4 1. FERRITES AND FERROELECTRICS
3. Lens Antennas with Beam Steering
Lens antennas are usually implemented as a stack of electroplated ferroelectric slabs [20], used
to apply the DC-control voltages across each slab. An appropriate tuning of these DC voltages
yields the desired directive beam pointing-steering.
4. Parallel Plate Tunable Capacitors or Varactors
e tunable dielectric constant "
r
of ferroelectrics yields the desired variation in capacitance.
Even though their losses are lower than those of semiconductor varactors, the high bias voltage
limits their applicabilities.
1.3.2 THIN FILM FERROELECTRICS
1. Varactors
Varactors based on ferroelectric films are made either as planar structures (planar capacitors) or
as triple-layered “sandwich” structures [2]. For triple-layered structures, a DC voltage of 1–20
volts is sufficient for effective tuning.
2. Microwave Ferroelectric Phase Shifters
e advancement in phased and adaptive antenna arrays stimulated an impressive research effort
in electronically controllable planar phase shifters and especially in integrated circuit form. e
purpose of a phased array is to steer its beam toward an arbitrary direction, while adaptive arrays
steer the nulls of their radiation patterns toward the direction of interfering sources. e emerg-
ing technology of multiple access like SDMA (Space Division Multiple Access) and OFDMA
(Orthogonal Frequency Division Multiple Access) in communications and the advanced multi-
functional radar require highly integrated modular RF front ends (above the intermediate, or IF,
stage). Within this topology, each antenna element will be fed by a module of transmit-receive
(Tx-Rx), which includes microwave phase shifters [19–21]. e phase shifters are used to con-
trol the phase of the signal (channel), exciting each element of the array, which, in turn, steers
the beam and, in general, modifies the radiation pattern.
e implementation of phase shifters using ferroelectric films enables their integration
with the microwave circuits on the same substrate, preferably as monolithic microwave inte-
grated circuits (MMICs). is primarily serves miniaturization since the antenna dimensions
are reduced. In addition, their mass and cost decrease substantially. Moreover, these devices are
accompanied by the advantages of ferroelectric films, e.g., control with low DC voltage, low
DC-power consumption and increased response speed.
Ferroelectric phase shifters are realized either as analog devices controlled by a DC voltage
(which can be digitally controlled through a digital to analog converter (DAC)) or as switched
digital phase shifters providing a certain fixed phase shift (45
ı
, 90
ı
, 135
ı
, and 180
ı
). e latter
are the corresponding analog of latching ferrite phase shifters [22].
1.3. FERROELECTRICS 5
3. Microwave Tunable Filters
A variety of microwave applications in communications, signal intelligence, jamming, and elec-
tronic counter measure systems (ECM) requires band-pass filters with variable center frequency
and variable bandwidth [23, 24]. Usually, in these applications the frequency of the expected
signal (to be received) is somewhere within a very wide band (even a few octaves). e octave
range of a device is characterized by the expression flog
2
(Higher/Lower Frequency)g. Accord-
ing to the matched filter theory, the signal to noise ratio is optimized when the filter frequency
response H(f) has the same amplitude and opposite phase of the signal spectrum S(f), namely
H(f)=S*(f ). Otherwise, the noise level at the output of the filter (receiver) increases, causing a
serious reduction in receiver sensitivity. As an example, recall that a typical wideband receiver-
sensor has a sensitivity of the order of 60 dBm at best. is is not the case of a super-heterodyne
frequency selective receiver-sensor, which has a typical sensitivity of 110 dBm. Wideband
sensors are usually simple, of low cost and can cover a huge wideband. A typical example is
the IFM (Instantaneous Frequency Measurement) receiver used in ECM (Electronic Coun-
termesures) systems. In the classical approach, wideband sensors are cascaded with a bank of
narrowband filters connected in parallel (RF pre-selected filters). ey have successive center
frequencies so that these parallel channels cover the whole band. An alternative approach is the
use of a phase locked loop (PLL), but this is costly and complicated. e use of adaptive filters,
able to tune their center frequency to that of the signal and to control their bandwidth toward
approaching the matched filter, is found to offer an attractive solution. A vast research effort
has been devoted to this approach, employing different tunable components. Such components
are varactor diodes [25, 26], microwave transistors and, in particular, ferrite loaded microstrip
structures [27, 28], ferrite films [28], superconducting films [29], ferroelectric films [30], and
Ferrite/Ferroelectric multilayer film structures.
Focusing on the ferroelectric film implementation of tunable filters, we can see that their
main advantage is the excellent integration in monolithic microwave chips (miniaturization).
However, their high losses still constitute a major problem. For example, in a typical three-pole
filter the insertion loss is of the order of 10 dB [28].
4. Microwave Tunable Amplifiers and Oscillators
e ferrite tuned and, in particular, the YIG resonator tuned oscillators are very well established
in the microwave range. VCOs (Voltage Controlled Oscillators) based on other techniques (e.g.,
varactor tuned) offer a tunability of about an octave and any attempt to increase it results in worse
spectral purity. In contrast, the YIG possesses a wide-band tuning with good spectral purity [31].
Two disadvantages of YIG resonator tuned oscillators stem from the required strong biasing
DC-magnetic field. e first disadvantage is the slow variation whereas an almost instant change
is required. e second one comes from the fact that it is very difficult to provide such a strong
DC-magnetic field in a monolithic microwave circuit (MMIC) configuration. Moreover, the
tuning speed of YIG resonators is severely limited by magnetic hysteresis while modern systems
6 1. FERRITES AND FERROELECTRICS
require a tuning speed of more than 1 GHz/sec [32]. Ferroelectric tuned oscillators could be
used to overcome the above difficulties, but the most promising configuration is that of multilayer
ferrite-ferroelectric films. e latter are expected to exploit the well-established tunability and
spectral purity properties of the YIG (now in a film form) in conjunction with the convenient
and fast changing DC voltage tuning of ferroelectrics in MMIC configurations.
5. Multi–Octave Tunable Amplifiers
Wideband microwave amplifiers are usually realized by employing a balanced topology. When
a bandwidth of up to 40% (maximum 50%), referring to their center frequency, is desired, a
balanced topology with two hybrid couplers in quadrature or with branch line couplers is used,
e.g., [22]. When Lange couplers [33] instead of branch line couplers are used, a bandwidth of
the order of more than 2.5 octaves can be easy achieved. In both cases, the center frequency of
operation could, in principle, be tuned by using a ferroelectric substrate. e change of its per-
mittivity will be realized with the aid of a DC-voltage bias. Some attempts toward this direction
are described in [33–35].
A distributed amplifier constitutes a classical wideband topology with a bandwidth of
more than a decade, presenting a good input and output matching [22]. e amplifier cannot
provide a very high gain, or a low noise figure and it is larger in size than an amplifier of narrower
bandwidth and comparable gain.
6. Tunable Microwave Antennas
e research effort toward phased and adaptive arrays and their applications have already been
discussed in the section on tunable phase shifters. Within this approach, the maximum and/or
nulls of the radiation pattern are steered with the aid of a tunable phase shifter inserted at the
input port of each antenna element, e.g., [21]. An alternative approach is to use the tunable
material as a substrate supporting the radiator or just as a tunable inclusion (e.g., post) inserted
into a dielectric substrate. e variation in either permeability (ferrite) or permittivity (ferro-
electric) or both (e.g., ferrite/ferroelectric layers) yields either resonance frequency agility or a
beam steering of the antenna, but it may also serve both purposes. e main advantage of this
approach is that it eliminates the requirement of a complex beamforming network (BFN) and
is accompanied by substantial cost reduction. Futhermore, it may serve a variety of purposes
since these features are provided even by a single element. In particular, when traveling wave
antennas or the specialized form of “Leaky Wave Antennas” are employed, very directive radi-
ation systems with exceptional characteristics are realized. Moreover, the introduction of EBG
(Electromagnetic Band Gap) techniques in the structure of tunable antennas constitutes a state-
of-the art with highly ambitious expectations, e.g., super directive antennas with ultra low side
lobes, miniaturization, and integration in microwave circuits.
Research on tunable and/or beam steering based on ferrites started some decades ago.
However, the main drawbacks of ferrites, namely the difficulties in integration, in high drive
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