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.
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