The characteristics of a double exponentially tapered slot antenna (DETSA) as a function of the radius that the DETSA is conformed to in the longitudinal direction is presented. It is shown through measurements and simulations that the radiation pattern of the conformed antenna rotates in the direction through which the antenna is curved, and that diffraction affects the radiation pattern if the radius of curvature is too small or the frequency too high. The gain of the antenna degrades by only 1 dB if the radius of curvature is large and more than 2 dB for smaller radii. The main effect due to curving the antenna is an increased cross-polarization in the E-plane

Chevalier, Christine T.

Jordan, Jennifer L.

Ponchak, George E.

NASA Technical Reports Server

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Characteristics of Double Exponentially 
Tapered Slot Antenna (DETSA) Conformed in 
the Longitudinal Direction Around a Cylindrical 
Structure 
George E. Ponchak, Senior Member, ZEEE, J m i f e r  L. Jordan, and Christine T. Chevalier, Student 
Member, ZEEE 
Abstract-The chamcterislics of a double exponentially 
ta~ered slot antenna (DETSAI as a function ofthe radius that the 
DETSA is conformed'to in th; longitudinal direction is presented. 
I t  is shown through measurements and simulations that the 
radiation pattern of the conformed antenna rotates in the 
direction through which the antenna is curved, and that 
diffraction affects the radiation pattern if the radius of curvature 
is too small or the frequency too high. The gain of the antenna 
degrades by only 1 dB if the radius of curvature is large and more 
than 2 dB for smaller radii. The main effect due to curving the 
antenna is an increased cross-polarization in the E-plane. 
Inder Terms--Conformal antenna, Double Exponentially 
Tapered Slot Antenna (DETSA), slot-line antenna. 
I. INTRODUCTION 
T I ~ L  family of Tapered Slot Antennas (TSA's) are useful because they have a wide 10 dB return loss bandwidth, are 
easy to fabricate on a variety of substrates using simple printed 
circuit board fabrication processes, and have good radiation 
pattern characteristics with moderate gain of approximately 8 
dBi [I]. The fmt implementation of a TSA was a stripline fed 
antenna formed in the ground planes in 1974 [2], and the fmt 
printed slot antenna (1979) was an exponentially tapered slot, 
the Vivaldi Antema [3]. Various forms of TSA's have been 
summarized [4]. However, even though TSA's have been in 
the literature for 30 years, there are many questions about them 
that have not been addressed, including the physics behind 
their radiation [I]. Typically, TSA's are endfire antennas that 
are useful for phased array antennas. However, it has also been 
noted that because they may be printed on thn, soft substrates 
such Liquid Crystal Polymer (LCP), they may be conformal 
[ll, PI ,  [GI. 
In [ 5 ] ,  a brief summary of an experiment about wrapping a 
Maooscl-ipt rcccived June 2,2006. 
G. E. Ponchak and J. L. Jordan are with NASA Glenn Research Center. 
Cleveland, OH 44135 USA ~rorxe.~onchak@ie~.nrg,  tel: 216-433-3504; 
Jenni~~l..Jordan@grc.nasa.gov). 
C. T. Chevalier is with Analex Cnp at NASA Glenn Rcscarch Center, 
Cleveland, OH 44135 USA(Christine.Chcvalirr@grc.nasa.gov) 
TSA around a foam cylinder with the discontinuity in the 
transverse and longitudinal direction is presented. Only the 
effect on gain is described, with no discussion of the radiation 
pattern. A Double Exponentially Tapered Slot Antenna 
(DETSA), a variation of the Vivaldi Antenna, designed for 
Ultra Wide Band (UWB) applications conformed over a gently 
rounded foam shape in the longihldial direction is described 
in [6]. It was shown that the main beam is skewed towards the 
direction of curvature. However, there has not been an in depth 
exploration on the effect of conforming a TSA around a 
curved structure. 
In this paper, experimental and simulated characteristics of 
a DETSA designed for 3 to 10 GHz and conformed in the 
longitudinal direction around cylindrical, foam structures is 
presented. The gain, 3dB beamwidth, cross polarization level, 
and beam direction are presented for different cylinder radii. 
11. DETSA DESCRIPTION 
A DETSA was fabricated with 18 pm thick copper on a 200 
pm thick LCP substrate, which has a dielectric constant of 
~.,=3.1 and a loss tangent of 0.003 [7]. The physical 
properties of LCP provide flexibility to conform the antenna 
around tight radii. The geometry of the DETSA was optimized 
for the antenna to operate over the ftequency range of 4 to 
10 GHz when held in a perfectly flat shape. Figure 1 shows an 
artist drawing of the DETSA with all dimensions and the 
equations for the exponential tapers. The slotline width at the 
start of the taper, afier the rectangular section of uniform 
slotliie, is 240 pn Standard photolithography processes and 
copper etching are used to defie  the DETSA. 
The radii of curvature are given in Table 1. Note that, 
throughout the paper, the data is presented as a function of the 
angle that the DETSA wraps around a cylinder, with the 
uniform section of slotliie held flat. For example, for the 
cylinder with a radius of 6.685 cm, the DETSA wraps one 
quarter, or 90', around the cylinder. Figure 2 shows an artist 
rendition of the curved DETSA wrapped 90' around a 
cylinder. 
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" .  - .. 
than in the direction of maximum H-plane radiation. As will be 
shown in Section V, taking E-plane measurements in the 
direction of maxirmun H-plane would have required cutting 1 G  
blocks of Styrofoam at specific angles. Thus, simulated E- 
m. RETURN LOSS AND RADIATION PATTERN MEASUREMENT 
TECHNIQUE 
The form of the DETSA was held by placing the antenna 
between two pieces of Styrofoam, which also provided 
structural support. The Styrofoam shape is cut on a band saw, 
which introduces small variations (errors) to the shape. Tlie 
beginning of the taper is placed where the Styrofoam starts to 
curve. 
Two coax to slotline launchers were used in the paper. First, 
to measure the return loss of the antennas, an inline, coax to 
microstripIcoplanar waveguide launcher with three of the four 
ground connections sawed off was used. It was found that this 
yielded good impedance match, but symmetric currents were 
not established on the slotline. Therefore, for radiation pattern 
characteristics, the outer conductor of a semi-rigid coaxial 
waveguide is soldered onto one side of the slotline 
perpendicular to the slot at the edge of the board and the centcr 
0.024+ c conductor is extended to and soldered to the opposite side of 
c 1.948 + the slotlie [8]. Although the impedance match was not 
optimnm, simulations and measurements showed that 
symmetric currents are established on the slotline, which is 
critical for radiation pattern characterization. 
Figure I :  Schematic of DETSA with dimensions in em. Using an HP 8510 Vector Network Analyzer, ISllI, rehlrn 
loss measurements were taken to verify the bandwidth of eacll 
Table 1: Dimensions of DETSA radii of curvature. 
I I plane patterns in the direction of the H-plane maximum beam I are used. N. SMUTION METHOD 
DETSA. A piece of RF absorber was used to minimize 
radiation from nearby metal surfaces during SII return loss 
characterization. Although the symmetric excitation method 
described earlier produced good radiation patterns, the return 
loss measurements were poor. Therefore, the presented return 
loss measurements, seen in Fig. 3, use a coaxial feed. 
Far-field radiation E and H plane measurements were taken 
in a calibrated far-field antenna range with a wide bandwidth 2 
to 18 GHz rigid hom antenna transmitting to the DETSA. 
The transient solver of the software package CST 
Microwave Studio [9] was used to simulate the DETSAs. The 
substrate material is of the type Normal with 6 ,  = 3.1. The 
antenna is patterned with the material PEC (perfect electric 
conductor). All materials are lossless. The Styrofoam used in 
the experiment to hold the DETSA was not modeled. The 
complex antenna shape is drawn using the polygon curve tool 
Measurements were taken from 3 to 9 GHz in 2 GHz 
increments. E-plane measurements were taken in alisrunent 
with the olane of the excitation feed (vz  lane in Fie. 2). rather 
Angle DETSA wraps 
around circle 
(degree) 
135 
90 
45 
22.5 
Radius 
(cm) 
flat 
4.46 
6.685 
13.37 
26.74 
2: schematic o f ~ ~ T S A  wrapped 90. amund srmchne, which is then extruded to add metal thickness. Initially a coax 
with an extended center conductor and a pin from the outer 
Radius (L, at 6 
GHz) 
flat 
0.89 
1.38 
2.68 
5.35 
conductor (ground) was used in conjunction with a waveguide 
port to excite the antenna. However this yielded asymmetric 
currents and farfields. Therefore, a discrete S-parameter port 
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with an impedance of 50 0 was used to excite the antenna. 
Automatic meshing of eight meshlines per wavelength was 
used. For the flat DETSA this yields 555,540 mesh cells. With 
dual 2.8 GHz processors the simulation took 5.25 hours to 
complete. For the 45 degree curved DETSA this yields almost 
five million mesh cells, taking 15.5 horn to complete. The 
gain of each antenna and the fafields in several planes were 
monitored. 
The measured return loss is shown in Fig. 3 as a function of 
the degree of curvature. It is seen that the 10 dB bandwidth is 
from 3 to 10 GHz, regardless of the degree of curvature. 
-4 0 
0 2 4 6 8 10 12 14 16 
Frequency (GHz) 
Figure 3: Measured return loss of DETSA as a function of frequency and 
dcflee of curvalure. 
The radiation pattern for the flat DETSA and DETSA's 
conformed in the longitudinal direction 22.5, 45, and 90' 
around cylindrical structures are shown in Figs. 4, 5, 6, and 7 
respectively. The simulated cross-polarization values are too 
low to be plotted with the measured data and are not shown on 
the figures for clarity. First, it should be noted there is 
excellent agreement between the measured and simulated 
radiation patterns. This agreement permits the use of only 
simulated data for a few cases (E co-polarization at angle of 
maximunl H co-polarization) where it is too difficult to 
configure the antennas for measurements. By comparing the 
flat DETSA H plane pattern with the 22.5 and 45' curved 
DETSA patterns, it is seen the number of nulls and lobes does 
not change as the antenna is conformed. Furthermore, the 
location of the nulls and lobes relative to the main heam 
remains approximately equal as the antenna is conformed. 
Therefore, the H plane pattern appears to he a simple rotation 
towards the direction of curvature. As the radius of the 
cylinder decreases, or the frequency increases, difiction 
effects increase, and the pattem below the antenna is affected. 
This is seen for the 90' curved antenna shown in Fig. 7a; note 
the larger sidelobes in the direction of 285'. The H plane 
radiation pattern of the 135' curved antenna, not shown, is 
severely affected by diffraction. The angle that the H plane 
pattern rotates, or the angle of the main beam, is presented in 
Fig. 8 for all of the antennas. It is seen that the main beam is 
skewed towa~ds the direction that the antenna is curved 
towa~ds, but that it is not in the direction that is tangent to the 
end of the antenna. As an approximation, the main beam is 
pointed in the direction of the tangent of the DETSA at one 
third of its length from the feed line. A secondary observation 
is that the main beam points more towards the diiection 
tangent to the DETSA at the end as the frequency increases. 
This agrees with the observation in [6]. 
77n 
(b) 
Figure 4: Measured and simulated (a) H plane and @) E plane radiation 
pattern for flat DETSA at 7 GHz. 
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- Hco meas. 
--- 
------ Her meas. 
77n 
(a) 
90 
Eco meas. 0=0 
------ Eco sim. 0=0 
- - - - - - - - - 
0 
37n 
(b) 
Figure 5: Measured and simulated (a) H plane and (b) E plane radiation 
pattern for 22.5' DETSA at 7 GHz. 
90 
--- 
HCO meas. 
Hco sim. _ 
77n 
(a) 
90 
Ewmeas.@=O 7 
------ Eco sim. 0=0 
--- --- --- Eco sim. @=@ma~ 
Ecr meas. 0=0 
77n 
@) 
Figure 6: Measured and simulated (a) H plane and @) E plane radiation 
panm for 45' DETSA at 7 GHz. 
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90 
Eco meas. 0=0 
------ Eco sim. 0=0 
- - - - - - - - - Eco sim. @=Omax --d. 
77n 
(b) 
Figure 7: Measured and simulated (a) H plane and @) E plane radiation 
pattern for 90' DETSA at 7 GHz. 
2 3 4 5 6 7 8 9 1 0  
Frequency (GHz) 
Figure 8: Messured and simulated angle of maximum ladialion in H plane as 
a function of degree of curvature (solid symbols are simulated, open symbols 
are measured). 
The antenna gain is shown in Fig. 9. The measured gain has 
been corrected for the coaxial to slotline transition return loss. 
but not for any attenuation, while the simulated gain assumes a 
lossless antenna with ideal mode excitation. It is seen that the 
gain increases with frequency for each antenna as expected for 
TSA's. Fig. 9 also shows that the gain decreases by only 1 dB 
for gently curved antennas compared to the flat DETSA, 
DETSA wrapped 45' or less around cylinder, but if the radius 
of curvature is greater, the gain decreases by an average of 2 
dB across the t?equency band. Interestingly, the gain for 22.5 
and 45' curvature is almost the same. 
Flat 
r 22.5 deg. 
7 
Frequency (GHz) 
Figure 9: Measured and simulated gain of DETSA as a function of degree of 
curvature (solid symbols an simulated, open symbols are measured) 
The beam shape obviously changes as the antennas are 
curved, and this is summarized in Fig. 10, where the 3 dB 
beamwidth is plotted. Again, the c w e d  DETSA behaves 
similar to the flat DETSA, with 3dB beamwidth decreasing 
with 'equency. The beamwidth decreases as the rate of 
antenna cwature increases, with the main beam in the H plane 
decreasing by approximately 25'. The E plane beamwidth is 
large at 3 GHz, but it is comparable to the H plane beamwidth 
at higher frequency. 
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Figure 1 la  shows the measured maximum cross polarization 0 
level at 0=0' and the simulated cross polarization level at the 
angle of maximum beam direction. The E plane cross -5 
polarization level is nearly constant with frequency, but it is ID 
seen that it increases by approximately 15 dB as the radius of 
curvature decreases. The highest cross polarization level is at 8 -10 
an angle approximately 30 degree off axis in the forward 
-15 direction. The H plane cross polarization level is shown in Fig. ,-- 
I lb. It is seen that it remains below 15 dB across the entire 
frequency band, and below 20 dB over most of the band. It -20 
was also found that the H plane cross polarization does not 
have a preferential direction. -25 
h 0 2 4 6 8 10 
a, 90 
2? 
m 80 Frequency (GHz) 
70 
(a) 
- -1 0 
Z 60 
Flat 
22.5 deg 
*.-..-r o 
---------' 
r 
7 
0 flat 
v 22.5 deg 
45deg 
0 90deg 
Frequency (GHz) -30 
(a) 2 4 6 8 10 
r 22.5 deg. 
45deg. 
+ 90deg. 
0 Flat 
Frequency (GHz) 
(b) 
Figure 10: Measured and simulated (a) H plane and @) E plane 3 dB 
beamwidth (salid symbol are simulated, open symbol are measured) 
Frequency (GHz) 
(b) 
Figure 11: Measured and simulated maximum (a) E plane and (b) H plane 
cmss polarization (open symbols are measured, salid symbols are simolstcd) 
(measured E plane at 6=0, simulated E plane at 9 or maximum 1.1 co- 
polarization) 
Curving or conforming a tapered slot antenna around a foam 
cylinder in the longitudinal direction has been shown to rotate 
the radiation pattern in the direction of curvature, and that the 
pattern is pointed in a direction that is tangent to the antenna at 
a point approximately one third of its length from the feed. 
The gain decreases by only 1 dB for large radii of curvature, 
but the E plane cross polarization level increases by 15 dB for 
the same large radius of curvature. It is noted that a DETSA 
without the rectangular pad at the feed point, used for 
soldering the coaxial launcher, was simulated, and the 
simulations showed that the conclusions in the paper are not 
effected by the pad. The pad only caused a small ripple on the 
pattern. 
[ I ]  R Q. Lee and R. N. S i m s ,  Tnperd Slot Anlennn in Advances in 
Micmshio and Rinted Antennas, Kai Fong Lee and Wei Chen Editors. 
John Wiley and Sons, Inc, 1997, pp. 443-514. 
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L. R. Lewis, M. Fassen, and J. Hunt, "A bmadband stripline array 
element," in 1974 IEEE AP-SInt. Qmp. Dig.., Atlanta, GA, June 1974, 
pp. 335-337. 
P. J. Gibson, 'She vivaldi aerial," in 9" Eumpm Microwove Conj 
Dig., Brighton, England, ScpL 1979, pp. 101-105. 
K. S. Yngvesson, D. H. Schaubert, T. L koneniowski, E. L Kollkg ,  
T. T~ungren,  and J. F. Jahansson. 'Zndfire tapered slot antennas on 
dielectric substrates," IEEE Trans. Antennas ond Propagation, Vol. 
AP-33, No. 12,pp. 1392-1400, Dec. 1985. 
R. Q. Lee and R. N. Simons. "Effect of curvature on tapered slot 
antennas." in 1996 IEEE AP-S Antennas and Propagolion Int. Symp. 
Di~.,21-26July 1996,pp. 188-191,vol.l. 
S. Nikolaau, L Marcaccioli, G. E. Ponchak, J. Papapalymerou, and M. 
M. Tentzeris, "Conformal double exponentially slot antennas (DETSA) 
for UWB communications system' front-snds," ZOO5 IEEE lnr. Conj 
on Ultm-Widebnnd (ICU 2005) Dis ,  Zurich, Switzmland, Sept. 5-8, 
2005. 
D. C. Thompson, 0. Tantot, H. Jallageas, G. E. Ponchak, M. M. 
Tentzen'r, and J. Papapolymernu, "Characterization of liquid crystal 
polymer (LCP) material and tlansmission lines on LCP subshates from 
30-1 10 GHz," IEEE Tmns. Microwove Theory ond Tech, Vol. 52 ,No. 
4. pp. 1343 - 1352, April 2004. 
J ,  R. Knoir, "Slot-Line Transitions," IEEE Trons. Microwove l7teoy 
n,rd Tech., Val. 22, No. 5, pp. 548-554, May 1974. 
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Characteristics of Double Exponentially Tapered Slot Antenna (DETSA) Conformed in the Longitudinal Direction Around a Cylindrical Structure

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