1. A UWB Cavity-Backed Compound Power-Archimedean Slot Spiral
for Body Centric Wireless Communications Applications
Jayson Maldonado-Vargas¹, Rafael A. Rodríguez-Solís¹, Mohamed A. Elmansouri², Dejan S. Filipovic²
1 Department of Electrical and Computer Engineering, University of Puerto Rico at Mayaguez; jayson.maldonado@upr.edu , rafael.rodriguez19@upr.edu
2 Department of Electrical, Computer and Energy Engineering , University of Colorado at Boulder; mohamed.elmansouri@colorado.edu, Dejan.Filipovic@colorado.edu
Spiral antennas are attractive for variety of applications including
ultra-wideband (UWB) Body Centric Wireless Communications
(BCWC) since they are relatively small, lightweight, and can be made
in planar and conformal forms [1]. However one disadvantage
presented in this antenna is that they are inherently dispersive and
can cause severe distortion for the short pulses transmitted/received
by UWB systems. To overcome the dispersive issue, the power spiral
antenna topology developed to have low-dispersion characteristics is
used. In order to overcome the axial ratio deterioration of power
spiral at high frequencies, a compound spiral [2]-[3] combining an
Archimedean spiral at the center and a power spiral taper at the
outside is utilized.
In this work, we present the design and the performance of a cavity-
backed, compound, two-arm spiral slot antenna with resistive
termination designed to satisfy the specifications and requirements
for UWB body-centric communications. The antenna is fed with a
coaxial bundle and 180° hybrid coupler to avoid the use of long
vertical balun and allow for easier implementation of shallow cavity.
The electrical performance is investigated in free space and on a
simple human body model using finite element method implemented
in ANSYS HFSS.
Antenna Design Results with Proposed Human Body Model
Methodology
Antenna Structure
Three Tissue Layer Human Body Model
Antenna Design Results at Free Space Condition
Introduction
Conclusions
• The design and the performance of a two-arm cavity-
backed compound Power-Archimedean slot spiral antenna
were presented. The antenna behavior is investigated in
free space and in the presence of a simple three-layer
human tissues model. It is demonstrated that the cavity-
backed spiral is insensitive to the presence of the human
body as expected.
• The proposed antenna complies with the UWB Off-BAN
requirements with VSWR < 2, axial ratio < 3 dB and
unidirectional pattern having Gain > 5 dBic within 3.1-10.6
GHz for both cases.
References
[1] H. Lee, M. M. Tentzeris, and J. Geiger, “Flexible spiral antenna with
microstrip tapered infinite balun for wearable applications,” Proc. 2012
IEEE Int. Symp. Antennas Propag., pp. 1–2, Jul. 2012.
[2] Z. Yu-xiao, Z. Shun-shi, and X. Sai-qing, “Miniaturized compound sprial,” vol.
50, no. 11, pp. 952–954, 2008.
[3] M.A. Elmansouri, “Joint Time/Frequency Analysis and Design of Spiral
Antennas and Arrays for Ultra-Wideband Applications”, Ph.D. dissertation,
University of Colorado, Boulder, Colorado, 2013.
[4] P. S. Hall and Y. Hao, “Body-Centric UWB Communications” in Antennas and
Propagation for Body-Centric Wireless Communications, 2nd ed., Norwood,
MA: Artech House, 2012, pp. 139-157.
Objectives
The antenna must operate a frequency band of 3.1-10.6 GHz
and satisfy the following requirements at both free space and
near the presence of human body:
 VSWR < 2 for 𝑍 𝑜 = 50𝛺.
 Broadside gain of at least 4 dBi and symmetric radiation
pattern with respect to theta.
 Axial ratio less than 3 dB.
 Small Size
Resistors
Termination
CavityParameters Archimedean spiral Power spiral
Rout 18.5 mm 27.5 mm
Rin 3.15 mm 1 mm
Growth rate (a) 0.8143 9.954
Offset Angle or SMR
(δ)
35° 90°
Number of Turns (N) 3 3
Spiral coefficient (n) Set to 1 3
Dielectric constant 2.2
Dielectric thickness 0.79 mm or 31 mils
Dielectric Diameter 64mm or 2.52 in
Number of arms 2
Cavity height 10 mm
Cavity Thickness 2 mm
Coaxial Diameter 3.5 mm
• Fabricated using Rogers RT/Duroid 5880 substrate
and LPKF ProtoMat H100 Milling Machine. Coaxial
Bundle Feed
Summary of chosen parameters for the compound slot spiral
(a) (b)
Figure 1. Antenna Geometry in (a) HFFS and (b) prototype of the compound slot
spiral with a total of twenty 0603 resistors termination and cylindrical cavity.
Two 50Ω SMP adapters are connected together by their outer conductor at the
center of the spiral acting as the balun feed for the antenna.
• Forearm dimensions:
 Width = 100 mm
 Length = 100 mm
• Separation between human body model
and antenna of 0.5 mm.
• Contains frequency dependent
electromagnetics parameters for each
tissue taken from [4].
• Dimension of each Tissue:
 Skin = 2 mm
 Fat = 5 mm
 Muscle = 10 mm
(a) (b)
(c) (d)
Figure 5. Comparison between simulated free space (red curve) and three-layer human tissue model (violet) for the
compound slot spiral in HFSS. The antenna parameter results are: (a) VSWR, (b) axial ratio in dB, (c) total broadside
gain in dB at θ=180° and radiation pattern with respect to total gain at (d) 3GHz, (e) 7 GHz and (f) 10.5 GHz.
(e) (f)
Figure 2. Studied three layer human tissue model that consists of skin
(creme color), fat (orange color) and muscle (red color) with 2, 5 and 11
mm of depth respectively.
Figure 4. Comparison between simulated in HFSS (red curve) and measured (violet curve) S parameter results in dB
of compound slot spiral at free space without the 180° hybrid coupler. (Top Left) Port 1 return loss or S11, (Bottom
Left) Port1 Insertion or S21, (Top Right) Por2 insertion loss or S12 and (Bottom Right) port 2 return loss or S22.
Figure 3 Comparison between simulated in HFSS (red curve) and
measured (violet curve) VSWR results of compound slot spiral with
non-ideal (Top) and ideal (Bottom) 180° hybrid coupler.