Pifa array2 rg

PIFA Passive Array Based Low Profile Spatial-Pattern Diversity Antenna for Mobile Terminals

(Technical Report)

I. INTRODUCTION or Abstract

In this work we report a very compact, low cost, spatial-pattern diversity antenna system for 2.45GHz ISM band. The overall size of the diversity antenna (including ground) is 95×49×6.4mm3, which is suitable for use in mobile terminals like mobile phone, laptop, etc. The geometry of the proposed antenna is depicted in Figs. 1 and 2 and the dimensions are listed in Table I. The proposed diversity antenna system consists of two sets of passive arrays, A 1 and A2

each of which consists of a probe fed planar inverted ‘F’ antenna (PIFA) and an open circuited PIFA. Since the open circuited PIFA does not have the feeding probe, henceforth it would be referred as planar inverted ‘L’ antenna or PILA. The PIFA and PILA radiators are placed anti-symmetrically on a small slitted ground of size 47×24mm2. The anti-symmetric placement consists of short-end of PILA placed close to the radiating end of PIFA, and introduces strong mutual coupling between them. The PIFA and PILA antennas are separated by a longitudinal slit which contributes to PILA work as a director. The directive nature of the passive array increases as we reduce the size of PILA.

In this work, first we design a directive passive array. Next we arrange two such passive arrays in such a way that they radiate in complementary regions of space. Besides, we take necessary measures to keep mutual coupling between driven PIFAs as low as possible. The proposed antenna solution has been realized using copper sheet of 0.397mm thickness. 50Ω

RG405 coaxial cables (d = 0.51mm, D = 1.7mm and rε = 2.1) have been used to feed the PIFAs.

Give important results about bandwidth, back-lobe radiation, S12, pattern correlation factor, etc?

Fig. 1 The proposed pattern (spatial) diversity antenna

Fig. 2 Top view of the proposed pattern (spatial) diversity antenna. Separation distance between PIFA and PILA?

A2

A1

HH

HH

WP

A2

A1

LAPW

G

LG

WP

LPP

∆LFeed W

Slot

WSlot

WG

LG

WP

LPP

LAP

WP

∆LFeedGap-X

Gap-Y

TABLE-IList of Structural Parameters of the Pattern Diversity Antenna

Copper sheet thickness: 0.397mm, Wp = 5mm, H = 6mm

Ground plane

LG 47 mmW 24 mm

WSlot 4 mmGap-X 1 mmGap-Y 1 mm

PIFALAP 24 mm

∆LFeed 3 mmPILA LPP 18 mm

Transverse distance

between PIFA andPILA

II. PRINCIPLES OF OPERATION OF THE PROPOSED PASSIVE ARRAY

In order to understand the principles of operation of the proposed antenna, first we study the behavior of the PIFA alone on a continuous ground, 50×24mm2. The antenna structure is shown in Fig. 3 and all its dimensions are summarized in Table II. |S11| of this antenna is plotted in Fig. 4, which shows that the antenna resonates at about 2.504 GHz. It may be pointed out that the antenna resonant frequency is sensitive to the size of the ground plane and any slit cut into it.

Fig. 3: PIFA on a continuous ground (50mm×24mm) define the various parameters

TABLE-IIList of Design Parameters of the PIFA Shown in Fig. 3

Copper sheet thickness: 0.397mm

LG

WLAP

WP

H∆LFeed

50 mm24 mm25 mm5 mm6 mm3 mm

Fig. 4 |S11| of a PIFA on continuous ground plane

Next we place an identical sized PILA very close to the PIFA as shown in Fig. 5. The transverse separation between the antennas is ? The open ends of PIFA and PILA are arranged on the opposite sides. The geometry is similar to that of a dipole which has been cut into two identical halves and arranged as shown. The asymmetric placement produces strong mutual coupling between the monopoles [1]. Due to the coupling, the original mode of resonance of the PIFA gets split into two coupled modes of operation at f0_Low = 2.08 GHz and f0_High = 2.77 GHz, Fig. 6. The inductive mutual coupling through the current on the common ground between the resonators might be responsible for this phenomenon.

Fig. 5 Driven and passive PIFA placed asymmetrically on continuous ground (50mm×24mm)Define the separation between PIFA and PILA

TABLE-IIIList of Design Parameters of the Structure Shown in Fig. 5


Ground PlaneLG 47 mmW 24 mm

PIFALAP 25 mm

∆LFeed 3 mmPILA LPP LAP

Transverseseparationbetween PIFA

and PILA?

Fig. 6 |S11| of PIFA and an identical PILA on continuous ground plane (as shown in Fig. 5)

Figs 6 and 3 may be combined.

In Yagi-Uda type antennas, the sizes of the passive elements are made larger or smaller than the active driven element so that the passive elements work as reflector or director, respectively. Electrically, the director current leads and the reflector current lags the current of the driven element [2], [3]. However, it is found for the passive array shown in Fig. 5 that the change in the length of PILA does not make the passive element work as director or reflector. Instead, the change in the length of PILA gives rise to change in the resonant frequencies of coupled modes. Fig. 7 shows |S11| plot for different values of ΔL, where ΔL = (LAP – LPP). These studies imply that the longer PILA adds more capacitance to the coupled system and the coupled mode resonant frequencies shift to lower values. The radiation patterns for this array show an interesting

behavior. The pattern in the xz-plane )0( °=φE is plotted in Fig. 8 for different values of LPP

at the lower resonant frequency, and is found to be monopole-like. The size of PILA influences the back-lobe radiation and directivity. The higher directivity is achieved at the cost of larger

back-lobe radiation. The radiation pattern ( )E 0φ = °u

at the higher resonant frequency is dipole-like

with maxima at °±= 90θ and near null at °=0θ , Fig. 9.

Fig. 7 |S11| of the passive array shown in Fig. 5 for different values of ΔL. (—) line is for ΔL = 6mm, (—) line is for ΔL = 3mm, (—) line is for ΔL = 0mm, (—) line is for ΔL = – 3mm, (—) line is for ΔL = – 6mm.

Fig. 8 ( )E 0φ = °ur

(in dB) of the passive array shown in Fig. 5 for different values of ΔL. (—) line is for ΔL = 6mm,

(—) line is for ΔL = 3mm, (—) line is for ΔL = 0mm, (—) line is for ΔL = – 3mm, (—) line is for ΔL = – 6mm. For all values of ΔL, the pattern is simulated at the corresponding lower resonant frequency.

Fig. 9 ( )E 0φ = °ur

(in dB) of the structure shown in Fig. 5 for different values of ΔL. (—) line is for ΔL = 6mm, (—)

line is for ΔL = 3mm, (—) line is for ΔL = 0mm, (—) line is for ΔL = – 3mm, (—) line is for ΔL = – 6mm. For all

values of ΔL, ( )E 0φ = °ur

is simulated at their corresponding upper resonant frequency.

For the PILA to work as a director element we cut a narrow slit in between the PIFA and PILA [see Fig. 10 and Table IV]. This slit serves two purposes. First, it splits the ground plane between driven and passive antennas. As a result, the coupling between the antennas gets modified and the antenna with slitted ground and identical sized PIFA and PILA resonates at three frequencies, 2.2 GHz, 2.5 GHz, and 3.1 GHz[Fig. 11]. The resonance at 2.5 GHz corresponds to that of the uncoupled mode, and the coupled mode resonances have moved to higher frequencies due to the capacitive effect of slit.

The other effect of the coupling slit is to introduce capacitance in the current path between driven and passive antennas. Generally, director elements are loaded capacitively by using lumped capacitors or tunable varactor diodes to make the director current lead the current of the driven element [4]. In the proposed structure, this capacitance is introduced by the slit. The slit is easier to fabricate along with rest of the antenna and is a low cost solution . The slit capacitance

may be determined by modeling the slit as a slot line. Figure 12 depicts the normalized ( )E 0φ = °ur

pattern at 2.5 GHz? In this array, the power radiated in -x direction (towards PILA) is 2.96dB higher than the power radiated in +x direction. Since the radiation gets enhanced towards PILA, it therefore acts as a director. We, next study the effect of variation of slit width on the directivity of radiation pattern.

Fig. 10 Active and passive PIFA placed asymmetrically on slitted ground (Ground size is 50mm×24mm and slit width is 0.5mm). Transverse separation between PIFA and PILA?

TABLE-IVList of Design Parameters of the antenna Structure Shown in Fig. 10



WSlot 0.5 mm

PIFALAP 25 mm

∆LFeed 3 mmPILA LPP LAP

Transverseseparation

Fig. 11 |S11| of the structure shown in Fig. 10

Fig. 12 Normalized ( )E 0φ = °ur

pattern (in dB) at 2.5GHz or 3.1GHz?

A. Variation of Array Performance with Change in Wslot

In the proposed passive array, the slit width Wslot is an important design parameter. The slit width influences both the directivity and the resonant frequencies of the antenna. The variation of

|S11| for various values of Wslot is plotted in Fig. 13. It shows that the resonant frequency for the uncoupled mode decreases with increase in slit width. The resonant frequencies for the coupled modes however remain mostly unchanged. The uncoupled mode behavior may be explained by a simple equivalent circuit model for the antenna presented in Fig. 14. Here, PIFA is modeled as a parallel resonating network where R represents radiation resistance and conductor losses in PIFA. LPatch is the inductance and C is the capacitance associated with PIFA, LG_Active is the inductance of the ground underneath PIFA, LG_Passive is the inductance of the rest of the ground associated with the ground underneath PILA, and CSlot is the slit capacitance. The effective inductance Leffective for this mode is therefore given by

Leffective = LPatch + [(LG_Active – ω2LG_ActiveLG_PassiveCSlot)/( 1 – ω2(LG_Active + LG_Passive)CSlot)]. ...(1)

This expression indicates that for the continuous ground case (i.e. for W slot = 0, CSlot = ∞ ), Leffective = LPatch + (LG_ActiveLG_Passive), which is the minimum value of Leffective. Equation (1) also indicates that Leffective increases with increase in Wslot or decrease in CSlot. Increase in Leffective brings down the resonant frequency. Therefore (1) predicts that resonant frequency decreases with increase in slit width Wslot. This prediction agrees with the results presented in Fig. 13 for the uncoupled mode. The tuning of uncoupled mode resonant frequency may also be realized by attaching a biased varactor diode across the slit. The effect of slit width on radiation pattern is studied next.

CSlot is the capacitance in the current path between the driven and passive element. The capacitive reactance of CSlot increase with increase in slot width Wslot influences the directivity of the radiation pattern, Fig. 15. As we increase Wslot from zero value, the directivity increases initially. For Wslot larger than 4 mm, directivity starts decreasing. This is expected because increasing Wslot beyond a certain limit weakens coupling between PIFA and PILA. The size constraint of the antenna also dictates small slit width. The figure of merit for the directive passive array is the back-lobe radiation. The smallest value of back-lobe radiation is

-8dB corresponding to Wslot =?

CSlot

R

LG_Active LG_Passive

C

Fig. 13 |S11| of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm, (—) line is for WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm.

Fig. 14 Simplistic Lumped Circuit Model of the structure shown in Fig. 10. The encircled portion of the circuit represents equivalent circuit of the slitted ground.

LPatch

Equivalent circuit of the slitted ground

Fig. 15 ( )E 0φ = °ur

(in dB) of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm,

(—) line is for WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm. ( )E 0φ = °ur

is simulated at

resonant frequency of each structure.

B. Variation of Array Performance with Change in Length ΔL of PILA

The resonant frequency and radiation pattern of the proposed passive array can also be controlled by tuning the length of PILA. It is found that the uncoupled mode resonant frequency is not affected by the change in length ΔL of PILA. However, the radiation pattern at this frequency becomes more directive as we decrease ΔL i.e. as we decrease ΔL, the PILA works as a better director. This is shown in Fig. 17. In addition, there is a beam squint of about °+15 . This is due to the fact that PIFA and PILA act as travelling wave array and the phase difference between them produces the squint. The minimum value of back-lobe is found to be ?dB corresponding to ΔL=? Determine the optimum values of Wslot and ΔL for maximum suppression of back-lobe and use it in the next section.

C. Variation of Array Performance with Change in Position of Slit??

One may carry out parametric studies on the effect of slit position on array performance. Can the equivalent circuit model of Fig. 14 explain the effect of slit position on the resonant frequency of uncoupled mode?

Fig. 16 |S11| of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm, (—) line is for

WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm. Should plot variation with ΔL?

Fig. 17 ( )E 0φ = °ur

(in dB) of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm,

(—) line is for WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm. ( )E 0φ = °ur

is simulated at

resonant frequency of the structure at 2.5GHz?. Why there is a plot for WSlot = 0 mm? Should show variation with

ΔL?

III. DESIGN OF THE PROPOSED PASSIVE ARRAY

In the last section we have shown that the resonant frequency decreases with increase in W slot. In presence of the PILA (with ΔL = 0), the resonant frequency of the driven PIFA (on slitted ground with 0.5 mm slit width) was 2.49 GHz [Fig. 11]. But for 6mm slit width, resonant frequency decreased to 2.38 GHz [Fig. 13]. Hence, after designing the antenna for desired radiation pattern, one needs to readjust the dimensions LG and LAP to make the antenna radiate in the desired band of frequency. In spite of this design complexity, the proposed structure has the advantage that widening of slit makes the antenna more compact.

In this work, after setting Wslot = 4 mm and ΔL = -6 mm, we have adjusted LG and LAP to make the passive array radiate at 2.45GHz ISM band. In the final design LG = 47 mm, WG = 24 mm [see Fig. 18 and Table V]. Figure 19 shows that the antenna operates in 2.45 GHz ISM band and its return loss bandwidth is 100MHz, which is more than the bandwidth of 2.45 GHz WLAN. What

about other resonances? Figure 20 shows Eur

pattern in xz plane and Fig. 21 shows Eur

pattern in

xy plane. In this antenna, the power radiated in -x direction is 7.61dB higher than the power radiated in +x direction [Fig. 20].

Fig. 18 Final design of the passive array

TABLE-VList of Design Parameters of the Structure Shown in Fig. 18



WSlot 4 mm

PIFALAP 25 mm

∆LFeed 3 mmPILA LPP 19 mm

Fig. 19 |S11| of designed passive array shown in Fig. 18. What about the other two modes?

Fig. 20 Normalized Eur

pattern (in dB) of passive array (shown in Fig. 18) in xz plane


pattern (in dB) of passive array (shown in Fig. 18) in yz plane

IV. DESIGN OF THE PROPOSED SPATIAL-PATTERN DIVERSITY ANTENNA

Next we arrange two identical passive PIFA arrays (A1 and A2) in such a way that they radiate in complementary regions of space and their mutual coupling becomes small. For this, the arrays

are placed in diagonally opposite quadrants. Figs 1 and 2 show the proposed arrangement. Dimensions of the array are summarized in Table I. The array A2 is the translated and shifted version of array A1. In this arrangement A2 is positioned with respect to A1 such that it is not illuminated by the radiation of A1, and vice versa. This is achieved by placing the arrays in diagonally opposite quadrants and positioning the open ends of A1 and A2 away from each other as shown. The passive arrays are separated by a distance of 1 mm along x- and y-axis. Due to this arrangement we achieve high isolation between two arrays (|S12| < -18 dB over the -10 dB return loss bandwidth) [Fig. 23]. The figure should include other resonances also? Result shows that the proposed antenna operates in 2.45 GHz ISM band [Fig. 22]. Bandwidth of WLAN (in 2.45 GHz band) is 80 MHz. In this work we got 100 MHz band width. Figure 24 to Fig. 27 shows

normalized radiation Eur

pattern in xz and yz planes for A1 driven (A2 terminated in matched

load), and A2 driven (A1 terminated in matched load). Results show that power radiated by A 1 in +x direction is –7.52dB lower than the power radiated in –x direction and power radiated by A2 in -x direction is –8.4dB lower than the power radiated in +x direction. Hence, the proposed antenna has very low antenna pattern correlation, how much?

The major contributions of this work are as follows:

(a) The PIFA antenna may be tuned electrically by cutting a slit in the ground plane in the longitudinal direction. The amount of capacitive reactance of the slit can be varied by mounting a reverse-biased varactor diode across it. Further studies on this aspect may produce a publication if not already reported.

(b) Although PIFA is a monopole antenna, a dipole like radiation pattern in its °=0φ plane may be obtained by placing an identical sized PILA in its close proximity, Fig. 5 and 9.

(c) A passive array of PIFA and PILA on a slitted ground may be employed as an end-fire antenna, the directivity and back-lobe radiation of which can be controlled by means of slit width or fixed slit plus a varactor diode across it, Fig.15.

(d) A passive array of PIFA and PILA on a slitted ground may be employed as an end-fire array which can be scanned electronically, Fig. 17. This may be further investigated by increasing the number of elements and slits.

(e) Spatial pattern diversity antenna may be realized by employing a pair of passive arrays of PIFA and PILA on a slitted ground, Figs. 1,2, 24 and 25.

Fig. 23 |S11| and |S12| (in dB) of the proposed spatial-pattern diversity antenna. Other resonances?


pattern (in dB) of the proposed spatial-pattern diversity antenna (A1 driven and A2 terminated

in matched load) in xz plane

( ) |S11

|

( ) |S12

|



in matched load) in xz plane



in matched load) in yz plane



in matched load) in yz plane

REFERENCES

1. K. L. Virga and Y. Rahmat-Samii, “Low-profile enhanced-bandwidth PIFA antennas for wireless communications packaging,” IEEE Transaction on Antennas And Wireless Propagation Letters, vol. 45, No. 10, pp. 1879–1888, Oct. 1997

2. J. D. Kraus, “Antennas”

3. C. A. Balanis, “Antenna Theory Analysis and Design”

4. Y. Yusuf and X. Gong, “A low-cost patch antenna phased array with analog beam steering using mutual coupling and reactive loading,” IEEE Antennas And Wireless Propagation Letters, vol. 7, pp. 81–84, 2008.

Engineering

Pifa array2 rg