A Miniaturized Cavity backed Archemedian Spiral
Antenna for mm-Wave Applications
M. Saad Khan
Research Institute for Microwave
and Millimeter-Wave Studies
(RIMMS),
National University of Sciences
and Technology (NUST),
Islamabad, Pakistan
13mseemkhan@seecs.edu.pk
Farooq A. Tahir
Research Institute for Microwave
and Millimeter-Wave Studies
(RIMMS),
National University of Sciences
and Technology (NUST),
Islamabad, Pakistan
farooq.tahir@seecs.edu.pk
Ultra Wide
Bi
?
and
M. Umar Khan
Research Institute for Microwave
and Millimeter-Wave Studies
(RIMMS),
National University of Sciences
and Technology (NUST),
Islamabad, Pakistan
umar.khan@seecs.edu.pk
Hammad M. Cheema
Research Institute for Microwave
and Millimeter-Wave Studies
(RIMMS),
National University of Sciences
and Technology (NUST),
Islamabad, Pakistan
hammad.cheema@seecs.edu.pk
Atif Shamim,
King Abdullah University of Science and
Technology
(KAUST), Thuwal, Saudi Arabia,
atif.shamim@kaust.edu.sa
Abstract— Design of a UWB circularly polarized, cavitybacked Archimedean spiral antenna is presented in this paper.
The antenna is designed for 18- 40 GHz frequency range with a
binomially tapered parallel-strip to microstrip balun and RF foam
absorbent for controlling the backscattering. The peak-measured
gain is 4.5 dBi and is greater than 2 dBi for the entire band. The
antenna is characterized by greater than 50 degrees 3 dB gain and
axial ratio beam-widths which makes it highly suitable for
airborne applications. Axial ratio is below 2 dB for the band of
interest. The height of the antenna cavity is 19 mm and the
diameter is 23 mm giving the antenna a low profile meeting the
space requirements of any RF system.
installed on any required platform. However, Reflections from
the cavity destructively interfere with the spiral antenna;
therefore, an absorber is needed to be filled in cavity to absorb
the back radiation of the antenna [1-3]. Furthermore, bringing
any material inside the near field of antenna distorts the radiation
pattern. Hence, a cushion material with air like properties is also
used in the near-field of the antenna. In this paper, a small sized,
compact, low profile and wideband ASA is presented with high
gain and excellent AR beam-width.
Keywords— Spiral; 5G; Millimeter-wave; Circular-polarization;
Cavity; RF-absorbers; Binomial-balun; Beam- width
I. INTRODUCTION
The frequency spectrum below 10 GHz is highly congested
due to the presence of a plethora of applications. This has
necessitated the exploration and utilization of higher frequency
bands for new applications. Besides, the current requirement of
higher data rates also demand broadband systems that can
support gigabits of wireless transmission. As an integral
component of such a system, antennas, which are broadband,
circularly polarized, simple, miniaturized, lightweight and low
cost, are highly desirable. Self-complimentary Archimedean
spiral antenna (ASA) is an important candidate when all these
features are desired.
Theoretical, mathematical & geometrical explanation and
electromagnetic working principles of ASAs are given in [1-3].
Input impedance of a balanced spiral antenna is 188 Ω, to
unbalance the input feed and transform this impedance to 50 Ω
for a practical antenna, a parallel strip to microstrip balun and
impedance transformer is presented in [4]. Other balun
configurations are presented in [5] and [6]. Archimedean spiral
antennas require a cavity, which is needed to house the radiator,
balun, RF absorber and connector. Furthermore, this cavity is
Fig. 1 Models of ASA: (Left) Standalone (Right) Antenna with balun
II. ANTENNA DESIGN
The systematic design of cavity backed ASA is presented
in this section. The design of the antenna is done in multiple
stages leading to the final optimized design of the antenna
system. In the beginning, a spiral antenna is designed
separately; following this, a multi-section binomially tapered
balun is designed for impedance transformation and feeding of
the spiral antenna. In the next step, the antenna and balun are
co-designed. Furthermore, the cavity for the antenna is modeled
and absorbing materials to be used are identified and realized
in the simulation model. Finally, the complete assembly of the
antenna system is again simulated and optimized for the desired
results.
Authorized licensed use limited to: CALIF STATE UNIV NORTHRIDGE. Downloaded on September 25,2022 at 21:21:47 UTC from IEEE Xplore. Restrictions apply.
The simulation model of ASA is generated using ANSYS
designer kit and is placed over Rogers RT/Duriod 5880
substrate with 5 mils thickness as shown in figure 1. No. of
turns and inner-radius of the antenna is adjusted according to
the space availability. Subsequently, a parallel-strip to
microstrip balun and impedance transformer is designed for
188Ω to 50Ω impedance centered at 30 GHz. The useful
bandwidth of a six-section binomial transformer is 100% [7],
which in this case, covers a frequency band from 15 GHz to 45
GHz. A balun is a transformer used to convert the signals from
an unbalanced circuit structure to a balanced configuration. A
modified planar back-to-back tapered microstrip to parallel
strip balun is presented; the simulation model is shown in
Figure 2. The simulation of back-to-back balun is used to check
the transmission and reflection properties of the balun itself,
which is to be integrated with the antenna. ASA integrated with
this balun is shown in Figure 1.
RT-Duriod 5880
Cu
Fig. 3: Geometry & Dim. of Top and Side Views of the Cavity
cavity
Fig. 2: Top and bottom views of simulation model of parallelstrip to microstrip balun
At one end of the balun is the parallel-strip, which is
connected to the antenna, where the impedance of the parallel
strip equal to the antenna’s input impedance. The width of the
ground plane is gradually reduced to eventually resemble a
parallel strip. The taper accomplishes the mode and impedance
transformation [4]. At the other end, which is the microstripend a lumped or waveguide port of 50 Ohm is placed. The
length of each section of impedance transformation balun is
equal to quarter-wavelength of the guided wave calculated by
the effective dielectric constant, as referred in (1). Where; ‘λeff’
is the effective wavelength of the medium and εr is the dielectric
constant of the balun substrate.
λ
λeff = °
√ε𝑟
(1)
An Aluminum cavity is designed for housing the antenna
and balun. The design and dimensions of the top and side views
of the antenna cavity are given in Figure 3. Specialized RF foam
Absorber, ECCOSORB LS-28 [8], is selected to fill the cavity
to absorb back radiations of the ASA and also reflections from
the metallic cavity to destructively interfere with antenna
radiations. By filling the cavity with absorber foam, the
effective dielectric constant of the cavity is changed. To
compute the effective dielectric constant of the boundary
condition of the balun substrate surrounded by ECCOSORB
LS-28, equation (2) is used, as discussed in [9].
𝜀𝑒𝑓𝑓 = √
𝑓𝜀1 +(1−𝑓)𝜀2
𝑓 1−𝑓
+
𝜀1 𝜀2
(2)
Where; ε1 is the permittivity of Rogers RT/Duroid 5880,
which is the balun substrate and ε2, is the frequency-dependent
dielectric constant of ECCOSORB LS-28. The width and
lengths of the sections of the balun are calculated for the
effective permittivity value at 30 GHz. ‘f” is the volume
fraction occupied by the balun inside the RF foam material.
Hence, calculated binomial impedances and microstrip track
widths of six sections of the balun are used in the design.
To prevent the near field of the radiator to be disturbed by
the absorber, an air like material C-STOCK RH-10 [10] is
sandwiched between the antenna and absorber. Finally, Radom
is also placed over ASA at a height of 15mm, which is multiple
Authorized licensed use limited to: CALIF STATE UNIV NORTHRIDGE. Downloaded on September 25,2022 at 21:21:47 UTC from IEEE Xplore. Restrictions apply.
of λ/4guided wavelength at 30 GHz. ABS-material, as given in
[11]. The complete simulation and fabrication models of ASA
are shown in Figure 4.
III. FABRICATION, TESTING & RESULTS COMPARISON
Antenna simulation is done using CST Studio Suite [12].
Fabrication of antenna is done in the same order as the
simulation design. Firstly, the spiral antenna is fabricated using
an LPKF machine on single-sided 0.127mm thick Roger’s 5880
substrate using the Gerber file exported from the simulation
design. Similarly, the balun is fabricated on double-sided
0.787mm Roger’s 5880 substrate. Subsequently, the Aluminum
cavity is modeled on CNC machine using the SAT file.
Moreover, a commercial RF absorber material and antenna
cushion material with ‘air-like’ properties are machined
accordingly in order to fit in the antenna and balun inside the
cavity. The top ends of the balun are soldered to the center feed
of the spiral antenna. At the other end, a 2.4mm RF connector
is attached to the cavity and the inner conductor of the
connector is soldered to the bottom-end of the balun. The final
fabricated and simulated antenna models are shown in Figure
4.
Fig. 5: Simulated and Measured VSWR
Fig. 6: Simulated and Measured Bore-sight Gain vs. Frequency
Fig. 4 Models of ASA (a) Simulation Model (b) Fabricated Antenna
Comparison of simulated and measured results for VSWR
vs. frequency, boresight gain vs. frequency, axial ratio vs.
frequency, 3dB axial ratio (AR) & Gain beam-widths (BW) and
Gain Radiation pattern at 28 GHz, are presented. Antenna’s
VSWR measured and simulated results are below ‘2’ in the
entire bandwidth from 18 to 42 GHz, as shown in Figure 5. The
gain of the antenna in boresight direction (Theta 0̊ & Phi 0̊) is
above 2 dB in the entire band from 18 to 42 GHz. Peak
simulated gain is 6.63 dB at 21 GHz; whereas peak measured
gain is 4.6 at 27 GHz. Measured and simulated results of boresight gain are shown in Figure 6. The depreciated measured
gain could be because of unaccounted losses in the RF absorber.
The axial-ratio of the antenna is below 2.5 dB from 18 to 42
GHz, which also meets less than 3dB axial ratio requirement as
specified. Axial Ratio results are shown in Figure 7.
Fig. 7: Simulated and Measured Axial Ratio vs. Frequency at Bore-sight
Radiation patterns of measured and simulated gain vs.
theta, at Phi=0̊ and frequency=28 GHz, are compared in
Figure 8. At theta 0, the simulated gain is 6.63 dBi and the
measured gain is 4.6 dBi. The front-to-back ratio is 30 dB in
simulation and 50 dB in measured results at 28 GHz.
Authorized licensed use limited to: CALIF STATE UNIV NORTHRIDGE. Downloaded on September 25,2022 at 21:21:47 UTC from IEEE Xplore. Restrictions apply.
Fig. 8: Simulated and Measured 2D Radiation pattern Phi=0̊ (28 GHz)
Fig. 10: Simulated and Measured 3dB Gain & Axial Ratio BW
Similarly, the radiation pattern of measured and
simulated gain vs. theta, at Phi=90̊ and frequency=27 GHz is
compared in Figure 9.
IV. CONCLUSION
Design of a simple, ultra-wideband, small size and circularly
polarized cavity-backed Archimedean spiral antenna is
presented. Furthermore, design & fabrication issues and
comparison of measured and simulation results are presented.
Antenna has ultra-wide gain and AR HPBW which makes the
scan angle of the antenna fairly large which is required for
airborne applications. Size reduction of ASA antennas at low
frequencies is yet another design challenge to which this work
can be further extended.
REFERENCES
[1]
Fig. 9: Simulated and Measured 2D Radiation pattern Phi=90̊ (28 GHz)
At theta 0, the simulated gain is 6.44 dBi and the
measured gain is 3.69 dBi. The front-to-back ratio is 30 dB in
simulation and 60 dB in measured results at 27 GHz. By
comparing both cross and co-poles, it can be seen that the axial
ratio is 0.9 dB for the measured gains in the bore-sight
direction.
Comparisons of measured and simulated half-power
beam-widths (HPBW) of gain & axial ratio against frequency
are shown in Figure 10. Measured AR & Gains HPBWs are
more than 50̊ for the entire frequency band making the antenna
suitable for wide-area coverage. Measured gain HPBW of the
antenna ranges from 57 to 84 degrees and measured axial ratio
HPBW ranges from 68 to 180 degrees. Hence, the comparison
of simulated and measured results of the ASA is completed.
Constantine A. Balanis, "Frequency Independent Antennas: Spirals and
Log Periodics," in Modern Antenna Handbook, 1, Wiley Telecom
[2] P.E. Mayes, “Frequency-independent Antennas and Broadband
Derivatives Thereof,” Proceedings of the IEEE, Vol. 80, 1982: 103–112
[3] Steven Shichang Gao; Qi Luo; Fuguo Zhu, "Case Studies," in Circularly
Polarized Antennas , 1, Wiley-IEEE Press, 2014, pp.328
[4] K. Vinayagamoorthy, J. Coetzee, D. Jayalath, “Microstrip to Parallel Strip
Balun as Spiral Antenna Feed”, Vehicular Technology Conference (VTC
Spring), IEEE 2012
[5] Chen, T.K. and G.H. Huff. Stripline-fed Archimedean spiral antenna,
IEEE Trans. Antennas Propagation., 10:346–349, 2011
[6] Mao, S.G., J.C. Yeh, and S.L. Chen. Ultra-wideband circularly polarized
spiral antenna using integrated balun with application to time-domain
target detection, IEEE Trans. Antennas Propagation., 1914–1920, 2009
[7] Pozar, David M. Microwave Engineering. Hoboken, NJ: Wiley, 2012.
[8] Emerson & Cuming Microwave Products, Inc. ECCOSORB® LS,
http://www.eccosorb.com/products-eccosorb-ls.htm
[9] B. Sareni, L. Krahenbuhl, and A. Beroual. “Effective dielectric constant
of random composite materials,” in Journal of Applied Physics, 81.5
(1997): 2375-2383.
[10] Cuming Microwave Corporation, 264 Bodwell Street, Avon, MA 02322,
C-STOCK RH, https://www.cumingmicrowave.com
[11] RFbeam Microwave GmbH. Schuppisstrasse 7 CH-9016 St. Gallen
Switzerland, RADOME (Radar Enclosure), Application Note AN-03,
https://www.rfbeam.ch/files/products/18/downloads/AN-03-Radome.pdf
[12] CST – Computer Simulation Technology AG, Bad Nauheimer Str. 19,
64289
Darmstadt,
Germany.
https://www.3ds.com/productsservices/simulia/products/cst-studio-suite/
Authorized licensed use limited to: CALIF STATE UNIV NORTHRIDGE. Downloaded on September 25,2022 at 21:21:47 UTC from IEEE Xplore. Restrictions apply.