Printed Inverted-F Antenna Array

1. 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, A1 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 r e = 2.1) have been used to feed the PIFAs.
Give important results about bandwidth, back-lobe radiation, S12, pattern correlation factor, etc?

2. H
H
Fig. 1 The proposed pattern (spatial) diversity antenna
A2
A1
H
H
WP
LG
WP
LG
WP
LPP
Gap-Y
Fig. 2 Top view of the proposed pattern (spatial) diversity antenna. Separation distance between PIFA and PILA?
A2
A1
W LAP G
LPP
ΔLFeed WSlot
WSlot WG
LAP
WP
ΔLGap-X Feed

3. TABLE-I
List of Structural Parameters of the Pattern Diversity Antenna
Copper sheet thickness: 0.397mm, Wp = 5mm, H = 6mm
Ground plane
LG 47 mm
W 24 mm
WSlot 4 mm
Gap-X 1 mm
Gap-Y 1 mm
PIFA LAP 24 mm
ΔLFeed 3 mm
PILA LPP 18 mm
Transverse
distance
between PIFA and
PILA
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

4. TABLE-II
List of Design Parameters of the PIFA Shown in Fig. 3
Copper sheet thickness: 0.397mm
LG
W
LAP
WP
H
ΔLFeed
50 mm
24 mm
25 mm
5 mm
6 mm
3 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.

5. Fig. 5 Driven and passive PIFA placed asymmetrically on continuous ground (50mm×24mm)
Define the separation between PIFA and PILA
TABLE-III
List of Design Parameters of the Structure Shown in Fig. 5
Copper sheet thickness: 0.397mm, Wp = 5mm, H = 6mm
Ground Plane LG 47 mm
W 24 mm
PIFA LAP 25 mm
ΔLFeed 3 mm
PILA LPP LAP
Transverse
separation
between
PIFA
and PILA?

6. 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.
E 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 ( f = 0 ° ) 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
u
back-lobe radiation. The radiation pattern E(f = 0°)
at the higher resonant frequency is dipole-like
with maxima at q =±90°and near null at q =0°, Fig. 9.

7. 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.

8. ur
Fig. 8 E(f = 0°)
(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.
ur
Fig. 9 E(f = 0°)
(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
ur
values of ΔL, E(f = 0°)
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
ur
may be determined by modeling the slit as a slot line. Figure 12 depicts the normalized E(f = 0°)
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.

9. 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-IV
List of Design Parameters of the antenna Structure Shown in Fig. 10
Copper sheet thickness: 0.397mm, Wp = 5mm, H = 6mm
Ground Plane
LG 47 mm
W 24 mm
WSlot 0.5 mm
PIFA LAP 25 mm
ΔLFeed 3 mm
PILA LPP LAP
Transverse
separation

10. Fig. 11 |S11| of the structure shown in Fig. 10
ur
Fig. 12 Normalized E(f = 0°)
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

11. |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 Wslot = 0, CSlot = ¥),
Leffective = LPatch + (LG_Active║LG_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 =?

12. 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.
CSlot
R
LPatch
LG_Active LG_Passive
C
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.
Equivalent circuit of
the slitted ground

13. ur
Fig. 15 E(f = 0°)
(in dB) of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm,
ur
(—) line is for WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm. E(f = 0°)
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?

14. 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?
ur
Fig. 17 E(f = 0°)
(in dB) of the structure shown in Fig. 10 for different values of WSlot. (—) line is for WSlot = 0 mm,
ur
(—) line is for WSlot = 2 mm, (—) line is for WSlot = 4 mm, (—) line is for WSlot = 6 mm. E(f = 0°)
is simulated at

15. 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 Wslot.
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

16. TABLE-V
List of Design Parameters of the Structure Shown in Fig. 18
Copper sheet thickness: 0.397mm, Wp = 5mm, H = 6mm
Ground Plane
LG 47 mm
W 24 mm
WSlot 4 mm
PIFA LAP 25 mm
ΔLFeed 3 mm
PILA LPP 19 mm
Fig. 19 |S11| of designed passive array shown in Fig. 18. What about the other two modes?

17. Fig. 20 Normalized Eur
pattern (in dB) of passive array (shown in Fig. 18) in xz plane
Fig. 21 Normalized Eur
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

18. 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 A1 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 f =0° 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.

19. Fig. 23 |S11| and |S12| (in dB) of the proposed spatial-pattern diversity antenna. Other resonances?
Fig. 24 Normalized Eur
pattern (in dB) of the proposed spatial-pattern diversity antenna (A1 driven and A2 terminated
in matched load) in xz plane
( ) |S11|
( ) |S12|

20. Fig. 25 Normalized Eur
pattern (in dB) of the proposed spatial-pattern diversity antenna (A2 driven and A1 terminated
in matched load) in xz plane
Fig. 26 Normalized Eur
pattern (in dB) of the proposed spatial-pattern diversity antenna (A1 driven and A2 terminated
in matched load) in yz plane

21. Fig. 27 Normalized Eur
pattern (in dB) of the proposed spatial-pattern diversity antenna (A2 driven and A1 terminated
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.