A Mechanically Tuned Via-Patch Loaded Compact ET-PIFA

A Mechanically Tuned Via-Patch Loaded

Compact ET-PIFA

Anirban Sarkar*, Sourav Pal and Monojit Mitra

Department of Electronics and Telecommunication Engineering

Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, W.B, India.

E-mail: [email protected]

Abstract- An idea of a compact tunable 30° sectoral

equilateral triangular planar inverted-F antenna

(ET-PIFA) is introduced and fabricated. In the

proposed approach, the conventional ET-patch

geometry is reduced to 30° sectoral ET-PIFA by

via-loading the zero voltage plane and miniaturized

by incorporating two slits and capacitive loading to

lower the operating frequency. The proposed

antenna size is reduced by 93.37 % in comparison

with the conventional ET-patch size and it exhibits

a 10 dB return loss bandwidth of 1.1 % suitable for

2.4 GHz ISM band applications. By simply

replacing the via from via patch with a screw and

adjusting the height of the via patch by turning the

screw, a tuning range of 2.4 GHz from 3.35 GHz

can be achieved which is suitable for mobile

WIMAX application. Both simulation and

experimental results are presented together with

parametric studies on proposed ETMSA structure,

which shows a good agreement between two

results.

Index Terms- Planar inverted-F antenna (PIFA),

slit loading, capacitive loading, size reduction,

mechanical tuning.

I. INTRODUCTION

Antennas are one of the most important

components for the transmission and reception of

electromagnetic waves in wireless

communications. Over the past decade, many

techniques for compactness of patch geometry

have been proposed, but in our structure we have

utilized via-loading, meandering and capacitive

loading technique to reduce the ET-PIFA [1]

geometry. We have chosen the PIFA [2] structure

because of its ease of design, low profile and low

cost. Conventional PIFA with either shorting wall

or shorting pin at one end of the radiating

element gives its length reduction of about a

quarter of a wavelength (λ0/4) at the center of the

operating frequency band. Moreover, designing a

small antenna (smaller than a quarter

wavelength) is always a challenging task. Studies

on microstrip antenna with equilateral triangular

patch geometry reveal the versatility of the same

for compactness and miniaturization of physical

dimensions with respect to the other common

shapes like circular or rectangular geometry and

thereby reducing the area and weight of the

antenna configuration. Space requirement for an

antenna installation in any device always a

constraint, therefore the aim of this paper is to

design an antenna geometry that takes a very

small space in comparison with its conventional

shape for its mounting. Recently it is found that

meandering technique [3],[4] and capacitive

loading technique are used to the PIFA structure

for further reduction in the antenna operating

frequency.

In this paper the alternative patch geometry is

chosen as ET-PIFA in comparison with other

geometries [5],[6]. In the proposed antenna, an

extra plate in between patch and ground plane

does the job of capacitive loading [7]. The via

patch is connected to the ground through via

while the coaxial feed is directly connected to the

radiating element. Also use of meandering

technique, i.e. cutting multiple slits in the patch

geometry increases the electrical path length that

reduces the operating frequency of the antenna.

With parametric studies on the location of the

vias, position of the slits on the patch and

location of the via-patch, structure of this paper

provides a design guideline to the proposed ET-

PIFA. The proposed design can provide a very

compact frequency tunable ET-PIFA through a

simple mechanical tuning to cover mobile

WIMAX as well as ISM band applications.

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II. CONVERSION OF CONVENTIONAL ET-

PATCH STRUCTURE TO 30º SECTORAL ET-

PIFA STRUCTURE

In general, microstrip antennas are half-

wavelength structures and are operated at the

fundamental resonant mode TM01 or TM10. Based

on the cavity model approximation, the

fundamental or first resonant frequency of the

triangular patch [8],[9-10] is given by

12 2 22 (m mn n )

3 e

mnl mn

e

cf f

S ε= =

+ + (1)

where Se is the effective side length, εe is the

effective permittivity of the dielectric i.e air and

the integers m, n, l satisfy the condition m + n + l

= 0. Instead of using m, n, and l , only m, n has

been used for simplicity, it is implied that l = -

(m + n), where eS is

4

e

eh

S Sε

= + (2)

where S is the original side length of the ET-

patch and h is the height of the radiating patch

from ground plane. We would like to

demonstrate the degree of size reduction obtained

by shorting the zero voltage plane [11].

In our antenna design, We have started with

conventional ET patch geometry with side length

70mm and it resonates at 2.4 GHz. After that we

choose the side length as 40 mm and by posting

vias in proper locations, geometry is changed to

basic ET-PIFA which is resonating at 3.73GHz.

We have shorted along the zero voltage line

which is 2/3rd

of height from the triangle tip to

the bottom edge of the triangle, by several vias

and separate the whole structure by two parts.

This 600

sectoral ET-PIFA whose electrical

length is λ/4, will resonate at the same resonant

frequency as the basic ET-PIFA. By applying the

same shorting technique the 600

sectoral ET-

PIFA is converted to the smallest 300 sectoral ET-

PIFA structure which resonates almost at the

same frequency which has shown in Fig. 1 and

table 1.

Fig. 1. Conversion of Conventional ET-patch

Structure to 30° Sectoral ET-PIFA Structure.

(a)

(b)


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(c)

Fig. 2. Configuration of the proposed PIFA with slits

and via-patch loading (units: mm). (a) Top view. (b)

3-D view. (c) Side view

Table 1. Conversion to 30º sectoral ET-PIFA structure

III. PIFA WITH SLIT LOADING

In this part, antenna radiating frequency is

reduced by meandering [12] the excited patch

surface current path by loading several narrow

slits at the patch’s non-radiating edge. In our

antenna geometry two slits are cut at the same

non-radiating edge. The upper slit and the lower

slit has the length 12.68 mm and 7.54 mm

respectively. The width of both of these slits is

0.5 mm. This slit loading reduces the resonating

frequency as well as bandwidth from 3.73 to 3.18

GHz and 3.1% to 2.26% respectively. The

corresponding simulated S11 plot in comparison

with 30° sectoral ET-PIFA return loss plot is

shown in Fig. 3.

IV. PIFA WITH VIA-PATCH LOADING

A. Via-patch Loading

Further reduction in the operating frequency of

the proposed PIFA can be done by capacitive

loading. The via- patch is 0.4 mm separated from

the radiating patch to provide the capacitive

effect [13],[14]. In fact, the capacitive load could

be adjusted by changing the size and height of the

via-patch, and this will not increase the overall

dimensions of the original structure. Both the

radiating patch and via-patch are made of 0.02

mm thick copper plate and they are 3.4 mm and

3.0 mm in height above the ground plane

Fig. 3. Comparison of resonant frequency between

30° sectoral ET-PIFA and slit loaded 30° sectoral ET-

PIFA.

respectively. The substrate of the whole antenna

is air and a 1.2 mm diameter coaxial probe is

directly fed to the radiating patch as depicted in

Fig. 2. The radiating element and via-patch are


slit loaded 30° sectoral ET-PIFA and slit and via-

patch loaded 30° sectoral ET-PIFA.

placed at the middle of the ground plane

(100×100 mm2). There are three pins in total in

the proposed antenna and they function

differently. First, the two 1.2 mm diameter

shorting pins are connected to the main upper

Type of the

structure

Resonant

frequency(fr) in

GHz

S11 (dB)

Basic ET-PIFA 3.73 -36

60° sectoral ET-PIFA 3.7 -24.52

30° sectoral ET-PIFA 3.74 -25


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Table 2. Variation of resonant frequency and

capacitance with separation between two patches

Table 3. Variation of resonant frequency by varying

via-patch area

radiating patch and they also support the patch in

the air. Next, another 1.2 mm diameter pin is

built in via-patch, which supports the lower patch

in air and provides a capacitive load eventually

making that radiating. Putting a cylindrical via on

a patch is easier in fabrication than a shorting

wall, and we can adjust the resonance of the

antenna by varying the via location. Capacitive

loading tunes the operating frequency from 3.18

GHz to 2.4 GHz as shown in Fig. 4 but

impedance bandwidth reduces to 1.1%.

B. Parametric Studies

In Fig. 5 and Fig. 6, different resonant

frequencies of antennas are obtained by

simulation for different separation between via

patch and radiating patch and area of the via-

patch respectively (All dimension values shown

in Fig. 5 and Fig. 6 are relative to the antenna

model described in Fig. 2). In table 2 and 3, it is

seen that the resonant frequency is more sensitive

to the separation than the area of the via-patch.

As capacitance increases, the resonant frequency

decreases and this is shown in Fig. 7. On the

other hand, in Fig. 8, an equation can be obtained

from 3 sets of data. When the actual separation of

0.4 mm, 1.4 mm and 2.4 mm and their

corresponding resonant frequencies are put into

(3), the parameters a, b and c could be computed

and the calculated values are 3.84, -20 and 26.3,

respectively. The simulation result

Fig. 5. S11 vs resonant frequency plot with varying

the separation between the via-patch and radiating

patch by keeping both of areas constant.

Fig. 6. Variation in resonant frequeny by varying the

via-patch effective area.

( ) ( ) ( )2

. . 3separation a freq b freq c= + +

shows that the separation between the two

patches and the resonant frequency of the antenna

Separation between radiating patch and via-patch

(mm)

Capacitance (pF)

Resonant frequency

(GHz)

Bandwidh (%)

2.4 0.047 3.35 2.18

1.9 0.06 3.27 2.07

1.4 0.08 3.15 1.87

0.9 0.125 2.91 1.33

0.4 0.28 2.4 1.1

Variation of area of the via-patch (mm

2)

Resonant frequency

(GHz) S11(dB)

Capacitance (pF)

7.91 2.68 -18 0.17

9.42 2.59 -23.2 0.20

12.82 2.4 -38.5 0.28

15.92 2.27 -24 0.35


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Fig. 7. Resonant frequency versus capacitance values.

can be represented by a quadratic equation. The

units of (3) for separation and frequency are mm

and GHz, respectively. Faster changes in the

resonant frequency are observed when the

separation is getting smaller. Therefore, the via-

patch should be placed close to the radiating

element in order to obtain an optimum antenna

size reduction. In fact, enlarging the via-patch

size or placing the via-patch closer to the

radiating element could further reduce the size of

the radiating element. The capability of lowering

the resonant frequency by loading a via- patch

under the main radiating patch has been

demonstrated above. However, we found that the

degree of size reduction of the antenna is also

closely related to the location of the via. Fig.

10(a) shows five via locations with different

distance, away from the right edge of the via-

patch. Actually, the antenna described in Fig. 2 is

the one with x equals to 1.34 mm. Referring to

Fig. 10(b), we noticed that the larger the value of

x, the higher the resonant frequency of the

antenna will be. This could not be explained by

the introduction of a capacitive load to the PIFA

only. The simulated surface current distribution

of two extreme cases with x equals to 1.34 mm.

and 6.14 mm are shown in Fig. 11(a) and (b). It is

clearly seen that the surface current is flowing

from the radiating patch to the ground plane

along the shorting pins, and then flowing up to

the via-patch through the via. By moving the via

to the right hand side (smaller value in x), longer

current paths are provided on the ground plane.

This will help in lowering the resonant frequency

of the antenna. Although this will provide the

freedom in miniaturizing the antenna, but reduces

the impedance bandwidth. The top view of the

fabricated antenna structure are shown in

Fig 12(a).

Fig. 8. Quadratic curve fit between the separation and

resonant frequency.


simulated result in IE3D simulation software [15] and

measured result in Agilent Technologies N5230A, 10

MHz - 20 GHz , PNA- L Network Analyzer of the

fabricated PIFA.

C. Frequency Tuning Capability

In principle, a ET-PIFA is not allowed to change

the resonant frequency because the resonance is

mainly governed by the current paths of the


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radiating element. Once the length of the

radiating patch is estimated, the resonant

frequency is fixed. However, the resonant

frequency of our proposed PIFA also depends on

the amount of capacitance introduced by the via-

patch and the radiating patch, and the capacitance

in turn depends on the separation

(a)

(b)

Fig. 10. (a) Via locations with different x. (b) Return

losses of via-patch loaded PIFAs with different via

locations.

Fig. 11. Simulated surface current distribution (a) with

via placing on x equals to 1.34 mm at 2.27 GHz and

(b) with via placing on x equals to 6.14 mm at 2.5

GHz.

between them and the area of the via-patch. The

effect on tuning of resonating frequency is more

for separation between the two patches than the

area variation of the via-patch. Therefore, the ET-

PIFA becomes frequency tunable simply by

adjusting the height of the via-patch. This can be

easily realized by replacing the via with a screw

as depicted in Fig. 12(b).

(a)

(b)

Fig. 12. (a) Top view of Fabricated proposed PIFA

(left side) & (b) Configuration of proposed PIFA with

a screw as via(right side).

V. CONCLUSION

In this paper we have presented a new approach

for a ET-PIFA with slit and capacitive loading.

The proposed antenna gain and directivity

obtained are 3.29 dbi and 4.31 dbi respectively.

We also found that meandered current paths on

the radiating patch and the capacitive load are the

main reasons for antenna size reduction. As

triangular geometry is chosen and modified to

30º sectoral geometry, more compactness is

achieved compared to other geometry like

rectangular or circular and it is possible to creat

more compact structure by converting the

proposed antenna to 15º sectoral geometry with

an electrical length of λ0/16. Measured and

simulated results show that the proposed ET-


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PIFA covers the 2.4 GHz ISM and mobile

WIMAX bands with an electrical length of λ0/8.

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A Mechanically Tuned Via-Patch Loaded Compact ET-PIFA

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