Compact reconfigurable antenna for LTE, WLAN and WiMAXapplications

ANURADHA A PALSOKAR-DESHPANDE* and S L LAHUDKAR

Imperial College of Engineering and Research, Pune, India

e-mail: panuradha2010@gmail.com

MS received 24 April 2021; revised 30 July 2021; accepted 7 August 2021

Abstract. A compact rectangular patch antenna is presented in this work, which is able to change the

frequency of operation and also the radiation pattern. The designed antenna can be used for long-term evolution

(LTE), wireless local area network (WLAN), and microwave connectivity worldwide interoperability (WiMAX)

applications. A PIN diode is used to connect or disconnect the L-shaped strip from the rectangular patch for

reconfigurable operation. Pattern search algorithm is used to optimize the position of the antenna feed for better

impedance matching. Testing results of the fabricated prototype show appreciable matching with the simulated

return loss and voltage standing wave ratio (VSWR). Also, good impedance matching (VSWR \1:4) and a

good frequency tuning ratio (2.26) are achieved for the prototype.

Keywords. Frequency reconfigurable antenna; pattern reconfigurable antenna; PIN diode; WLAN; WiMAX;

LTE.

1. Introduction

Mobility allows users to physically switch when using an

appliance, such as a portable personal computer or data

collector. Many professions require staff to be mobile;

these include inventory clerks, healthcare workers, police-

men, and emergency care specialists. With the high demand

for Wi-Fi networks, far more mobile devices can support

wireless services like the time-division–long term evolution

(LTE), wireless local area network (WLAN), and micro-

wave connectivity worldwide interoperability (WiMAX).

The frequency reconfigurable antenna covers multiple

services with a single device, while the polarization or

pattern reconfigurable antenna can increase system capacity

and efficiently receive signals in multi-path environments.

In particular, a reconfigurable pattern antenna may regulate

a primary beam or null direction in a particular direction.

The reconfiguration of patterns can enhance the capacity of

communication systems and assist the antenna to receive

signals in a multi-path environment efficiently. The ability

to alter its radiation pattern dynamically increases the

diversity that fixed antennas do not allow. Hence, these

antennas may be applied to decrease interference and take

advantage of multipath phenomena to significantly enhance

wireless connection quality and expand system capability.

A single-element antenna that can be reconfigured in fre-

quency and radiation pattern is undoubtedly a good choice.

Reconfigurable multiband antennas will also minimize the

number of antennas and eliminate band-pass filters in a

multiband system which will lead to overall volume

reduction and cost-saving. Hence, the design of a recon-

figurable multiband antenna has attracted a lot of interest in

academia and industry due to lower cost, low volume, or

multi-band operations.

In the literature, we find many designs of reconfigurable

antennas. Also, various techniques are observed in the lit-

erature to change the antenna properties to achieve recon-

figurability. Most of the designs presented used switching

diodes to make the structural changes in the geometry of

the antenna and change the electrical length and current

direction to achieve frequency and pattern tunability.

The antenna in [1] was a folded slot antenna resembling

a T shape with a stub at the bottom side of the T-shaped

slot. Three PIN diodes are used on the antenna to change its

operating frequency by adjusting both positions and arm’s

length. A simple monopole was presented in [2], which

used two switches to achieve frequency agility by altering

the electric length of the monopole. A dual-band switchable

vertical folded monopole antenna was designed in [3].

Antenna’s radiator was a small stem with two branches

acting as two monopoles and was being designed to res-

onate at two different frequency bands.

A V-shaped tapered slot antenna was described in [4] for

different frequency bands in the 1–4 GHz range. The

tapered slot was connected with two T-shaped and two

C-shaped resonators using PIN diodes. The length of the

stub for a T-shaped resonator was greater than the stub for a

C-shaped resonator. The antenna could work in various*For correspondence

Sådhanå (2021) 46:228 � Indian Academy of Sciences

https://doi.org/10.1007/s12046-021-01729-7Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

frequency modes by switching diodes. A Vivaldi antenna

with five slots on both sides and a circular patch at the end

of the tapered section was presented in [5]. The antenna

was made reconfigurable by adding an ideal switch in the

tapered portion of the Vivaldi structure. A trapezoidal

antenna was modified into a split ring structure by having a

loop on one side and the splits on the other in [6]. A single

switch was used with a split ring resonator to change the

frequency. A microstrip line fed antenna with two rectan-

gular patches etched with complementary split-ring ele-

ments could change the frequency with a diode connected

between the feed and one of the patches. The ground of this

antenna was truncated to support the operation and the

switch was ideal [7]. A C-shaped monopole designed in [8]

was capable of changing the frequency in two bands by

changing the electric length using a PIN diode and a chip

inductor.

An antenna capable of operating at multiple frequen-

cies in the range of 3.4–3.7 GHz and also capable of

steering the beam was presented in [9]. The design was a

square patch monopole with six shorting vias in the

center. Four stubs on four sides were connected to the

monopole using varactor diodes. DC bias applied to the

varactor is used to get the reconfigurable operation. A

modified version of this monopole was given in [10],

where the frequency band was changed to 2.6–3.81 GHz

using three stubs, each on two sides of the antenna. The

design in [11] has an inverted U-shaped rectangular patch

around the inset feed patch, these two patches are sepa-

rated by a slot with a PIN diode to achieve frequency

and pattern change.

From the study and analysis of various reconfigurable

antennas in the literature, it is noted that reconfigurability

is achieved using electronic switches. More number of

switches not only leads to multiple operating conditions

but also leads to the complexity of the biasing network

and increases the associated losses. The proposed antenna

in this work is a compact rectangular patch frequency

and pattern reconfigurable antenna for handheld devices

for LTE, WLAN, and WiMAX applications. The antenna

is designed using only one switching diode and the

biasing elements are placed away from the radiating

element. The design of a multi-functional reconfigurable

antenna with reduced complexity is the focus of this

work.

The paper is organized as follows. Section 1 elaborated

the motivation for the work presented in this paper, it also

elaborated the literature survey done to understand various

designs in the literature. Section 2 details the design of the

proposed antenna and the simulation results for the same. In

section 3, the testing results of the fabricated prototype are

given. This section also compares the performance of the

proposed antenna with the designs presented in the litera-

ture. The last section concludes the paper with a concluding

note.

2. Antenna design

The microstrip patch antenna is commonly used for wire-

less applications, owing to its lightweight, planar configu-

ration, and convenient integration with microstrip

technologies. Hence, the design started with a rectangular

patch antenna designed for 5.5 GHz with the footprints of

27� 34� 1:6mm3; the design equations for the rectangu-

lar patch antenna are taken from [12]. L-shaped strip is

added to the rectangular patch antenna to get dual-band

operation. This dual-band antenna resonates at 2.4 and

5.5 GHz. A rectangular patch antenna is modified by add-

ing a side stub to the patch antenna. The length of this stub

(Ls) is calculated using equation (1):

Ls ¼ k4

ffiffiffiffi

�rp ð1Þ

To convert the antenna into a frequency reconfigurable

antenna a PIN diode is connected between the rectangular

patch and L-shaped strip. With the operating condition of

the PIN diode, the antenna achieves a frequency of 5.5 GHz

under the off switch condition and 2.37 GHz under the on

switch condition. Figure 1(a) shows the geometry of the

antenna and figure 1(b) shows return loss for the antenna

with L-shaped strip.

The geometry of the antenna is further modified by

cutting a slot in the rectangular patch and by truncating the

left corners of the patch. With this modified geometry

shown in figure 2; the antenna is able to represent the fre-

quency and pattern agility for 2.36, 3.3, and 5.32 GHz,

respectively. The length of the rectangular slot is obtained

using equation (2), where the k value is calculated at

3.3 GHz

L ¼ k4

ffiffiffiffi

�rp ð2Þ

While designing an antenna, impedance matching is an

important criterion, as it decides the coupling effect and

return loss. At the location of the feed point, the input

impedance must be 50 X for the resonant frequency.

Finding this position for the feed is usually done by the hit

and trial method. But this manual method is cumbersome

and time-consuming. To avoid this cumbersome method of

trial and hit, the location of the feed position is optimized

using the pattern search (PS) algorithm. The PS algorithm

is a direct nonrandom method for searching minima of a

given function which is not necessarily differentiable,

stochastic, or even continuous. It can therefore be used

effectively to deal with optimization problems without any

knowledge on the gradient of the fitness function [13]. In

contrast to conventional optimization approaches, which

use gradient or higher derivatives information to look for an

optimum point, the PS algorithm looks for a collection of

points around the current point where the fitness function’s

228 Page 2 of 8 Sådhanå (2021) 46:228

value is lower than the present one. Hence, the PS algo-

rithm proves to be faster than the conventional algorithms.

The aim of this optimization is to obtain the feed position

for better impedance matching. Figure 3(a) shows the

convergence curve obtained during the optimization

process, the optimized feed position is the one that gives

minimum cost function. Figure 3(b) presents the graph of

the reflection coefficient of the antenna before and after

optimization.

After applying the PS algorithm, a noticeable change is

found in the value of return loss and impedance of the

antenna. Before application of the optimization procedure,

the impedance at 2.35 GHz was 42.9 X with �17:87 dB

return loss, which changed to the impedance of 49 X and a

return loss of �16:3 dB. At 5.32 GHz, after the application

of the optimization algorithm, the impedance is changed

from 45 to 50 X, and the return loss is improved from

�14:34 to �18:71 dB. The antenna has used one PIN

diode, so there will be two modes of operation according to

the state of the switch.

2.1 Mode I

When the switch is in the OFF state, the frequency obtained

is 3.3 GHz. The gain obtained here is 1.8 dBi, which is

shown in figure 4(a). The electric field distribution is given

in figure 4(b), which shows that the maximum intensity of

Figure 1. Simulation results for an antenna with a side stub.

(a) The geometry of the antenna. (b) Return loss.

Figure 2. Geometry of antenna

Figure 3. Effect of optimization. (a) Optimization of feed

position. (b) Reflection coefficient of the antenna before and after

optimization.

Sådhanå (2021) 46:228 Page 3 of 8 228

the electric field is along the left top corner of the rectan-

gular patch. The obtained band covers the 3.3 GHz WiMax

802.16.e standard.

2.2 Mode II

In this configuration, the antenna operates at two bands

centered around 2.35 and 5.32 GHz, which can be used for

LTE and WLAN applications. During the second mode of

operation when the diode is in the ON state; the antenna

shows a multiband operation with 2.35 and 5.32 GHz. The

gain of the antenna during the simulation as shown in fig-

ure 5 is 1.33 dBi at 2.35 GHz and 5.32 dBi at 5.4 GHz,

which is sufficient for handheld devices.

As the switch is turned ON the stub is connected with the

rectangular patch and the electric length of the antenna is

changed. Also, the electric field distribution is different in

the ON switch condition as compared to the OFF switch

condition. Hence, the radiation pattern is different in both

conditions. Also, when the switch is in ON state figure 6

shows that maximum electric field is along the stub for

2.35 GHz and for 5.32 GHz the electric field is along the

stub and the corner of the rectangular patch, therefore, the

radiation pattern will also differ for these frequency values.

2.3 Reconfigurable operation

As shown in the antenna geometry, the PIN diode is used to

obtain the frequency and pattern agility. As the diode is

turned off it is equivalent to an open switch; hence the

L-shaped strip is disconnected from the rectangular patch.

This operation changes the electrical length of the antenna

and as a result frequency of operation is also changed. The

switching operation of the PIN diode redistributes the sur-

face currents and thus alters the antenna radiating structure

topology or radiating edges. From the electric field distri-

bution on the antenna during ON and OFF mode of the PIN

diode, it is clearly observed that the radiation pattern for the

antenna will also change during these modes of operation

and pattern agility is observed

Figure 5. Gain for an antenna in mode II. (a) At 2.36 GHz.

(b) At 5.32 GHz.

Figure 4. Effect of optimization. (a) Gain for the antenna.

(b) Simulation results for an antenna in mode I.

228 Page 4 of 8 Sådhanå (2021) 46:228

3. Results and discussion

The antenna is fabricated using a low-loss glass epoxy FR4

substrate. The PIN diode used is SMP1320 SC79; the

equivalent circuit for this diode is shown in figure 7. For

biasing purposes, two inductors of 33 nH and two capaci-

tors of 10 pF are used. The fabricated antenna is shown in

figure 8(a). Result validation is done by testing the fabri-

cated antenna using a VNA 6000 vector network analyzer

for the values of return loss and voltage standing wave ratio

(VSWR). The testing setup used to test the prototype is

shown in figure 8(b). A Comparison of simulated and

measured results is given in table 1. The graph for the

simulated and measured return loss for the antenna proto-

type is depicted in figure 9. The simulated and measured

radiation patterns are shown in figures 10, 11 and 12. Due

to the inverted L-shape structure, the radiation pattern is

tilted at 45�. This is how pattern diversity is achieved.

Table 1 shows that the measured results appreciably

agree with the simulated results. Minor discrepancies in the

values are due to imperfections during the manufacturing

process or due to losses in biasing and DC blocking ele-

ments. The extended cables of the fabricated antenna for

activation of the PIN diode also contribute to the difference

in the values. The comparison of the antenna performance

is also done with various reconfigurable antennas designed

in the literature as given in table 2.

From the comparison given in table 2, we observe that

the antenna presented in this work is able to exhibit

Figure 6. Electric field for antenna in mode II. (a) At 2.36 GHz.

(b) At 5.32 GHz.

Figure 7. Equivalent electric circuit for the PIN diode. (a) ONstate. (b) OFF state.

Figure 8. Antenna prototype and testing set up. (a) Fabricatedantenna prototype. (b) Antenna testing set up using VNA 6000.

Sådhanå (2021) 46:228 Page 5 of 8 228

frequency and pattern reconfigurability for LTE, WLAN,

and WiMAX applications. The antenna achieves the

required operation and good frequency tuning ratio (2.25)

with a small profile and only a single PIN diode. Some of

the antennas in table 2 (e.g. [2, 3, 6, 7]) are having com-

parable size as the proposed prototype but those are only

frequency reconfigurable antennas. For a similar range of

frequencies the presented antenna achieves frequency and

pattern reconfigurability with a comparatively smaller

footprint and good frequency tuning ratio. The use of a

single diode leads to a simple biasing network and minimal

losses associated with the biasing network. Also, the use of

the PS algorithm helps to avoid manual efforts for finding

the optimized position of the feed.

Table 1. Comparison of simulated and measured result.

Switch condition Results Frequency (GHz) Return loss (dB) VSWR Bandwidth (MHz)

Switch in OFF state Simulated results 3.30 - 15.90 1.38 95

Measured results 3.34 - 17.65 1.30 85

Switch in ON state Simulated results 2.35 - 16.25 1.37 100

5.32 - 18.71 1.37 250

Measured results 2.40 - 16.04 1.37 95

5.37 - 14.88 1.4 280

Figure 9. Comparison of simulation and testing results. (a) Dur-ing switch OFF state. (b) During switch ON state.

Figure 10. Simulated and measured radiation pattern for OFF

switch condition. ( simulated co-polarization, measured co-

polarization, – simulated cross polarization, – – measured cross

polarization).

228 Page 6 of 8 Sådhanå (2021) 46:228

Figure 11. Simulated and measured radiation pattern for ON

switch condition at 2.35 GHz ( simulated co-polarization,

measured co-polarization, – simulated cross polarization, – –

measured cross polarization).

Figure 12. Simulated and measured radiation pattern for ON

switch condition at 5.32 GHz ( simulated co-polarization,

measured co-polarization, – simulated cross polarization, – –

measured cross polarization).

Table

2.

Antennaperform

ance

comparison.

Ref.

Size(m

m3)

Frequency

(GHz)

Frequency

tuningratio

Pattern

recon.

Switches

Max.gain(dBi)

[1]

30�40�1:524

3.4,5.27and2.42,5.79

2.39:1

Yes

3(PIN

diodes)

5.2

[2]

20�35�1:6

2.45,3.5

and5.8

2.36:1

No

2(PIN

diodes)

NA

[3]

30�40�1:6

2.4,3.3,5.1

and5.6

2.33:1

No

3(PIN

diodes)

2.8

[4]

80�30�1:6

2.4,3.5

and5.8

2.41:1

No

4(PIN

diodes)

4.5

[5]

80�30�1:6

2.4,3.5

and5.2

2.16:1

Yes

4(Idealsw

itches)

5.4

[6]

27�25�1:6

2.4,3.5

and5

2.41:1

No

1(PIN

diode)

2.98

[7]

32�26�0:64

2.4

and5

2.08:1

No

1(Idealsw

itch)

6

[8]

40�35�1:6

2.45,3.52,4.67,6.1

2.49:1

NA

2(Idealsw

itches)

NA

[9]

32�42�1:6

3.38,3.5,5.8

1.71:1

Yes

5(PIN

diodes)

4.97

[10]

45:8�80�12:5

2.4,5.4

2.25:1

Yes

5(PIN

diodes)

5

[11]

66�58�1: 6

2.47,3.8,5.36

2.17:1

Yes

1(PIN

diode)

5.34

Prop.Design

27�34�1:6

2.35,3.3

and5.32

2.26:1

Yes

1(PIN

diode)

5.54

Sådhanå (2021) 46:228 Page 7 of 8 228

4. Conclusion

A compact frequency and pattern reconfigurable antenna

for LTE, WLAN, and WiMAX applications is demon-

strated in this work. The antenna presented has many

advantages. The use of the PS algorithm for feed position

optimization is the novelty of this work. The required

reconfigurability is obtained using only one PIN diode. The

use of a single PIN diode decreases the complexity and

associated losses of the biasing circuit. Also, the biasing

circuit is not placed on the rectangular patch; this

arrangement can further reduce the effect of biasing circuit

on the radiation properties of the antenna.

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