chapter 3 broadband l-probe fed quarter-wave...
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CHAPTER 3
BROADBAND L-PROBE FED QUARTER-WAVE
MICROSTRIP ANTENNA
3.1 INTRODUCTION
The prototype antenna is designed to improve the bandwidth by the
novel method of feeding technique. This antenna is a derivative of rectangular
microstrip antenna. In general, coaxial probe feed is used. The major
difference between the half-wave patch and the quarter-wave patch is that
quarter-wave patch has one radiating edge compared to those of the half-wave
patch. This physical difference is responsible for all the differences in antenna
characteristics. The prototype antenna is electromagnetically excited by L-
probe feed.
3.2 ANTENNA DESCRIPTION
Figure 3.1 shows the geometry of the prototype antenna. The
electric field distribution of the rectangular patch is given by E0 cos( x/L). A
rectangular microstrip antenna has a maximum electric field at one of the
radiating edge of the patch, zero in the middle at x = L/2, and again maximum
at the other radiating edge of the patch. At x = L/2 plane, a wall is erected
without disturbing the field distribution. The other half portion of the patch is
discarded.
The patch is still resonant at the designed frequency of the half-
wave rectangular patch. This type of rectangular patch geometry is called
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Quarter-Wave patch (Zaid 1999), as the separation between the radiating edge
and the electric wall is about g/4. Compared to a rectangular patch, a quarter-
wave patch has only one radiating edge and this physical difference is
responsible for all the differences in the antenna characteristics. The antenna’s
novelty is by the feeding technique of the prototype antenna; that is an L-
shaped probe feed than the usual coaxial feed, which is placed along the
diagonal line beneath the radiating patch. This gives rise to the right-hand
circular polarization. The antennas compactness is achieved by discarding one
half portion of the rectangular microstrip antenna.
Figure 3.1 Geometry of broadband L-probe fed quarter-wave
microstrip antenna
(a)Side View (b) Top view
Figure 3.2 Simulated model of broadband L-probe fed quarter-wave
microstrip antenna
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3.3 ANTENNA DESIGN
The prototype antenna is designed for the resonant frequency of
4.5 GHz. The antenna with a width W and a length L is supported by an air-
filled substrate with the dielectric constant of r=1 which is sandwiched by the
radiating patch and the ground plane. The total thickness of the substrate is
6.6mm, were h1 is 5.5mm and h2 is 1.1mm. The radiating patch is fed by an
L-shaped probe with length Lp=10.5mm, width Wp=1mm and with the height
of 5.5mm. As one of the bandwidth enhancement technique of the patch
antenna, air or foam is used as the substrate material with the thickness of
6.6mm. The design details are listed in Table 3.1.
The design procedure is outlined in the following steps:
Step 1: The width of the microstrip patch antenna is given as
0
( 1)2
2r
cW
f
(3.1)
Step 2: The Effective dielectric constant ( reff) is given as
0.51 1(1 12 )
2 2
r rreff
h
W (3.2)
Step 3: The Effective length (Leff ) is given as
02eff
reff
cL
f (3.3)
Step 4: The length extension ( L) is given as
( 0.3)( 0.264)
0.412
( 0.258)( 0.8)
reff
reff
W
hL hW
h
(3.4)
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Step 5: The actual length of rectangular patch (LRP) is given as
(3.5)
Step 6: The actual length of quarter-wave patch (LQRP) is given as
(3.6)
Table 3.1 Design details of broadband L-Probe fed Quarter-Wave
microstrip Antenna
Design Parameters Design Values in mm
W (Patch Width) 30.0
L (Patch Length) 12.5
Lp (L-probe length) 10.5
Wp (L-probe width) 1.0
h1 (upper substrate height) 5.5
h2 (lower substrate height) 1.1
h (Total Substrate Height) 6.6
D Probe (Probe dia) 1.0
The sequence of the design, simulation and measurement of the
compact, broadband L-probe fed Quarter-wave microstrip patch antenna is
shown in Figure 3.3.
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Figure 3.3 Design Flow diagram of broadband L-probe fed quarter-
wave microstrip antenna
The quarter-wave patch for r =1 has lower quality factor than that
of the half-wave patch resulting in larger bandwidth. The L-shaped probe
forms an open circuit stub that introduces inductance effect. This inductance
is compensated with the capacitance effect introduced in discarding one half
of the patch. The stored energy in a quarter-wave patch is one-half that of the
half-wave patch because of the identical field distribution over half the area.
The quarter-wave patch has a radiation resistance that is twice than that of the
half-wave patch
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3.4 PARAMETRIC STUDY AND OPTIMIZATION
3.4.1 Effect of Height, h
Increase in the substrate height, increases the fringing fields from
the edges, which increases the extension length L and hence the effective
length, thereby decreasing the resonant frequency. The gain and bandwidth
of the antenna is increased to 8.51 dBi and 59.7% for the substrate height h =
0.09 0 (6.6 mm). Figure 3.4 shows the variations of gain with frequency.
Figure 3.4 Gain variations of broadband L-probe fed quarter-wave
microstrip antenna for various values of substrate height
3.4.2 Effect of r
The decrease in r increases the bandwidth due to increase in the
fringing field. It is observed that the bandwidth and gain increases with
decrease in the substrate dielectric constant. Good results are obtained for
r=1. Figures 3.5 and 3.6 show the gain and bandwidth variations with
frequency for various values of r
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Figure 3.5 Gain variations of broadband L-probe fed quarter-wave
microstrip antenna for various vales of r
Figure 3.6 Bandwidth variations of broadband L-probe fed quarter-
wave microstrip antenna or various vales of r
3.4.3 Effect of Patch Width, W
Bandwidth of the patch is largely affected by the patch width.
Increasing the patch width increases the bandwidth of the patch. Also,
aperture area of the patch increases resulting in increasing the directivity,
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efficiency and hence the gain of the patch. As patch width increases feed
point is shifted due to the input Impedance decreases. Usually patch width is
chosen greater than the patch length. Figure 3.7 shows the bandwidth
variations for the patch width, 0.45 0<W<0.6 0. Good bandwidth is obtained
for W = 0.45 0. The bandwidth is increased from 36% to 59.7% and gain is
increased to 8.51 dBi.
Figure 3.7 Bandwidth variations of broadband L-probe fed quarter-
wave microstrip antenna for various values of patch width
3.5 SIMULATED RESULTS
The proposed antenna has been simulated using Zeland software’s
IE3D simulation package. Figure 3.8(a) shows the variation of VSWR with
frequency. The frequency from 3.32 GHz to 5.85 GHz for the input VSWR is
2; the total impedance bandwidth of 2.69 GHz is available. Figure 3.8 (b)
shows the variation of return loss with frequency and for the proposed
antenna –34.19dB return loss is available.
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Figure 3.8 (a) VSWR plot of broadband L-probe fed quarter-wave
microstrip antenna
Figure 3.8 (b) Return loss plot of broadband L-probe fed quarter-wave
microstrip antenna
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Figure 3.8(c) shows the Smith chart. The impedance locus lies in
the VSWR circle, indicating that the input signal is coupled properly. The
gain against frequency is shown in Figure 3.8(e).
Figure 3.8(c) Smith chart of broadband L-probe fed quarter-wave
microstrip antenna
Figure 3.8 (d) Gain plot of broadband L-probe fed quarter-wave
microstrip antenna
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Figure 3.8 (e) 2D Radiation pattern of broadband L-probe fed quarter-
wave microstrip antenna
Figure 3.8(f) 3D Radiation pattern of broadband L-probe fed quarter-wave
microstrip antenna
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3.6.1 Parametric Study
An L-probe fed quarter-wave microstrip patch antenna of length
L=25mm and width W=30mm is considered to study the effects of various
parameters on its performance. The substrate parameters are r =1(air/foam)
h=6.6mm. The variations in Lp cause the impedance loci to shift towards right
side and downward in the smith chart It also shifts the resonant frequency of
the antenna. The width of the quarter-wave microstrip antenna has significant
effect on the input impedance, bandwidth and gain of the antenna. With
increase in width the input impedance decreases, thereby shifting the feed
point. With increase in h, the bandwidth of the antenna increases as the fringe
fields are increased. When r is decreased to one, the resonant frequency
increases and hence the bandwidth of the antenna. The simulated results of
the L-probe fed quarter-wave microstrip patch antenna are given in Table 3.2.
Table 3.2 Simulated Results of the broadband L-Probe fed Quarter-
Wave microstrip Antenna
Parameter Results
Return Loss (dB) -10
VSWR 2
Resonant Frequency (GHz) 4.5
Upper Resonant Frequency (GHz) 6.01
Lower Resonant Frequency (GHz) 3.32
Frequency Range (GHz) 2.69
Bandwidth (%) 59.77
Maximum Gain (dBi) 8.51
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3.7 MEASUREMENT
The prototype model of the L-probe fed Quarter-wave microstrip
patch antenna is constructed on a (75 × 75) mm ground plane. With the
substrate height of 6.6 mm filled air. The photograph of the fabricated patch
antenna is shown in Figure 3.9. The return loss of the antenna is measured
using a network analyzer (E8363 B PNA – Network Analyzer by Agilent
Technologies) with frequency specification of 10 MHz to 40 GHz. The
measured returned loss of broadband L-probe fed quarter-wave microstrip
antenna is shown in Figure 3.10. The lower frequency is 3.67 GHz and the
upper frequency is 6.15 GHz. The bandwidth obtained is 2.45 GHz which
gives an impedance band width of 54.44%. The measured return loss data are
given in Appendix 1.
Figure 3.9 Photograph of broadband L-probe fed quarter-wave
microstrip antenna
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Figure 3.10 Measured return loss of broadband L-probe fed quarter-wave
microstrip antenna
3.7.1 Radiation Pattern Measurement
Radiation pattern is the graphical representation of radiation
properties of the antenna as a function of space coordinates. Radiation pattern
of an antenna are basically three dimensional thus can be represented in
spherical coordinate system as a function of (r, ). The pattern provides the
complete information regarding the characteristics of an antenna. To represent
the pattern in two dimensional, it is a common practice to cut the pattern in
either XY plane or XZ plane i.e., either azimuth or elevation. The electric
field consists of and components that are perpendicular to each other.
Proper radiation is possible with a separation of r 2D2
.
3.7.2 Measurement Setup
Figure 3.11 shows the block schematic arrangement used for
radiation pattern measurement. A horn antenna in the frequency range of
1.6 GHz to 12 GHz was chosen to be the transmitting antenna. The L-probe
fed quarter wave patch antenna was mounted on a positioner connected with
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tripod setup which is controlled by a position controller (MI-4190, MI
Technologies). The position controller is used to rotate the test antenna from
-180 to +180 in the azimuth plane. The transmitting horn antenna is kept
fixed on the mounting pad. The height of the test antenna is adjusted to the
axis of the transmitting horn antenna. The feed signal is given to the reference
antenna using signal generator (E8257, Agilent Technologies). A magnitude
level of -10 dBm was set in the signal generator and maintained constant
throughout the measurement period. The received signal is amplified and fed
to a spectrum analyzer of frequency range 3 Hz to 44 GHz (E4446A – Agilent
Technologies) and then to the pattern recorder, which directly plots the
radiation pattern either in polar or in rectangular plot.
Figure 3.11 Antenna radiation pattern measurement setup
The polarization of a rectangular patch antenna is linear and
directed along the resonating dimension, when operated in the dominant
mode. The radiation pattern and polarization for these modes can be different
from the dominant mode. The cross polarizations are due to the fringing fields
along the non-radiating edges. The cross polarization level increases with
substrate thickness. The cross-polarized component of a rectangular patch can
be minimized by a suitable choice of patch width W.
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The return loss with frequency of the antenna was measured using a
E8363 B PNA – Network analyzer and the far-field patterns were measured
with antenna-measurement system available at SAMEER – Centre for
Electromagnetics, Chennai, India. The patterns are separately measured for
horizontal and vertical polarizations. Measured H plane and E plane radiation
patterns for frequencies from 3.6 GHz to 6 GHz are shown in Figures 3.12
(a-f) and 3.13 (a-f). The horizontal and vertical patterns are unidirectional.
The radiation patterns are measured in the E (yz) and H (xz) planes at six
frequencies. It is seen that the copolarization patterns are stable. The half-
power beamwidth in the E plane is 49 at 3.6 GHz.
The measured H plane pattern shows that the copolarization level is
stable for all the frequencies except for 5.5 GHz and 6 GHz. At 3.6 GHz, the
Copolrization level in H plane pattern is broader conpared to other
frequencies. The maximum appears at 0o with the HPBW of 91
o for 4 GHz
frequency, maximum appears at 0o and it is greater than the cross
polarization level. At 4.5 GHz, the maximum apears at 30o wih the HPBW of
84o. At 5 GHz, copolarization level is maximum at 318
oand cross polarization
level maximum appears at 308o. At 5.5 GHz, there are ripples in
copolarization level. The pattern size is reduced and the maximum appears at
points 12o and 300
o. At 6 GHz, the copolarization level is reduced with the
maximum appearing at 3o
and the cross polarization level is increased than the
coplarization level which has to be minimized. The measured H plane pattern
and E plane pattern data are given in Appendix 1.
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
3. 6 GHz_VP_co pol
3.6 GHz_HP_x pol
Figure 3.12(a) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency, 3.6 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
4 GHz_VP_co pol
4 GHz_HP_x pol
Figure 3.12(b) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency, 4 GHz
50
-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
4.5 GHz_VP_co pol
4.5 GHz_HP_x pol
Figure 3.12 (c) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency,4.5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
5GHz_VP_copol
5GHz_HP_xpol
Figure 3.12 (d) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency, 5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
5.5GHz_VP_copol
5.5GHz_HP_xpol
Figure 3.12 (e) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency,5.5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
6GHz_VP_copol
6GHz_HP_xpol
Figure 3.12 (f) Measured H-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antenna for frequency, 6 Ghz
The measured E plane pattern shows that the copolarization level
is stable for all the frequencies except for 5.5 GHz and 6 GHz. At 3.6 GHz,
the Copolrization level in E plane pattern is broader conpared to other
frequencies. Copolarization levels maximum appears at 330 and cross
polarization level maximum appears at 357 . At 5.5 GHz, there are varitions
in copolarization level with the maximum appearing at 300 and a null at
230 . The cross polarization is reduced and the maximum appears at 40 and
185 , null at 0 and 180 . At 6 GHz , the copolarization level is reduced and a
null appears at 10 and 90 .
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The HPBW of H plane pattern and E plane of broadband L-probe
fed quarter–wave microstrip antenna are given in Table 3.3.
Table 3.3 HPBW of broadband L-probe fed quart –wave microstrip
antenna
Frequency (GHz) H Plane pattern E plane pattern
3.6 91 92
4 67 75
4.5 84 74
5 147 105
5.5 136 153
6 59 85
-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
3.6GHz_VP_Xpol
3.6GHz_HP_co pol
Figure 3.13(a) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 3.6 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
4 GHz_VP_Xpol
4 GHz_HP_copol
Figure 3.13 (b) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 4 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
4.5GHz_VP_Xpol
4.5GHz_HP_copol
Figure 3.13 (c) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 4.5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
5 GHz_VP_Xpol
5 GHz_HP_co pol
Figure 3.13 (d) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
5.5GHz_VP_X pol
5.5GHz_HP Co pol
Figure 3.13 (e) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 5.5 GHz
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-40
-30
-20
-10
00
30
60
90
120
150
180
210
240
270
300
330
6 GHz_VP_X pol
6 GHz_HP_co pol
Figure 3.13(f) Measured E-plane pattern of Broadband L-probe fed
Quarter-wave microstrip antennafor frequency, 6 GHz
3.8 CONCLUSIONS
The design demonstrates that an L-probe fed quarter-wave
microstrip patch antenna provides a simulated impedance bandwidth of
59.77% with gain of 8.51dBi. The measured bandwidth is 54.44%. The
difference may be due to fabrication discrepancies. This novel antenna is truly
compact and hence finds applications where small-sized antennas with wider
impedance bandwidths and considerably good gain are needed, and were
deterioration in cross-polarization characteristics can be tolerated. Table 3.4
gives the comparison of simulated and measured results.
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Figure 3.14 Return loss and gain variations of broadband l-probe fed
quarter-wave microstrip antenna
Table 3.4 Results of the broadband L-Probe fed quarter-wave microstrip
antenna
Parameters Simulated Measured
Return Loss (dB) -10 -10
VSWR 2 2
Resonant Frequency (GHz) 4.5 4.5
Upper Resonant Frequency (GHz) 6.01 6.15
Lower Resonant Frequency (GHz) 3.32 3.7
Frequency Range (GHz) 2.69 2.45
Bandwidth (%) 59.77 54.44
Maximum Gain (dBi) 8.51 7.17