This Letter demonstrates a new design of a probe-fed patch antenna with a modified antenna ground, and a constructed prototype ideal for applications in a 2.4-GHz WLAN access point is presented. The antenna has a thick air substrate for broadband operation and is fed by an inclined probe pin at the edge of the patch bent portion. The antenna ground comprises different portions and is in the shape of a step. With the proposed probe feed and ground configuration, good impedance bandwidth with VSWR below 1.5 over the 2.4 GHz WLAN band can be obtained. In addition, good broadside radiation characteristics have also been observed.

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© 2008 Wiley Periodicals, Inc.
A PROBE-FED PATCH ANTENNA WITH
Figure 1 (a) Geometry of the proposed probe-fed patch antenna with a
A STEP-SHAPED GROUND PLANE FOR step-shaped ground plane for 2.4-GHz WLAN operation. (b) Side view of
2.4 GHz ACCESS POINT the proposed antenna. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com]
Fa-Shian Chang,1 Yung-Tao Liu,2 Saou-Wen Su,3 and
Jui-Hung Chou3
1
Department of Electronics, Cheng Shiu University, Kaohsiung
County, 83347, Taiwan which makes it very difficult to well match the antenna to imped-
2
Department of Electrical Engineering, R.O.C. Military Academy, ance bandwidth within VSWR of 2. In addition to the use of a short
Fengshan 83059, Taiwan probe pin, the matching mechanism in these studies also demon-
3
Network Access Strategic Business Unit, Lite-On Technology strates some modification to the structure of the antenna feed.
Corporation, Taipei County 23585, Taiwan; Corresponding author:
susw@ms96.url.com.tw
These designs include the use of a bevel-feed transition [1], a
cylinder-feed transition [2], a T-shaped probe feed [3], an L-
shaped probe feed [4], an edge-fed patch with an L-shaped ground
Received 26 May 2008
[5], a ridge-shaped ground [6], and so forth. In this letter, we
introduce a new design of a probe-fed patch antenna capable of
ABSTRACT: This letter demonstrates a new design of a probe-fed
broadband operation. The antenna is backed by a step-shaped
patch antenna with a modified antenna ground, and a constructed proto-
type ideal for applications in a 2.4-GHz WLAN access point is pre-
ground plane and can be fed by a long probe pin at one of the patch
sented. The antenna has a thick air substrate for broadband operation radiating edges. Simply by tuning the probe-pin length, good
and is fed by an inclined probe pin at the edge of the patch bent por-
tion. The antenna ground comprises different portions and is in the
shape of a step. With the proposed probe feed and ground configuration,
good impedance bandwidth with VSWR below 1.5 over the 2.4 GHz
WLAN band can be obtained. In addition, good broadside radiation
characteristics have also been observed. © 2008 Wiley Periodicals, Inc.
Microwave Opt Technol Lett 51: 139 –141, 2009; Published online in
Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.
24019
Key words: probe-fed patch antennas; WLAN antennas; access-point
antennas
1. INTRODUCTION
Many patch antennas utilizing thick air substrates have been de-
veloped and studied to achieve impedance bandwidth exceeding
10% defined by 2:1 VSWR or 5% by 1.5:1 VSWR for broadband
operation [1– 6]. Among these researches, a short probe pin of the
probe feed is mostly employed in conjunction with various match- Figure 2 Measured return loss for the design prototype of various
ing mechanism. That is because for patch antenna with such thick probe-pin lengths. [Color figure can be viewed in the online issue, which
air substrate, using a long probe pin can cause large inductance, is available at www.interscience.wiley.com]
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 139

2. For matching the input impedance of the patch antenna, the
length of the probe pin was carefully tuned. When a thick air
substrate is utilized for the radiating patch, large inductance intro-
duced by the probe pin may occur, which can be compensated for
additional capacitive reactance arising between the vertical plate
(of the ground) and the patch bent portion and between the inclined
plate and the bent portion. Thus, the probe pin of the probe feed
can be relatively long in this study, compared with the short probe
pin used in broadband patch designs reported in [1– 6]. The related
results will be elaborated more in the next section.
3. EXPERIMENTAL RESULTS AND DISCUSSION
On the basis of the antenna configuration shown in Figure 1, a few
prototypes have been constructed and experimentally studied. Fig-
ure 2 shows the measured return loss for the design prototype of
Figure 3 Measured input impedance for the design prototype of various
various probe-pin lengths (d). It is first seen that with d equal to 8
probe-pin lengths. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com]
mm, good impedance bandwidth defined by 1.5:1 VSWR (14 dB
return loss) can be obtained and reaches 140 MHz (2370 –2510
MHz), easily covering the 2.4 GHz band. The sum of d and L
impedance matching over frequency band with 1.5:1-VSWR band- (patch length) also corresponds to a half wavelength of the center
width of about 6% can be obtained. Several design prototypes have operating frequency at 2442 MHz targeted in this study. In addi-
been built, and a design example aimed for operation in the 2.4 tion, compared with a short probe pin, usually 2% wavelength of
GHz (2400 –2484 MHz) WLAN band has been implemented too. the center operating frequency, used for broadband patch antenna
The proposed antenna is designed for access-point applications in designs [1– 6], the probe pin of 8 mm here is relatively long and
the WLAN environment. Details of the antenna design are de- about 6.5% wavelength at 2442 MHz. Notice that when the probe-
scribed, and experiment results are discussed. pin length d changes from 5 to 9 mm, the thickness of the air
substrate slightly increases from 8 to 11 mm. In general, with an
2. ANTENNA DESIGN increase in d, the antenna operating frequency moves to lower
Figure 1(a) shows the geometry, in detail, of the proposed antenna frequencies, where the behavior is the same as that a thicker
for operation in the 2.4 GHz band. The radiating patch is in the substrate can result in a lower frequency band as reported in [5].
shape of a rectangle with the dimensions 54 mm (L) 60 mm (W) The measured input impedance, including real (resistance) and
and has a small bent portion (4 mm in length) at one of the patch imaginary (reactance) parts, for the antenna of various probe-pin
radiating edges. The antenna ground plane is bent three times into lengths (d) is shown in Figure 3. It can be seen that for operating
a step-shaped structure and consists of four portions: two horizon- frequencies of interest at about 2442 MHz, the curve of imaginary
tal plates, one vertical plate, and one inclined plate, all with the part is closest to 0 with the real-part curve approaching 50 when
same width of 80 mm. The angle between the inclined (5 mm in a near optimal value d of 8 mm is chosen.
length) and horizontal (60 mm in length) plates is 135°. In the Figure 4 gives the measured radiation patterns of the E-plane
center of the inclined plate , below a via hole is located a 50- (x-z cut) and H-plane (y-z cut) at 2442 MHz for the constructed
SMA connector of the probe feed. In this case, the probe pin prototype studied in Figure 2 with probe-pin length d 8 mm.
(length d) of the probe feed is inclined at an angle of 45° [see Fig. Measurements at other frequencies in the 2.4 GHz band were also
1(b)]. The radiating patch is edge-fed by this inclined probe feed, taken, and the results were similar radiation patterns as those
and the probe pin is coplanar with the patch bent portion. The plotted here. In the x-z cut was seen cross-polarization radiation
thickness of the air substrate is set to be 10 mm in this study, which below about 20 dB in the E-plane patterns for the frequencies
is about 0.08 times the free-space wavelength at 2442 MHz, the over the 2.4 GHz band; as for H-plane patterns in the y-z cut, the
center frequency of the 2.4-GHz WLAN band. cross-polarization radiation is below about 15 dB. The measured
Figure 4 Measured radiation patterns at 2442 MHz for the patch antenna studied in Figure 2 with the length of the probe pin equal to 8 mm. [Color figure
can be viewed in the online issue, which is available at www.interscience.wiley.com]
140 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 DOI 10.1002/mop

3. ACCURATE DESIGN OF A COMPACT
SUPERCONDUCTING MICROSTRIP
FILTER
Huili Peng,1 Xubo Guo,1 Shichao Jin,1 Yunlong Piao,1
Xiaoping Zhang,1 Bin Wei,1 Baoxin Gao,2 and Bisong Cao1
1
Department of Physics, Tsinghua University, Beijing, People’s
Republic of China; Corresponding author:
guoxb05@mails.tsinghua.edu.cn
2
Department of Electrical Engineering, Tsinghua University, Beijing,
People’s Republic of China
Received 26 May 2008
ABSTRACT: This article presents a six-pole high-temperature super-
conducting (HTS) filter at 2.5 GHz with a compact size of 10 mm 15
mm on MgO substrate. Twin-spiral resonators with high simulation ac-
Figure 5 Measured peak antenna gain against frequency. [Color figure curacy are used in the design procedure. EM simulations verify that the
can be viewed in the online issue, which is available at www.interscience. filter response is insensitive to the variations of the cell sizes and the
wiley.com] substrate parameters. The untuned measurement of the HTS filter at 70
K shows a high performance. The minimum return loss is 14.7 dB with-
out any tuning. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol
peak antenna gain is presented in Figure 5, and the peak-gain level Lett 51: 141–144, 2009; Published online in Wiley InterScience (www.
is seen to be about 8 dBi. interscience.wiley.com). DOI 10.1002/mop.24018
4. CONCLUSION Key words: HTS filter; microstrip; accurate design
A patch antenna fed by an inclined probe pin with a step-shaped
ground plane has been proposed, and several prototypes have been 1. INTRODUCTION
studied. The impedance matching with VSWR below 1.5 of the High-temperature superconducting (HTS) thin films with ex-
antenna for WLAN operation in the 2.4 GHz band can easily be tremely low surface resistance have enabled compact and excellent
obtained. The measured results show that although a long probe bandpass filters, which exhibit very low insertion loss, steep band-
pin is in use, large probe inductance is well compensated for edges, and high out-of-band rejection. These attractive perfor-
additional capacitive coupling arising between the vertical and mances are of great help to improve the sensitivity and selectivity
inclined plates of the ground and the patch bent portion. The of communication systems, such as wireless and satellite commu-
proposed patch antenna along with good radiation characteristics is nications [1–5]. However, the measured responses of HTS filters
suitable for WLAN access-point antennas, especially for the use in usually deviate from the designed results due to the fabrication
panel antennas. errors and the variations of the substrate properties [6 – 8]. Thus,
tuning is usually a routine procedure to produce a high-perfor-
REFERENCES mance HTS filter. As tuning is a time-consuming and repetitious
1. F.S. Chang and K.L. Wong, A broadband probe-fed patch antenna for work, researchers have made great efforts to produce high-perfor-
a DCS base station, Microwave Opt Technol Lett 30 (2001), 341–343. mance HTS filters without tuning [9, 10].
2. F.S. Chang and K.L. Wong, A broadband probe-fed planar patch Spiral resonator has been known to have several advantages,
antenna with a short probe pin and a conducting cylinder transition, including compact size, good simulation accuracy, and insensitiv-
Microwave Opt Technol Lett 31 (2001), 282–284.
ity to the tolerance of fabrication procedures and substrate param-
3. C.L. Mak, K.F. Lee, and K.M. Lu, Broadband patch antenna with a
T-shaped probe, IEE Proc Microwaves Antennas Propagat 147 (2000),
eters [11–15]. In this article, we choose a half-wavelength twin-
73–76. spiral resonator to design a high-performance HTS filter. The
4. S.L. Yang and K.M. Luk, Wideband folded-patch antennas fed by linewidth of the resonator is 100 m, so that a compact six-pole
L-shaped probe, Microwave Opt Technol Lett 45 (2005), 352–355. filter can be created on a 10 mm 15 mm MgO substrate. The EM
5. F.S. Chang and K.L. Wong, Broadband patch antenna edge-fed by a simulations verify that the filter performance is insensitive to the
coplanar probe feed, Microwave Opt Technol Lett 31 (2001), 287–289. variations of the cell sizes and the substrate parameters. In addi-
6. J.W. Zhang, S.S. Zhong, and Q. Wu, Large-bandwidth patch antenna tion, the photolithography process was carefully carried out, and
with ridge-shaped ground plate, Microwave Opt Technol Lett 48 there is no defect observed in the filter pattern. As a result, the
(2006), 487– 488. measured results of the HTS filter show a high performance
without any tuning.
© 2008 Wiley Periodicals, Inc.
2. FILTER DESIGN PROCEDURE
The layout of the six-pole microstrip filter is shown in Figure 1.
Half-wavelength twin-spiral resonators with six turns are used in
the design, which occupy a size of only 1.1 mm 5.05 mm. The
linewidth of the resonator is 100 m, and the space between the
tracks is 100 m. We use an MgO substrate with a thickness of
0.51 mm and a relative dielectric constant of 9.73. The total area
of the six-pole filter is 15 mm 10 mm.
The filter is designed by the general procedure of coupled-
resonator filters [16]. The required coupling coefficients according
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 1, January 2009 141