PRINTED TUNABLE MINIATURIZED METAMATERIALS WITH BST/POLYMER COMPOSITE VARACTORS
Mahdi Haghzadeh, Craig Armiento and Alkim Akyurtlu
Center for Photonics, Electromagnetics, and Nanoelectronics (CPEN)
Raytheon UMass Lowell Research Institute (RURI)
ECE Department at UMass Lowell
MATERIALS & METHODS
MOTIVATION
A FSS structure comprised of square loops and wire grids on opposite sides of 2
mil Kapton film is modeled in CST Microwave Studio. The dimensions are
optimized for a resonance frequency at 10 GHz for FSS with IDCs (see Fig. 2). To
realize the tunable element, a composite is prepared by mixing micro and nano
size particles of sintered Ba0.55Sr0.45TiO3 with a thermoplastic polymer, which has a
very low loss tangent at high frequencies. It is found that when the key parameters
– particle size and volume fraction – are tailored, dielectric tunability up to 30% can
be obtained for the composite, while the dielectric constant can be as low as 30 for
high RF frequencies. To avoid the low permittivity limitation, we introduced the filled
IDC configuration that requires a ferroelectric media with a dielectric constant
below 40 for a full frequency sweep at X-band, instead of the conventional IDCs
with fingers on top of a BST ceramic layer that would require epsilon of few
hundreds (see Fig. 3). In order to enable DC biasing of the BST material, a
diagonal biasing network is proposed and modeled in CST. A 10x10 unit cell FSS
with biasing network is printed on Kapton by Optomec printer (see Fig. 4)
3
10
Filled
IDC
Permittivity
This work investigates the realization of printed tunable Frequency Selective
Surface (FSS) structures, a subset of metamaterials, at X-band utilizing a novel
version of Interdigitated Capacitors (IDCs), in which the space between fingers
are filled with a ferroelectric Barium Strontium Titanate (BST)/polymer
composite. The model of FSS on 2 mil Kapton with IDCs is simulated using CST
Microwave Studio and the dimensions are optimized for operation at 10 GHz. It
is shown that the performance of FSS with the BST/polymer composite filled
IDCs is similar to that of FSS with conventional IDCs. A biasing network to apply
DC bias to BST material in IDCs is proposed. Finally, a sample of FSS with IDCs
and biasing network is printed.
RESULTS
2
IDC
On
BST
10
1
10
Filled IDC
IDC
0
10
-30
-20
-10
0
Vol t age ( v )
10
20
30
14
Tunabi li t y
12
f [GHz]
ABSTRACT
10
8
6
4
-40
Filled IDC
IDC
-20
0
20
40
Vol t age ( v )
One key application of Frequency Selective Surfaces (FSS) is in radomes, where
they are placed on curved surfaces, thus requiring that FSS are angle
independent, i.e. have the same resonance frequency for all incident angles. An
example of a miniaturized FSS at X-band is shown in Fig.1 (a). The full-wave
electromagnetic model and the measured transmission results are shown in Fig. 1
(b) and (c), respectively. Tunability is another highly desirable feature for FSS and
other metamaterials. Currently, tunability is implemented by solid state varactors
based on ferroelectric ceramics.
Figure 3. Performance comparison of FSS with BST material filled IDC vs FSS with conventional IDC
on BST (a) permittivity vs applied field, (b) Tunability vs applied field.
However these varactors have low Q factor and are expensive to fabricate; and
use ceramics that are brittle in nature and require very high processing (a)
temperature (>850°C).
(a)
On the other hand, tunable ferroelectric polymers such as Polyvinylidene Fluoride
(PVDF) are lossy and slow in switching at RF frequencies. However, a BSTpolymer composite promises both excellent ferroelectric characteristics of the BST
in its paraelectric phase and the flexible processing of a polymer.
(b)
(b)
(a)
(a)
(b)
V2
(d)
(b)
V1
(b)
(e)
Figure 4. Printing of metamaterials (a) the CAD file, (b) the double side printed sample, (b) close up view of
squared loops with IDCs on top and the wire grid at the back, (d) printing of wire grids in process by Optomec
printer, € close up of IDCs printed for tunability.
CONCLUSIONS
(d)
Figure 1. Miniaturized FSS at X-band (a) a fabricated sample from literature, (b) the full-wave
electromagnetic model, (c) The CST model of FSS-IDCs with biasing network, (d) the measured
transmission results.
(c)
Figure 2. Implementation of miniaturized FSS with BST/polymer composite filled IDCs (a) the unit cell of
FSS, (b) the values of parameters that define the unit cell, (c) periodic structure of FSS with BST
material filled IDCs, (d) simulated S11 curves for FSS without IDCs, with IDCs, and with BST/polymer
composite (ε=20) filled IDCs.
In this study, we design and analyze metamaterial inspired (miniaturized)
Frequency Selective Surfaces on Kapton films. We introduce the BST/Polymer
composite filled IDC as a reliable alternative for the tunable element in FSS
structures. The tunability of the varactors are calculated and analyzed. This
tunable ferroelectric composite is compatible with low temperature fabrication
processes on flexible films. Although, we demonstrate the implementation of this
tunable filled IDC on FSS structures, this varactor can be utilized in several other
applications.