Bandwidth enhancement through slot design on RLSA performance

Abstract

Radial Line Slot Array Antenna is a slot planar antenna. The slot has been arranged in radials to create a concentration of wave into a single point. The body of this antenna normally a metal with hollow cavity and slots. However, recent development on RLSA has utilized the FR4 as a substrate material. The copper is attached to the substrate and the slot is not hollow as compare to the conventional approach. This research is studying the effect of slot with dielectric substrate to the Radial Line Slot Array Antenna (RLSA). From the measurement, it is found that the slot without dielectric provide a better return loss as compare to slot with dielectric. It is also improve the bandwidth of the RLSA. However, slot with dielectric performed better on radiation pattern characteristic.

Full text

Bandwidth Enhancement through Slot Design on
RLSA Performance
I.M. Ibrahim
Faculty of Electronics and Computer
Engineering,
Universiti Teknikal Malaysia
Melaka, Malaysia
T. A. Rahman, M. I. Sabran
Wireless Communication Centre,
Universiti Teknologi Malaysia,
Skudai, Johor,
Malaysia
M. F. Jamlos
Advanced Communication
Engineering Centre (ACE), School
of Computer & Communication
Engineering, UniMAP Malaysia
Abstract-Radial Line Slot Array Antenna is a slot planar
antenna. The slot has been arranged in radials to create a
concentration of wave into a single point. The body of this
antenna normally a metal with hollow cavity and slots. However,
recent development on RLSA has utilized the FR4 as a substrate
material. The copper is attached to the substrate and the slot is
not hollow as compare to the conventional approach. This
research is studying the effect of slot with dielectric substrate to
the Radial Line Slot Array Antenna (RLSA). From the
measurement, it is found that the slot without dielectric provide a
better return loss as compare to slot with dielectric. It is also
improve the bandwidth of the RLSA. However, slot with
dielectric performed better on radiation pattern characteristic.
Keywords- RLSA antenna; FR4 substrate; air gap structure;
Wideband Slot Antenna
I. INTRODUCTION
RLSA antenna for point to point microwave link based
on 802.11a standard currently a popular candidate for this
application due to its capability of carrying high speed signal
[1]. RLSA prototypes has been designed and developed at the
frequency range of 5725 – 5875 MHz by few researchers [1-
3]. The classic design was using an air gap as a separator
between radiation surface and ground plane [4]. Then, the
polypropylene has been used as a slow wave element in the
RLSA structure. This material normally gives 2.3 dielectric
value [5-6].
Recent development on RLSA has utilized FR4 board as a
part of the antenna structure [7-11]. The copper is attached to
the substrate and the slot is not hollow as compare to the
conventional way. This research is studying the effect of slot
with dielectric substrate to the Radial Line Slot Array Antenna
(RLSA)
II. ANTENNA STRUCTURE
In this research, the FR4 board with air gap distance to the
ground plane has been introduced as shown in Figure 1. The
thickness of overall cavity is 9.6mm where the thickness of
open air gap is 8mm. A 50Ω single coaxial probe coated with
Teflon is used to feed the signal into the cavity. The aluminum
plate is used as a platform to hold the antenna and also become
a ground plane. The FR4 with 1.6mm thickness with 5.4
permittivity value is used as a first layer substrate to the
radiating surface. Figure 2 shows the front view of the
antenna.
Fig.1. Fabricated open ended Air Gap RLSA Structure from side view
Fig.2. The slots arrangement on the surface of Air Gap RLSA
(a) Simulated, (b) Fabricated
III. ANTENNA DEVELOPMENT
The slot has been constructed as shown in Figure 3 base on
FR4 board. The arrow showed the wave movement to the air.
However, the wave has to pass through the dielectric substrate
that might provide the resistant value to the circuit. Another
approach to maximize the wave excitation to the air is to
provide the hollow structure on the slot. The slot hole has been
constructed as shown in Figure 4. It is expected that the wave
will be fully excite to the air because the resistant element has
been remove.
The hollow slot has been constructed using DMF CNC
Machine. The computerized machine has made the fabrication
process become easier. The FR4 board with hollow slot has
shown in Figure 5.
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Fig.3. Slot antenna with dielectric substrate
Fig.4. Slot antenna without dielectric substrate
Fig.5. Slot antenna without dielectric substrate fabricated using DMF
Milling Machine
Fig. 6: RLSA antenna with slot without dielectric substrate
A 2mm drill has been used to create the slots. The machine
read the CATIA drawing file from it computer. The drawing
originally drew using CST software before converted to
CATIA drawing file. Using this high end machine, an accurate
slot dimension was successfully constructed. The hollow slot
board then installed into the antenna platform and become a
complete antenna as shown in Figure 6.
IV. RESULT AND DISCUSSION
The concept of slot with and without dielectric was
simulated using CST software. The simulated results show a
smooth graph on slot with dielectric but not the slot without
dielectric. The slot with dielectric performed better on
reflection coefficient. The return loss of antenna slot with
dielectric is from 5.3-6.9 GHz while the return loss antenna
slot without dielectric are from 5.1-6.3GHz. The return loss at
5.8GHz antenna slot with dielectric is 16dB while antenna slot
without dielectric is 12dB. However, the return loss on
antenna slot without dielectric seems close to real
environment. The researchers decided to proceed to
fabrication and measured the real performance of the antenna.
Fig.7. Simulated reflection coefficient of RLSA antenna slot with and
without dielectric substrate
Fig.8. Simulated vs measured reflection coefficient of RLSA antenna
slot without dielectric substrate
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The fabricated antenna without slot has been measured it
performances. A comparison on simulated and fabricated
result for antenna slot without dielectric has been performed
and presented in Figure 8. The fabricated results show a better
performance. The return loss of the antenna is from 4.2-6.6
GHz. The 44% bandwidth recorded. It is 23% better than
simulation. The return loss at 5.8 GHz for fabricated result is
24dB which is 12 dB better than simulated result. The antenna
slot with and without dielectric are measured their
performance and presented in Figure 9. The reflection
coefficient result on antenna slot without dielectric
demonstrate better performance as compare to antenna slot
with dielectric.
Antenna slot without dielectric performed return loss from
4.2-6.6 GHz while antenna slot with dielectric performed from
4.2-6.2GHz. The 6% bandwidth improvement recorded for the
antenna slot without dielectric. The return loss on that region
show the antenna slot without dielectric is more dominant
while antenna with dielectric demonstrated a few band notch
and fluctuation. The return loss at 5.8 GHz for antenna
without slot is 24dB which is only 2dB better than antenna slot
with dielectric.
The overall results are compressed on Table 1. It is show
that the simulation performance show the antenna with
dielectric slot perform better than antenna without dielectric
slot. However, the measured performance show antenna
without dielectric slot performed better.
Fig.9. Measured reflection coefficient of fabricated RLSA antenna slot
with and without dielectric substrate
TABLE I.
COMPARISON ON SIMULATED VS MEASURED PERFORMANCES OF
RLSA ANTENNA WITHY DESIGN SLOT WITH AND WITHOUT DIELECTRIC
SUBSTR ATE
Design Slot with Dielectric
Slot Without
Dielectric
Parameter
Measured
Bandwidth Return
Loss at
5.8GHz
Bandwidth Return
Loss at
5.8GHz
Simulation 5.3-6.9
GHz,
(26%)
16dB 5.1-6.3
GHz,
(21%)
12dB
Measurement 4.2-6.2
GHz,
(38%)
22dB 4.2-6.6
GHz,
(44%)
24dB
The radiation pattern test has been performed in Radiation
Laboratory, Universiti Malaysia Perlis. Figure 10 illustrated a
comparison between measured radiation pattern on RLSA
with and without dielectric. The RLSA slot with dielectric
show 1dB higher gain compare to slot without dielectric. The
side lobe level for slot without dielectric also 2dB higher
compared to slot with dielectric. Furthermore, the radiation
pattern for slot with dielectric show better directivity
characteristic compare to slot without dielectric.
Fig.10. Measured radiation pattern of RLSA antenna slot with and without
dielectric substrate
V. CONCLUSION
It is show that the antenna without slot performed a better
bandwidth and return loss. The return loss on the perform
region also very dominant. The dielectric substrate has
contributed a resistance element to the circuit. It has created
resonant and band notch to the result. When this resistance
element been removed from the circuit, it is show the wave
has been smoothly travel through the slots. However, the
antenna with dielectric slot show better gain and radiation
characteristic compare to antenna without slot. In overall, this
research concluded that the slot design without dielectric has
improve the bandwidth and return loss of the RLSA antenna
but not the gain and radiation pattern.
ACKNOWLEDGMENT
The authors would like to acknowledge and express
sincere appreciation to Universiti Teknologi Malaysia and
Ministry of Higher Education (MOHE) for financing the
research project. Appreciation also goes to Universiti Teknikal
Malaysia Melaka and Ministry of Higher Education Malaysia
for funding the author’s scholarship. The authors also would
like to acknowledge Advance Manufacturing Centre,
Universiti Teknikal Malaysia Melaka and Radiation
Laboratory, Universiti Malaysia Perlis for the equipment
assistance on this research.
2014 IEEE Region 10 Symposium
978-1-4799-2027-3/14/$31.00 ©2014 IEEE 219

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