APUSNCURSINRSM.2017.8072317

Super-Directive, Efficient, Electrically Small, LowProfile Antenna based on Compact Metamaterial
Nebil KRISTOU, Jean-François PINTOS
Kouroch MAHDJOUBI
CEA-LETI, MINATEC Campus, 38054 Grenoble, France
Nebil.KRISTOU@cea.fr
IETR, Université de Rennes 1, 35042 Rennes, France
kouroch.mahdjoubi@univ-rennes1.fr
Abstract— Artificial Magnetic Conductors (AMC) present an
attractive substrate for low profile antennas which can take
advantage of intrinsic zero reflection phase response to boost
antenna performance without the need for thick quarter wave
backplane. In this paper, a modified version of the Egyptian Axe
Near-Field-Resonant-Parasitic (NFRP) dipole antenna is placed
over compact AMC based on square spiral metallic patches and a
capacitor network beneath the ground plane. Simulation results
demonstrate that the NFRP antenna is electrically small
(ka1=0.498) with a low-profile (total height = 0.07Ȝ0) when placed
over compact AMC (ka2=1.29). It achieves super-directivity (6.52
dBi), high realized gain (5.7 dBi) and high radiation efficiency
(83.7%).
II.
A. Antenna and AMC design
The selected antenna is a modified version of the Egyptian
Axe Dipole (EAD) antenna as proposed in [2], it consists on a
driven element printed on the bottom-side of the substrate and a
near field resonant parasitic (NFRP) element printed on the topside. The resonant behavior is dependent on the capacitive load;
the dipole’s self-inductance and capacitance; and the resonant
lengths of the dipoles arms [2]. The NFRP element takes the
same shape as the driven element for impedance matching
purposes and their dimensions are co-designed to achieve the
desired reactive and resistance matching. Finally, two lumped
capacitors of 0.1 pF are introduced on both antenna sides. These
capacitors are used to adjust the impedance value once the
antenna is placed over the metasurface. The proposed modified
NFRP antenna shown in Fig. 1 is based on the Rogers Duroid
3003 (İr = 3, tan δ=0.0013) substrate with a thickness of 0.75
mm. The dimensions of the NFRP element are given in Fig. 1
and summarized in Table I.
Keywords—Artificial Magnetic Conductor (AMC); metasurface;
Electrically Small Antenna (ESA); Super-directive antenna; high
gain antenna; low-profile antenna.
I.
INTRODUCTION
Electrically small antennas (ESA), because of their compact
sizes, continue to be of great research interest for potential
wireless applications. There have been many efforts to find the
best tradeoff between size and performances for ESAs,
Successful miniaturized antenna designs, with high directivities,
have been reported recently based on some type of metastructure [1]. Artificial Magnetic Conductors (AMC) present an
attractive substrate for low profile antennas which can take
advantage of intrinsic zero reflection phase response to boost
antenna performance without the need for thick quarter wave
backplane. However, for an effective AMC structure, several
unit cells are required. As a result, the whole antenna system
usually has a relatively large electrical size.
Instead of the typical square or rectangular shape, a circular
AMC structure is designed to minimize the volume (radius) of
the smallest sphere enclosing the antenna structure. The selected
unit cell, as detailed in [3], is based on square spiral metallic
patch and a capacitor network beneath the ground plane.
In this paper, a modified version of the Near Field Resonant
Parasitic (NFRP) antenna [2] is chosen to be positioned over a
compact AMC based on high miniaturized unit cell proposed in
another work [3]. The NFRP antenna is selected because of its
smal electrical size (ka < 0.5), high efficiency (ηrad > 90%),
operational bandwidth (1.8%), and dipole radiation
characteristics. The aim of this paper is to propose a superdirective, efficient, electrically small, low-profile NFRP antenna
based on compact AMC metasurface. We note that, the term
“electrically small” will mean ka < 0.5, where a is the radius of
the smallest sphere enclosing the entire antenna system and
k=2ʌ/Ȝ, Ȝ being the free-space wavelength. Note that all of the
numerical simulations reported in this paper were performed
using ANSYS Electronic Desktop (HFSS).
‹,(((
NFRP ANTENNA OVER COMPACT AMC
(a)
(b)
Fig. 1. Top view of the proposed system: (a) Egyptian axe NFRP dipole
antenna with key parameters. (b) NFRP antenna over compact AMC
TABLE I.
DIMENSIONS (MILLIMETERS) OF THE NFRP ANTENNA DESIGN
NFRP outer radius: a
24.8
Driven outer radius: e
21.8
NFRP inner radius: b
21.8
Driven inner radius: f
20.8
NFRP gap length: c
10
Driven gap length: g
37
NFRP strip width: d
1.5
Driven strip width: h
2
$36
Unit cell dimensions are 11.8×11.8 mm2. The same substrate
(RO 3003) is used for top and bottom layers of the cell. The air
gap layer is used to ensure the minimum thickness needed with
a lightweight structure. The overall thickness is about 7.75 mm
(Ȝ0/40). All capacitances at the bottom layer are fixed to C=0.2
pF with serial ohmic resistance Rres=1 Ohm. The AMC structure
is optimized to be an in-phase reflector, i.e., to act as an artificial
magnetic conductor, near 960 MHz, the frequency of interest.
antenna over the compact AMC and confirm the superdirectivity behavior.
0
-5
|S11| [dB]
-10
-15
-20
-25
NFRP antenna over AMC
B. Results and discussion
The simulated return loss of the NFRP antenna alone is
shown in Fig. 2(a) (black curve). It resonates at the frequency
fr= 990 MHz, where the minimum |S11| = -19 dB and the -10 dB
bandwidth is 18 MHz (1.8%). Radiation efficiency is 96%, peak
directivity and peak realized gain are respectively 2.16 dBi and
1.91 dBi. At f0=960 MHz, the NFRP antenna alone is not
matched to 50 Ÿ with a minimum |S11| = -2 dB. Radiation
efficiency is 90% while peak directivity and peak realized gain
are respectively 2.17 dBi and -2.62 dBi.
-30
0.95
1
Frequency [GHz]
1.05
(a)
0
30
330
10
60
5
[dBi]
0
300
10 5
-5
0
90
-5 -10
270
-10
Directivity
Realized Gain
-15
The NFRP antenna, with a ka1 = 0.498, is placed 15 mm
(Ȝ0/20) above the AMC structure as shown is Fig. 1(b). The
AMC structure has a finite diameter equal to 128 mm (Ȝ0/1.6).
This gives ka2 = 1.29 at the resonance frequency f0 = 960 MHz.
Simulated return loss of the NFRP antenna over the AMC
surface is shown in Fig. 2(a) (red curve). The resonance
frequency is shifted from 990 MHz to 960 MHz because of the
presence of the AMC surface in the extreme near field of the
NFRP antenna. The return loss of the entire system reach -32 dB
with 9 MHz (1%) of -10 dB bandwidth at the operating
frequency f0. Radiation results are shown in Fig. 2(b) and Fig.
2(c). At frequency of interest f0, high peak directivity (6.52 dBi)
and peak realized gain (5.7 dBi) more than double the peak
realized gain of the NFRP alone (1.91 dBi without AMC) are
achieved, while the radiation efficiency and total efficiency are
83.7% and 83.5%, respectively. Fig. 2(c) shows the simulated
radiation pattern of the entire system compared to that of the
NFRP antenna alone.
-20
0.9
0.95
1
Frequency [GHz]
120
240
1.05
NFRP antenna over AMC
150
210
NFRP antenna
180
(b)
(c)
Fig. 2. Simulation results: (a) Comparison of S|11| of the NFRP antenna aver
AMC and NFRP antenna alone (b) Peak directivity and realized gain of the
proposed system. (c) Radiation patterns at 960 MHz for phi=0°.
Directivity [dBi]
15
10
5
0
Harrington Limit
NFRP antenna over AMC
-5
-10
In Fig 3. the peak directivity of the designed electrically
small NFRP antenna over the proposed compact AMC surface
at f0 = 960 MHz is compared to Harrington limit of directivity
[4]. Considering Fig 3. It is observable that the proposed system
achieves a very high directivity (6.52 dBi) which exceeds the
Harrington limit. Then, the proposed system can be considered
as a super-directive antenna.
III.
NFRP antenna
-35
0.9
0
0.5
1
1.5
ka
2
2.5
3
Fig. 3. Peak directivity of the proposed NFRP antenna over the AMC
compared to the normal directivity Harrington limit [4].
REFERENCES
[1]
CONCLUSION AND FUTURE WORK
[2]
In this work, a super-directive (6.52 dBi), efficient (ηrad=
83.7%), electrically small (ka1=0.498), low-profile (total height
= 0.07 Ȝ0) NFRP antenna based on Compact AMC (ka2=1.29) is
designed and simulated. Nevertheless, a deterioration in the
frequency bandwidth of the NFRP antenna is noted which drops
from 1.8 % without AMC to 1 % with the AMC. The next step
will be the prototyping and characterization of the NRFP
[3]
[4]
P. Jin and R. W. Ziolkowski, "High-Directivity, Electrically Small, LowProfile Near-Field Resonant Parasitic Antennas," in IEEE Antennas and
Wireless Propagation Letters, vol. 11, no. , pp. 305-309, 2012.
P. Jin and R. W. Ziolkowski, "Linearly and circularly polarized, planar,
electrically small, metamaterial-engineered dipole antennas," 2010 IEEE
Antennas and Propagation Society International Symposium, Toronto,
ON, 2010, pp. 1-4.
N. Kristou, J-F. Pintos, K.Mahdjoubi , "Miniaturized Tunable Artificial
Magnetic Conductor for Low LTE Band,” 2017 IEEE International
Symposium on Antennas and Propagation (APSURSI), San Diego,
California, USA, 2017.
R. F. Harrington, "Effect of antenna size on gain bandwidth and efficiency
" J. Res. Nat .Bur., vol 64-D, pp 1-12, Jan/Feb. 1960.