Tải bản đầy đủ

Nghiên cứu anten bowtie

CHAPTER - 2
BASIC FRAME WORK FOR ANTENNA DESIGN
INTRODUCTION
Antenna is a primary and necessary component of all wireless communication
systems. It enables the transition of energy between a guiding device, such as coaxial line or a
waveguide to the free-space. It transforms the electric energy to electromagnetic energy and
vice versa. By the IEEE Standard Definitions, antenna is defined as “a means for radiating or
receiving radio waves." In a transceiver system, the antenna is the final block in the
transmission region and is the first block in the receiving region. So, the fundamental
understanding of antenna parameter and working are prerequisite to develop an antenna
solution for smart systems. So, in this chapter basics of microstrip patch antenna are
elaborated to define the common antenna parameter and terminology. In order to complement
the next chapters the design description for UWB technology, UWB antenna and
reconfigurable antenna are also detailed. As PIN diode is the back-bone of this research work
which is used as switching element, therefore a detailed description, working principle and
characterization of PIN diode is also described in this chapter.

2.1. MICROSTRIP PATCH ANTENNA
Microstrip antenna is the best choice for modern wireless and mobile applications due
to many considerable advantages like simple, lightweight, simple and economical and
compatible with MMIC designs etc. [1-5]. The basic shape of microstrip antenna can be

rectangle, square, ellipse, circle, triangle, ring, pentagon, or their complex variations to meet
particular design demands [1-6].

Fig. 2.1 Basic Rectangular Patch antenna (b) Patch antenna showing E- field distribution

22


Chapter 2
A basic rectangular patch is considered here to understand the basics of antenna which
consists of ground plane, dielectric substrate and radiating patch as shown in Fig. 2.1(a). This
rectangular microstrip patch antenna (L,W) is designed on a substrate with relative dielectric
constant =

and substrate with height = h. The CAD formulae [1] for the calculation antenna

dimension (L,W) at resonating frequency f0 are listed below:
Effective dielectric constant  re   r  1   r  1 1  12

2

2



1

h2
W 

Patch width: W =
Patch length: L =

(1)
(2)

.

(3)


Extended length (ΔL) of patch due to fringing field
ΔL =

(4)

Effective patch length: Leff = L + 2ΔL

(5)

If the input impedance of antenna is 50Ω at a particular frequency then antenna will
efficiently matched with the input impedance (50 Ω) of input port. For efficient antenna
design, impedance distribution should be known so that antenna can be easily matched to 50
Ω impedance. To study the impedance distribution over a patch it is necessary to study the
electric and current distribution. The feed probe couples electromagnetic energy in and or out
of the patch as shown in Fig. 2.1(b). The electric field is zero at the center of the patch,
maximum on one edge and reverses its direction on opposite edge. This field distribution
continuously reverses its direction according to the instantaneous phase of the RF signal. Fig.
2.2 shows the current, voltage and impedance behavior in the radiating patch; the current
(magnetic field) is maximum at the center of patch and minimum on the opposite sides of
patch, while the voltage (electrical field) is zero in the center and maximum on one edge and
reverses its direction (minimum) on opposite edge.

Fig. 2.2 Voltage, Current and Impedance distribution along patch resonant length

23


Chapter 2
Hence the distribution of impedance is minimum at the center and maximum on both edges
of patch. So there is a point lie inside the surface of radiating patch where the impedance is
50Ω. The simplest method for impedance matching is to locate the position of 50 Ω point on
the antenna surface and connect the input RF port at this point.
The input impedance of rectangular microstrip patch antenna is calculated by
Transmission line model. The equivalent transmission line model of a microstrip fed
rectangular patch [1-2] is shown in Fig. 2.3 which consists of a parallel-plate transmission
line connected with two radiating slots (apertures), each of width W and height h, separated
by a transmission line of length L. Each radiating slot of microstrip patch antenna is
represented as a parallel equivalent admittance Y=G + jB.

Fig. 2.3 Transmission line model for rectangular patch antenna as radiating slot [1-2]

Since both slots are identical, the total resonant input impedance [1-2] becomes Zin=1/2 G.
The conductance (G) of single radiating slot-1 is associated with the power radiated and is
given by eq. (6)
(6)
Where W = patch width and λ= resonant wavelength. B is susceptance due to energy stored
in the fringing field near the edge of the patch and given by eq. (7)
(7)
If G12 is the mutual conductance between two slots, Jo is Bessel function of first kind than
(8)
So, the total input impedance is given by eq. (9)
(9)

24


Chapter 2
So using formula given in eq. (9) input impedance for microstrip patch antenna can be
accurately calculated. This is more reliable method to calculate the input impedance of a
rectangular patch antenna.
After calculating the input impedance, it should be matched to 50Ω because the facts
that almost all the microwave sources and lines are manufactured with 50Ω characteristic
impedance [6-8]. After calculating the input impedance various impedance matching
techniques can be applied. These impedance matching techniques can be categorized in two
broad categories i.e. distributed method and lumped element method. In distributed method
[9-12], impedance matching can be done by doing some structural modifications through the
use of stubs [9-10], quarter wave transformer [11], tapered line [12], balun and active
components as shown by Fig. 2.4. The main advantage of distributed method is that there is
no requirement to modify the geometry of radiating structure. Therefore, radiation
performance of the radiating structure is independent to the matching network and results in
easy design optimization. However, this method increases the size of antenna and not
recommended for the design of practical array systems. Also system efficiency degrades due
to the increase in spurious radiation losses from extra circuitry of matching network.

Fig. 2.4 Matching techniques (a) Distributed impedance (b) Lumped element

In the second method, a lumped network [13-14] is introduced to realize impedance matching
between antenna and feed structure. This method can be implemented either by inserting a
separate network without changing the antenna structure or by etching slots or notch in the
antenna geometry as indicated in Fig. 2.4(b). The most important advantage of placing the
impedance matching network between antennas and feeding structure is the enhancement in
the impedance bandwidth. This method allows incorporating last minute design change by
allowing freedom in choosing the values of discrete components, independently.

25


Chapter 2
2.1.1. Antenna Feeding Techniques
Microstrip patch antennas can be excited by a number of methods [1-5]. These
methods can be categorized into two types: Contacting and Non-contacting. In the contacting
method, the RF power is fed directly to the antenna using a connecting part such as a
Microstrip line/Coaxial Cable. In the non-contacting method, electromagnetic field coupling
is provided to transfer the RF power between the microstrip line and the patch such as
aperture coupling and proximity coupling.
The four most popular feeding techniques for microstrip patch antenna are: coaxial feeding,
Microstrip feeding, proximity feeding and aperture feeding. Each is explained below briefly:
(a) Coaxial feeding: It is one of the basic techniques used in feeding microwave power to
the antenna. The coaxial cable is connected to the antenna such that it’s outer conduct or is
attached to the ground plane while the inner conductor is soldered to the metal patch. Coaxial
feeding is simple to design, easy to fabricate, easy to match and have low spurious radiation.
However, coaxial feeding has the disadvantages of requiring high soldering precision. There
is difficulty in using coaxial feeding with an array since a large number of solder joints will
be needed. Coaxial feeding usually gives narrow bandwidth and when a thick substrate is
used a longer probe will be needed which increases the surface power and feed inductance.
(b) Microstrip feeding: In Microstrip feed, the patch is excited by a microstrip line that is
located on the same plane as the patch. In this technique, impedance matching is required
between patch and 50 Ω feed line. The main disadvantage of this technique is that antenna
suffers from narrow bandwidth and the introduction of coupling between the feeding line and
the patch which leads to spurious radiation.
(c) Proximity coupled feeding: In this type, feeding is conducted through electromagnetic
coupling that takes place between the patch and the Microstrip line. The patch antenna is
located on the top of the upper substrate and the Microstrip feeding line is located on the top
of the lower substrate. The two substrates can be chosen different than each other to enhance
antenna performance. The proximity coupled feeding reduces spurious radiation and increase
bandwidth. However it needs precise alignment between the two layers in multilayer
fabrication.
(d) Aperture coupled feeding: It is a non contacting feed; the feeding is done through
electromagnetic coupling among antenna and the microstrip line through the slot etched in the
ground plane. It consists of two substrate layers with common ground plane in between the
two substrates, the Microstrip patch antenna is on the top of the upper substrate and the
26


Chapter 2
Microstrip feeding line on the bottom of the lower substrate and there is a slot cut in the
ground plane. The slot can be of any size or shape and is used to enhance the antenna
parameters. The two substrates can be chosen different than each other to enhance antenna
performance. The aperture feeding reduces spurious radiation. It also increases the antenna
bandwidth, improves polarization purity and reduces cross-polarization.
2.1.2 Basic Antenna Parameters
An antenna is a device that converts a guided electromagnetic wave on a transmission
line to a plane wave propagating in free space. Thus, one side of an antenna appears as an
electrical circuit element, while the other side provides an interface with a propagating plane
wave. Antennas are inherently bi-directional which can be used for transmitting and receiving
as well. The power, gain and the directivity define the ability of the antenna to concentrate
energy in a particular direction. Some important antenna parameters [15-16] concerning the
radiation performance of antenna are described here.
1. Resonant Frequency (fr): The antennas are tuned to work at one particular frequency and
are operative only over a range of frequencies centred on this frequency, called the resonant
frequency. So, when driven at its resonant frequency, large standing waves of voltage and
current are excited in the antenna elements. These large currents and voltages radiate the
intense EMW, so the power radiated by the antenna is maximum at the resonant frequency.
2. Reflection coefficient (RL): is a measure of effectiveness of power delivered to antenna.
If the power incident on the antenna is Pin and the reflected power from the antenna to the
source is Pref. The degree of mismatch between the reflected and incident power is given by
Reflection coefficient =
3. Bandwidth (BW): It is defined as the range of operating frequencies within which the
performance of the antenna conforms to a specified standard. BW is the difference of either
side of frequencies in accordance to the center frequency where the antenna characteristics
such as radiation pattern, polarization, gain, are close to those values which have been found
at the center frequency. The BW of a UWB antenna can be demarcated as the relation of the
upper to lower frequencies of acceptable operation. The BW of a narrowband antenna is the
percentage of the frequency difference over the center frequency. So, it can write in terms of
equations as under:
BWWB=

BWNB =

x100

(10)
27


Chapter 2
If FH/ FL= 2 then antenna is assumed to be UWB. The method of trying how capably an
antenna is operating over the required range of frequencies is to calculate its VSWR. A
VSWR≤ 2 ensures good performance.
4. Voltage Standing Waves Ratio (VSWR):- The antenna will operate efficiently when the
maximum transfer of power must take place between the transmitter and the antenna. The
Maximum power transfer can only take place when the impedance of the antenna is matched
to that of the transmitter. The VSWR can be expressed as
SWR=

(11)

The VSWR expresses the degree of match between the transmission line and the antenna.
When the VSWR is 1 to 1 (1:1) the match is perfect and all the energy is transferred to the
antenna prior to be radiated.
5. Antenna Efficiency (η): The radiation efficiency of an antenna is defined as the ratio of
the power radiated by the antenna to the power at its input terminals. It is a measure of how
efficiently an antenna radiates its input power as RF energy. When given in terms of a
percentage, an antenna efficiency of 0% means all power absorbed by the antenna at its input
is effectively lost within the device and no useful radiation occurs. An efficiency of 100%
refers to a perfectly radiating antenna wherein all power absorbed at the input is radiated.
6. Gain (G): The gain of an antenna is a measure of the ability to focus power into a narrow
angular region of space. If an antenna is transmitting with a positive gain is used as a
receiving antenna, it will also have the same positive gain for receiving. The energy
propagated in the direction compared to the energy that would be propagated if the antenna
were Omni-directional are said to be gain of antenna. It is related to directivity and efficiency
by Gain (G) = directivity (D) * efficiency (η)
7. Directivity (D): The ratio of the radiation intensity (U) in a given direction from the
antenna to the radiation intensity of an isotropic antenna (U0) is known as the directivity D of
an antenna [1]. In mathematical form, it can be written as
D=

(12)

8. Radiation Pattern: The Radiation Pattern of an antenna is a 3-dimensional graphical
representation of the relative strengths of the fields emitted by the antenna. It can also be
thought as the locus of points around the antenna which have the same electric field. The
pattern consists of a main lobe and several minor lobes. These minor and side lobes are
28


Chapter 2
always unwanted because they represent wasted energy for transmitting antennas and
potential noise sources for receiving antennas. The radiation pattern is determined in the farfield region and is represented as the power radiated or received by an antenna in a function
of the angular position and radial distance from the antenna. The two or three dimensional
pattern of spatial distribution of radiated energy can be constructed using multiple twodimensional patterns. For a linearly polarized antenna, performance is often described in
terms of its principal E- and H-plane patterns. The E-plane is defined as “the plane containing
the electric field vector and the direction of maximum radiation,” and the H-plane as “the
plane containing the magnetic-field vector and the direction of maximum radiation”.
9. Polarization: It is the property of an electromagnetic wave describing the time-varying
direction and relative magnitude of the electric-field vector. According to the electric field
vector behavior polarization may be classified as linear, circular, or elliptical.
10. Input impedance (Zin): It is the impedance presented by an antenna at its terminals and
can be written as: Zin = Rin + jXin where Zin is the antenna impedance at the terminals, Rin is
the antenna resistance which consisting of radiation resistance Rr and the loss resistance RL.
The imaginary part Xin is the antenna reactance and represents the power stored in the near
field. The power associated with the radiation resistance is the power actually radiated by the
antenna, while the power dissipated in the loss resistance in the form of heat is due to
dielectric or conducting losses.
2.2 UWB TECHNOLOGY
Since, FCC declared a bandwidth of 7.5GHz (from 3.1GHz to 10.6GHz) designated as UWB
spectrum platform i.e. wireless communications for public uses [17-19], the UWB technology
is rapidly advancing as a short range high-speed high data rate wireless communication
technology. UWB is defined as any wireless plan that occupies either a fractional bandwidth
greater than 20% or more than 500 MHz of absolute bandwidth. This technology has been
engaged into our daily lives with minimal interference. This technology is an unlicensed
service that can be used anywhere, anytime, by anyone. UWB communications transmit
signal without interfering with other traditional narrow bands operating in the same frequency
band. Fig. 2.5 displays the behavior between Emitted signal powers versus frequency in GHz.
UWB signal is noise-like signal with low energy density, hence its detection is quite difficult.
Additionally, the “noise-like” UWB signal has a particular shape compared to real noise
signal (no shape). So, it is almost unfeasible for real noise signal to destroy the UWB pulse

29


Chapter 2

Fig. 2.5 Comparison of various communication standards [18]

because interference would have to spread uniformly across the entire spectrum to obscure
the pulse. UWB pulse behaves as a wideband noise source for other NB systems operating in
that frequency range; but it doesn’t affect them because of its low signal power. It only
increases the SNR requirement of those systems. By using PN (Pseudo Random) codes UWB
system can be made undetectable for hostile receivers and can be protected from jamming.
Hence, UWB is possibly the most safe and secure means of signal transmission. The unique
characteristics of UWB technology present a more powerful solution to wireless broadband
than other technologies [17-19]. The UWB devices operate by employing a series of very
short electrical pulses that result in very wideband transmission bandwidths. In addition,
UWB signals can run at high speed and low power levels. It also enables various types of
modulation scheme to be employed, including on–off keying, pulse-amplitude-modulation,
pulse-position-modulation, phase-shift-keying, as well as different receiver types such as the
energy detector, rake, and transmitted reference receivers. Another strong candidate for UWB
is multicarrier modulation by using orthogonal frequency division multiplexing (OFDM).
The unique characteristics of Ultra Wide band technology are listed below:
1. Capacity: Since UWB has an ultra wide frequency bandwidth, so a huge capacity as high
as hundreds of Mbps or even several Gbps can be obtained.
2. Low power transmission: UWB systems operate at extremely low power transmission
levels. By dividing the power of the signal across a huge frequency spectrum, the effect upon
any frequency is below the acceptable noise floor. For example, 1 watt of power spread
across 1GHz of spectrum results in only 1nW of power into each hertz band of frequency.
Thus, UWB signals do not cause significant interference to other wireless systems.

30


Chapter 2
3. Fading Robustness: It is channel fading resistant, due to the large number of resolvable
multipath components. Wide band nature of the signal helps it in avoiding the problem of
time varying amplitude fluctuations. It is also immune to Multipath Delays where various
version of same signal appear at the receiver which have undergone a variety of diffraction,
reflection, scattering effects as time delay introduced is generally more than the signal
duration.
4. Short Range: Its normal range of operation is within 10m, so its power requirement is low
and interference with other short range devices is less. It comes under WPAN protocol.
5. Security Aspects: UWB provides high level security and reliable communication.
6. Low Cost: UWB system has low cost and low complexity because it does not modulate
and demodulate a complex carrier waveform, so it does not require components such as
mixers, filters, amplifiers and local oscillators.
7. Large Bandwidth: The FCC allocated an absolute bandwidth more than 500 MHz up to
7.5 GHz which is about 20% up to 110% fractional bandwidth of the center frequency. This
large bandwidth spectrum is available for high data rate communications as well as radar and
safety applications.
8. Very Short Duration Pulses: Ultra-wideband pulses are typically of nanoseconds or
picoseconds order. Transmitting the pulses directly to the antennas results in the pulses being
filtered due to the properties of the antennas. Due to using UWB systems those very short
duration pulses, they are often characterized as multipath immune or multipath resistant.
9. Resolution: High resolution localization, due to the very short pulse duration.
10. Multiple accesses: UWB technology provides multiple access capabilities, due to the
wide bandwidth of transmission.
11. Target Detection: UWB antenna is used as target detection in RADAR.
All these unique features of UWB technology make it suitable for many different
applications such as geo positioning, radar and sensor applications e.g. vehicular, marine,
GPR, imaging, wall-imaging, sense-through-the-wall (STTW), surveillance systems etc.
2.2.1 UWB Antenna Design Challenges
UWB antennas exhibit very large bandwidth compared to general antennas [20-26]. There
are two criteria available, for identifying when an antenna may be considered as UWB. A
definition given by DARPA says that a UWB antenna has a fractional bandwidth greater than
0.25. Whereas, the United States Federal Communications Commission (FCC), places this

31


Chapter 2
bandwidth limit to 0.2. Additionally, the FCC provides an alternate definition whereby an
UWB antenna any antenna may have a bandwidth greater than 500 MHz.
There are several known antenna topologies that are said to achieve broadband
characteristics, such as the horn antenna, biconical antenna, helix antenna and bowtie
antenna. All these antennas have been proven to have excellent broadband characteristics, but
they are large, non-planar and physically obtrusive, therefore ruling them out as a possibility
for use with small UWB integrated electronics. Another antenna design approach is to use
frequency independent antenna which uses Babinet’s Equivalence Principle of duality and
complementarity for meeting the requirements of very wide impedance bandwidth. The
Archimedian spiral antenna, logarithmic spiral antenna, fractal antenna are used for UWB
operation because they possess small size, light weight and thin shape for portable devices.
The design of a UWB antenna is very difficult, because the fractional bandwidth is
actually big, and antenna must cover multiple octave bandwidths in order to transmit pulses
that are of the order of a nanosecond in duration. Since data may be contained in the shape of
the UWB pulse, antenna pulse distortion must be kept to a minimum value. A non-dispersive
characteristic in time and frequency domain, provides narrow pulse duration to enhance a
high data throughput. Antennas in the frequency domain are typically characterized by
radiation pattern, directivity, impedance matching, and bandwidth.
The following are important challenges in designing UWB antennas.
1. UWB antenna must possess ultra wide frequency bandwidth.
2. The performance of a UWB antenna is required to be consistent over the entire operational
band. Ideally, antenna radiation patterns, gains and impedance matching should be stable
across the entire band. Sometimes, it is also demanded that the UWB antenna provides the
band-rejected characteristic to coexist with other narrowband devices and services occupying
the same operational band.
3. UWB antenna must possess directional or omni-directional radiation properties depending
on the practical application. Omni-directional patterns are normally desirable in mobile and
hand-held systems. For radar systems and other directional systems where high gain is
desired, directional radiation characteristics are preferred.
4. UWB antenna needs to be small enough to be compatible to the UWB unit especially in
mobile and portable devices. It is also highly desirable that the antenna's feature should be
low profile and compatible for integration with PCB.

32


Chapter 2
5. UWB antenna should be optimal for the performance of overall system. For example, the
antenna should be designed such that the overall device (antenna and RF front end) complies
with the mandatory power emission mask given by the FCC or other regulatory bodies.
6. UWB antenna is required to achieve good time domain characteristics. For the narrow
band case, it is approximated that an antenna has same performance over the entire
bandwidth and the basic parameters, such as gain and return loss, have little variation across
the operational band. In contrast, UWB systems often employ extremely short pulses for data
transmission.
The UWB antenna design is the major dimension in the progress of UWB technology.
The main challenge in UWB antenna design is achieving the wide impedance bandwidth
while still maintaining high radiation efficiency. UWB antenna should be designed focussing
on various parameters such as frequency of operation, substrate height, dielectric constant to
be used. To cater to all these requirements the microstrip antenna has been gaining
popularity. Owing to its narrow bandwidth, many solutions have been introduced, which
offer the impedance bandwidth across the entire UWB range. Some of these solutions are
described in next sections.
2.2.2 UWB Antenna Design Techniques
There are many methods for broadening the impedance bandwidth of antenna.
Different techniques are applied for good impedance matching over the UWB range which
includes different combination of specially designed patch or feed line with partial or
optimized ground plane. The descriptions of some techniques are given below:
(a) Combination of different patch geometry with partial ground:
UWB operation can be achieved by using either partial ground or CPW feed in different
shapes of patch structures. By using different shapes of the patch, accommodate multimode
surface current waves, which in turn lead to resonating at multiband frequencies and finally
widen the impedance bandwidth [20-29]. The geometries shown in Fig.2.6 includes step
rectangular patch in which steps are slotted from original patch structure, circular, octagon
monopole, U-shaped monopole, knight’s helm shape monopole and two steps circular
monopole are used to modify the impedance bandwidth.

33


Chapter 2

Fig. 2.6 Broad Banding methods by varying patch geometry

(b) Combination of different feeding structure with slotted ground
The shape of feed line is optimized for broad banding the UWB behaviour. Addition of
different shape stub and wide slot under the feed line also enhance the bandwidth. Fig. 2.7
shows the different shaped stub that can be used for broad banding antenna like T-shaped stub,
three offset stub, fan shaped stub, rectangular [29-31]. Fig. 2.7(b) shows the different shape
slot under feed line like tapered, circular hexagon, semicircular, elliptical to enhance the
impedance bandwidth.

Fig. 2.7 Broad banding method by (a) varying Feed and stub (b) various shapes of wide-slots

(c) Modified partial ground structure: The structure of partial ground plane can be
optimized to make a broad band antenna [32-37]. This method improves the gain and

34


Chapter 2
bandwidth, reduces the reflection coefficient. Different ground structure are used for board
banding like notched, saw tooth, rounded, trapezoid form as shown in Fig. 2.8.

Fig. 2.8 Broad banding methods by modifying ground structure [38-43]

2.3. RECONFIGURABLE ANTENNA
In modern times, Wireless devices are not limited to one standard and can operate at
multiple frequencies. Multi-mode terminals have received great attention and have increased
in popularity because by this single terminals or devices could have many applications such
as, GPS, GSM, WLAN, Bluetooth, etc. Reconfigurable antennae have their ability to modify
fundamental characteristics, including operating frequency, impedance bandwidth, radiation
pattern, and polarization or even a combination of these features in real time [38-42].
Reconfigurable antenna have the potential to add substantial degrees of freedom and
functionality to mobile communication and phase array systems by allowing us for spectrum
reallocation in multi-band communication systems, dynamic spectrum management, therefore
reducing the number and size of antenna in a system. Compared with conventional antennas,
reconfigurable antennas have more advantages for example, saving energy; reducing the
number of antennas thus reducing the mutual interferences between them.
Different types of reconfigurable antenna help to overcome the problem like spectrum
congestion, interference between different users etc. Reconfigurable antenna support new
desired capabilities [43-49] to cope with extendable multiservice and multi standard,
multiband operation, as well as with efficient spectrum and power utilization. Some
important consequences of reconfigurable antennas are:
1. Allows spectrum reallocation and dynamic spectrum management,

35


Chapter 2
2. Meets flexible multi-radio wireless platform requirements i.e. multiple services in a single
device.
3. Reduced number of antennas in the system resulting reduced overall device size and cost.
4. Provides good isolation between different wireless standards and bands.
5. Reconfigurability in antenna radiation patterns achieves spatial diversity for interference
cancellation.
6. Frequency reconfigurable antennas are useful to support many wireless applications,
where they can reduce the size of the front end circuitry.
7. Polarization reconfigurable antenna are valuable to solves the various problems like
signal fading due to multipath propagation, sensitivity of signals to transceiver antenna
orientation, limited channel capacity; security etc.
8. Radiation pattern reconfigurable antennas are useful to improve the coverage area and
system performance by redirecting the main beam.
9. Low front end processing circuits as there is no need for front end filtering, good out-ofband rejection.
10. Enable cost-effective SDR, MIMO and Cognitive Radio implementations.
11. Compact reconfigurable antenna allows efficient radio implementations in reduced formfactor mobile platforms.
2.3.1 Classification
The reconfigurable antenna can be classified into four different categories viz.
frequency reconfigurable antenna, radiation pattern reconfigurable antenna, polarization
reconfigurable antenna, and combination of any two.
(a) Frequency Reconfigurable antenna: A radiating structure that is able to change its
operating frequency by hoping between different frequency bands is called frequency
reconfigurable antenna [38-42]. This is achieved by producing some tuning in the antenna
length. Frequency reconfigurable antennas have attracted significant attention due to their
ability to cover multiple frequency bands so significantly reduce the number of antenna
required for multi-mode communication.
(b) Radiation pattern Reconfigurable antenna: A radiating structure which is able to tune
its radiation pattern and maintain its operating frequencies is called radiation pattern
reconfigurable antenna [43-49]. Manipulation of an antenna’s radiation pattern can be used to
avoidance of noise source, improved security by directing signals only toward intended users,
avoidance of signal fades, improved beam steering capability of phased array systems, and
36


Chapter 2
increased diversity gain. So there is a great demand for pattern reconfigurable antennas in the
fields of wireless communications, satellite communications, radars, etc. The antenna is
designed to be able to reconfigure its radiation pattern during real time operation such that it
maintains its broad pattern in the absence of interferences, and is capable of narrowing its
pattern beam width, when the interfering signals arrive at the antenna, to suppress these
undesired signals as much as possible.
(c) Polarization Reconfigurable Antenna: A radiating structure that can change its
polarization (horizontal/vertical, slant 450, left hand or right-hand circular polarized, etc.) is
called polarization reconfigurable antenna [50-58].
(d) Bandwidth Reconfigurable Antenna: A radiating structure that can change its
bandwidth from NB to WB in different frequency range and vice -versa according to user's
requirement is called as bandwidth reconfigurable antenna [59-60].
2.3.2 Reconfigurable Methods
Reconfigurablity of antenna is normally achieved in one of four ways as summarized below.
(a) Electrical Method: In this method, the switching or tuning of antenna to redirect their
surface currents is achieved by means of PIN diodes, GaAs FETs, MEMS devices or
varactors [61-66]. The MEMS devices have the advantage of very low loss, but the
disadvantages are high operating voltage, high cost and lower reliability than semiconductor
devices. GaAs FET used in switching mode, with zero drain to source bias current, have low
power consumption but poorer linearity and higher loss. PIN diodes can achieve low loss at
low cost, but the disadvantage is that in the ON state there is a forward bias dc current, which
degrades the overall power efficiency. Varactor diodes have the advantage of providing
continuous reactive tuning rather than switching, but suffer from poor linearity. One of the
major advantages of such components is their good isolation and low-loss property. The
incorporation of switches increases the complexity of the antenna structure due to the need
for additional bypass capacitors and inductors which will increase the power consumption of
the whole system. The activation of such switches requires biasing lines that may negatively
affect the antenna radiation pattern and add more losses.
(b) Optical Method: In this method, an optical switch is integrated to reconfigure the
antenna structure. These switches are integrated into the antenna structure without any
complicated biasing lines which eliminates unwanted interference, losses, and radiation
pattern distortion [67-69]. The activation or deactivation of the photoconductive switch by
shining light from the laser diode does not produce harmonics and inter-modulation distortion
37


Chapter 2
due to their linear behavior. Despite all these advantages, optical switches exhibit lossy
behavior and require a complex activation mechanism. Table 2.1 shows a comparison of the
characteristics for the different switching techniques used on electrically (RF-MEMS/PIN
diodes) and optically reconfigurable antennas.
Table 2.1 Comparisons of different switching scheme
Electrical property

RF MEMS

PIN diode

Optical switch

Voltage[V]

20-100

3-5

1.8-1.9

Current [mA]

0

3-20

0-87

Power consumption[mW]

0.05-0.1

5-100

0-50

Switching speed

1-200 μ sec

1-100 n sec

3-9 μ sec

Isolation[1-10GHz]

Very high

High

High

Loss{1-10 GHz}[dB]

0.05-0.2

0.3-1.2

0.5-1.5

(c) Physical Method: Antennas can also be reconfigured by physically altering the radiating
structure by using mechanical motor. The tuning of the antenna is achieved by a structural
modification of the antenna radiating parts. The importance of this technique is that it does
not rely on any switching mechanisms, biasing lines, or optical fiber/laser diode integration
rather it use some mechanical rotational part (stepper motor). The main drawback of
mechanical switching is bulkier structure of antenna and hard to implement in small devices.
(d) Material Based Method: Antennas are also made reconfigurable by changing the
substrate characteristics, using special materials such as liquid crystals, or ferrites and Meta
material [70-72]. These materials have property to change their relative electric permittivity
or magnetic permeability under different operating conditions. In fact, a liquid crystal is a
nonlinear material whose dielectric constant can be changed under different voltage levels, by
altering the orientation of the liquid crystal molecules. As for a ferrite material, a static
applied electric/magnetic field can change the relative material permittivity/permeability.
In the present research work electrical method is used to achieve
reconfigurablity of antenna in which PIN diode is used as a switching element. So complete
functioning, working principle of PIN diodes is important to elaborate and it is discussed in
the next section.
2.4 PIN DIODE
This section presents a general overview of PIN diode, its operating condition as a RF
switch and its characterization to form an adequate basis for the subsequent chapters. PIN

38


Chapter 2
diode is used as a switch to controls the path of RF signals [73]. A PIN diode is constructed
by sandwiching a wide, intrinsic semiconductor region between a P-type semiconductor and
an N-type semiconductor region. The P-type and N-type regions are typically heavily
doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an
ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the
normal function of a diode), but it makes the PIN diode suitable for attenuators, photo
detectors, and high voltage power electronics application.
Drawing of a PIN diode chip is shown in Fig. 2.9 (a). The PIN diode is generally
constructed using a PIN chip that has a thicker I-region, larger cross sectional area. The PIN
diode has small physical size compared to a wavelength, high switching speed, and low
package parasitic reactance; make it an ideal component for the use in RF applications. The
performance of PIN diode primarily depends on chip geometry and the nature of the
semiconductor material used in the finished diode, particularly in the I region.

(a)

(b)

(c)

Fig. 2.9 (a) Cross section of a basic diode (b) Forward bias (c) Reverse bias

2.4.1 Equivalent Circuit parameters
When the diode is forward biased, it can represent as basic electrical characteristics of series
resistance (RS), and a small Inductance. If the PIN diode is reverse biased, there is no stored
charge in the I-region and the device behaves like a Capacitance (CT) shunted by a parallel
resistance (RP) as shown in Fig 2.9(c). These equivalent circuit parameters are defined in
detail as follows.
a) Under forward bias
PIN diode behaves as a current controlled resistor when forward biased. The equivalent
circuit for the forward biased is shown in Fig. 2.9(b) which consists of a series combination
of the series resistance (Rs) and a small Inductance (L). The Rs inversely proportional to the
stored charge Q = If τ where If is the forward current and τ is the recombination time or
carrier lifetime and Inductance (L) depends on the geometrical properties of the package such

39


Chapter 2
as metal pin length and diameter. The resistance (Rs) of the I region under forward bias is
given by
μ

μ

(13)

W = I-region Width, If = forward bias current, τ = minority carrier lifetime, μ , μ = electron
and hole mobility
The eq. (13) is valid for frequencies higher than the transit time of the I-region
(f in MHz and W in microns). At lower frequencies, the PIN diode rectifies the RF signal just
as any PN-junction diode.
b) Under Reverse Bias
The reverse bias equivalent circuit consists of a parallel combination of capacitance (CT) and
resistance (Rp). The defining equation for CT is
(14)
Which is valid for frequencies above the dielectric relaxation frequency of the I-region, i.e.
Where = dielectric constant of silicon, A =diode junction area, = resistivity
of silicon. At frequencies much lower than ƒ, the capacitance characteristic of the PIN diode
resembles a varactor diode. Due to changes and variations in the capacitance PIN diode
switches have low frequency limitations.
2.4.2 Working Operation of PIN diode
A switch is an electrical component for opening and closing the connection of a circuit or for
changing the connection of a circuit device. An ideal switch exhibits zero resistance to
current flow in the ON state and infinite resistance to current flow in the OFF state. A
practical switch design exhibits a certain amount of resistance in the ON state and a finite
resistance in the OFF state. A PIN diode obeys the standard diode equation for low frequency
signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for
large signals) resistor.
When the diode is forward biased, the carrier concentration is much higher than the
intrinsic level carrier concentration in I region. Due to this high level injection level, the
electric field extends deeply (almost the entire length) into the region. This electric field helps
in speeding up the transport of charge carriers from P to N region, which results in faster
operation of the diode, making it a suitable device for high frequency operations. Diode
40


Chapter 2
doesn't turn off until the stored charged removed and I region provide plenty of store charge
at low DC voltage.
So

Qs>>> QRF (IRF / ω)

Stored charge >>> RF induced charge

(QRF added or removed from the I-region cyclically by the RF current).
At high frequencies the stored carriers within the intrinsic layer are not completely swept by
the RF signal or recombination because there is not enough time to remove the stored charge
so always ON in negative cycle also (not rectify in case of PN diode at low bias condition).
Under zero or reverse bias, PIN diode has a low capacitance and very high impedance which
resist the flow of RF signal. Under a forward bias of 1 mA, a typical PIN diode will have an
RF resistance of about 1 Ω, making it a good RF conductor. Consequently, the PIN diode
makes a good RF switch. At RF frequency the PIN diode resistance is governed by the DC
bias applied. In this way it is possible to use the device as an effective RF switch or variable
resistor for an attenuator producing far less distortion than ordinary PN junction.
2.4.3 Important Features of PIN diode
1. A microwave PIN diode is a semiconductor device that operates as a variable resistor at RF
and microwave frequencies. The value of resistance varies from 1Ω (ON) to10 kΩ (OFF)
depending on the amount of DC current flowing through it.
2. Due to lightly doped I layer it has high carrier life time, high breakdown voltage low
junction capacitance, high switching speed and poor reverse recovery time.
3. A PIN diode is a current controlled device in contrast to a varactor diode which is a voltage
controlled device.
4. When the forward bias control current of the PIN diode is varied continuously, it can be
used for attenuating, leveling, and amplitude modulating an RF signal.
5. When the control current is switched on and off, or in discrete steps, the device can be used
for switching, pulse modulating, and phase shifting an RF signal.
6. PIN diodes are used to control RF power in circuits such as switches, attenuators,
modulators and phase shifters.
7. High voltage current controlled RF resistor for RF attenuator and switches.
2.5. CHARACTERIZATION OF PIN DIODE AND ITS BIASING COMPONENT
PIN diodes are often used as a switch that controls the path of RF signals. The
fundamental parameters that describe PIN diode switch performance are: Isolation and
41


Chapter 2
Insertion loss. Physically, Isolation is a measure of the RF power through the switch that is
not transferred to the load, when the switch is OFF. But practically, isolation is a measure of
how effectively a PIN diode switch is turned OFF. Insertion Loss (IL) is measure of
transmission loss through the physical structure of a PIN diode switch. This is a measure of
large values of bias current plus RF current may flow through the switch structure, causing
significant ohmic loss under the ON state [79].
Working operation of PIN diode as a switch can be easily explained by Fig. 2.10 (a).
To bias the PIN diode accurately, it is necessary to provide some degree of isolation between
DC signal and the RF signal. Otherwise, RF current can flow into the power supply's output
impedance, causing unfavourable effect to the efficient operation of the power control circuit.
The DC bias supply is isolated from the RF circuits by inserting an RF inductor in series with
the bias line and a RF by-pass capacitor, in shunt with the power supply output impedance
the RF control circuit. In Ansoft HFSS simulation, PIN diodes are modeled using lumped
RLC boundary PIN diodes. For forward bias, Infelon diode is modelled as a forward
resistance of 2.1 Ω, and lead inductance= 0.6 µH as shown in Fig. 2.10 (b) and in reverse bias
it is modeled as a reverse parallel resistance = 3 KΩ, capacitance = 0.17 pF and lead
inductance = 0.6 µH as shown in Fig. 2.10 (c). The simulated S-parameter for PIN diodes is
shown in Fig. 2.11. It is observed that in ON condition, insertion loss is 0.1 dB from 1GHz to

Fig.2.10 The biasing circuit of PIN diode (b) equivalent circuit in ON state (c) OFF state

Fig.2.11 S-parameter of PIN diode in OFF and ON condition

42


Chapter 2
8 GHz hence diode would offer low impedance and acts as short circuit for RF signal. When
PIN diode is OFF; insertion loss is greater than 18 dB as shown in Fig. 2.11; hence it exhibits
high impedance so there is no propagation of power from source to load terminal.
It is important to figure out the insertion loss of each component when actually
embedded in the fabricated prototype. In this research Coil Craft Inductor [74], Murata SMD
ceramic multilayer capacitor [75] and Infineon PIN diodes [76] are used in testing. To find
out the working behaviour and frequency response of these components, some prototype for
each biasing component is fabricated and tested. The detail for the characterization of each
component is described as follows.
2.5.1 Testing of a Simple 50Ω Microstrip line
A simple microstrip structure is designed on 20x20mm2 sized FR4 substrate having
thickness 1.57 mm and relative dielectric constant

= 4.4. The schematic for microstrip line,

electric field distribution over it and the fabricated photograph are shown in Fig. 2.12. The
characteristic impedance of microstrip through line is 50Ω. Fig. 2.13 shows the simulated and
measured S-parameter of microstrip line.

Fig.2.12 (a) 50Ω Microstrip Line (b) E-field Distribution (c) fabricated photograph

(a)

(b)

Fig.2.13 (a) S-parameter vs. frequency of a microstrip line (b) measurement setup

43


Chapter 2
Ideally 50Ω microstrip line offer 0 dB insertion loss for RF signal but practically there
is some insertion loss. In simulation, value of insertion loss is 0.5dB which is nearly constant
from 1 to 10 GHz whereas measured results show that the insertion loss is better than 0.1 dB
from 1 to 6 GHz and 0.4dB from 6 to 9 GHz.
2.5.2 Testing of SMD Capacitor as a RF bypass element
The RF bypass/DC Block capacitor offer minimum resistance for RF signal but
blocks the DC signal. Therefore when a capacitor is placed in between the 50 Ω microstrip
line, it should bypass the RF with minimum loss. The 30pF SMD Ceramic Multilayer
Capacitor is used for characterization of capacitor. To figure out the insertion loss of a 30 pF
capacitor; it is mounted in the 0.5 mm wide gap of 50 Ω microstrip line as shown in Fig. 2.14.
In HFSS simulation, capacitor is assigned as 30 pF using lumped boundaries condition. Fig.
2.14 (a) shows the geometry of the proposed layout, electric field distribution and the
fabricated photograph. Simulated and measured S-parameters are shown in Fig.2.15.

Fig. 2.14 (a) Layout for Capacitor under test (b) E-field Distribution (c) fabricated photograph

(a)

(b)

Fig. 2.15 (a) S-parameter vs. frequency for Capacitor under Test (b) measurement setup

44


Chapter 2
Simulated value of insertion loss is 0.5dB from 1GHz to 7GHz whereas measured value is
0.2 dB from 1-7 GHz. So, the SMD capacitor is used to bias the PIN diode and also to isolate
the different DC voltage regions while maintaining RF signal continuity from 1-7 GHz.
2.5.3 Testing of Coil Craft Inductor as a RF choke element
Now, to check the behaviour of inductor in the circuit, a prototype for DC block bias
line has been designed as shown in Fig.2.16. The Inductor and capacitor used here for
characterization are coil craft Inductor L=0.3μH, Murata 0.3nF ±5% 50V dc Dielectric SMD
Ceramic Multilayer Capacitor. The prototype consists of two SMD capacitors in 0.5mm wide
gap and two inductors connected with bias pads via thin interconnecting lines. From this we
are checking that RF capacitors offer minimum impedance to RF signal in the presence of
DC voltage and the inductor offer low impedance towards DC signal. When +5V signal is
applied on DC pad, the SMD capacitor will offer minimum impedance for RF signal but
maximum impedance for DC signal.

Fig. 2.16 (a) RF choke under test (b) E-field Distribution (c) fabricated photograph

(a)

(b)

Fig. 2.17 (a) S-parameter vs. frequency of a DC block under test (b) measurement setup

45


Chapter 2
So, the transmitted RF signal must have low insertion loss. Fig. 2.17 show the simulated as
well as measured S-parameter for it which clearly shows that insertion loss is better than 2dB
and return loss is better than 13dB from 1 to 7 GHz. So, it can be is conclude that presence of
DC voltage don’t affect the transmission of RF signal if the value of capacitor and inductor is
chosen accurately for specific frequency range.
2.5.4 Testing of Infineon PIN diode as a RF switch
The prototype to characterize PIN diode as a switch is fabricated and tested as shown in Fig.
2.18. The biasing scheme is very simple, requiring only two RF choke coil and a two DC
blocking capacitors. The PIN diode used for characterization is Infineon BAR-64-02,with
operating parameter: Diode reverse voltage =150V, total breakdown current = 5μA, Diode
forward current = 100mA, operating voltage = 1.1V, forward current at 1.1 V = 50mA, Diode
capacitance at 0.17pF, Reverse parallel resistance = 3KΩ, Forward Resistance =2.1Ω,
Insertion loss =0.16dB, Isolation =22dB.

Fig.2.18 (a) PIN diode under test (b) E-field Distribution (c) fabricated photograph

(a)

(a)

(b)

(b)

Fig.2.19 (a) S-parameter vs. frequency of PIN diode under test (b) measurement setup

46


Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay

×