IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 1, JANUARY 2001
45
The Dependence of the Input Impedance on Feed
Position of Probe and Microstrip Line-Fed Patch
Antennas
Lorena I. Basilio, Student Member, IEEE, Michael A. Khayat, Student Member, IEEE,
Jeffery T. Williams, Senior Member, IEEE, and Stuart A. Long, Fellow, IEEE
Abstract—The impedance of a rectangular patch antenna fed by
an inset microstrip transmission line was measured for various feed
positions. The dependence found was then compared to theoretical
predictions both for this geometry and for the similar case of an
inset coaxial probe feed.
Index Terms—Impedance, inset feed, microstrip antennas, resonant frequency.
I. INTRODUCTION
R
ECTANGULAR microstrip patch antennas have received
much attention because of their low cost, low profile, and
lightweight properties. Microstrip patch antennas can, for many
applications, be fed using a simple coaxial probe feed. This type
of feed can be advantageous because of the ease of fabrication.
However, in array applications, a microstrip line feed may often
times be more appropriate. It has been typically assumed that the
input resistance of a conventional rectangular microstrip patch
antenna has the same dependence on the feed position for both
the probe feed and the inset microstrip feed [1]–[3]. The input
resistance that is consistent with the cavity model for a probe-fed
rectangular microstrip patch is proportional to a cosine-squared
type distribution as the feed position is varied from the edge in
toward the center of the radiator.
In this paper, an experimental characterization of the input
resistance as a function of feed position for an inset microstrip
feed is undertaken. The measurement techniques are first
demonstrated by characterizing the input resistance behavior as
a function of feed position for the probe-fed patch, confirming
the predicted cosine squared behavior.
Fig. 1. Probe and microstrip line-fed rectangular patch antennas.
II. MEASUREMENT TECHNIQUES
Probe-fed and microstrip line-fed rectangular patches, such
as the ones shown in Fig. 1, were used to perform the measurements. A copper clad Duroid substrate with a thickness of
cm and an approximate dielectric constant of
was used. Several patches were fabricated with widths of
cm and resonant lengths of
cm, which
produced radiators that were resonant near 2.3 GHz. This geometry is illustrated in Fig. 2, where the microstrip line width
cm to provide a 50- characteristic
was chosen to be
impedance on this substrate. Although it has been shown that
Manuscript received August 5, 1999.
The authors are with the Department of Electrical and Computer Engineering,
University of Houston, Houston, TX 77204-4793 USA.
Publisher Item Identifier S 0018-926X(01)02288-8.
Fig. 2.
Geometry of inset microstrip fed patches.
the input resistance is a function of the spacing between the line
and the patch conductor, a common design of one microstrip
line width spacing on both sides of the inset feed was adopted.
cm was chosen for all cases.
Thus, the spacing
The input impedance measurement of the probe-fed patch required that the outer and inner conductor of the probe were prop-
0018–926X/01$10.00 © 2001 IEEE
46
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 1, JANUARY 2001
Fig. 3. Measured impedance of probe-fed patch versus feed position with
curve of cos (x =L) normalized at x = 0:5(L=2).
erly secured to the ground plane and the patch conductor, respectively. In order to obtain the input impedance measurements for
the inset fed patch, a fixed length line to the feed position on the
patch was used. By experimentally characterizing the characteristic impedance of the microstrip line along with the transition
between the coaxial feed and the microstrip line, it was then possible to extract the input impedance of the patch at the junction of
the microstrip line and the patch. It was experimentally verified
that the spacing between the microstrip line and the patch was
sufficiently large as to not alter the characteristic impedance and
propagation characteristics of the feed line.
Throughout this investigation, the resonant frequency of the
antenna was defined as the frequency at which the maximum
input resistance occurred. Results for the input impedance were
then taken at this frequency. For the purposes of minimizing
experimental error, measurements were made on two antennas,
which were fabricated to be as identical as possible. The results
given here for both the probe-fed and microstrip line-fed antennas reflect the average of the two measurements (typical difference between the two values was less than 1%).
III. RESULTS AND DISCUSSION
To begin the investigation, a set of experimental measurements was made for the probe-fed patch. These results are
shown in Fig. 3, where the resistance and reactance are plotted
versus a normalized feed position. This position is chosen such
that the full range of the graph represents feed positions from
to the center of the patch
.
the edge
The functional behavior of the resistance is evident when these
curve on the same
results are compared to the
, where the
graph. This curve is normalized at
measurements are quite stable. The only minor discrepancy
occurs for the data point at the edge, which is expected since
the cavity model tends to break down there, and hence the
impedance at this position is always more difficult to predict.
The results for the reactance are also shown and are seen to be
relatively independent of feed position.
Fig. 4 shows the measured resonant frequency for each position of the probe feed. This frequency was defined as the frequency for which the resistance was a maximum, and its value
remained quite constant over the entire range of feed positions,
varying less than 1% from 2.295 to 2.316 GHz.
Next, the impedance was measured for the inset microstrip
fed patches, and the experimental results are shown in Fig. 5. As
Fig. 4.
Measured resonant frequency of probe-fed patch versus feed position.
Fig. 5. Measured impedance of microstrip line-fed patch versus feed inset
distance.
with the probe-fed case, the maximum input resistance occurs
at the edge of the patch and decreases as the inset distance is increased toward the center of the radiator, while the reactance remains reasonably constant. However, the rate at which the resistance decreases with changing feed position is seen to be much
more rapid, and the resistance is also seen to rise slightly as the
inset approaches the center of the patch. The latter behavior is in
contrast to the probe-fed case, where the resistance became essentially zero near the center. For comparison purposes, a curve
normalized at
is also shown and
of
is seen to accurately model the behavior except near the center
where the measured resistance begins to rise again. Empirically,
this seems to be the best fit for the experimental data, but there
is no real theoretical justification.
Most early derivations of the input resistance as a function
of feed position depended on the cavity model and assumed a
probe feed. As the inset microstrip line feed was developed, it
was always assumed that the behavior could continue to be modeled as a cosine-squared type function. Modern textbooks routinely assume that this is the case [4]. Some very early measurements by Weinschel [5] could have given a hint that the inset microstrip feed produced a more rapid decrease in the resistance,
but no other actual measurements seem to have been made. The
resistance measurements are shown again in Fig. 6 along with
and
curves normalized at
Weinschel’s data and both
. Weinschel’s measurements show a more rapid
decline than the cosine squared function, but less than the cosine to the fourth power. Since his data cover only a small range
of inset distances, no definite behavior can be necessarily confirmed.
The resonant frequency for the microstrip fed patch is shown
in Fig. 7 and was found to vary between 2.253 and 2.282 GHz
BASILIO et al.: DEPENDENCE OF INPUT IMPEDANCE ON FEED POSITION
47
Lorena I. Basilio (S’00) was born in Pasadana,
TX, on June 22, 1970. She received the B.S. degree
(magna cum laude) and the M.S. degree in electrical
engineering from the University of Houston, TX,
in 1996 and 1998, respectively. She is currently
working toward the Ph.D. degree at the same
university.
She has been a Research Assistant at the University of Houston since September 1996 and was an Instructor in the summer of 1999. Her current research
interests are in the areas of theoretical and applied
electromagnetics, with a focus on frequency-tunable printed circuit antennas.
Fig. 6. Measured resonant resistances with comparison to different powers of
cos(x =L).
Fig. 7. Measured resonant frequency of microstrip fed patch versus feed inset
distance.
if the single measurement very near the center of the patch is
neglected. Without this data point, the range of resonant frequencies is about the same as that for the probe-fed case (about
1.2%).
IV. CONCLUSION
An experimental investigation has shown that the dependence of the input resistance on the feed position of a patch
antenna differs when using a probe or a microstrip line feed.
Although the measured input resistance for the probe-fed
structure demonstrated the familiar cosine squared behavior,
the input resistance for the microstrip line-fed patch was found
to decrease at a more rapid rate. If a model is normalized to
the measured input resistance at a point between the edge and
the center of the radiator, it is found that a cosine to the fourth
dependence best represents the experimental data.
REFERENCES
[1] K. R. Carver and J. W. Mink, “Microstrip antenna technology,” IEEE
Trans. Antennas Propagat., vol. AP-29, pp. 2–24, Jan. 1981.
[2] D. M. Pozar, “Input impedance and mutual coupling of rectangular microstrip antennas,” IEEE Trans. Antennas Propagat., vol. AP-30, pp.
1191–1196, Nov. 1982.
[3] D. H. Schaubert, “A review of some microstrip antenna characteristics,”
in Microstrip Antennas, D. M. Pozar and D. H. Schaubert, Eds. New
York: IEEE Press, 1995, ch. 2, pp. 59–67.
[4] C. A. Balanis, Antenna Theory: Analysis and Design. New York:
Wiley, 1997.
[5] H. D. Weinschel, “Measurements of various microstrip parameters,” in
Proc. Workshop Printed Circuit Antenna Tech., Las Cruces, Oct. 1979,
pp. 2/1–2/15.
Michael A. Khayat (S’00) was born in Houston, TX, in 1972. He received the
B.S.E.E. and M.S.E.E. degrees from the University of Houston, TX, in 1996 and
1999, respectively. He is currently working toward the Ph.D. degree in electrical
engineering at the same university.
During the summer of 2000, he was with Compaq Computer Corporation
researching wireless technologies for the Portable PC Division. His current interests include microstrip antenna design and computational electromagnetics.
Jeffery T. Williams (S’85–M’87–SM’97) was born in Kula, Maui, HI, on July
24, 1959. He received the B.S., M.S., and Ph.D. degrees in electrical engineering
from the University of Arizona, Tempe, in 1981, 1984, and 1987, respectively.
He joined the Department of Electrical and Computer Engineering, University of Houston, Houston, TX, in 1987, where he is now an Associate Professor.
Prior to that, he was a Schlumberger–Doll Research Fellow at the University
of Arizona. He spent four summers (1983–1986) at the Schlumberger-Doll Research Center, Ridgefield, CT, as a Research Scientist. From 1981 to 1982, he
was a Design Engineer with Zonge Engineering and Research Organization,
Tucson, AZ, and a Summer Engineer at the Lawrence Livermore National Laboratory, Livermore, CA. He is a past associate editor for Radio Science. His
research interests include the design and numerical analysis of high-frequency
antennas, antenna measurements, the application of high-temperature superconductors in antenna systems, and leaky-wave propagation.
Dr. Williams is a past Associate Editor for IEEE TRANSACTIONS ON
ANTENNAS AND PROPAGATION. He is a member of URSI Commission B.
Stuart A. Long (S’65–M’74–SM’80–F’91) was born in Philadelphia, PA, on
March 6, 1945. He received the B.A. (magna cum laude) and M.E.E. degrees
in electrical engineering from Rice University, Houston, TX, in 1967 and 1968,
respectively, and the Ph.D. degree in applied physics from Harvard University,
Cambridge, MA, in 1974.
From 1968 to 1969, he was an Aerosystems Engineer in the Antenna Design Group of General Dynamics, Ft. Worth, TX. From 1970 to 1974, he was
a Teaching Fellow and Research Assistant in applied mathematics and applied
physics at Harvard University. He was also a Research Assistant at Los Alamos
Scientific Laboratories, Los Alamos, NM, in 1970 and 1971. In 1974, he joined
the Faculty at the University of Houston. He was Chairman of the Department
of Electrical and Computer Engineering (1984 to 1995 and 1998 to 1999) and
Associate Dean of the College of Engineering (1995 to 1998). Presently, he
is a Professor in the Department of Electrical and Computer Engineering. He
teaches a variety of undergraduate and graduate-level classes in applied electromagnetics.
Dr. Long is a membera of Phi Beta Kappa, Tau Beta Pi, Sigma Xi, and Commission B of URSI. He is a member of the Electromagnetics Academy and was
an IEEE Antennas and Propagation Society Distinguished Lecturer from 1992 to
1994. He is a registered Professional Engineer. He was a member of the Administrative Committee (AdCom) of the IEEE Antennas and Propagation Society
(AP-S) for a three-year term in 1981 and again in 1989. He was the Organizer
and General Chairman of the 1983 IEEE AP-S/URSI International Symposium,
Houston, TX, and presently is the National Meetings Coordinator of AP-S. He
was Vice-President in 1995 and President in 1996 of AP-S. He also served on the
IEEE Technical Activities Board (TAB), was TAB Magazines Chair, and was a
member of the Periodicals Review Committee from 1997 to 1999. He presently
is a Member-at-Large of the IEEE Publications Activities Board and is on the
Spectrum Editorial Board. He received the Halliburton Award of Excellence as
the Outstanding Teacher in Engineering at the University of Houston in 1983,
the University Teaching Excellence Award in 1991, the Engineering Alumni Association’s 1992 Distinguished Faculty Award, and the Senior Research Award
from the College of Engineering in 1995. He was chosen as the outstanding
teacher in electrical engineering by the IEEE/HKN students in 1994.