A simple, bent monopole antenna well useful for WLAN applications in the 2.4 GHz band is presented. The monopole antenna has a rectangular radiating plate in general and is short-circuited to a small antenna ground and an assembly plate. The assembly plate is not only used as a supporting plate for antenna installation but also regarded as antenna ground. With a low profile of the monopole and use of the coaxial-line feed, the antenna has much flexibility in the placement inside a wireless device. Good radiation characteristics have been observed too.
2. been studied in academia. Among many of them, the coaxial-line-
fed antennas of a small form factor have favorably been introduced
[1–7]. Owing to their flexibility and mobility, these kinds of
antennas are very attractive to fit into many kinds of WLAN
devices. In this letter, we demonstrate a new design of a low-
profile, short-circuited monopole antenna for WLAN operation in
the 2.4 GHz (2400 –2484 MHz) band with good impedance band- Figure 2 Photo of a constructed prototype made of a 0.3-mm-thick alloy
width. The antenna is fabricated by stamping a single, flat metal and fed by a 50- mini-coaxial cable of length 30 mm. [Color figure can
plate only, which consists of a bent monopole, a shorting portion, be viewed in the online issue, which is available at www.interscience.
a small antenna ground, and a flat assembly plate that is connected wiley.com]
to the antenna ground. The assembly plate in this design is not only
used as a supporting plate for affixing the antenna to a wireless to meet the bandwidth requirement of the WLAN 2.4 GHz oper-
device’s internal surface but also treated as antenna ground. De- ation.
tails of a design example of the proposed antenna are described, To test the design prototype in experiments, a short 50-
and the experimental results thereof are elaborated and discussed. mini-coaxial cable of length 3 mm with an I-PEX connector is
utilized (see photo of a working sample in Fig. 2). The inner
2. ANTENNA DESIGN conductor of the coaxial cable is connected to feed point A, and the
Figure 1(a) shows the configuration of the proposed, bent mono- outer braided shielding is connected to ground point B. Notice that
pole antenna in detail. The antenna mainly comprises a bent a small portion of 2 mm 4 mm protruding from the antenna
monopole, a shorting portion, an antenna ground, and an assembly ground is used to accommodate ground point B. In addition, the
plate. The bent monopole, formed by bending a radiating plate, is near optimal value of the feed gap in between the bent monopole
fed at one corner and short-circuited, at a partial side of the and antenna ground is 1 mm in the design.
monopole, to the antenna ground and assembly plate through the
shorting portion. Both the bent monopole and shorting portion are 3. EXPERIMENTAL RESULTS AND DISCUSSION
5 mm in height and also in width and located above the antenna Figure 3 shows the measured and simulated return loss of a design
ground (5 mm 40 mm). The assembly plate with dimensions 10 prototype. It can be first seen that in general, the experimental data
mm 40 mm is then perpendicularly connected to the antenna compare favorably with the simulation results, which are based on
ground. In this case, the proposed antenna can firmly be affixed to the finite element method (FEM). The measured impedance band-
the internal surface of a wireless device by various mechanic width, defined by 10 dB return loss, reaches about 195 MHz
methods. Detailed dimensions of the antenna in a flat plate struc- (2368 –2563 MHz) and meets the bandwidth specification for 2.4
ture are also given in Figure 1(b). Notice that for matching the GHz WLAN operation easily. Notice that the impedance matching
input impedance of the antenna, a small gap of distance d between is even better than 14 dB (about 1.5:1 VSWR). Further, when there
the antenna feed (point A) and shorting portion is carefully tuned is no assembly plate (see inset in Fig. 4), the achievable bandwidth
can still cover the 2.4 GHz band with 10-dB return-loss require-
ment. The obtained result is highly beneficial because it helps relax
constraints on the mechanic structure in the assembly plate for
affixing the antenna to the internal surface of some wireless device.
For example, the assembly plate can be perforated by a few small,
circular holes for holding welding posts with the antenna operating
band remaining the same.
Figure 5 gives the far-field, 2D radiation patterns at 2442 MHz
in E and E fields. The measurement was conducted at a 3 3
7-m3 fully anechoic chamber, with the great-circle method, at
Figure 1 (a) Geometry of the proposed, bent, shorted, planar monopole
antenna for 2.4 GHz band. (b) Detailed dimensions of the antenna unbent Figure 3 Measured and simulated return loss; d 2 mm. [Color figure
into a flat plate structure. [Color figure can be viewed in the online issue, can be viewed in the online issue, which is available at www.
which is available at www.interscience.wiley.com] interscience.wiley.com]
456 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 2, February 2009 DOI 10.1002/mop