366
Wg
Tze-Meng, Geok, and Reza
Geometry
W
2
Parameter
Size
29 mm
12.5 mm
8.8 mm
24 mm
3 mm
51 mm
16 mm
“L” Shaped
Element
L
1
L
1
L
2
W
2
L
3
W
3
L
2
Rectangular Shaped
Element
Ground Element
Lg
Overall Length
Overall Width
L
3
L
g
W
g
W
3
Figure 2.
Geometry and dimension of the proposed antenna.
wavelength dipole antenna with the upper and lower portions each of
having an electrical length of about
λ/2.
As presented in the figure,
the antenna structure consists of two conductive portions, first is the
radiating element (upper portion) and second is the ground element
(lower portion). The geometry of the proposed antenna occupies a
volume of 51 mm
×16
mm, which mainly consists of two radiating arms
and a rectangular element in between that is connected to the tapering
feature.
The antenna is fabricated onto a single-sided FR-4 printed circuit
board (PCB) with substrate thickness of 0.8 mm, tangent factor of
0.025, and permittivity (dielectric constant) of 4.3. Theoretically, a
quarter wavelength (without considering losses) can be calculated using
following equation.
3
×
10
8
c
= 125 mm
(1)
λ
= =
f
2.4
×
10
9
where
λ
is the wavelength,
c
is the speed of light in vacuum, and
f
is
the frequency of 2.4 GHz. Therefore, the quarter wavelength obtained
Progress In Electromagnetics Research, Vol. 106, 2010
367
= 31.25 mm
≈
L
1
.
Since the antenna is printed on the surface of a substrate, the
substrate’s permittivity will influence the resonant length. To get a
good performance, it is required that the resonant dipole to be designed
slightly less than half a wavelength long. A good assumption is 0.47
times the wavelength as reported in [16]. The length of the resonant
dipole
rd
can be calculated by using the following Equation (2).
v
rd
= 0.47
×
λ
= 0.47
×
(2)
f
is
where
v
denotes the actual propagation speed on the dipole radials.
This speed depends on the effective dielectric constant of the substrate.
The speed can be calculated by using Equation (3) as follows.
c
(3)
v
=
√
ε
eff
where
ε
eff
is the effective dielectric constant of the substrate. Using
Equations (2) and (3), a printed dipole for 2.4 GHz is designed.
For
f
= 2.4 GHz, the speed on the radials can be calculated using
Equation (3) as follows.
3
×
10
8
= 1.45
×
10
8
m/s
v
=
√
4.3
With this speed, the length of the resonant printed dipole
rd
can be
obtained using Equation (2) as follows.
1.45
×
10
8
= 28.39 mm
≈
L
1
rd
= 0.47
×
2.4
×
10
9
Therefore, the overall length of “L” shaped element is
L
1
= 29 mm,
which is approximately equal to the value of
rd.
In summary, the characteristics of this antenna can be enumerated
as follows:
(i) By varying the length of
L
1
and
L
2
, the wideband operation of
the microstrip printed dipole antenna can be excited with good
impedance matching. Based on the obtained simulation results
of Figures 3 and 4, as the length of
L
1
(“L” shaped element)
and
L
2
(rectangular shaped element) increases, both low band
and high band resonant frequencies will be shifted to the right.
Therefore, the length of
L
1
and
L
2
is tuned for the optimum
antenna performance.
(ii) At low band frequency range of 2.4 GHz, the antenna is operable
such that the “L” shaped radiating element has electric length of
about
λ/4.
However, the electrical length of rectangular element
λ
4
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