, considerable pattern distortion can be tolerated at the endsof the operating band- to show a multiple beam effect with corresponding irwidth; hence, the antenna can be operated t o a smaller regularities or "ears." It is worth noting that this relasize. For such applications Fig. 7 gives the active region tionship between aperture size and antenna directivity
It is brought about
bounds as radii a3- and ala+, where ala+ is the radius at is exactly opposite to the usual case.
which theunperturbednear-fieldamplitude
is 10 d B because the phase distribution across the effective radibelow the recorded maximum, and patterns number (3) ating aperture is such that all parts of the aperture are
in Fig. 4 are typical at the lowest frequencies of opera- phased to radiate in progressively different directions.
Increaseddirectivity
is obtainedfortheperiodic
tion. At these frequencies the typical axial ratio
on axis
structurebyextendingtheradiatingapertureover
shouldbe 4.7 d B or less. Foroperationonlytothe
is applicable
bounds shown in Fig.6, the axial ratio on axis should bemore elements. In principle, this concept
to the tapered periodic structures and may be realizable
approximately 3 dB orless a t t h elowest frequencies.
Hence, the antenna operating bandwidth B is given by
Uyson: L'onzcal L o g - 8 p r d Antenna
1965
4Y3
for a slight taper. As the taper is increased to obtain a wide beam. T h e backfire phasing of the 2" cones, for a
greater than TO", is clearly evident.
practical size antenna that will scale over a wide range
The approximate half-power beamwidths of the radiof frequencies, i t is no longer possible t o maintain the reated fields of the conical antennas are plottedin Fig. 11.
quired phasing for backfire radiation over a large aperture. The larger turns, or more widely spaced elements,The patterns recorded by orthogonally oriented receivingantennas Eo and E , differ typically 8" t o 10" in
are phased to radiate energ17 a t a n angle from the axis
beamwidth. The values given in Fig. 11 are the average
of the antenna. Hence, for any given ccne angle, the
net effect is a n inversion of theusualaperture
size- of these beamwidths. I n addition, this mean level of the
beamwidth must be evaluated in conjunction with the
directivity relationship.
This effect is shown in Fig. 9 where the relative mag- indicatedvariationinbeamwidtharoundthislevel
nitude and phase of the tangential magnetic field meas- shown in Fig. 12. This indicated variation in beamwidth
ured along the surfaceof three log-spiral antennas, each is caused by the fact that the radiated beam, which may
with an included cone angle of 15", is shown. When the not be rotationally symmetric, rotates about the axis
arms are wound fairly tightly,
a = 80", the leading phase of the antenna as the frequency of operation is changed
over a relatively narrow aperture concentrates the radi-[14]. This effect, and the fact that the practically conated energy in the backward direction. -4s the rate of structed antennawill not scale exactly a t all frequencies,
spiral is relaxed the relative radiating efficiency of the causes a variation in beamwidth in a fixed plane of observation as the antenna is operated over a very wide
aperturedecreases,andhencetheaperturesizeincreases. The slope of the phase curve is decreased and range of frequencies. This variation can be minimized
the last few turns in the active region radiate apprecia- for operation over a reduced bandwidth and would be
ble energy a t a n angle from the axis of the cone. Yi7hen on the order of that for any conventional antenna when
of a few percentfor
the angle of wrap a is decreased t o 45", a major portion operatedoverthebandwidths
which such antennas are designed.
of the active region is phased for broadside radiation.
The approximate directivity with respect to a circuTo maintain good backfire patterns the leading edge of
11 was
the effective aperturemusthavean
excess of phase larlypolarizedisotropicsourceplottedinFig.
obtainedfromthe
use of theseaveragehalf-power
shift for backfire radiation, and this aperture must be
beamwidths in the expression [IS]
an efficient radiator.
If n7e recall from Figs. 3 and 5 that the ratio of the
32 600
D = 10 loglo
dB,
velocity of propagation of the surface wave to a freespacewavewasapproximately
cos a! cos Bo, and
solve for the intersectionof this asymptotic line and the where q51 and O1 are the average half-power beamwidths
region boundary given by
in two orthogonal planes.
Thefront-to-backratios
of theradiated fields are
ka
pa
-- - I - plotted in Fig. 13. This is the typical minimum value
tan a
tan a
t h a t should be expected for a well-constructed antenna
operated at a frequency such that there is no effect due
we obtain the expression
to the truncated base or tip. The decrease in front-toa
1 sin Q cos Bo
back ratio with increasing cone angle is to be expected
--x 2 1 C O S Q cos00 .
since the planar structure is bidirectional.
The antenna is elliptically, and essentially circularly,
This and similar expressions for the intersection of the
polarized in any direction in which there is substantial
asymptote and thelines pa = t a n a! (broadside radiation)
radiation. Typical valuesof axial ratio recorded
a t angles
and
from the axisof a 20" antenna are shown in Fig.14. T h e
Pa
ka
lon7er curve is indicative of the increased beamwidth and
-- - l + increased energy radiated in directions approaching that
tan Q
tan CY
perpendicular t o t h e axis of the antenna as the rate of
(end-fireradiation)havebeenplottedasthedashed
wrap a! is decreased.
curves in Fig. 10.
The conical log-spiral antennas do not havea unique
The radii ax- and a15+that outline the active region center of phase,howeveroveraportion
of the main
have also been plotted for
2" and 20" cones. Although
beam an "apparent phase center" may be defined [16].
this is only an approximate indication
of the relative
As shown in Fig. 15, the apparent phase centers of the
phasing of the radiating aperture,
it clearly indicates the smallerconeantennas
lie wellbelow
theslow-wave
phenomena observed in Fig. 9. Recalling the amplitude region boundary on the Brillouin diagram. For 200 5 20"
distribution from Figs. 4 and 9, we see t h a t for the 20" the position of the apparent phase center can be given
cones the major part
of the radiating aperture of the by the straight-line approximation in Fig. 15. In terms
tightly wrapped antennas is phased for backfire radiaof the cone radius this m a g be expressed as:
tion. From these we get relatively narrow well-formed
1
1.2 sin a
beams.However,
as theangle a decreases,thenet
alx r result of the1radiation from the wider active region is a
2n 1.4
cos 01
~
+
+
IEEE TRANSACTIOA'S OAT dATTENAiAS AND PROPAGATION
494
July
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DISTANCE ALONG CONICAL SURFACE L"/A)
,
3
0
4
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SPIRAL
Fig. 9. Relative amplitude and phase of magnetic fields measured
along the surface of conical antennas (260=15, 6=90°), and corresponding far-field radiation patterns.
Fig. 1I.
5
0
MIOLE
m
m
m
3
0
a
Averagehalf-powerbeamwidth
andapproximate
tivity of the conical log-spiral antennas (6=90°).
direc-
Theheavyweighting
of theportion of theaperture
nearest the apex of the cone, due to the high amplitude
fields present there, tends t o keep the phase center in
thisgeneralregioneventhoughtheradiationregion,
albeit a t lower amplitudes, extends considerably farther
down the antenna.
THEANTENKA
FEEDSYSTEM
SPIRAL ANGLE a
Fig. 12. .Approximate variationinhalf-powerbeamwidthtobe
expectedwhen antenna is operatedoverwidebandwidths.
28, = 15"
28. = 20°
29, = 300
30
25
20
I5
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IO
5
0
90
8
0
180'0
90
1880
6
90
l80"O
90
6
180"
6
ANGULAR ARM WIDTH
Fig. 13.
a
Minimumfront-to-backratio of radiationpatternsas
function of ea, a,and 6.
2.0,
6
ANGLE
e
IN DEGREES
Fig. 14. Typicalvariation of axial ratio off the axis of antenna.
Measured for one antennawith 200=20", 6 =go".
.4
-
.3-
\ \
0
.I
.2
.3
.4
.5
.6
.7
.8
0
9
.9
Bo
tan
D
Fig. 15. Measured "apparent phasecenters"forseveralconical
antennas ( 200 = 20").
1
LO
The two-arm conical antenna is a symmetrical structure, and when excited in a balanced manner it radiates
a beam on the axis of the cone without squint or tilt.
I n all cases a tilt in the radiation pattern can be traced
t o t h ephysical construction or to imbalance in the feed.
The antenna may be excited b y bringing a balanced
transmission line along the axis, to the apex
of the cone,
and connecting one wire to each arm. I t is preferable,
although not necessary, that this be
a shielded line. T h e
presence of a metallic shield or cylinder o n axis has a
minimum of effect on the antenna characteristics if the
diameter of this cylinder (or metal cone)
is no more than
one third the diameter of the antenna at any point on
the axis.
The transition from an unbalanced coaxial line to a
balanced line may be made by placing a balun inside the
antenna if the balun is nonradiating and not affected by
the fields inside the cone. Conventional baluns may fall
into this class, but are very limited in bandwidth. The
tapered line balun [I71 with its extremely wide bandwidth wou!d seem to be ideally suited for use with this
antenna.When placedinside
theantenna,however,
there appears to be an interaction
or interference between the fields around the balun and those of t h e a n tenna, with the result that a truly balanced feed is seld
realized. For some purposes, however, the degradation
of thepatternduetothe
use of thisbaluncanbe
tolerated.
T h e excellent coaxial hybrids now on the market make
satisfactory baluns [lS]. These units provide equal amplitude out-of-phase signalsat the side arms. Fifty-ohm
coaxial cables connected t o these arms may be carried
along the axisof the cone, and the two center conductors
of the cables thus become a shielded balanced 100-ohm
transmission line. For maximum symmetry, and to preventthetwooutersheathsfrombecomingatransmission line for any unbalanced currents, these cables
should be bonded together or placed insidea metal tube
orcylinder.Most
of thesehybridsarelimitedtoan
octave bandwidth and provision must be made for replacing or switching of hybrids. In the VHF and UHF
range,however,low-powerhybridshaverecentlybecome available foruse over ten-to-one and greater bandwidths.
To overcome the limiting bandwidths of the baluns,
the method of feeding the antenna by carrying the coaxial cable along one arm was devised[14]. T o maintain
phqFsica1 symmetry a similar cable must be placed on the
other arm. This method,
whichhasbeentermedthe
"infinitebalun"feed,remainsthemostsatisfactory
496
IEEE TRANSACTIONS ONPROPAGATION
ANTENNAS
AND
method if the physical presence of the cable does not
limit the sizeof the truncated apex region, and
if the loss
in the relatively long length of cable can be tolerated.
I t is a truly balanced feed under the condition that the
antenna currents have decayed to a negligible value a t
the truncated base of the cone and a symmetrical connection is maintained a t t h e feed point.
INPUTIMPEDANCE OF THE
lhTEXNd
Ju2y
ance levelsof these antennas have not yet been
precisely
defined. Figure 1 7 does indicate approximate levels for
design andthetrend
in thislevelwithangulararm
width.
The presence of cable on the arms tends to give the
narro\v arms near the apex an effectively greater cross
section and, hence, shifts the impedance level. An approximate curve of the measured input impedance for
an antenna with280 = 20", a = 60, and a tip truncated at
The geometry of the truncated tip, including thepossible presence of the feed cable on the antenna arms,
has a markedeffectuponthecharacter
of the input
impedance of the antenna. Since there is an unlimited
combination of cable diameter-truncated tip combinations, it appears to be meaningful to present only the
impedance of the basic antenna when fed on axis, and
indicatethetrend
as youdepartfromthisbasic
structure.
Figure 16 indicates the effect that the feed geometry
can exert on the measured impedance. SlTmmetry, balance, and tapered leads from the axis to the surface of
the cone are required.
The use of the coaxial hybrids as baluns makes possi- Fig. 16. Effect of the feed region geometry on the input impedance.
ble a very convenient method
of making balanced im500
pedance measurements, if identical slotted lines are inserted in the coaxial lines between the antenna and the
hybrid [191.
T o determine the impedance level
of the antennas,
measurements referred to a reference plane at the tips
of the tapered leads on the truncated apex of the cone
were made over a five-to-one bandwidth. The normalized impedances plotted on the Smith Chart were enclosed by a circle. The "center" of this circle, chosen t o
make the hyperbolic distance toall parts of the circle a
constant [20], was considered to be the characteristic
impedance of the antenna. The maximum
VSWR referred to this characteristic impedance was typically 1.3
t o 1.4 t o 1.
The variation of the characteristic impedance with
arm width is shown in Fig. 17. The input impedance of
ANGULAR ARM WIDTH ( 8 )
the antenna is primarily controlled by the arm width,
17. Approximate characteristic impedance level of the conical
varying from around 320 ohms, for the very narrow armFig.log-spiral
antenna as a function of the angular arm width.
antennas, to around80 ohms, for the corresponding very
wide arm antennas. The impedance increases as the includedconeangleincreases,with
thatforthe
selfOr-----7
complementaryantennas
(6 = 90") approachingthe
theoretically predicted 60a or 189 ohms for the planar
antenna [21]. T h e measured impedance variation with
changeinthespiralangle
a wassmall,althougha
tendency for this impedance to increase with increasing
a was observed.
The apparently clear trend in impedance level with
cone angle shown in Fig.
1 7 was not observed for all
angles, and the impedance levels for cone angles
less
than 15" were not lower than those shown for 15". For
example,for 280 = 5", a = 80", the characteristic impedance measured for 6 = 16, 90, and 164" u-as approxi- Fig. 18. Approximateratio of the radius at the baseend of the
active region for very narron- or very wide arm widths to that for
mately 300, 1S5, and 72 ohms, respectively. The impedthe self-complementary antenna.
I
I
6.16'
t
Q
-
Q
..
Q
Q
Fig. 19. Typical electric field radiation patterns indicating general change in shape with cone angle
eo, spiral angle a,and angular arm width 6.
,
0.75 inch, fed with RG141jU cable of diameter 0.141
inch is shown. This curve indicates the general impedance level forthisparticularcable-truncationcombination only. The maximum VSM'R referred to this impedance level was approximately 1.75 t o 1. The tapered
leads at the truncation are still required if the width of
the arm on the surface of the cone a t this truncation is
substantially wider than the diameter of the cable.
The impedance bandwidth
is consistently greater than
the radiation pattern bandwidth and hence the active
region was defined in terms of the latter.
EFFECTOF ARM IVIDTH
The angular width of the expanding antenna arms is
defined in terms of 6 where 0 <6 <T.
In nearlyall cases, thecharacteristicsof these antennas
have been investigated for very narrow arm structures
(6= 16"), for very wide arm structures
(6= 164"), and
for 6 =90". This latter structure is defined t o be selfcomplementary in the sense that the geometry
of the
arms and the space between armsis identical except for
a rotation of 90" about theaxis of the structure.
'Lt:ith the exception of the impedance whichis directly
related to the arm width, nearlyall d a t a presented thus
farhave
beenfortheself-complementaryantenna.
Changes in the other characteristics do occur as this
width is changed. For example, the rate of decay of the
near fields becomes less as 6 departs from 90". Thus uI5+
becomeslarger. I n Fig. 18 a factor 114 indicating the
ratio of the maximum radius required for veq- narrow
or very wide arms to that required for arms with
6 = 90"
has been plotted. For values
of 6 between 90" and 16" or
16-1" linear interpolation from :71=1.0 to the indicated
value should be sufficient. Since us- changes little compared to the change in uljf, this modification factor M
can also be taken to be the ratio
of the active region
bandwidths B,,, for 6 = 16" and 164", to that for6 = 90".
For
5 20" the variation with6 is less for a 2 55", and
hence, for these small-cone fairly tightly wrapped antennas it is possible to use thin arms and the constant
width wire or cable arms version
of the antenna [3], with
only a small change in characteristics
of the radiation
pattern. The change in the electric field radiation patterns with change in arm width is indicated in Fig. 19.
T h e principalchange
is a n increase in beamwidth
(So t o 8" for a = SO") brought about by the increase in
active region width.
These patterns are typical
of those to be expected
when the antenna is operated a t frequencies such t h a t
there is no distortion due to the truncation of the base
or tip. As the frequency of operation is decreased and
the lower edge of the activeregion becomes distorted by
the truncated base, the amplitude of the back lobe will
increase rather rapidly.As the leading edgeof the active
region moves to the truncated tip, the pattern beamwidth may at firstbecome narrower and the pattern become rough. 4 n y lack of precision in construction a t t h e
apes region will cause pattern tilt or distortion.A further
increase in frequency may cause the pattern to broaden
with a tendency to break into lobes.
Radiation patterns for the very loosely wrapped antennas,a = 45", with self-complementar). width arms only
are shown. As the arm width deviates from 6 = 90" for
antennas with 2 0 0 2 15' and aS45", the pattern breaks
into many lobes with
a majorportion of theenergy
radiated in the direction of the base.
A decrease in the front-to-back ratio of the radiation
patterns, as indicated in Fig. 13, may be noted as 6 departs from 90".
AXTEXXA
DESIGNAND COKSTRUCTION
The antenna engineer is usually interested in designing an antennawhich has a given directivity or radiation
pattern beamwidth and input impedance over a given
frequencyband. A satisfactorydesignshouldbeobtained from the previously plotted data. The data was
obtained for antennas constructedof 2-mil thick copper,
etched on 10-mil teflon-impregnated fiber glass, with a
balanced two-coaxial line feed carried along the axis of
the cone. Hence, the data should be representative
of
the basic antenna structure. The
use of a n infinite balun
feed or a cable approximation to the expanding arms
should modify the antenna characteristics in a predictable manner, since the cable gives an effectively greater
width to narrow arms.
Half-power beamwidths on the order
of 40" t o 50" can
beobtainedwiththesmallconeantennas.However,
whenverywidebandwidthsare
t o becoveredthese
antennas may become quite long. Physical requirements,
such as allowablelengthor height, may dictate thechoice
of the wider cone angles and a compromise of antenna
characteristics. The antenna length or height in terms
of the longest wavelength of operation X L may be expressed as
k
_
--~ 1
XL
2 tan&
;
--__
3
.
In constructing these antennas it must be
realized
that they are potentially extremely wide-band antennas,
andareexpected
to performwithconstantcharacteristics w e r these bandwidths. To do this, the active
region must be able to scalet o a constant geometry and
toconstantphysicalparametersexpressedinwavelengths at all frequencies. Hence, all such parameters
should scale in size with distance from the apex. Symmetry, balance, and detail and precisionof construction
are important. Violation of any of these principles can
only lead to degradation of the antenca characteristics
as the frequency of operation is varied.
CONCLUSIONS
T h e conical log-spiral antenna canbe considered to be
a locally periodic structure, with slowly varying period.
4 study of the antenna in this context has led to ready
identification of many of its characteristics.
This study indicates that the fairly tightly spiraled
self-complementary antenna tends to be the “optimum
antenna,” in the sense t h a t it has the narroxest active
region and, hence, the greatest operating bandwidth
for a
given physical size, and the most compact symmetrical
radiated beam with the least energy radiated
in back
lobes. The input impedance,
however, depends primarily
upon arm width, with typical valuesof 140 t o 165 ohms
for these self-complementary structures.
-4CKSOWLEDGhiEXT
The author gratefully acknowledges the assistance of
A. L. Davidson, G. R. Fruehling, 31. D. Johnson, V. P.
Rash, R. J. Kopczyk, and other members of the University of Illinois Antenna Laboratory,x7ho helped with
themeasurementprogramcarriedonduringthisinvestigation.
REFERENCES
[l] Dyson, J. D., Frequency-independentantennas-survey
of
development, Electronics, 1-0135,Apr 20,1962, pp 39-44 (includes extensive list of early references).
[2] Jordan, E. C., G. A. Deschamps, J. D. Dyson, and P. E. Mayes,
Developments in broadband antennas, ZEEE Spectrum, vol 1,
Apr 1964, pp 58-71.
[3] Dyson, J . D.,The unidirectionalequiangularspiral
antenna,
I R E Trans. on Antenms and Propagation, vol AP-7, Oct 1959,
pp 329-334.
[4] Mayes, P. E., G. A. Deschamps, and \V. T. Patton, Backwardwave radiation from periodic structures and application to the
design of frequency-independentantennas, Proc. I R E (Correspondence),vol 49, May 1961, pp 962-963.
[5] Oliner, A. .I., Leaky waves in electromagnetic phenomena, in
Electron~ugneticTheory and Antennas, E. C. Jordan, Ed. Iiew
York: Pergamon, 1963, pp 837-856.
Rumsey, V. H., Propagationover a sheet of sinusoidal mires
.ibid.,
and its application to
frequencyindependentantennas,
pp 1011-1030.
glittra, R., and E;. E. Jones, Theoretical Brillouin ( k - 0 ) diagram
for monopole and dipole arrays and their application to
logperiodic antennas, 1963 ZEEE Iizternat’l C o m . Rec., pt 1, pp
118-128.
Hudock, E., and P. E. Mayes, Propagation along periodic monopole arrays. 1963 G-AP Internat‘l S p z p . Digest, pp 127-132.
Klock, P. \V.,and R. Mittra, On the solution of the Brillouin
( k - p ) diagram of the helix and its application tohelical antennas,
1963 G-AP Internut’l Symp. Digest, pp 99-103.
Ishimaru, A., and H. S. Tuan, Theory of frequency scanning
of antennas, I R E T a n s . on Antennas and Propagation, vol
4P-10, Mar 1962, pp 144-150.
Dyson, J. D.,The coupling andmutual impedancebetween
conical log-spiralantennas in simple arrays, 1962 I R E Znlewut’l
Corn. Rec., pt 1, pp 165-182.
Dyson, J. D., and G. L. Duff, Kear field measurements on the
conical log-spiral antenna, 1963 G-AP Ifzternat’l Synzp. Digest,
pp 137-142.
Carrel,R.,The
design of log-periodicdipole antennas, 1961
I R E I n t e n d l Conv. Rec., pt 1, pp 61-75.
Dyson, J. D., The equiangular spiral antenna, IRE Trans. on
Antennas and Propagation, uol AP-7, .lpr 1959, pp 181-187.
Stegen,R.J.,Thegain-beamwidthproduct
of anantenna,
I E E E Trans. o?z Antennus and Propagation (Correspndence),
vol XP-12, Jul 1964, pp 505-506.
Dyson, J. D., and R. E. Griswold, Measurement of the phase
centers of antennas, Tech Rept 66, Antenna Lab., University
of Illinois, Urbana, Dec1963.
and 1‘. P. Minema. 1OO:l bandwidthbalun
Duncan, J. ij:.,
transformer, Proc. I R E , vol 48, Feb 1960, pp 156-164.
Alford, A,, and C. B.\4:atts, Jr., -4 wide band coaxial hybrid,
1956 I R E COHV.
Rm., pt 1, pp 171-179.
Dyson, J. D., Balanced impedance measurements with a coaxial
of Illinois, Ursystem, Tech Rept,AntennaLab.,University
bana, to be published.
TeleDeschamps, G. A., X hyperbolic protractor,Federal
commuaication Labs., h-utley? X . J.; 1953.
1957 I R E
Rumsep, V. H., Frequencyindependentantennas,
ITat’l Cono. Rec., pt 1, pp 114-118.
Antenna and Wave Theories of Infinite Yagi-Uda Arrays
R. J. ?dL$ILLOUX,SENIOR
MEXBER, IEEE
,
A b s t ~ ~ ~ t - Aintegral
n
equationforthepropagationconstant
along an M t e l y long Yagi structure is derived by expanding the
vector potential function for such an array in terms of the spatial
be
harmonic solutions of wavetheory. This equation is shown to
on the basis of array
identical with the integral equation derived
theory and transformed by the Poisson summation formula.
this newwavetheory
Withtheidentity
of arraytheoryand
formation established, the wave theory is used to discuss allowed
wave solutions and the physical characteristics required of dipoles
in order that they support a wave solution.
The fundamental integral equation is solvedby means of the
array theory of King and Sandler; the numerical results are found to
agree quite well with previously published data.
Finally, the problem of two parallel nonstaggered Yagi arrays is
considered,andit
is shown that the propagationconstant of the
composite structure either decreases or increases over that of the
isolated array depending upon whether the symmetric
or the antisymmetricmodeisexcited.Somepeculiareffects
are notedwith
IiIanuscript received September 14, 1961; revised March 1, 1965. respect tothis antisymmetric solution, and these lead the
to existence
The research in this paperwas supported in part by National Science of conditions under which no unattenuated wave solution is possible.
Foundation Grant GP85l and
in part by joint Services Contract This is referred to as the“cutoff condition.’!
Nonr 1866(32).
Numerical results are acbiaved which agree very well with exof A3pplied
The author is with the Gordon McKay Laboratory
perimental data obtained a s part of this research.
Science, Harvard UniuersitJ-, Cambridge, Mass.
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