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8.1 FSA (Functionally Small Antennas)	279

Return loss (dB)

Measured

Simulated

1.0

1.2

1.4

1.6

1.8

Frequency (GHZ)

Figure 8.14 Measured and simulated return loss ([6], copyright C 2010 IEEE).

Gain dBic

12	Measured
	Measured
10	Simulated
10
8
6
4
2
0
−2

1.0	1.2	1.4	1.6	1.8
		Frequency (GHz)

AR, dB

Measured

Simulated

1.0

1.2

1.4

1.6

1.8

Frequency (GHz)

(a)

(b)

Figure 8.15 (a) Simulated and measured gain at the boresight and (b) measured and simulated axial ratio at the boresight ([6], copyright C 2010 IEEE).

be so complicated that it may hardly be recognized as a PIFA. Use of slots or slits is, however, considered as the best means to realize a multiband antenna that accommodates the need of the recently deployed wireless communication systems, even with small size that can be ﬁt within the extremely limited space in the small equipment.

8.1.2.1.1.2.1 Microstrip slot antennas

Microstrip monopole slot antennas with different shapes such as straight, L, and T, have been investigated and demonstrated that a wide bandwidth could be obtained by proper

280	Design and practice of small antennas II

	Slot	G	w
			w
		Sw
	Ls		Lf	Microstrip
	Ls			Microstrip
				feed line
	St		Fl	50 ohm
				SMA probe
GL				Ground
GL				plane
				Y
				X
				Z
h	εr
				X
		Slot

50 ohm SMA probe

Figure 8.16 Geometry of microstrip monopole slot antenna on a small ground plane fed by a microstrip line ([7a], copyright C 2005 IEEE).

selection of antenna parameters, length of feed line, position of the feed over the slot, and stub length [7a]. These antennas are placed on a small ground plane, which is about the size of a typical PC wireless adapter card.

Geometries of microstrip monopole slot antennas on a small ground plane fed with a microstrip line are illustrated in Figure 8.16 (straight), Figure 8.17 (L-shape) and Figure 8.18 (T-shape), in which dimensional parameters are also given. In Figure 8.17, three different feeding shapes are shown. The size of the ground plane is width GW = 50 and length GL = 80 (in mm). The slot in each antenna is open at its end, positioned at the center of the narrow ground plane edge, and fed with a 50microstrip transmission line.

The straight slot has length Ls = 30, width Sw = 30, and length of feed lines F1 = 24, and S1 = 11. In the optimized design, the substrate with εr = 2.5 and tan δ = 0.001, having thickness h = 1.57 is used (measurements in mm). The simulated return loss (–10 dB) bandwidth of this antenna is 58.8% (2.46 GHz to 4.51 GHz) as can be observed in Figure 8.19.

Regarding the L-slot antenna, simulated return loss bandwidth is shown in Figure 8.20, where comparisons depending on the feed shape, straight, inclined, and bent, are provided. The geometrical parameters of the antennas fed with different shapes of feed line, respectively, are L1 = 18.5, L2 = 11.5, Sw = 7, Gw = 50, GL = 80 as

8.1 FSA (Functionally Small Antennas)	281

Slot		Gw					Slot		Gw
		Gw							Gw
	L1	Sw	Lf		Microstrip				Sw
			Lf		Microstrip		L
					feed line		L	1
					feed line			1
S	t			Fl	50 ohm				St
S				Fl
						Microstrip
				L2	SMA probe	Microstrip			L2
				L2	Ground	feed line		Fl	L2
GL					Ground			Fl
GL					plane				G	L
										L
						50 ohm
						SMA probe
						Ground
						plane
					Y
					X
		(a)			Slot				(b)
					Slot	Gw
						Gw
						Sw
					L1
						St
					Microstrip	L2	Ground
					feed line	Fl	Ground
					feed line	Fl	plane
					GL		plane
						50 ohm
						SMA probe

(c)

Figure 8.17 Geometry of L-slot antenna fed by a line with different shape: (a) straight,

(b) inclined, and (c) bent ([7a], copyright C 2005 IEEE).

common dimensions and Lf = 11, Ft = 18.5, St = 9 in the straight feed line, Ft = 16.5, St = 7.5 in the bent feed line, and Ft = 17, St = 7.5 in the inclined feed line. The substrate used is common to all antennas, with εr = 4.5 and tan δ = 0.002, having thickness h = 0.81 (in mm). The impedance bandwidths of antennas with straight feed line, bent feed line, and inclined feed line, respectively, are 82% (2.24 GHz to 5.36 GHz), 75.7% (2.48 GHz to 5.5 GHz) and 75.7% (2.47 GHz to 4.97 GHz). By replacing the substrate with a low loss one, with εr = 2.5 and tan δ = 0.001, having thickness h = 0.79, an improved bandwidth of 82% (2.42 GHz to 5.78 GHz) can be obtained.

282	Design and practice of small antennas II

Slot	Gw
L1	Sw	Lf		Microstrip
				feed line
S1			F1	50 ohm
				50 ohm
	L2			SMA probe
				Ground
				plane
GL

Figure 8.18 Geometry of T-slot antenna fed by a microstrip line ([7a], copyright C 2005 IEEE).

Return loss, S11 (dB)

–5

–10

–15

–20

–25

–30

2.5

3.5

4.5

Frequency (GHz)

Figure 8.19 Simulated return loss of the antenna shown in Figure 8.16 ([7a], copyright C 2005 IEEE).

With the T-slot, having Sw = 7, Lf = 75, Fl = 23.5, St = 10, L1 = 7, L2 = 23 mm, using the same substrate as the other slot-type antennas, simulated return loss is depicted in Figure 8.21. The impedance bandwidth of 78% (2.56 GHz to 5.8 GHz) is obtained, while 80% (2.74 GHz to 6.4 GHz) resulted with low-loss substrate.

A novel compact open-end slot antenna fed by a short-circuited microstrip line is introduced in [7b]. The antenna has reduced dimensions by virtue of placing two matching capacitors at both sides of an electrically small open-end slot. With the proposed conﬁguration, the antenna resonance frequency is 2.45 GHz, having the slot of

8.1 FSA (Functionally Small Antennas)	283

−5

+++

+ ++

(dB)

+++

−10

++++

+++++

−15

+ ++++

loss,

−20

Return

−25

Straight feed line

−30

Inclined feed line

−35

Bent feed line

2.5

3.5

4.5

5.5

Frequency (GHz)

Figure 8.20 Simulated return loss of the L-shape antenna shown in Figure 8.17 ([7a], copyrightC 2005 IEEE).

		0
		–5
	(dB)	–10
	(dB)	–15
	11	–15
	S
	loss,	–20
	Return	–30
		–25

–35

–40

2.5

3.5

4.5

5.5

6.5

Frequency (GHZ)

Figure 8.21 Simulated return loss of T-slot antenna shown in Figure 8.18 ([7a], copyright C 2005 IEEE).

9 mm long and 1.5 mm wide, originally a 4.8 GHz antenna in a conventional design. The equivalent circuit model is developed, and input impedance of the antenna for variations in the feed position is calculated. The results of calculation agree well with full wave simulation. By adding two capacitances to the slot for cancelling the inductive reactance of the slot at lower frequencies, the resonance frequency can be lowered. The antenna length becomes shorter than one-eighth the wavelength at the operating frequency. The measured bandwidth of 109 MHz at 2.45 GHz and peak gain of 1.89 dBi demonstrate the performance of the proposed small-sized slot antenna design.

284	Design and practice of small antennas II

U-shaped slot	W
U-shaped slot
	b
L	Ls
	Fp
	c

Ground plane	Ws
Ground plane

Rectangular patch

εr	Dprobe	t
Coaxial feed		Ground plane
		Ground plane

Figure 8.22 The basic geometry of U-slot patch antenna ([8b], copyright C 2003 IEEE).

8.1.2.1.1.2.2 Microstrip patch antennas 8.1.2.1.1.2.2.1 Patch antennas with U-slot

A patch antenna with a U-slot was introduced in 1995 [8a] as an antenna capable of providing a wide impedance bandwidth in a range of 25–30%. Since then, a number of studies followed and recent interest has arisen to design a U-slot patch antenna for multiband operation and circular polarization as well as wideband operation. Size reduction is also a signiﬁcant requirement, as the resonance length of a conventional patch antenna was about a half-wavelength, which was too large for small wireless portable devices. To meet the requirement, various techniques have been developed to decrease the size of the U-slot patch antenna, keeping its wide bandwidth unchanged. Typical examples are use of an L-shaped probe to feed [8b, 8c], a shorting wall [8b, 8d], a shorting pin [8b, 8e], or reduction of a U-shape to a half-U-shape [8b, 8f, 9], which corresponds to an L-shape.

The basic geometry of a U-slot antenna is illustrated in Figure 8.22, where dimensional parameters are given [8b]. Basically, the slot dimensions determine the resonance frequency as well as the patch dimensions. By appropriately designing slot and patch dimensions, wideband or multiband resonance can be attained. The U-slot cancels out the feed inductance and currents around the U-slot create additional resonance that makes the antenna broadband as a result of combination with the resonance of the patch.

A reduced-size U-slot patch antenna of the basic structure (Figure 8.22) by increasing the substrate εr is introduced. Design parameters are provided in Table 8.2, which shows cases for three values of εr, including an air substrate case εr = 1. The operating

8.1 FSA (Functionally Small Antennas)	285

Table 8.2	Design parameters for 900 MHz U-shaped patch antenna
[cm] ([8b], copyright C 2000 IEEE)
	εr	= 1.0		εr = 2.33		εr = 4.0
W	21.97		(0.659λ0)	12.40	(0.372λ0)	9.29 (0.201λ0)
L	12.45		(0.374λ0)	8.96	(0.269λ0)	6.71 (0.201λ0)
Ws	6.86			4.82		3.61
Ls	8.22			6.20		4.65
b	1.94			1.38		1.03
c	0.89			0.69		0.52
Fp	6.22			4.48		3.36
t	2.69		(0.081λ0)	2.76	(0.083λ0)	2.40 (0.072λ0)
Dprobe	0.3			0.34		0.17
				L
			D
					W
Ground plane				Lp
Ground plane
L-shaped probe						Rectangular patch
εr		t1
Air or foam		t2				t
Coaxial feed						Ground plane
Coaxial feed
Figure 8.23 Geometry of L-probe-fed patch antenna ([8b], copyright C 2003 IEEE).

frequency f0 of the antenna is 900 MHz. The patch length L is initially 0.374λ0 with εr = 1.0, but reduced to 0.201λ0 with increased εr = 4.0. The patch size, deﬁned here as the normalized area in comparison with the U-slot patch area (21.97 × 12.45 = 273.53 mm2) when εr = 1, is also decreased to 23% (εr = 4.2) of that with εr = 1. Size reduction is achieved with sacriﬁce of the bandwidth, being decreased to 22.1% (εr = 4.2) from 42% (εr = 1), although it is still substantially wideband.

By using an L-probe to feed the U-slot patch as shown in Figure 8.23 [8b, 8c], nearly the same amount of size reduction with the increase in εr can be achieved,

286	Design and practice of small antennas II

		Table 8.3(a)	Design parameters of L-probe patches (mm) ([8b], copyright
		C 2000 IEEE)

			εr = 1.0		εr = 2.32
		W	15.0	(0.383λ0)	15.0	(0.315λ0)
		L	13.0	(0.332λ0)	13.0	(0.273λ0)
		Lp	7.0		7.6
		D	3.0		3.0
		t1	−		5.0
		t2	−		1.58
		t = t1 + t2	6.0	(0.15λ0)	6.58 (0.14λ0)
		dprobe	1.0		1.0

VSWR or gain/dBi

6
6
5.5
5.5
5				Gain
5
4.5


4
4
3.5
3.5
3		VSWR (Experiment)
3
2.5
			VSWR (Simulation)

2


1.5
1.5
1
4.5	5	5.5	6	6.5	7	7.5	8

Frequency (GHz)

Figure 8.24 Simulated and measured VSWR and measured gain of an L-probe fed patch antenna ([8b], copyright C 2003 IEEE).

while substantially wide bandwidth is kept. The L-probe-fed U-slot patch antenna is fabricated practically by embedding a horizontal arm on the foam substrate beneath the dielectric substrate, on which the patch is embedded. An L-probe-fed U-slot patch antenna with two-layer conﬁguration to operate at the 6-GHz band is designed with the antenna parameters given in Table 8.3(a) for cases of two values of εr. The simulated and measured VSWR and measured gain for the case of εr = 2.32 are shown in Figure 8.24 and the results are provided in Table 8.3(b) with the patch dimensions.

As another way to reduce the size of the U-slot patch, a shorting wall is introduced on the substrate as shown in Figure 8.25 [8b, 8d], in which geometrical parameters are also given. A U-slot patch antenna with a shorting wall is fabricated on a substrate with εr = 4.4. The center frequency f0 = 2.5915 GHz and the design parameters are; W = 14, L = 14, Ws = 9, Ls = 12, b = d = 1, c = 2, Fp = 2, Dprobe = 1, and the thickness t = 12.1 (units are mm). The patch is a square with the sides measuring 0.121λ0 and

			8.1 FSA (Functionally Small Antennas)				287

	Table 8.3(b) Measured results of two-layer L-probe patch ([8b], copyright C					2000 IEEE)

		Patch Dimensions	Thickness	f0	Normalized
		(mm)	(mm)	(GHz)	f0	BW	Gain
1.0		15.0 × 13.0	6.0 (0.15λ0)	7.66	1.0	36%	6.5 dBi
	2.32	15.0 × 13.0	6.0 (0.15λ0)	6.3	0.82	36%	4.5 dBi

Ground plane

Rectangular patch

Shorting wall

t εr

Dprobe

Coaxial feed

Ground plane

Figure 8.25 Geometry of U-slot patch antenna with a shorting wall on a substrate ([8b], copyrightC 2003 IEEE).

thickness of 0.105λ0. The area occupied by the patch is 0.01446λ2, a 94% area reduction compared to a square half-wave patch. Figure 8.26 illustrates the simulated and measured VSWR.

Another way to reduce the patch size is to use a shorting pin, which is added in close proximity to the probe feed as shown in Figure 8.27 [8b, 8e]. The shorting pin and the feed are placed at the opposite sides of the patch. The shorting pin couples equivalently capacitively with the resonance circuit of the patch, consequently effectively increasing the permittivity of the substrate. A U-slot patch antenna with a shorting pin is designed for operation at approximately f0 = 4 GHz. The design parameters are; W = 18, L = 15, Ws = 16, Ls = 10, b = 4, c = 4.5, Fs = 4, Fp = 14.5, Dprobe = 1, Dshort = 2, and the thickness t = 7 (units are mm). Figure 8.28 illustrates simulated and measured VSWR and measured gain. The bandwidth (VSWR ≤ 2.0) obtained through simulation is 30% and the average gain is 2 dBi. The patch is a 0.200λ0 by 0.240λ0 rectangle supported

288	Design and practice of small antennas II

VSWR

Measurement

Simulation

2.25

2.5

2.75

3.25

Frequency (GHz)

Figure 8.26 Simulated and measured VSWR for the U-slot patch antenna with a shorting wall ([8b], copyright C 2003 IEEE).

		L
			Fs
	W	Fp	Ws
		c
		b
Ground plane		Lp
Ground plane
Rectangular patch			Shorting pin
t	εr	Dprobe	Dshort
	εr

Coaxial feed

Ground plane

Figure 8.27 Geometry of a U-slot patch antenna with a shorting pin ([8b], copyright C 2003 IEEE).

by a foam substrate with 0.093λ0 thickness. The area occupied by the patch is 0.048λ2, 80.8% smaller than that of a square half-wave patch.

A full-size U-slot is halved to form a half-U-slot or an L-slot by removing the patch area along the line of symmetry as is shown in Figure 8.29 [8b, 8f, 9]. In the ﬁgure,

8.1 FSA (Functionally Small Antennas)	289

VSWR or gain/dBi

10
9
8				VSWR (Experiment)
7				VSWR (Simulation)
7				Gain (Experiment)
6
5			VSWR			Gain
4			VSWR			Gain
4
3
2
1
2.5	2.75	3	3.25	3.5	3.75	4	4.25	4.5	4.75	5

Frequency (GHz)

Figure 8.28 Simulated and measured VSWR and measured gain of the U-slot antenna with a shorting pin ([8b], copyright C 2003 IEEE).

(a)		W = 42
y			23
			23	10
	x
	L = 70	48.8	44
	L = 70			16.4
				16.4

(b) y

	28
47.7
	y
	air
17.6	z
17.6
Shorting pin
W = 84

L = 70

Shorting pin

Line of symmetry

Figure 8.29 Geometry of a half-U-slot patch antenna (in [mm], not to scale), (a) half-U-slot and

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