# Hexagonal Monopole Strip Antenna with Rectangular

Authors: A. A. Pramudita; A. Kurniawan; A. Bayu Suksmono
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

A printed ultra wideband (UWB) monopole antenna is proposed for applications on stepped frequency continuous wave (SFCW) ground penetrating radar (GPR) within a frequency range of 100–1000 MHz. The proposed antenna consists of a hexagonal strip line with resistive loading and a rectangular slot that is added to the ground plane side of the printed antenna implemented on FR4 epoxy materials. The resistive loading at the hexagonal monopole is effective to increase bandwidth in the higher frequency region, while the rectangular slot is used to improve bandwidth characteristic in the lower frequency region. This paper investigates the characteristic improvement in the lower frequency region by applying a parametric study on the rectangular slot that is added at the ground plane side of the UWB monopole antenna. Computer simulation was conducted and measurements were carried out to validate the result.


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## Body

## 1. Introduction

GPR systems are used for subsurface investigations. They are
used for detection of objects buried beneath the earth surface such as
landmines, cables, pipes, and hidden tunnels. One important component in any
GPR system is the transmitter and receiver antennas [1]. Desired features of a
GPR antenna include UWB operation capability, good impedance matching, and
small or compact in size. For SFCW-GPR applications, it is also important that
the antenna has a flat transfer function in the frequency domain. In this
paper, the antenna was designed for transmitting a monocycle pulse generated by
a 100–1000 MHz SFCW system. The antenna should have low VSWR (VSWR≤2), flat transfer function in the entire
bandwidth, and exhibits very small late-time ringing (in time domain).There are several methods previously
published that have been used to improve bandwidth. Broadband antennas can be
achieved by implementing a flare shape aperture [2–5]. An array of
several antenna elements with different dimensions has also been proposed for
bandwidth improvement [4, 6]. Resistive/capacitive loading was also reported to
increase antenna bandwidth in [2, 5, 7].In this paper, we proposed a new antenna
design. The design uses a resistive loading at the end of a hexagonal monopole
strip (HMS) that is utilized for bandwidth improvement. However, resistive
loading is only effective to increase the antenna bandwidth in the higher
frequency region. An additional rectangular slot at the ground plane side is required
in an attempt to improve the antenna bandwidth in the lower frequency region.A parametric study of a rectangular slot has
been conducted in order to investigate the influence of the rectangular slot dimension/configuration,
such as the length of the slot, the position of the slot, and the optimal value
of resistive loading on the antenna bandwidth. This will provide valuable information
for optimizing the antenna characteristics to meet the required specifications.This paper presents the simulation to
obtain the optimal dimensions of the rectangular slot that is added at the ground
plane side of the UWB HMS antenna. Simulation has been performed both in
frequency and time domains. The
method of moment (MoM) has been chosen for the numerical method. Measurements have been carried
out to verify the simulation results.The rest of the paper is organized as
follow. Section2 presents the antenna design. Section 3 discusses
parametric study, measurement setup, and antenna characteristic in optimum
dimension. Conclusion of this research is given in Section 4.


## 2. Antenna Design

The UWB antenna has been designed to have a dimension of100×86 mm2 and printed on an FR-4 epoxy dielectric substrate. The
dielectric substrate has thickness of 1.6 mm and a relative dielectric constant
of 4.4. Antenna is designed with resistive loading at the end of the
HMS that is used to improve bandwidth and to reduce the ringing level. Metal
film resistor is used as a resistive load. Applying resistive loading will of
course reduce the antenna efficiency. This has been shown in [8], that Wu-King loading
profile reduces the antenna efficiency until 50 percent. Simulation shows that
resistive loading reduces the antenna efficiency below 60 percent, as shown in
Figure 13. In addition, low ringing level (in time domain) is also important in
GPR applications. Therefore, despite the reduction of antenna efficiency,
resistive loading is used to obtain wide bandwidth and to suppress the ringing
level.The previous research
shows that a modified dipole antenna with a number of sharp edges increased the
efficiency [9]. At the sharp edges, radiation occurs more effectively because
of increasing displacement current. It becomes the reason why a hexagonal shape
was selected for the antenna discussed in this paper. The total length of the
HMS is related to the wavelength of the central frequency which corresponds to
the spectrum of the monocycle pulse that will be transmitted. The feed point
was connected with SMA connector.The bandwidth of the antenna in Figure1
should meet the requirement of 100–1000 MHz SFCW GPR application. The resistive
loading at the end of the hexagonal strip is only effective to enhance the
bandwidth in the higher frequency region.Figure 1
HMS antenna with resistive loading.A=10 mm, B=8 mm, and S=28 mm.Figure2 shows the simulated return loss
of the antenna in Figure 1. It can be seen that, the antenna transmits only the
high-frequency part in the 100–1000 MHz range. Thus, theoretically it will
differentiate the transmitted monocycle pulse.Figure 2
Simulated return loss characteristic of the HMS antenna in Figure1.The rectangular slot is added at the ground plane side of UWB
HMS antenna in order to improve the bandwidth characteristic in the lower
frequency region. The rectangular slot will perform as an effective resonator
in the lower frequency region. Therefore, the dimensions of the rectangular
slot must be larger than the dimensions of HMS.


## 3. Result and Discussion

Parametric study was carried out to
investigate the influence of some important parameters such as the length of
the slot(P,L), the position of the slot (K), and the resistive load (R).When the rectangular slot position from the
feed line is the same as the HMS position from feed line (K=0), the optimal
position of the rectangular slot is achieved. Figure 4 shows the effect of
varying K.Adjustment ofP has a significant influence
on the antenna characteristic at high frequencies and impedance matching the
pass band. Figure 5 shows the effect of varying P. It can be seen that lager P
does not affect the low frequency edge, but has much larger impact on the high-frequency
edge. Furthermore, it is observed that an increase in impedance matching will
be followed by a decrease in bandwidth.Adjustment ofL has significant influence
on the antenna characteristic at both low and high frequencies. Figure 6 shows
the effect of varying L. An increase in L will decrease the low-frequency edge
and will consequently cause a decrease in bandwidth.Figure7 shows the effect of varying the
resistive load value to the antenna in Figure 3. Adjustment of the resistive
load has an influence on the antenna impedance matching.Figure 3
HMS antenna with rectangular slot.Figure 4
Effect of varyingK to the antenna characteristic.Figure 5
Effect of varyingP to the antenna characteristic.Figure 6
Effect of varyingL to the antenna characteristic.Figure 7
Effect of varying resistive loads value to the antenna characteristic.The complete dimensions for optimum design of the HMS antenna
with rectangular slot areS=28 mm, A=10 mm, B=10 mm, K=0, L=56 mm, P=68 mm, and R=100 Ohm. The optimum dimension of UWB HMS antenna can be found by
performing parametric study for rectangular slot after determining the total
length of the HMS. The dimensions of the rectangular slot must be larger than
the dimensions of HMS as discussed in Section 2. The parametric study is
started from K and followed by L and P.The optimum design of the HMS antenna with
rectangular slot has bandwidth from 97 to 1200 MHz at the return loss level
less then −10 dB. Therefore, its bandwidth meets the bandwidth requirement of
the 100–1000 MHz SFCW GPR application. The simulation and experiment results
show that the antenna has relatively constant transfer function between 100 MHz
to 1000 MHz, as shown in Figures8 and 9. It produces minimum distortion
to the individual frequency series from the synthesizer. Figure 9 shows that
the antenna has relatively constant input impedance within the frequency range
of the SFCW GPR, and which would minimize the ringing level.Figure 8
Simulated input impedance HMS UWB antenna with rectangular slot.Figure 9
VSWR HMS UWB antenna with rectangular slot.Figure10 shows a comparison of transmitted
waveform between the HMS antenna and a strip dipole with the same length when
excited by a 5-nanoseconds pulse. It can be seen that the HMS antenna with
rectangular slot contributes to a lower ringing level in comparison with the
strip dipole.Figure 10
Transmitted waveform comparison between the HMS
antenna and strip dipole.The antenna system can be modeled as a linear
time invariant system. Therefore, the transmitted signal (Et) can be obtained as a product of the measured S21 and the spectrum of the
exciting pulse (Sf)
followed by inverse FFT operation [10]. The monocycle pulse that is synthesized
by the 100–1000 MHz SFCW has duration of 5 nanoseconds. Et can be written as
(1)Et=IFFT[S21⋅Sf].The transmit waveform of the antenna is shown
in Figure12(a). The simulation result has been verified with experiments
measurements. The HMS antenna with rectangular slot was measured at both
frequency and time domain using Vector Network Analyzer. The measurement setup
in time domain is illustrated in Figure 11. The measurement result at frequency
domain shows that the HMS antenna with rectangular slot meets the bandwidth
requirement. The time domain measurement result is shown in Figure 12. As shown in Figure 12(a), the transmitted waveform measurement result agrees with simulated transmitted waveform.Figure 11
Transmit waveform measurement setup.Transmitted waveform measurement (a) comparison between
simulation and measurement results, (b) comparison of transmitted waveforms
with different excitation pulses.
(a)(b)Simulated antenna efficiency and gain (a) simulated
antenna efficiency, (b) simulated antenna gain.
(a)(b)Figure 14
Photo of the HMS antenna with rectangular slot.In Figure12(b), the antenna is excited
with 3-nanoseconds, 4-nanoseconds, and 5-nanoseconds pulse and the transmitted
waveform is shown. The 3-nanoseconds pulse has higher distortion and produces
higher ringing level. This implies that the operation bandwidth of 100–1000 MHz is
only effective for 5-nanoseconds pulse transmission as a narrower pulse requires
larger bandwidth.


## 4. Conclusion

The UWB HMS antenna has been proposed for SFCW GPR
application. The proposed antenna meets the 100–1000 MHz SFCW GPR requirements
and has good performance for transmitting a 5-nanoseconds monocycle pulse excited
by the SFCW synthesizer.The rectangular slot that is added at the ground plane side
of the UWB HMS antenna improves the bandwidth characteristic in the lower
frequency region. Parametric study was carried out to investigate the influence
of the dimensions of the slot(P,L). Adjustment of P has significant influence
on the antenna characteristic at high frequencies and impedance matching in the
pass band. A larger P does not affect the low-frequency edge, but has much
larger impact on the high-frequency edge. An increase in L is able to decrease
the low-frequency edge and will be followed by a decrease in bandwidth. Increasing
L also increases the impedance matching in the high-frequency edge.



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*Source: 391386-2008-05-11.xml*