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New Tunable Coplanar Microwave Phase Shifter With Nematic Crystal Liquid
Fehim Sahbani 1,2
, N.Tentillier1, A.Gharsallah
2,A.Gharbi
2 ,C.Legrand
1
LEMCEL1 ,LPMM
2
E-mail: [email protected]
Abstract: The liquid crystals known for their
applications as viewers have interesting features
for a microwave circuit design’s so-called ”agile
frequency”. These circuits offer the advantage to
change their frequency response according to an
external command. Here we present a new phase shifter
agile frequency liquid crystal substrate. Indeed the nature
of viscous liquid crystal requires development
ad-hoc structures.The measurements are
characterised in [26-40]GHz band. This phase shifter
with agile frequency will be integrated into an
antenna network and will be used to control the
radiation pattern of the synthesised array
antenna.
Keywords:liquid crystals:LC , phase shifter,
antenna, microwave, frequency agility
1. Introduction
The devices are agile circuits whose frequency
response can be controlled through an external
command. A major application relates to the
miniaturisation of embedded systems, replacing
a set of circuits by another.In this sense we can
use a substrate product called ”agile” composed
of materials whose electromagnetic properties
can be changed through an electric or outside
magnetic field. Among these materials we cite the
ferrites [1] and ferroelectric ceramics [2] but also
the liquid crystal [3-6]. Indeed they are anisotropic
and therefore have different permittivity
depending on their orientation in relation to the
electromagnetic field. in this article we propose
to use LC in the design of phase shifter . As a
first step we will consider the phase nematic
liquid crystal.We present an new phase shifter
liquid crystal substrate a characterise it up to
40GHz. Finally we chose a possible application
of this phase shifter in an antenna structure in
order to change the orientation of the main lobe
of the radiation pattern.
2. Nematic liquid crystal
Here we present liquid crystal LC. Under the
applications we use liquid crystal nematic phase in
an ambient temperature. The nematic liquid crystals
are characterised by their center of gravity,the
molecules showing no order of position. However
molecules procure an orientation order incase of long
distance . their long-distance and long axis is
parallel to an average direction defined by the vector
director n (Figure 1).
x
y
z
Figure 1. Representation of molecules CL in phase
nematic
In this phase the liquid crystals are anisotropic
materials with complex permittivity presented in
the form of a tightening (Relationship 1). It then
defines the dielectric anisotropy by the
relationship2.
εε
εε
*/ /*
*
*
= ⊥
⊥
0 0
0 0
0 0 (1)
∆ε = ε’
// - ε’⊥ (2)
Measures in two different directions of the material are needed to determine each of its parameters. It
is possible to orient the liquid crystal by applying
an outside magnetic field of typical 0.4 Tesla.
therefore the molecules align their long axis in its
direction. The permittivity analysed by the
microwave field EHF measure is amended following this trend. Here we will choose two configurations in
measurement of permittivity ε∗r// and ε∗
r⊥ (Figure 2).
3. Characterisation of the microwave
dielectric nematic liquid crystal
To characterise the liquid crystal, we chose a new
method without calibration of the vector network
analyser developed in the laboratory [7]. This
method which is named ∆γ is based on two measures.
978-1-4244-3477-0/08/$25.00 ©2008 IEEE 78
BB
E HFConfiguration parallèle Configuration perpendiculaire
BBBB
E HFE HFConfiguration parallèle Configuration perpendiculaire
Figure 2. Configurations for measuring permittivity
ε∗r// and ε∗
r⊥. The LC is guided by an outside magnetic
field. .
The permittivity presented by the liquid crystal K15
is constant over the entire frequency range, with an
average direction perpendicular 2.52 and 2.95 to the
parallel direction.
4. Phase Shifter agile frequency
liquid crystal substrate
4.1.Order of liquid crystal within a microwave
sustrate
In the case of a line micro strip classic, the gap
reached between input and output is fixed at a given
frequency. This phase shift depends on effective permittivity and the line length of the following:
c
refffL ε
φ...360
= (3)
when we use a substrate of a liquid crystal structure
defined above, it will be possible to vary the effective
permittivity on the substrate using in addition to the
microwave signal, a low frequency voltage
command (Figure 3).
Figure 3. Influence of an electric field command on
the orientation of liquid crystal molecules
The electrodes by treating surface (planar ori-
entation). The permittivity seen by the microwave
signal is noted εreff(0). This permittivity is related mainly to the permittivity crystal liquid frequency
microwave signal εr⊥ . As a result of the electric field command, the molecules of liquid crystal will gradually move
perpendicular to the electrodes ( n // E ) to
saturation permittivity. The east view εreff(E) . The
saturation permittivity εreff(E) is related mainly to the
liquid crystal permittivity εr// This variation of permittivity will induce a change in the wavelength
guided, therefore a change in phase following the
following relationship:
c
reffErefffL −−
=∆)0()(
...360 εεφ (4)
4.2. Phase Shifter coplanar access
The main structure presented in Figure 3 is not
feasible because of the nature of viscous liquid
crystal. It is therefore necessary to develop a phase
shifter suited for this material. The phase shifter
adapted to this constraint is characterised by an access of coplanair lines engraved on a substrate
such as”RT / DUROD 5880” 381µm of depth and
relative permittivity near 2.16 .To achieve the party
agile, a ground (GND) copper form a cavity of 60
µm of arrivals in which the liquid crystal is inserted
by capillary (Fig. 4)
Figure 4. Structure of coplanar phase shifter.
HFSS simulations have confirmed in a mode of
spread of type micro-strip inside this cavity. As a
result, maintaining a characteristic impedance of 50 is obtained throughout the structure by changing the
width of the central ribbon taking into account the
characteristics of the dielectric liquid crystal made pre-
viously. That is why, in the coplanar access, it has a
width of 980µm and outside and inside of the cavity
its width is 180 µm. In this new structure, the
electromagnetic field is essentially confined to the
anisotropic material and is directly influenced by
changing the permittivity. So,from the point view of ”agility”, this structure should be effective. The
order is obtained by applying a voltage, adjustable
from 0 to 10 Volts between the tape and central layout.
A surfactant-based PVA (PolyVinylAlcool) was
deposited on the tape and in the cavity, in order to
give direction within the liquid crystal in the absence of electric field command. The maximum change
measured phase is 100° to 35 GHz for a voltage of 10
v in an electric field of 0.16v/µm.
The variation of this phase shifter is higher than
0.71°/GHz/cm (Fig. 5).
60 µm
3 cm
Plan de Masse
Cristal Liquide
79
0 5 10 15 20 25 30 35 40 45
-100
-80
-60
-40
-20
0
PH
AS
E S
21(°
)
Fréquency
Figure 5. Change phase maximum phase shifter
depending on the frequency orders for 10 Volts.
Moreover, the shifter in access change in phase is a
linear function of frequency. This phase shifter in
the range of insertion’s analysis has losses of
between -1 dB and -7 dB for a ROS average less than 2
(Figures 6 ,7).
0 5 10 15 20 25 30 35 40 45
-9
-8
-7
-6
-5
-4
-3
-2
-1
Mo
du
le S
21
(dB
)
Fréquency
Figure 6. Losses in reflection of the phase shifter
coplanair depending on the frequency.
0 5 10 15 20 25 30 35 40 45
-50
-40
-30
-20
-10
0
Mo
du
le S
11
(dB
)
Fréquency
Figure 7. Losses in transmission phase shifter coplanair
depending on the frequency.
4.3. Scanning Antenna The development of wireless telecommunications
(Wi-Fi technology, Bluetooth,) results in an increase
of electromagnetic pollution. One way to reduce it
is to issue only in the direction of the element with
which it communicates. Thus, a limited area is
affected by communication and power can be
reduced. In this sense we can use an antenna to scan.
Using the phase shifter associated with patch antennas, it is possible to achieve antenna scanning. In this case,
the network of antennas is devoid of mechanical
elements that leads to the lobe emission-reception.
The latter is amended by playing on the phase of the
microwave signal from each antenna. A prototype Ka-band is currently underway in the laboratory
and will soon be characterized.
5. Conclusion
This study focuses on the characterisation of a phase shifter ”agile frequency.” A dielectric prior
characterisation of liquid crystal in the frequency
band used is essential for an optimal size. Moreover,
the nature of viscous liquid crystal makes it necessary
to define an appropriate topology. Taking into account
these constraints, a new structure phase shifter”Frequency agile” liquid crystal substrate is
presented. Starting with the dielectric liquid crystal,
it was designed, produced and characterised until
40GHz. This phase shifter offers interesting
performances (0.71°/GHz/cm to 35 GHz) for a low
voltage command (about 10Volts to get 80% of agility). The integration of this phase shifter is planned and
will lead to the realisation of a scanning antenna
whose lobe radiation will be electrically controlled.
References
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