beat frequency can be tuned quite linearly over the very wide range
by using the all-optical method for obtaining the MWP carrier.
The tuning range of the VCO is approximately 16 GHz and the
VCO gain is approximately 25 MHz/mV.
5. SUMMARY AND CONCLUSION
We have shown that the all-optical heterodyne MWP scheme can
be used to obtain the MWP VCO. It has been shown that the
frequency of the heterodyne beat signal can be varied linearly with
applied voltage on the reference port, in which the frequency-
tuning range and VCO gain are determined by the optical param-
eters of the FPE, such as free-spectral range, finess, and so forth.
REFERENCES
1. N.M. Kwak, H.J. Huh, H.S. Jeong, and K. Cho, Frequency tunable
microwave-photonic carrier generation using heterodyne beat between
all-optically frequency locked two lasers, Conf Proc IEEE LEOS 1
(2001), 312–313.
2. U. Gliese, T.N. Nielsen, M. Bruun, E.L. Christensen, K.E. Stubkjaer, S.
Lindgren, and B. Broberg, A wideband heterodyne optical phase-locked
loop for generation of 3-18 GHz microwave carriers, IEEE Photon
Technol Lett 4 (1992), 936 –938.
3. L.G. Kazovsky, Balanced phase-locked loops for optical homodyne
receivers: performance analysis, design considerations, and laser line-
width requirements, and laser linewidth requirements, IEEE Lightwave
Technol 4 (1986), 182–195.
4. Y. Li, A.J.C. Vieira, S.M. Goldwasser, and P.R. Herczfeld, Rapidly
tunable millimeter-wave optical transmitter for lidar/radar, IEEE Trans
Microwave Theory Tech 49 (2001), 2048 –2054.
© 2004 Wiley Periodicals, Inc.
Figure 1 Geometry of the omnidirectional microstrip antenna
AN OMNIDIRECTIONAL PLANAR
radiating elements along the array. The widths of the array are
MICROSTRIP ANTENNA WITH LOW
varied in a linear manner to produce a ⫺6-dB taper on a pedestal.
SIDELOBES
Randy Bancroft and Blaine Bateman 2. TAPERED OMNIDIRECTIONAL MICROSTRIP ANTENNA
Centurion Wireless Technologies The geometry of a tapered omnidirectional microstrip antenna
6252 West 91st Avenue
(OMA), which produces improved sidelobe-level performance, is
Westminster, CO 80031
presented in Figure 1 [2].
The antenna is created using a set of upper and lower traces.
Received 15 December 2003
The upper trace starts with a narrow trace of width W 1 and length
L, a 50⍀ microstrip transmission line of length L connects this
ABSTRACT: An omnidirectional printed planar microstrip antenna
element to one of width W 3 . A 50⍀ microstrip transmission line of
with low sidelobes is described. The radiation pattern of this antenna is
length L connects the element to another element of width W 3 ,
analyzed numerically using the finite-difference time-domain (FDTD)
method and compared with the measurements of a prototype. The driv- which in turn is connected to an element of width W 1 .
ing point is unbalanced, which allows the antenna to be fed directly The lower trace begins with a short circuit in line with the
with a coaxial cable. © 2004 Wiley Periodicals, Inc. Microwave Opt center of the element W 1 of the upper trace. A 50⍀ microstrip
Technol Lett 42: 68 – 69, 2004; Published online in Wiley InterScience transmission line of length L leads from the short to an element of
(www.interscience.wiley.com). DOI 10.1002/mop.20210 width W 2 . This element is connected in turn to the center element
of width W 4 connected to another element of W 2 , which has a 50⍀
Key words: microstrip antenna; omnidirectional; printed antenna; low microstrip transmission line that terminates at a short in the center
sidelobes of the upper trace final element of width W 1 .
1. INTRODUCTION 3. SEVEN-SECTION LOW-SIDELOBE OMA DESIGN
An omnidirectional microstrip antenna, recently introduced by A seven-section OMA was designed on 0.762 mm (0.030⬙) Shel-
Bancroft and Bateman, is useful for 802.11b (2.40 –2.50 GHz) or dahl Comclad laminate material. The relative dielectric constant of
802.11a (5.15–5.35 GHz) wireless applications [1]. The antenna the substrate is r ⫽ 2.6 with a 0.0025 tan ␦. The dimensions of
has an approximately uniform aperture illumination, which pro- the antenna are: W 1 ⫽ 3.0 mm, W 2 ⫽ 7.32 mm, W 3 ⫽ 11.66
duces sidelobes as high as ⫺11 dB with respect to the main lobe. mm, W 4 ⫽ 16.0 mm, and L ⫽ 36.58 mm. The width of the 50⍀
In applications where the sidelobe level of an omnidirectional transmission line is 2.06 mm. FDTD analysis was used to optimize
pattern is critical, it will be shown that the current distribution the antenna. The center width W 4 was varied and the widths of the
along the OMA may be controlled by varying the width of the other elements were determined by using a ⫺6-dB taper on a
68 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 1, July 5 2004
pedestal. Shorting pins located on either end of the antenna have a
0.5-mm radius.
The antenna is fed at the junction of the first narrow line and the
next wide section meet (Fig. 1). The dielectric material extends out
2.0 mm from each side and 2.0 mm from each end.
The driving-point impedance of the prototype antenna was
found to have a real part of 50⍀ at 2.628 GHz with an inductive
reactance which produces an unacceptable mismatch. A 1.0-pF
surface-mount capacitor was used as a via, which provides series
capacitance at the driving point to match the antenna with better
than 25-dB return loss.
When compared with a uniform design, (that is, W 1 ⫽ W 2 ⫽
W 3 ⫽ W 4 ), which varies the width of the identical elements to
produce a 50⍀ driving point, the impedance bandwidth is much
narrower. The normalized impedance bandwidth of a comparable
uniform array is 14% to 15%, the impedance-matched nonuniform
array has a normalized impedance bandwidth of only 3.8%.
The finite-difference time-domain method (FDTD) was used to
compute the expected radiation patterns for optimization [3]. A
sinusoidal 2.628-GHz source was utilized to compute the radiation
patterns of the antenna. The optimized patterns are presented in
Figures 2(a), 2(b), and 2(c) with their corresponding measured
radiation patterns. The measured patterns correlate well with the
FDTD analysis. The maximum gain was predicted to be 5.39 dBi
versus 5.0 dBi (measured). The FDTD analysis predicts a sidelobe
level of ⫺22.5 dB. The measured antenna sidelobes are approxi-
mately ⫺20.0-dB below the main lobe.
4. CONCLUSION
A low-sidelobe omnidirectional microstrip antenna, which utilizes
tapered element widths in order to provide a desired aperture
distribution, has been described. The antenna has a very omnidi-
rectional pattern with a sidelobe level of approximately ⫺20 dB
and may be directly fed with a coaxial cable.
REFERENCES
1. R. Bancroft and B. Bateman, An omnidirectional microstrip antenna,
IEEE Trans Antennas Propagat (accepted).
2. US Patent Pending, US Patent Application Serial no. 60/461,689.
3. K.S. Yee, Numerical solution of initial boundary value problems in-
volving Maxwell’s equations in isotropic media, IEEE Trans Antennas
Propagat 14 (1966), 302–307.
© 2004 Wiley Periodicals, Inc.
Figure 2 (a) x–y plane, (b) y–z plane, and (c) x–z plane radiation
patterns of a low-sidelobe OMA computed by FDTD analysis (dashed) and
measurements (solid) using a sinusoidal 2.628-GHz source
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 1, July 5 2004 69
