01-11-2016, 02:58 PM
1463046875-AMiniaturizedAntipodalVivaldiAntenna.pdf (Size: 684.92 KB / Downloads: 9)
Abstract—In this letter, a modified antipodal Vivaldi antenna is
presented. A novel tapered slot edge (TSE) structure is employed
in this design. The proposed TSE has the capacity to extend the
low-end bandwidth limitation and improve the radiation characteristics
in the lower frequencies. A prototype of the modified antenna
is fabricated and experimentally studied as well. The measured
results show reasonable agreement with the simulated ones
that validate the design procedure and confirm the benefits of the
modification.
Index Terms—Antipodal Vivaldi antenna (AVA), tapered slot
edge (TSE), wideband antenna.
I. INTRODUCTION
AS THE demand of compact, smart, and multifunctional
antennas for modern communication in both military and
civil applications increases, wideband antennas attract more and
more interests in academic field recently. With a history of more
than 30 years, the tapered slot antenna (TSA) is still one of the
most widely used wideband antennas. The tapered slot antenna,
as its name suggests, is a class of antenna with a tapered radiator
profile. The first TSA was introduced by Gibson [1] with
exponential profile, which is also known as the ETSA or Vivaldi
antenna. Other forms, such as linear TSA, constant-width TSA,
parabolic TSA Fermi TSA, logarithmically TSA, etc., are introduced
afterward [2]–[6]. The dual exponentially tapered slot antenna
(DETSA, or also known as the Bunny ear antenna), which
supplies additional design degrees of freedom, is also presented
in [7] and [8]. Compared to other wideband antennas, the TSAs
have moderately high directivity, planer structure, low profile,
and symmetric beam in both E- and H-plane. Also, it is inexpensive
to fabricate and easy to integrate. All those characteristics
make the TSA a good candidate for phased array, remote
sensing, and short-rage communication.
The TSA belongs to the class of endfire traveling wave antennas,
which has theoretically infinite bandwidth. The conventional
TSA usually applies slotline as radiation fins. In the
practical situation, however, the operating bandwidth is limited.
First, the high-end working band is restricted by the transmission
structure between the microstrip to slotline. In the low-end working band, on the other hand, the bandwidth is limited by
the width of the antenna patch. According to the research in [9]
and [10], the width of a TSA should reach at least one half-wavelength
for effective radiation to occur.
In order to overcome the limitation, Gazit proposed an
antipodal Vivaldi antenna (AVA) [11]. A microstrip-to-symmetric-double-sided-slotline
transmission that has extremely
wide operating band was employed instead of the conventional
microstrip-to-slotline transmission. In [12]–[14], various kinds
of wideband AVAs are presented with compact size. The high
end of the working band can be extended to an extremely
high frequency, but the lower end cutoff is still limited by the
antenna aperture.
In this letter, a modified AVA is designed and measured. The
tapered slot edge (TSE) structure is applied to miniaturize the
size of the antenna so as to overcome the limitation proposed
in [9] and [10]. Also, the modification is proved capable of
improving the antenna’s low-end radiation characteristics. The
letter is organized as follows. In Section II, the configuration and
design procedure are proposed. Results are presented and discussed
in Section III, and the conclusion is given in Section IV.
II. ANTENNA DESIGN
Fig. 1 demonstrates the evolution of the antenna. The original
AVA is shown in Fig. 1(a). The dimensions of the antenna
are set to be 48 60 mm , which is approximately
, where is the wavelength of 3.1 GHz. The antenna is
microstrip-fed, while the feedline width is set to be 1.9 mm in
order to match the 50- coaxial line. FR4 is used as the substrate
(thickness mm, dielectric constant ). The exponential
profile curves employed in this design can be described by the
equation
where
mm mm mm mm (1)
Fig. 1(b) illustrates the primary modified structure. Two pairs
of symmetrical regular slots are etched on the fins inspired by
the corrugation edges structure reported by Rizk and Rebeiz
in [15], which was used to reduce the width of the antenna
without degrading the radiation patterns. In this design, the
lengths of the slots are set to be approximately 0.15 according
to experiment.
Although the regular slot edge (RSE) shown in Fig. 1(b) is expected
to be effective in improving the antenna’s performance,
it is worth noticing that this configuration is not able to take full use of the patch area for its uncoordinated regular shape with
the antenna slot profile. The TSE modification is therefore proposed,
expecting to further improve the performance of the antenna.
As shown in Fig. 1©, due to its coordinate profile with
the radiation fins, the TSE is a more efficient structure in lengthening
the overall effective electrical length of the antenna in the
limited patch area, which will be discussed later.
III. RESULTS AND DISCUSSION
The dimensions of the configurations proposed in Section II
are simulated and optimized by CST Microwave Studio. By
fine-tuning and optimization, the final optimal dimension values
are obtained and listed in Table I.
As a practical example, a prototype of the TSE AVA is fabricated
for the purpose of supporting the dwelled design. Fig. 2
shows the fabricated prototype. A 50- SMA connector is used
to feed the antenna.
Fig. 3 illustrates the variation of original AVA, RSE
AVA, and TSE AVA, respectively. As shown in the figure, the lower-end 10-dB limitation of the original AVA is
3.3 GHz, while the RSE AVA extends it to 2.8 GHz. The TSE
AVA further extends the limitation to 2.4 GHz. Additional
resonant points can be observed around the lower limitation of
working band in both RSE and TSE cases. It is observable from
the figure that the TSE is able to miniaturize the size of the antenna
by means of lowering the minimum working frequency.
Furthermore, the measured variation with frequency is also
plotted in this figure. The measured result agrees well with the
simulated one except for the slight degradation in the middle
band, which is possibly due to the introduction of the SMA
connector, the irregularity of soldering, or the inaccuracy in
fabrication.
Despite that, the two curves both show that the proposed design
has ultrawideband performance extending from 2.4 to more
than 14 GHz.
In order to further understand the behavior of the TSE structure
especially in the lower frequencies, current distribution of
both original and TSE AVA at 3.5 GHz is given in Fig. 4. Comparing
Fig. 4(a) and (b), it is easy to find that the proposed
modification is able to eliminate the unwanted surface currents
that radiate vertically with endfire direction in region A. On the other hand, by etching the slots, significant currents are observable
in region B along the slot edges, indicating that the effective
length of the current path on the antenna is lengthened
through the modification. These behaviors contribute to extend
the lower-end bandwidth and improve the radiation directivity.
Fig. 5 plots both E- and H-plane radiation patterns of the simulated
original AVA, simulated TSE AVA, and measured TSE
AVA at 3.5, 7, and 10 GHz, respectively. As the figure reveals,
the proposed antennas have endfire characteristics with the main
lobe in the axial direction of the tapered slot ( -direction in
Fig. 1). For the TSE AVA, reasonable agreement is also obtained
between the measured result and the simulated one, which validates
the design approach. In 3.5 and 7 GHz, both simulated
and measured TSE AVA results show significant improvement
in directivity compared to the original AVA. In 10 GHz, the TSE
AVA has a similar performance as the original one, which indicates
the TSE is more effective in lower frequencies. Also, it
is worth noticing that the TSE modification is more effective in
E-plane than in H-plane.
The realized gain variation with frequency of TSE AVA is
shown in Fig. 6. The curve in the figure suggests a considerable
directivity compared to its electrical dimensions. In the band
between 2.5 and 14 GHz, the minimum gain is at the frequency
2.5 GHz with a value 3.7 dB and maximum gain at 7.5 GHz with
a value 10.0 dB. The jitter behavior presented in the figure is
possibly due to the irregular currents around the slots, especially
at high frequencies, which can be considered as the acceptable
compensation of the low-end performance improvement.
Another critical parameter for an ultrawideband antenna, the
group delay, which measures the time signal distortion introduced
by the antenna, is measured in this design. As shown in
Fig. 7, two identical TSE AVAs are placed face to face over a
distance 300 mm. The measured result shows that the group
delay of the proposed design is around 2 ns in the whole working
band with variation less than 1 ns in most frequencies. In other
words, the result indicates that this antenna is reliable so that a
transmitted signal will not be seriously distorted by the proposed
antenna.
CONCLUSION
An antipodal Vivaldi antenna with tapered slot edge modifi-
cation is designed and experimentally studied in this letter. The
lower-end 10-dB limitation of the modified antenna
is extended to 2.4 GHz from the original 3.3 GHz. The expansion
of th elow-end working band indicates the proposed TSE is
able to miniaturize the size of the AVA. Moreover, the TSE AVA
behaves with higher endfire directivity in lower and middle frequencies
compared to the original design. Measured results in
both frequency and time domain illustrate the validation of the
design, which suggest the proposed antenna is a good candidate
for ultrawideband or other communication systems.