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Engineering, Technology & Applied Science Research Vol. 8, No. 5, 2018, 3479-3483 3479  

  

www.etasr.com Shastri et al.: Coplanar Stripline Loaded Reconfigurable Loop Antenna for WLAN and … 

 

Coplanar Stripline Loaded Reconfigurable Loop 

Antenna for WLAN and WiMAX Applications 
 

S. P. R. Shastri 

Pacific Institute of Technology, Pacific 

University, Rajasthan, India 

shailendra.shastri@thakureducation.org 

R. R. Singh 

Thakur Educational Trust, 

Mumbai, India 

ravishrsingh@yahoo.com 

K. V. Ajetrao 

K. J. Somaiya College of Engineering, 

Mumbai, India 

kiranjetrao@somaiya.edu 
 

 

Abstract—In this paper, a loop antenna loaded with coplanar 

strip (CPS) line is proposed as a multiband antenna. The CPS 

line is added with two switches to vary the antenna perimeter to 

cover seven different bands. The CPS line introduced into the 

loop is not only useful in reconfiguring antenna dimensions but 

also provides stationary radiation patterns for the all the covered 

bands. The proposed antenna works in single and dual-band 

modes. When the proposed antenna works as a single band 

antenna, it produces a band from 4.2GHz to 5.7GHz. Under dual-

band operation, it produces bands from 3.75GHz to 4.7GHz and 

from 6.4GHz to 7.8GHz. The other dual-band mode ranges from 

3.5GHz to 3.8GHz and from 5.58GHz to 7.4 GHz. The simulated 

and measured results are in good agreement and the proposed 

antenna can be used satisfactorily for W-LAN and WiMax 

applications. The proposed technique can also be used for size 

reduction of loop antennas. 

Keywords-coplanar stripline; loop antenna; multiband; 

wideband; wavelength 

I. INTRODUCTION 

UWB band allocation ranges from 3.1GHz to 10.6GHz. 
There are 12 different bands, each 528MHz wide [1]. The 
center frequency of each band from [1] is used as reference 
below. The electrical behavior of the wire loop antenna is 
examined in [2]. For a fixed radius of the loop, the impedance 
of the antenna depends on the wire thickness. The relation 
between the wire thickness and loop radius is referred to as 
thickness factor (Ω). The thickness factor is defined in (1): 

Ω = 2ln	(2�	 
⁄ )    (1) 

where Ω is the thickness factor, 	 is the radius of the loop in 
meters and 
 is the wire radius in meters, used to construct the 
loop antenna. The impedance variation of the loop as a function 
of the thickness factor and the loop circumference can also be 
seen in [2]. As the thickness factor decreases, antenna 
impedance also decreases. Therefore by varying thickness of 
the loop antenna, the impedance of the antenna can be varied 
easily. The loop antenna is categorized as thin or thick loop. 
The thin antenna has Ω>9 and resonates at more than one 
frequencies [2]. The thick antenna is capacitive in nature with 
the advantage of almost uniform resistance. The loop antenna is 
also categorized as small or large loop antenna. When the 
circumference of the loop is equal to or larger than the 

operating wavelength, the loop is called a large loop and small 
otherwise. In general, the small loop antenna has a 
circumference less than λ/10, where λ is the wavelength at 
which the antenna is designed. The small loop is a poor radiator 
while the large loop antenna is a good radiator as shown in [2]. 
The loop antenna resonates at a wavelength equal to the 
circumference of the loop. The circumference of the loop is 
calculated using (2) and (3): 


 = 2�	 = �     (2) 

� = � �⁄      (3) 

where C is the circumference of the loop antenna (m), r is the 
radius of the loop (m), � is the speed of light in free space 
(m/s), � is the design frequency of the loop (Hz).  

A multiband antenna should operate at different frequencies 
with stable radiation patterns. Since a thin and large loop 
antenna resonates at different frequencies [2], it can be used to 
operate at different frequencies. It is observed that the pattern at 
a smaller wavelength is different from the pattern at a larger 
wavelength for the specified band of operation. This happens 
due to the variation in the current distribution along the loop 
due to variation in the operating wavelength. A thin loop with 
Ω=12 resonates at C/λ=1.1 i.e. C=1.1λ or C=2.2λ. The second 
resonant wavelength is twice the first. Therefore we can design 
a loop to have the two desired bands using (2). But this 
technique suffers from poor stability in radiation patterns. 
Figure 1 explains this very well. A simple wire loop whose 
circumference is C=λ radiates normal to its plane as shown in 
Figure 1(a). When the same loop is excited at a wavelength 
double to that of the previous case, then the pattern does not 
remain the same (Figure 1(b)). The radiation pattern at 2λ is too 
random. Such an antenna cannot be claimed as a multiband 
antenna. A simple solution to this problem is that the 
circumference of the loop should vary in accordance with the 
desired wavelength. This arrangement will keep the 
circumference C=λ or near to λ.  

II. RESEARCH METHOD 

Since mechanical changes in the entire structure of the 
antenna are not possible to hold C=λ true for different 
wavelengths, the loop structure in Figure 2 is proposed. The 
proposed loop is square in shape and a coplanar strip (CPS) 



Engineering, Technology & Applied Science Research Vol. 8, No. 5, 2018, 3479-3483 3480  

  

www.etasr.com Shastri et al.: Coplanar Stripline Loaded Reconfigurable Loop Antenna for WLAN and … 

 

line is added to it. The CPS line is added with multiple 
electronic switches that can be turned ON and OFF to change 
CPS line length. The change in the CPS line length finally 
changes the overall circumference of the loop. 

 

 
(a) 

 
(b) 

Fig. 1.  The loop antenna showing radiation patterns at (a) λ, (b) 2λ. 

 
Fig. 2.  Proposed loop antenna with CPS line along with the arrangement 

of switches. 

The total length of the loop is 4×L, L representing the arm 
length of the loop. When switch 1 is closed and the others are 
open, the length of the loop is 4×L+2d, where d is the length of 
the CPS line. Therefore, the loop length can be varied from 
4×L to 4×L+2d. The loop length can be varied in small steps if 
the switches are placed close to each other. It is known that the 
transmission line only carries electrical energy from one point 
to another but does not radiate significantly. Therefore the four 
sides of the loop are the only radiating components of the 
proposed structure. The role of the CPS line is to only vary the 
overall length of the structure and eventually the generation of 
multiple bands. This structure ensures that the current 
distribution along the radiating portion will remain the same, 
therefore uniform radiation pattern is expected. A square loop 
of 16mm outer length and 0.7mm and 1mm width is simulated 
using CST Studio software in order to know the availability of 
possible bands with the corresponding antenna impedance as 
shown in Figure 3. It can be observed in Figure 4 that the 
antenna behaves differently for the two widths. There can be 
two bands available only after proper impedance matching. The 
larger width ensures larger bands since the antenna impedance 
is uniform. It can be observed in Figure 5 that the width of the 
loop antenna affects its impedance. When antenna width is 

0.7mm, the loop resonates at 4.55GHz. The resistance at this 
frequency is about 125Ω. The resistance drops to about 96Ω 
for 1mm width. The loop with 1mm width shows minimum 
reactance at 5.1GHz and the resistance at this frequency is 
about 176Ω. The loop resistance is more than 270Ω for the 
same frequency when the width is 0.7mm. The antenna 
impedance for these frequencies is larger than the reference 
impedance of 50Ω, therefore there is poor reflection coefficient 
at these frequencies (Figure 4). Since the magnitude of the 
reflection coefficient is more than -8dB at the lower frequency, 
the band availability cannot be ignored. Another possible 
location for the availability of the band is at 7.6GHz and 
7.9GHz for a loop with width 0.7mm and 1mm respectively. 
The reactance is larger at these frequencies. The loop with 
1mm width has less variation in reactance than the loop with 
0.7mm width.  

 

 
Fig. 3.  Proposed loop antenna with CPS line along with the arrangement 

of switches. 

 
Fig. 4.  Frequency v/s reflection coefficient of the loop antenna for 

different widths. 

 

Fig. 5.  Loop impedance as a function of width and frequency. 

In order to solve the impedance mismatch problem, a 
printed BALUN transformer can be used to feed the antenna. 
The inclusion of the BALUN increases antenna size and if 
possible should be avoided. Here, we are providing a simple 



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solution to bring antenna impedance down to the reference 
impedance of 50Ω. Above, it was demonstrated that the loop 
impedance can be varied by varying the loop width. Therefore, 
instead of having a uniform width the loop can be made to have 
non uniform width. The proposed loop antenna contains non 
uniform width which changes gradually from 1mm to 1.5mm 
and then to 2mm towards feed end. Since there is no standard 
technique to introduce nonlinearity in the loop antenna, the 
dimension for the different arms of the loop is chosen only 
after multiple simulations. The different dimensions are given 
to the loop for reducing antenna impedance so that the loop can 
be directly fed with a 50Ω source. The dimensional information 
of the loop is shown in Figure 6. The feed is connected to the 
broader width of the antenna since it offers low impedance 
while the narrow width offers high impedance. Here the 
broader width helps converting loop antenna to a thick antenna 
which has low resistance with high capacitive reactance. The 
narrow dimension of the loop is present at the opposite of the 
feed and is 1mm wide. The narrow dimension of the loop 
converts the loop to a thin antenna which has high resistance 
and inductive and capacitive reactance. Therefore this 
combination brings overall antenna impedance to a uniform 
level. Finally, a folded coplanar strip line (CSP) feed is added 
to the loop. The folded CPS feed not only helps in lowering 
antenna impedance to 50Ω but also it saves the onboard space 
as most of the part of the folded CPS is within the loop area as 
shown in [1]. The proposed feed can be useful in feeding the 
antenna directly with a coaxial feed. The dimensional details of 
the folded CPS line are shown in Figure 7.  

 

 

Fig. 6.  The proposed loop with dimensional details. 

 
Fig. 7.  The dimensional details of the feed. 

The CPS line not only plays important role in the 
transformation of impedance but also converts an unbalanced 
coaxial cable to balanced feed [3]. The proposed loop is added 
with additional CPS line of 10mm length exactly opposite to 
the feed as shown in Figure 6. Therefore the inclusion of the 
CPS line can increase the length of the loop by 20mm. The 
CPS length can be increased or decreased by turning ON and 

OFF the switches and the overall length of the loop can be 
varied to obtain different frequencies. 

III. LOOP RESPONSE ANALYSIS 

In this section, the proposed reconfigurable loop is analyzed 
for different switching conditions of the switches along CSP 
line. The ON state of the switch is realized by the short circuit 
and the OFF state by the open circuit. The switch is operated 
from the left of the CPS line towards the thin arm of the loop 
and the response of the loop is tabulated in Table I. It can be 
observed from Table I that there are two distinct modes of 
antenna operation: single and dual band modes. The antenna 
works as dual band antenna when the switch at locations 0mm 
and 3mm is ON from the left end of the CPS line. The bands 
covered are Band-6, Band-7, and Band-2, Band-8 respectively. 
Only one switch is ON at a time while the others are OFF.  

TABLE I.  BANDS GENERATED UNDER SWITCH ON STATE 

Mode 
Switch 

Location 

f1-f2 10dB BW 

(GHz) 

Available 

band [1] 

Center frequency 

(GHz) 

% Band-

width 

A 0mm 
3.5-3.8 No Band 3.65 8.2 

5.6-6.85 Band-6, 7 6.23 20 

B 3mm 
3.7-4.5 Band-2 4.1 19.5 

6.8-7.43 Band-8 7.12 9 

C 8mm 4.17-5.9 Band-3, 4, 5 5 34.4 

D All Off 4.2-5.75 Band-3, 4, 5 4.975 31.2 

A, B = dual band and C, D = single band, f1 and f2 are the lower and upper cut –off frequency 
respectively. The switch location is measured from the left of the CPS line shown in Figure 6. 

 

It can be observed in Table I that when the switch at 8mm 
(near to the thin arm of the loop), is ON, the loop generates a 
wider band from 4.17GHz to 5.9GHz. The loop produces 
Band-3, Band-4 and Band-5 for this particular position of the 
switch. The loop generates the same bandwidth when all the 
switches present along the CPS line are OFF. Therefore to get 
the maximum number of bands at the minimum expense of 
switches, switch at location 8mm can be discarded. If we 
operate the switch at 0mm, and 3mm, we can easily get 7 
different bands. It is also observed that the loop antenna 
generates larger bandwidth when it is operated under single 
mode (mode C and mode D). The maximum % bandwidth 
under C mode is 34.4%. It is also important to note that under 
C mode, CPS line is almost completely removed from the loop 
as the switch is closed. The lowest frequency observed under 
this mode of operation is 4.17GHz. When the switch at 0mm is 
ON, complete CPS line is added in series with the loop. The 
lowest frequency under this mode, i.e. mode A, is 3.5GHz. 
Since the area occupied by the proposed antenna is the same 
for the two frequencies, this technique can also be used to 
reduce antenna size. The different bands under the influence of 
the operating switches are shown in Figure 8, which gives a 
good idea of how well the proposed technique covers a wider 
band from 3.5GHz to 7.43GHz. In order to check the stability 
of the radiation pattern for claiming multiband operation, the 
radiation pattern at the center frequency of each band listed in 
Table I is analyzed. The radiation patterns are orthogonal to the 
plane of the loop and are shown in Figures 9(a) and 9(b). The 
patterns are plotted at the center frequencies of each obtained 
band listed in Table I. The plane of the proposed loop is 
aligned with X-Y plane for all radiation patterns. An antenna 



Engineering, Technology & Applied Science Research Vol. 8, No. 5, 2018, 3479-3483 3482  

  

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having such a stable radiation pattern for different frequencies 
must have a uniform current distribution. 

 

 

Fig. 8.  The reflection coefficient v/s frequency for different switches. 

 
(a) 

 
(b) 

Fig. 9.  The simulated radiation patterns at the center frequency of the 

bands listed in Table I. 

In order to verify this relation, current distribution for all 
the plotted radiation patterns is reproduced and analyzed in 
Figure 10. The current distribution in the proposed antenna can 
be divided into two parts: a) Current in vertical arms, parallel to 
Y-axis and b) Current in horizontal arms, parallel to X-axis. 
The variation in the magnitude of current along the antenna is 
shown with a dashed line and the direction of current is shown 
with a continuous line (Figure 12(a)-(b)). It can be observed 
that the current distribution along Y-arms is in phase, while out 
of phase along X-arms. X-arms radiations are mutually 
canceled. On the other hand, the current from Y-arms 
contributes to radiation. The inclusion of the CPS line is 
completely justified here.as it only helps in reconfiguring the 

antenna dimension and keeps the current distribution uniform 
along the loop for all the frequencies. A comparison of the 
proposed loop is done with other recent existing reconfigurable 
antennas. The summary of the comparison is shown in Table II. 

 

 
 

(a) (b) 

Fig. 10.  The current distribution along the loop for different center 

frequencies. (a) f=3.9GHz, (b) f=7.1GHz 

TABLE II.  PROPOSED ANTENNA COMPARATIVE ANALYSIS 

Reference Bands Frequency (GHz) 
Number of 

switches 

Number of 

antennas 

[4] 2 1.8, 2.4 8 1 

[5] 2 8.2, 10.4 
1, MEM 

capacitor 
1 

[6] 2 15.95, 17.1 
1, MEM 
capacitor 

1 

[7] 2 2.4, 5.8 Motor 2 

[8] 
3 1.58, 2.38 2 1 

5 1.51, 2.28 4 1 

[9] 5 2.16-9 4 1 

[10] 3 2.4, 3.5, 5.2 6 1 

[11] 4 2.45, 3.49, 5.13, 5.81 2 1 

Proposed 7 3.9-7.4 2 1 
 

The antenna given in [4] generates two distinct bands 
centered at 1.8GHz and 2.4GHz. These two bands are 
generated using eight different switches. The antennas 
proposed in [5, 6] are multiband antennas generating two bands 
at 8.2GHz, 10.4GHz and 15.95GHz, 17.1GHz respectively. 
Both antennas use MEM switch for reconfiguration. There are 
four antennas proposed in [7]. The two antennas are used to 
generate two different bands and the others are used to change 
pattern and polarization. The two antennas used to generate 
different bands are selected using a rotating mechanism so the 
reconfiguration is controlled mechanically. The antenna 
proposed in [8] is operated with two switches and depending 
on the ON and OFF states of diodes, the antenna generates 
three different bands from 1.58GHz to 2.38GHz. When the 
same antenna is added with four diodes and is made ON or 
OFF in a particular sequence then it generates five different 
bands from 1.51GHz to 2.28GHz. The antenna proposed in [9] 
uses 4 switches and depending on the ON and OFF state of 
switches, it is operated as single, double and triple band 
antenna generating a total of 5 different bands. The antenna 
proposed in [10] uses 3 pairs of the diode and generates six 
different bands under single, dual and triple band modes. A 
single monopole antenna with two optical switches generates 
four different bands in [11]. The proposed antenna uses only 
two switches and can be operated as single and dual band 
antenna. When both switches are OFF, the loop works as a 
wideband antenna. The total bands generated by the proposed 
antenna are seven and is higher than all the antennas compared 



Engineering, Technology & Applied Science Research Vol. 8, No. 5, 2018, 3479-3483 3483  

  

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in Table II. The proposed antenna uses a lesser number of 
switches. Other than this the antenna profile is very compact. 

IV. RESULTS AND ANALYSIS 

The proposed reconfigurable antenna is manufactured and 
tested using Vector Network Analyzer N9926A. A photograph 
of the entire four proposed antennas is shown in Figure 11. A 
comparison between the simulated and measured reflection 
coefficient is shown in Figure 12.  

 

 
Fig. 11.  Photograph of the reconfigurable square loops for (a) no switch, 

(b) switch at 0mm, and (c) switch at 3mm and (d) switch at 8mm. 

 
(a) 

 
(b) 

Fig. 12.  Comparison between simulated and measured reflection 
coefficient. 

TABLE III.  SIMULATED AND MEASURED RESULT COMPARISON 

Switch 

Location 

Simulated 

f1-f2 (GHz) 

Measured 

f1-f2 (GHz) 

Bands 

Available 

Center frequency 

(GHz) 

0 mm 
3.5-3.8 3.5-3.8 No Band 3.65 

5.6-6.85 5.58-7.4 Band-6,-7 6.23 

3 mm 
3.7-4.5 3.75-4.7 Band-2 4.1 

6.8-7.43 6.4-7.8 Band-8 7.12 

All Off 4.2-5.75 
4.2-5.7 Band -3, -4, 

-5 

4.975 

6.8-8.3 7.55 

f1, f2 lower and upper cut off frequencies respectively 

 

The simulated and measured results are in good agreement 
and are summarized in Table III. Measured results justify the 
importance of folded CPS feed and the role of the non-uniform 
loop arms with inner CPS line for frequency reconfigurablility. 
It also shows that the inclusion of two switches is sufficient to 
cover seven different bands. 

V. CONCLUSION 

The proposed structure shows that a loop antenna can be 
designed to have multiple bands. The included CPS line within 
the antenna ensures the reconfigurability in antenna structure to 
get multiple bands with uniform radiation pattern. Since the 
CPS line does not contribute to radiation, the patterns generated 
for different lengths of CSP line have the same orientation. The 
proposed loop can be operated as single and dual band antenna 
and generates a total of seven bands using minimum number of 
switches. Since the proposed antenna covers Band-2, Band-3, 
Band-4, Band-5 Band-6, Band-7 and Band-8 ranging from 
3.5GHz to 7.8GHz, it can be useful in WLAN 802.11 a/n and 
WiMAX applications. The proposed structure has the smallest 
dimensional profile. The proposed technique can be used for 
size reduction of the loop antenna. 

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