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THEORY MODELLING AND PARAMETRIC STUDY OF PLANAR
INVERTED-F ANTENNA(PIFA)
Vijesh.K.R 1, 2
, Hrudya B Kurup 1, 2, 3
,
P.Mohanan5, V.P.N.Nampoori
1,3, Bindu.G
3,4
1.International School of Photonics, 2.Swadesi Science Movement, 3.KMEA
EngineeringCollege, 4. Nansen Environmental Research Center, Cochin 5.Center for
Research in Electromagnetics and Antennas (CREMA) Department of Electronics,CUSAT,
Cochin- 22, Kerala, India.
ABSTRACT
Wireless communication has been experiencing an exceptional growth at the end of the last century. This growth
is likely to continue or even accelerate in the new millennium. PIFA antenna structure has emerged as one of the
most promising candidate in the category of antennas used in handheld devices and most built-in antennas
currently used in mobile phones include planar inverted F- Antenna (PIFA) due to its handy size, light weight
and built-in structure. Broad range of applications employs PIFA as their basic antenna. In this paper a detailed
theoretical study of Planar Inverted F Antenna is presented.
INTRODUCTION
Recent years has witnessed a very rapid expansion of wireless communications. The main
cause of this rapid expansion is the considerable advancements in the field of digital
communications. In comparison with its analogue counterpart, digital communications has a
greater immunity against noise and interference and provides greater security of information,
through encryption. These features assisted the rapid growth of cellular phone systems.
Now mobile phones have become an integral part of our day to day life. A wide verity of
mobile phones is available in market which can support various applications and provide
different types of services. Apart from the fact that today's mobile handsets are packed with
more functions than ever, they are becoming increasingly lighter. With respect to its working
one of the most important element in a mobile handset is its antenna. The various capabilities
of a mobile phone such as Bluetooth, Wi-Fi, CDMA etc. cannot be functional without a good
performing antenna . In addition, the antenna is generally required with small size and light
weight.
As antennas are becoming highly integrated in terms of design and their volume, the need for
antennas featuring wideband, compact, high efficiency and multiband characteristics is
growing. As a result, antennas have gone from external to internal. They have also become
subject to numerous constraints in size and function. Currently a lot many researches are
going on in the field of mobile phone antennas.
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The mobile phone manufacturers have increasingly focused on low profile, compact, multi-
band capabilities for antennas. Recently, there has been a great demand for mobile devices
and antennas that have small size with multiband operation because of widespread use of
Bluetooth, GSM, and Wi-Fi which can be easily fabricated with low manufacturing cost.
BACKGROUND
PIFA antenna structure has emerged as one of the most promising candidate in the category of
antennas used in handheld devices and most built-in antennas currently used in mobile phones
include planar inverted F- Antenna (PIFA). Broad range of applications employs PIFA as
their basic antenna. The main reasons for this are:
1) They are of low profile in comparison to standard micro strip antennas because the short
favors resonance for electrical dimensions smaller than half-a-wavelength (λ/2)
2) Their radiation patterns are near omnidirectional;
3) They are installed above the phone circuitry, ‘reusing’ the space within the phone to some
degree [1]
4) They exhibit a low specific absorption rate (and less loss to the head) [2].
5) Easy fabrication
6) Small volume
5) Low manufacturing cost.
6) PIFA structure is easy to hide in the casing of the mobile handset as compared to
monopole, rod & helix antennas.
7) They can resonate at much smaller antenna size and by cutting slots in radiating patch;
resonance can be modified.
8) Proper changes results in multiband operation without much increase in volume.
A detailed theoretical discussion, modelling and parametric study of planar inverted-F
antenna is done in this paper. Next section discusses the theory of PIFA in detail. Section-A
explains basic structure of a simple PIFA, Section-B quarter wavelength operation, Section-C
design equations and the relationship between various parameters, Section-D feeding
techniques. The other topics discussed are impedance matching, parametric study of antenna
dimensions and return loss, electric and current distributions, radiation in PIFA and its multi-
band operations.
PIFA THEORY
A. PIFA Structure
Planar inverted-F antennas evolved gradually from other antennas in order to overcome
certain limitations of its preceding structure. The first in the line is a monopole antenna.
Monopoles were the antenna of choice for the earliest mobile phones. They have the
advantage of providing significant clearance between the antenna and the head, which allows
low SAR and, perhaps most importantly, high efficiency to be achieved. The drawback of this
type of antennas is that it makes the device larger and bulkier.
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In order to achieve compactness the monopole structure was transformed into a structure as
shown in fig.1. Since the structure resembles an inverted L the antenna is called an inverted-L
antenna. The inverted-L antenna consist of a short vertical monopole and a long horizontal
arm above it .Because of the addition of inverted L segment above the ground plane, it is
difficult to match inverted L antenna to the feed line.
Inorder to reduce the mismatch losses the next step was to develop an antenna with nearly
resistive load. For this kind of operation the inverted – F antenna (IFA) was designed. As seen
from fig. 1 the structure resembles an inverted F so the antenna is called an inverted-F
antenna. It adds a second inverted – L segment to the end of an ILA structure. This additional
segment gives a convenient option for tuning original ILA.
Fig.1: From monopole to PIFA
But the limitation with IFA is that it has very narrow bandwidth [3]. In order to improve the
bandwidth characteristics, antenna has transformed the horizontal element from a wire to a
plate resulting in the so called planar inverted-F antenna (PIFA). It has a self-resonating
structure with purely resistive load impedance at the frequency of operation.
Fig.2 shows a basic PIFA structure which is fed at the base by a feed wire. It consists of a
ground plane and a top radiating patch. The top radiating patch is of length L1 and width L2.It
is connected to the ground plane using a vertical wall of height h and width w. This vertical
wall is commonly known as short pin or short post. The whole structure is exited by a feed
probe which is located at a distance d from the shorting wall
Fig.2: Basic PIFA Structure
B. PIFA as a quarter wavelength antenna
As seen in the previous section the PIFA structure comprises of a ground plane, radiator, feed
line, and short pin. This structure resembles a short-circuit MSA .Therefore PIFA can be
thought of as a shorted micro strip patch antenna with air as dielectric.[4] The side view of a
micro strip antenna is shown in fig. It consists of two radiating slots.
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A micro strip patch antenna is approximately a half wavelength long section of micro strip
transmission line .The current and voltage distribution along the patch length is as shown in
fig.3. From fig it is seen that the impedance of the antenna is maximum at the ends and
minimum at the middle. There is a virtual short circuit at the middle. If we replace this virtual
short circuit with a physical short circuit we get a quarter wave length antennas. This halves
the size of the antenna.
The side view of PIFA is as sown in fig 3. It consists of only a single radiating slot. Hence the
gain of PIFA is low compared to conventional micro strip antenna.PIFA is therefore a micro
strip antenna which radiates at quarter wavelength. The current and voltage distribution along
the patch length is as shown in fig.3.
Fig.3: PIFA as a quarter wavelength antenna
The shorting post near the feed point of PIFA structure is a good method for reducing the
antenna size, but this result into the narrow impedance bandwidth which is one of the
limitations for its use in wireless mobile devices.
C. Basic Design Equation
The frequency at which PIFA resonates can be calculated by using a basic formula as given
below
L1+L2-W = λ/4 …………………………………………………………………………… (1)
Where L1 is Top patch length
L2 is Top patch Width
λ is wavelength corresponding to resonant- frequency
This equation stems from the theory that if we arbitrarily take a point far away from the short
circuit edge on the top radiating patch , and calculate the current path , on an average it will be
equal to L1+L2-W
But λ = c/f …………………….……………………………………………………….….. (2)
Also PIFA sits on top of a dielectric substrate with permittivity
Therefore the above equation can be written as
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L1 + L2-W = c /4f√ ………………………………………………………………………. (3)
Where c is the speed of light,
f is the resonant frequency
Above equation represents that the resonant frequency is dependent on width and length of
the top plate, the width of the shorting plate and the substrate used. Further more if the height
of PIFA is taken as a parameter and if permittivity is taken as the effective permittivity
respective to each substrate then the equation can be modified as [5]
L1 + L2-W+h = c /4f√ ………………………………………………………...…………..(4)
Where the effective permittivity respective to each substrate is approximated using Equation:
……………………………………………………………………………(5)
From Equation 4, it should be observed that not only is the resonance frequency of PIFA is
dependent on various parameters like the physical dimensions of the radiating element (L1
and L2), the width of the short as well as the thickness of the substrate. As width of the
shorting plate also affects resonant frequency of the antenna so reduction in the width of
shorting plate results in lowering the resonant frequency and vice versa.
The substrate electrical properties as well as thickness affect the performance of PIFAs in
terms of gain and bandwidth. Substrates with high loss tangent are very lossy and result in
low gain. Substrates of high permittivity or of narrow thickness lead to poor radiators of
narrow bandwidth. Hence antennas are often designed with thick substrates of low
permittivity. However, the thickness of the substrate should be limited so that surface waves,
which deteriorate the radiation efficiency, are not generated [1]. The height of the shorting
plate plays an influential role in broadening the bandwidth of PIFA structure. Various
techniques can be employed and the most widely used method is to increase the height of the
shorting plate. But this finally results in increase of volume [5].
However this approximation is rough and does not cover all the parameters that significantly
affect the resonant frequency of the antenna. For instance the performance of the antenna can
be enhanced by varying ground plane length. Optimum length of the ground plane is 0.4λ at
the operating frequency [6]. In several designs, position of the antenna on the dielectric
substrate is important as enhancement in the operating bandwidth can be achieved to few
more percentage. Location of feed point and the type of feed used and position of shorting pin
or plate etc are some other significantly influencing parameters.
D. Feeding Technique
The Coaxial feed also called probe feed is one of the most commonly used feeding techniques
for PIFA.Microstrip line feed is also used but in such cases the inverted F structure is not
maintained. The inner conductor of the coaxial cable extends through the dielectric substrate
and is soldered to the radiating patch; while the outer conductor is connected to the ground
plane .The antenna is fed through feeding pin which connects to the ground plane. This type
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of feeding technique allows designer to place it at any desired location in the patch for getting
good impedance matching.
E. Impedance Matching
For an antenna to radiate all of the power it must be resistive and the resistance must match
the source resistance. In the case of an antenna the system source is usually though not always
the end of a transmission line which has an impedance of 50 ohm.
As stated above PIFA is a microstrip antenna which radiates at quarter wavelength length. If
we take the case of a microstrip antenna, which is approximately a half wavelength antenna,
the antenna impedance in theory at the center is zero ohms and the end is infinite ohms and is
resistive .Similarly if we take the case of a PIFA, as per theory the impedance from the short
pin increase from zero to infinity with either end of the quarter wave having a purely resistive
impedance. Anywhere between either ends, the impedance of PIFA increases or becomes a
combination of resistance and reactance.
Therefore somewhere between the ends lies the 50 ohm impedance. By optimizing the
spacing between feed point and shorting point, impedance matching of the PIFA can be
obtained [8]. By analyzing the resonant frequency and bandwidth characteristics we can
determine the optimum location of the feed point, at which minimum return loss is to be
obtained. The shorting pin and shorting plate allows good impedance matching achieved with
the patch above ground plane of size less than λ/4. Resulting PIFA structure is of compact
size than conventional λ/2 patch antennas.
The main idea designing a PIFA is to don’t use any extra lumped components for matching
network, and thus avoid any losses due to that. Dependent of the antenna type there are
several possibilities to obtain optimum impedance at the correct frequency. The size of
ground plane, distance from antenna to ground plane, dimensions of antenna elements, feed
point etc are factors that can affect the impedance.
F. Electric field distribution
The electric field under the planar element of the PIFA is z-directed. The dominant
component of the electric field, Ez, is zero at the short circuit plate and maximum at the free
end of the planar element. The electric fields, Ex and Ey are generated at all open edges of the
planar element. Means that the electric line of force is directed from feed source to the ground
plane. These are commonly known as fringing fields.[11]
G. Current distribution
PIFA has very large current flows on the undersurface of the planar element and the ground
plane compared to the field on the upper surface of the element. Due to this behavior PIFA is
one of the best candidate when is talking about the influence of the external objects that affect
the antenna characteristics (e.g. mobile operator’s hand/head). PIFA surface current
distribution varies for different widths of short-circuit plates. The maximum current
distribution is close to the short pin and decrease away from it. [11]
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H. Radiation in PIFA
The patch acts approximately as a resonant cavity (short circuit walls on top bottom and side,
open-circuit walls on the other side). If the antenna is excited at a resonant frequency, a strong
field is set up inside the cavity and a strong current on the surface of the patch.The electric
fields that extend out from the open circuit edge of PIFA (called fringing felids) causes PIFA
to radiate. These fringing fields are the radiating sources in PIFA.
I. Multiband PIFA
As a PIFA antenna is a distributed radiating system, too many parameters play a role in
deciding the antennas characteristics. There are several ways to design a multi-band PIFA
antenna. The most popular way is to cut slits on the patch. Those are used: to create new
resonances, to lengthen the electric lengths,and to create new resonators. Here lowest
frequency means the biggest wavelength in comparison to other higher frequencies and hence
the respective current path should be the longest. The main advantage of this technique is that,
it allows multi-band operation without much increase in the volume. When the antenna area is
big enough, a better design approach is to use different independent branches to cover each
band.
Using one slit with multiple bands or multiple slits are ways to expand PIFA antenna
bandwidth. Another way is to use parasitic element. In such cases there will be a main
radiator and a parasitic element. The parasitic element usually resonates at the higher band
increasing the bandwidth of the higher band. As higher frequency means smaller parasitic
radiator, this also makes antenna design easier. [12] In all cases the resonant frequency is
affected by various factors like the position of the slit, its length and width , the shape of the
slit etc There are so many unknowns when designing an antenna for a real device, no formula
can give an accurate prediction. The best way is to find out what the effect of each parameter
is, then tune the antenna accordingly.
J. Parametric study
In a PIFA structure there are several design variables which can be varied and the
performance of the desired antenna is achieved [9]-[10]. Some of the design variables are
width, length and height of the top radiating patch, width and position of shorting pin or plate,
location of the feed point, dimensions of the ground plane.
Fig.4: 3-D view of simulated antenna structure
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To know the dependency of return loss on these variables, a parametric study was done on a
basic PIFA structure. The procedure adopted for this study is that only one parameter is
changed at a time to observe its effects on the PIFA characteristics while all other parameters
are held constant. The software used for simulation is High Frequency Structure Simulator
(HFSS) based on the Finite Element Method.
The simulated structure is as shown in fig.4. It consists of main radiating patch, a rectangular
ground plane, a shorting plate, coaxial feed and a ground plane. Total dimensions of the
radiating parts of the antenna are 50 x 21 mm2. And that of ground plane are 120 x 50 mm
2.
Fig.5: Simulated return loss for different heights Fig.6: Simulated return loss by varying the
top-radiating patch length
The height of the antenna is varied from 10 mm to 4 mm and the return loss characteristics are
simulated. As shown in fig.5 increasing height results in a better return loss characteristic.For
a height of 10 mm keeping the initial dimensions of the antenna as such and varying the top
radiation patch length as 50mm, 40mm and 25mm, the return loss characteristics simulated is
as shown in fig.6. It can be seen that as L1 increases resonant frequency and return loss is
lowered.
Fig.7: Simulated return loss by varying the top Fig.8: Simulated return loss by
varying the radiating patch width shorting pin width
For a height of 10 mm , and L1 of 50mm changing the width of the top radiation patch as
20mm,30mm and 40mm the return loss characteristics simulated is as shown in fig.7.As L2
increases return loss increases.For a height of 10 mm , L1 of 50mm and L2 of 20 mm
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changing the width of the shorting plate as 1mm,2mm and 3mm the return loss characteristics
simulated is as shown in fig.8. As W increases return loss also increases.
For a height of 10 mm , L1 50mm, L2 20 mm, W 1mm , changing the return loss
characteristics simulated is as shown in fig.9. FR4, Duroid, Mica and Alumina are used as
substrate materials.
Fig.9: Simulated return loss by varying permittivity
CONCLUSION
New high performance antennas are being developed to satisfy the competing demands of
emerging wireless applications for small size, internal and low profile applications, and broad
bandwidth or multi-band capability to support multiple services. One such structure is the PIFA
antenna. The distinguishing fact of PIFA from other antennas is that they exhibit a low
specific absorption rate and hence less hazardous when health issues related to exposure of
electromagnetic radiations is considered.
FUTURE TRENDS
Compact reduced size antennas are mainly required for mobile communication equipment to
meet its miniaturization requirement along with light weight. These applications demand
some features like, compactness, wideband/multiple band operation, high gain, diversity
reception, uniform radiation pattern, reduced radiation hazards. etc. The radiation hazard is an
important issue from the point of view of user’s health. In planar inverted-F antenna research,
a new trend is found where the researcher try to improve antenna characteristics by
introducing different structures within the antenna geometry. With the development of
computational electromagnetics new approaches of analysis has become another branch of
activity. Optimization of patch geometry is an ideal technique to have single or more
optimized figures of merit like, impedance bandwidth, efficiency and gain. Genetic Algorithm
(GA) based optimization and application of Frequency Selective Surfaces (FSS), reducing the
specific absorbtion rate etc is some other recent trends.
ACKNOWLEDGEMENT
Authors are thankful to KSCSTE for financial assistance through emeritus scientist
scheme. They are also thankful to Director ISP and Head of the Department ISP CUSAT for
providing facilities to carry out the project.Authors also acknowledge interest in the project by
Swadesi science movement Kerala.
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