IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO.

9, SEPTEMBER 2007 2473

Ultrawideband Band-Notched Folded Strip Monopole

Antenna
Tzyh-Ghuang Ma, Member, IEEE, and Sung-Jung Wu

Abstract—We propose a new band-notched folded strip low-noise amplifier (LNA) with respect to its noise, linearity,
monopole antenna for ultrawideband applications. This antenna and group delay variation [5]. The antenna in an UWB radio
is composed of a forked-shape radiator and a 50

microstrip also involves considerable extra design constraints. In such

line. To achieve band-rejected filtering property at the WLAN
bands, the forked-shape strips are folded back and result in a pair a system, the antenna behaves more like a bandpass filter in
of coupled lines on the radiator. The length and gap width of the both spatial and frequency domains. Any nonideal variation
coupled lines primarily determine the notched frequency of the of the magnitude or phase response of an UWB antenna will
antenna. Based on the band-notched resonance, an equivalent inevitably introduce signal distortion and hence seriously de-
circuit model is proposed for the antenna and the calculated an- teriorates the overall performance [6]. Generally speaking, an
tenna input admittance agrees with the full-wave simulation data.
With the help of the dimensionless normalized antenna transfer UWB antenna should fulfill all the critical requirements of ul-
function, the radiation characteristics are investigated thoroughly. trawide bandwidth, stable radiation pattern and gain, consistent
The transmission responses of a transceiving antenna system and group delay, high radiation efficiency and low profile. Various
their corresponding transient analysis are discussed at the end of literatures have been devoted to evaluating the performance of
this paper. an UWB antenna [6]–[11]. Chen et al. discussed the consider-
Index Terms—Antenna transient analysis, equivalent circuits, ations for source pulses and antennas in UWB radios [6], and
monopole antennas, ultrawideband (UWB) antennas. Ma and Jeng proposed a new measurement arrangement for
measuring wideband antenna transfer functions [7]. Time-do-
main, frequency-domain as well as spatial-domain antenna
I. INTRODUCTION
characterizations have been also studied in various literatures
[9]–[11].
Among the newly proposed UWB antenna designs, the planar
O WING TO THE rapid development of modern communi-
cation and semiconductor technologies, a wide variety of
wireless services have been successfully introduced worldwide
monopole antennas should be the most promising candidate for
future applications due to their remarkably compact size and
in the past few years. While the wireless local area networks stable radiation characteristics [12]–[15]. Microstrip-fed square
(WLANs) aim at providing reliable and secure outgoing data monopoles [12], [13] as well as circular monopoles [14] have
stream over air interface in the range of hundreds of meters, the been studied extensively in terms of their impedance bandwidth,
IEEE 802.15.3a task group is currently engaged in finalizing the radiation patterns and transient response. A CPW-fed U-type
standards for wireless personal area networks (WPANs), a pro- monopole antenna demonstrating similar performance with re-
tocol which promises extremely high transmission rates over a duced antenna size has been proposed in [15] as well. Such
very short distance, say, 10 meters [1]. Ultrawideband (UWB) fork-shaped structure is known to have the ability to increase the
technology, one of the core technologies in WPANs, experi- antenna bandwidth [16]. A LTCC planar monopole with slotted
ences a blooming growth recently. Two distinct schemes, the di- back ground plane also gives the possibility of building highly
rect sequence-UWB (DS-UWB) [2] and the multiband orthog- integrated circuits in a single substrate [17]. Other UWB an-
onal frequency division multiplexing (MB-OFDM) [3], are now tenna design like the double-sided printed bow-tie antenna has
competing for the IEEE 802.15.3a standard. For the moment, also been proposed [18].
both schemes support transmission rate of as high as 480 Mbps, In addition to the above-mentioned design considerations, it
and undoubtedly illustrate the fascinating future of UWB radios. is also noted that owing to the overlapped spectra of UWB and
Due to the inherently ultra-wide operating bandwidth from WLAN radios, ultrawideband antennas with filtering property
3.1 to 10.6 GHz, the circuit components to be implemented in at the 5–6 GHz band have been proposed not only to mitigate
an UWB radio face quite different challenges than those applied the potential interference but also to remove the requirement
to a conventional narrowband system. Wang et al. proposed a of an extra bandstop filter in the system [19], [20]. It is noted
new UWB bandpass filter with very compact circuit size [4], that keeping such bandstop filtering property invariant over all
and Park et al. discussed the design considerations of UWB spatial directions is one of the crucial issues in designing such
antennas.
Manuscript received March 6, 2007; revised April 22, 2007. This work was
In this paper, we propose a new microstrip-fed folded strip
supported by the National Science Council, R.O.C., under Grant 95-2221-E- monopole antenna with band-notched characteristics. The an-
011-021. tenna is formed by a microstrip-fed forked-shape radiator along
The authors are with the Department of Electrical Engineering, National with a pair of coupled lines. The coupled-line sections act as
Taiwan University of Science and Technology, Taipei 10607, Taiwan R.O.C.
(e-mail: tgma@ ee.ntust.edu.tw). parallel resonators and prevent the antenna from radiating at the
Digital Object Identifier 10.1109/TAP.2007.904137 targeted rejection band. An equivalent circuit model is extracted
0018-926X/$25.00 © 2007 IEEE
2474 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 9, SEPTEMBER 2007

to explain this band-notched phenomenon, and the calculated

antenna input admittance using this circuit model agrees with
the full-wave simulation data. The antenna geometry and de-
sign guideline are first introduced in Section II. The parameters
in a traditional sense including the return losses and antenna ra-
diation patterns are followed. The equivalent circuit model is
detailed in Section III while the antenna radiation characteris-
tics are further examined in terms of the normalized antenna
transfer functions in Section IV. At the end of this paper, the
transmission responses of a transceiving antenna pair and their
corresponding transient analysis are investigated as well.
Fig. 1. The proposed antenna configuration.
II. ANTENNA CONFIGURATION AND PERFORMANCE
The configuration of the proposed antenna is shown in Fig. 1.
The antenna was fabricated on a 1.6-mm FR4 epoxy substrate , , , , and
with dielectric constant and loss tangent . The proposed antenna has an overall size of 30
. As shown in the figure, a forked-shape radiator is fed by 30 .
by a 50 microstrip line of length . The included angle The simulated and measured return losses are shown in Fig. 2.
between the radiator and the ground plane primarily determines The simulation was performed using Ansoft HFSS 9.2 while the
the in-band impedance matching and is given by measurement was taken by an Agilent E8362B performance net-
work analyzer. The agreement between the simulation and mea-
surement is fairly good except near the lower band edge. This
(1) discrepancy is believed to be caused by the interference of the
connector and feeding cable in the measurement. The simulation
In designing the antenna, the lowest operating frequency can result of the antenna without the folded strips, i.e., ,
be empirically approximated by is also shown in Fig. 2. From the figure it is evident that the
desired filtering property is introduced by the folded strips as
expected. Table I summarizes the simulation results regarding
(2) the center frequencies of the rejection band for several values
of and . It is noted from the table that the notched fre-
(3) quencies move downward to the lower frequency range as
lengthens or widens. The in-band impedance matching, al-
(4) though not shown here for simplicity, remains roughly the same
for both cases. In addition, it is also noted that the lowest an-
where is the estimated longest current path along the outer tenna operating frequency decreases slightly as increases. It
edges of the radiating strips, and and are the speed of light can be readily accounted for by (2)–(4). The bandwidth con-
and the approximated effective dielectric constant, respectively. trollability of the proposed band-rejected UWB antenna is an-
The highest antenna operating frequency, on the other hand, is other issue of interest. As shown in Fig. 2, the return loss of the
principally determined by the distortion of radiation patterns. It antenna with folded strips reveals somewhat degradation in the
is also worthwhile mentioning that according to the simulated frequency range of 4 to 5 GHz. It is likely a result of the slow
current distributions, the current on the metal plate of a square roll-off rate (or poor frequency selectivity) of the band-notched
monopole is in essence concentrated along the outer edges of resonator loaded to the antenna. To the best of author’s knowl-
the radiating plate. As a result, the interior metal has minor ef- edge, the bandwidth controllability of most recently proposed
fects on the antenna performance and can be removed for design band-notched UWB antennas is relatively limited. It is owing
purpose, which are just the cases in the proposed scheme as well to the fact that in such design only a single pair of resonators is
as in [15]. loaded to the antenna structure. Accordingly, the notched band-
To achieve the required band-notched filtering property, the width of a UWB antenna is simply inversely proportional to the
forked-shape strips are folded back to form a pair of coupled quality factor of the resonator, and has limited tunability if one
lines. By simply adjusting the length and gap width of also wants to keep the center frequency of rejection band un-
the coupled-line sections, an additional resonance with desired changed. It is, in essence, similar to the case of a first-order
band-rejected properties can be introduced to the antenna re- microwave bandstop filter. However, a multipole band-notched
sponse. It should be emphasized that here the coupled lines UWB antenna with good frequency selectivity has been pro-
neither support TEM mode nor form coupled microstrip lines posed recently [21]. This antenna demonstrates bandstop filter-
since the lines are operated in inhomogeneous media without like response at the notched band, and is now under further in-
back ground plane. It complicates the analysis and the simpli- vestigation.
fied equivalent circuit model will be discussed in Section III. The antenna radiation patterns were measured in a
The final design parameters are , , 7 3.2 3 anechoic chamber in National Taiwan Uni-
, , , , versity of Science and Technology. The measurement was
MA AND WU: UWB BAND-NOTCHED FOLDED STRIP MONOPOLE ANTENNA 2475

TABLE I
SIMULATED CENTER FREQUENCIES OF THE REJECTION BAND

Fig. 3. Measured radiation patterns at 4 and 8 GHz. (a) xy -plane. (b) xz -plane.

Fig. 2. Simulated and measured return losses.

discussion on the frequency dependence of the antenna radi-
ation characteristics will be addressed in Section IV with the
help of normalized antenna transfer functions.
performed by an Agilent E8362B network analyzer along with
NSI 2000 far-field measurement software. In the measurement
III. EQUIVALENT CIRCUIT MODEL
the connecting cables were carefully shielded by absorbers to
reduce the multiple-reflection interference. Fig. 3(a) and (b) In this section, a lumped equivalent circuit model is discussed
illustrates the measured radiation patterns in the - and - for the proposed antenna. Conceptually, the coupled-line section
planes at the center frequencies, 4 and 8 GHz, of the UWB low in the forked-shape radiator can be represented by the schematic
and high bands. Here the UWB low band is referred to as the diagram in Fig. 4(a). In the figure, and are the even-mode
mandatory band from 3.1 to 5.1 GHz whereas the UWB high characteristic impedance and complex propagation constant, re-
band stands for the optional band from 5.85 to 10.6 GHz. The spectively, of the coupled lines and and are the cor-
frequency band from 5.1 to 5.85 GHz, on the other hand, is not responding odd-mode parameters. For correct modeling of the
used in modern UWB systems due to the potential interference folded strip, the coupled lines in Fig. 4(a) are connected at one
problem. The patterns in the -plane are quite omnidirectional end and open-circuited in one of the two lines at the other end.
as expected whereas in the -plane the radiation patterns The input impedance of the resultant one-port network can be
remain roughly a dumbbell shape over the frequency band of readily expressed by [22]
interest. The cross-polarization levels are generally much lower
than the co-polarization ones. The effect of antenna ground
plane is also studied since its size is comparable to the radiator (5)
and contributes to the radiation. The ground plane size in the
original design, i.e., by , is 11.5 by 30 . It is observed It is evident from (5) that as the length of the coupled lines
through HFSS simulation that as varies from 11.5 mm to approximating one-quarter wavelength long, it behaves like a
26.5 mm, the antenna input matching at the UWB mandatory parallel resonator which gives rise to the band-notched filtering
band experiences somewhat degradations whereas the return property at the targeted rejection band.
loss at the optional band remains better than 10 dB. The ground However, the antenna modeling using the schematic diagram
plane width , on the other hand, is found to have limited in Fig. 4(a) is not as straightforward as expected owing to the
effects on the antenna matching when varying from 30 mm to difficulties in accurately estimating the radiation loss of the cou-
45 mm. The shapes of radiation patterns remain principally pled lines in the forked-shape radiator. To tackle this problem,
unchanged despite the variations in the antenna ground plane. in this paper we attempt to first deal with the one-port network
Nevertheless, it is indeed noted that the dumbbell-shaped in Fig. 4(b) which consists of a coupled-line section and a fi-
pattern in the -plane tilts gradually as lengthens. Further nite-size ground plane. Here the dimensions of the coupled lines
2476 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 9, SEPTEMBER 2007

TABLE II
ELEMENT VALUES OF THE LUMPED EQUIVALENT CIRCUIT MODEL

Fig. 6. Comparisons between the simulated input admittance of the proposed

antenna by HFSS and by the equivalent circuit model in Fig. 4(d). (a) Conduc-
tance. (b) Susceptance.

(6)–(8) are summarized in Table II. Noted that corresponds

to the real part of the input impedance at the center frequency .
Fig. 5 compares the input impedance of the one-port network in
Fig. 4. (a) Schematic diagram of the coupled lines. (b) Circuit layout for ex- Fig. 4(b) to that of the equivalent circuit in Fig. 4(c). Fairly good
tracting the lumped equivalent circuit model in (c). (c) Lumped equivalent cir-
cuit model of the coupled-line section. (d) Equivalent circuit model of the pro- agreement can be observed over the frequency band of interest.
posed antenna. With the lumped equivalent model in Fig. 4(c), we further
note that the proposed antenna can be modeled by an equivalent
circuit in Fig. 4(d). Here the two lossy parallel resonators rep-
resent the pair of folded strips in the fork-shaped radiator. The
flared metal plate between the folded strips and the feeding mi-
crostrip line behaves quite similar to a quarter-wavelength trans-
former, and can be therefore modeled as a pair of -inverters.
The -inverter is cascaded with the parallel resonator and trans-
forms it into a series one at the antenna feeding point. The value
of the -inverter is determined by equaling the admittance levels
of the antenna and the equivalent circuit model, and is given by
Fig. 5. Comparisons of the input impedance of the one-port network in 0.0154 mho (i.e., 65 ohm). The conductance represents the
Fig. 4(b) to that of the lumped equivalent circuit model in Fig. 4(c). (a)
Resistance and (b) reactance.
antenna radiation conductance, and is selected to be 0.02 mho
(i.e., 50 ohm) to account for the wideband nature of the antenna.
The input admittance of the antenna equivalent circuit is hence
and ground plane are the same as those of the proposed an- given by
tenna, and a delta source is served as the excitation. By means
of full-wave simulation, the network parameters of this one-port
network can be easily achieved and transformed into the lumped (9)
equivalent parallel resonant circuit in Fig. 4(c) with
(10)

(6) Fig. 6 compares the antenna input admittance simulated by

HFSS to that calculated using (8). In the full-wave simulation
(7) the antenna response has been de-embedded to the feeding
point in advance. Referring to the figure, the trends of curves
(8) agree reasonably well over the operating frequencies, and both
responses behave like series resonators at the targeted notched
Here, represents the fractional bandwidth with the magni- frequency band. The discrepancy between the curves can be
tude of the input impedance dropping to 0.707 of its peak value, mostly attributed to the inaccurate modeling of the coupled-line
is the quality factor, and is the center angular frequency. sections on the radiator. That is, the one-port network in
The calculated element values of the lumped circuit model using Fig. 4(b) is still too lossy to accurately model the coupled lines
MA AND WU: UWB BAND-NOTCHED FOLDED STRIP MONOPOLE ANTENNA 2477

in the fork-shaped radiator. It in turn results in an equivalent

circuit with lower- and wider rejection bandwidth than the
actual one. Despite the error in the equivalent circuit model,
the results still provide valuable information of the antenna
behavior and sustain the correctness of the proposed equivalent
circuit model.

IV. ANTENNA TRANSFER FUNCTIONS

To further examine the radiation characteristics of the pro- Fig. 7. Measured dimensionless normalized antenna transfer function of the
proposed antenna. (a) Gain responses. (b) Group delays.
posed folded strip monopole antenna, the dimensionless nor-
malized antenna transfer function in [7] is adapted. This dimen-
sionless normalized antenna transfer function is defined by Following the measurement procedure, the gain responses
and group delays of the proposed antenna were measured from
2 to 12 GHz with a 1.5625 MHz step. Fig. 7(a) illustrates the
(11) measured gain responses in the -plane at four specific an-
gles, i.e., , 90 , 180 , and 270 . The masked spec-
where represents the normalized impulse response trum at the WLAN band is shown in the figure for compar-
of an antenna. With this definition, the conventional two-an- ison purpose. The gain responses in Fig. 7(a) are quite stable
tenna gain measurement method can be applied to evaluate the over the frequency band of interest. It implies that the antenna
antenna transfer function without additional effort as long as the will introduce limited distortion to incoming signals. In addi-
standard antenna for calibration purpose is well matched to the tion, the band-notched phenomenon can be readily observed at
measuring system and with constant group delay. The experi- the WLAN bands, and the gain suppression at the center fre-
mental arrangement for evaluating this antenna transfer function quency of the rejection band is at least 10 dB and can be as high
has been carefully discussed in [7]. It should be emphasized that as 30 dB. It is also noted that despite the slight shift in the center
the magnitude in dB of this antenna transfer function is exactly frequency, the band-notched phenomenon remains exist as the
the absolute gain of an antenna, and therefore will be referred to observation angle varies. Similar radiation characteristics can be
as the gain response in the following discussion for simplicity. also observed in the -plane but not shown here for simplicity.
In this measurement arrangement, the transmission scattering The measured group delays are shown in Fig. 7(b) at the same
parameter of a transceiving antenna system was first angles. As shown in the figure, the antenna group delay is about
measured using a vector network analyzer whose reference 450 ps and remains invariant as the spatial angle varies. The
planes have been calibrated to the antenna terminals in advance. dramatic variations of group delays at the notched band can be
The dimensionless normalized antenna transfer function of the mostly attributed to the resonance behavior of the coupled lines
antenna under test (AUT) can be then determined by in the fork-shaped radiator.
The transmission responses as the proposed antenna serving
as both the transmitting and receiving antennas in a transceiving
(12) system is then experimentally evaluated and compared with
the calculation results using the normalized antenna transfer
In (12), is the measured transmission scat- functions in Fig. 7. In the measurement the antenna pair was
tering parameter as the receiving AUT oriented to a specific mounted on bakelite supports and aligned face-to-face or
angle , is that of the standard antenna in its side-by-side. The separation between the antennas was 1 m and
maximum gain direction, and is the dimensionless the measurement was again performed in an anechoic chamber
normalized antenna transfer function of the reference standard with an Agilent E8362B network analyzer. To improve the
antenna. In this paper, an EMCO 3115 double-ridged horn measuring accuracy, the reference planes were calibrated to the
antenna was chosen to be the standard antenna. This antenna antenna terminals and suitable time-gating function was imple-
is well matched to the measuring system, and with a constant mented as well. Fig. 8 compares the calculated and measured
group delay of 630 ps. In our measurement the distance between transmission responses, , of the antenna pair with two
the transmitting and receiving antennas was 3.6 m. different arrangements. Noted that the calculated transmission
The measuring reliability of the antenna group delay can be responses in the figure are achieved by [7]
significantly improved with the help of the time gating technique
in a vector network analyzer. In our measurement the gating
window is set to 4.5 ns, i.e., from 9 ns to 13.5 ns. This corre-
sponds to a propagation distance of 2.7 to 4 m in free space.
Meanwhile, to increase the signal-to-noise ratio, the IF band- (13)
width of the vector network analyzer is reduced from 3 kHz to
700 Hz. With the phase noise in the connecting cables and in- In (13), and are the
struments being successfully suppressed, the measuring accu- measured dimensionless normalized antenna transfer functions
racy can be significantly improved. of the transmitting and receiving antennas, respectively, and
2478 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 9, SEPTEMBER 2007

Fig. 8. Transmission responses with the transmitting and receiving antennas

being aligned (a) face-to-face and (b) side-by-side.

and are the associated orientations of the an-

tennas. Here, and are both equal to
for the face-to-face arrangement, and equal to and
, respectively, in the side-by-side case. It is shown
in the figure that the agreement between the measured and
calculated responses is fairly good over the frequency band
of interest. The discrepancy at the band edges is most likely
a result of the interference from connecting cables, bakelite Fig. 9. Transient responses with the transmitting and receiving antennas being
aligned (a) face-to-face and (b) side-by-side.
supports and the imperfect shielding under the supports.
Finally, the transient responses are studies using the above-
mentioned calculated transmission responses of a transceiving side-by-side arrangement, and can be as high as 0.9661 in the
antenna system. Here the input pulse is chosen to be a modu- face-to-face arrangement. In the test the template pulse selected
lated Gaussian monocycle of second order is the same as the input one. As a summary, the discussion in
this section further demonstrates the suitability of the proposed
design for UWB radios.
(14)

(15) V. CONCLUSIONS
In this paper a new UWB microstrip-fed folded strip
where is the carrier frequency, is a normalized constant, monopole antenna has been proposed and discussed. This
and is the pulse duration parameter. The constants and antenna demonstrates ultra-wide impedance bandwidth along
are equal to 0.7 and 0.95 ns, respectively, for low band with a rejection band at the WLAN frequencies. The antenna
DS-UWB operation, and the carrier frequency is equal to 4.1 performance has been carefully investigated, and an equivalent
GHz. As for the pulse in high band DS-UWB, the parameters circuit model based on the resonance phenomenon of the folded
are equal to 2, and 0.375 ns, and 8.3 GHz, respectively. The strips has been proposed and verified. The antenna radiation
pulse spectrum, although not shown here for simplicity, fully characteristics and their associated transient responses have
comply with the FCC emission regulation. The power density also been examined in detail in terms of the normalized an-
spectrum of the pulse in (14) is obtained using a Fourier trans- tenna transfer functions of an individual antenna as well as
form, and then multiplied by the calculated transmission re- the transmission responses of a transceiving antenna system. It
sponses in (13). An inverse Fourier transform is then has been shown that the proposed antenna possesses consistent
performed to achieve the required transient response. The output radiation properties over the whole frequency band of interest.
waveform at the receiving antenna terminal can therefore be ex- Accordingly, this antenna is expected to find applications in
pressed by various UWB systems.

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