Thin-Film HTS Planar Antennas
Michael J. Lancaster, Member, IEEE, Hanyang Y. Wang, and Jia-Sheng Hong, Member, IEEE
Fig. 1. The “H” antenna and feed network. The dimensions of the antenna used in the examples below are a = 5:5 mm, b = 1:5 mm, c = 2:5
mm, and d = 0:5 mm, and the aperture is 5.5 by 2.5 mm. For the copper antenna t = 1:27 mm, "r = 10:8; and for the YBCO superconducting
antenna the upper substrate has t = 1 mm and "r = 9:8.
Abstract—The “H” microstrip antenna is suitable for use as
an efficient small antenna when it is constructed out of superconducting
thin-film materials. An aperture feed and matching
network is described which provides a convenient enhancement
of the capabilities of the “H” antenna. Methods of prediction
of the center frequency are given. In addition, an analytical
expression is developed for the farfield radiation pattern of the
“H” antenna and the efficiency and Q of superconducting and
copper antennas are described using this expression. It is likely
that superconducting antennas will only have significant application
when they are used in arrays. Three arrays are described
demonstrating multiband self-diplexing multifrequency-enhanced
bandwidth and multifrequency beam forming.
Index Terms— High-temperature conductors, patch antenna,
superconductor.
I. INTRODUCTION
ASUPERCONDUCTING antenna was one of the first
microwave components to be demonstrated as an application
of high-temperature superconducting material [1].
Since then, there has been considerable work on new types
of superconducting antennas, with patch antennas looking like
an interesting possibility for a number of applications [2]–[7].
The advantage of using superconducting materials in the
development of antennas is the increase in efficiency or
Manuscript received December 8, 1997; revised October 8, 1998. This work
was supported by the U.K. EPSRC. The work of M. J. Lancaster was supported
by the Nuffield Foundation.
The authors are with the School of Electronic and Electrical Engineering,
The University of Birmingham, Edgbaston, Birmingham, B15 2TT U.K.
Publisher Item Identifier S 1051-8223(98)09646-8.
gain. There are a number of ways for this improvement in
efficiency to occur [2]: 1) For small antennas, the power
losses in the metallic parts of conventional metallic antennas
can dominate over the power radiated. Hence reducing these
losses by using a low surface resistance material, such as a
superconductor, increases the efficiency. 2) Losses are not
only important in the antenna element itself, but also the
losses in the matching network can contribute to the overall
efficiency. A superconducting matching network reduces the
matching network loss. 3) Superdirectional antenna arrays
become more efficient with the use of superconductors [2].
4) In complex high-frequency ( 20 GHz) arrays, losses in the
feed network may contribute to reduced efficiency, but making
the network out of superconductors effectively removes this
problem. and 5) There are now many applications, other
than antennas, in which superconductors are used. Integrating
antennas with additional functionality within these systems can
offer significant additional benefits.
A potential problem for electrically small and superdirective
antennas is the high radiation quality factor which together
with high efficiency, in the case of superconductive realization,
results in narrow bandwidth operation. This is due to the low
loss superconducting materials and the difficulty of designing
and constructing potentially complex matching networks. The
cooling requirements are of course a problem to practical
implementation, but cooler technology has improved vastly
over the last few years and will continue to do so. The
additional power requirement required by the cooler must of
course be considered carefully for any application. The power1051–8223/98$10.00
 1998 IEEE
LANCASTER et al.: THIN-FILM HTS PLANAR ANTENNAS 169
Fig. 2. Return loss as a function of frequency for copper and HTS antennas
compared with the full-wave simulation.
handling capability of the superconducting material may also
be of concern for high-power transmit applications.
This paper addresses some of the perceived problems with
superconducting antennas. Firstly, the “H” antenna is studied
in detail with new analytical expressions developed for its
performance. This antenna has a simple feed and matching
network. Arrays of this antenna type are demonstrated which
show the flexibility and the multifunctionality of the structure;
methods of increasing the bandwidth are also demonstrated.
Planar arrays with capabilities of filtering, dual polarization,
circular or elliptical polarization, self-duplexing, and multifrequency
operation are discussed.
II. THE “H” ANTENNA
The “H” antenna shown in Fig. 1 was first demonstrated in
1985 [8], and it has been investigated in its superconducting
form for a number of years [9]–[12]. The feed networks for the
early antennas were coaxial cables. However, more recently an
aperture-fed version has been demonstrated [13], [14]. This
is far more suitable for implementation in arrays [15]. The
miniaturization of the antenna comes about from both the
shape of the “H” element and the high dielectric constant of
the substrate on which it is placed.
A. Aperture Feed
The aperture coupled “H” antenna shown in Fig. 1 consists
of two dielectric layers and three metallic layers. On the
bottom of the lower dielectric substrate is the microstrip feed
network, shown as the dotted line in the figure. The ground
plane for this microstrip is on the upper side of the lower
dielectric. In Fig. 1 this is separated from the dielectric for
Fig. 3. Experimental results of the resonant frequency of the “H” antenna
compared with different methods of analysis. The antennas are square, and
with the length and width shown on the ordinate they have a=b = 3:67;
a=c = 2:2; a=d = 11:0 a=e = 1.
clarity. This ground plane has a rectangular aperture cut into it;
this is the coupling aperture. The “H” antenna is patterned onto
the upper surface of the upper dielectric and energy is coupled
to it through the aperture. The aperture resonance frequency is
much higher than the resonance frequency of the “H” patch.
Matching is achieved by the correct positioning of the “H”
over the aperture and adjusting the length of the microstrip
which extends beyond the aperture.
There are a number of distinct advantages of having this
particular type of structure, where the patch is isolated from the
feed network: 1) there is no spurious radiation from the feed;
2) the substrate can be a different material from the feed; 3) the
implementation of the matching circuit is straightforward;
4) because of the additional area available complex feed
networks can be produced; and 5) the symmetrical nature of
the feeding structure will result in lower cross polarization in
comparison with conventional probe or edge fed microstrip
antennas.
The caption for Fig. 1 gives the dimensions of an example
antenna used for experimental and theoretical results in the
sections below. In the case of the copper antenna, it is produced
on RT-Duroid 6010. The superconducting YBa Cu O
(YBCO) antennas are produced by laser ablation on MgO
substrates. The surface resistance used on the theoretical
graphs are [2] 8.7 m for copper at 77 K, 0.1 m for YBCO,
and 26.1 m for copper at 300 K. All these values are at a
frequency of 10 GHz.
To investigate the resonant properties of the H-shaped patch
antennas and examine the differences between the copper patch
and high temperature superconductor (HTS) patch antennas,
we have simulated and measured the antennas using full-wave
analysis (Sonnet Software em) and an HP network analyzer.
The return loss as a function of frequency for both copper
and HTS antennas is shown in Fig. 2. The dimensions of
the H-shaped patches are the same as those given in Fig. 1.
If allowance is made for fabrication and measurement error,
170 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 8, NO. 4, DECEMBER 1998
(a)
(b)
Fig. 4. (a) Current distribution and (b) charge distribution on the “H” antenna with the dimensions given in the caption of Fig. 1.
the agreement between predicated and measured is good. The
matching of the antenna is excellent. In comparison with the
copper patch, the resonant frequency for the HTS patch is
slightly higher. This is attributed to the lower relative dielectric
constant of MgO.
B. Resonance Frequency
There are a number of different ways to estimate the
resonance frequency of the antenna. Three ways are described
below, ranging from a very simple model to the use of full
wave analysis software. The results from each of the models
are plotted in Fig. 3 and compared with experimental results.
It is possible just to consider the “H” antenna as a simple
lumped element circuit, with a capacitance and inductance in
series. The resonance frequency of this LC circuit can then
be determined. Expressions for the capacitance of a patch
and the inductance of a narrow piece of transmission line are
available in the literature [16]. This simple model gives the
results shown in Fig. 3, and these are compared with the other
models and a set of experimental results. Measurements are
of 16 antennas each of a different size.
If a straight piece of transmission line is an open circuit
at both ends, it behaves as a resonator, with a resonance
frequency corresponding to the length of the transmission line
being a half wavelength. The “H” antenna can be approxi-
LANCASTER et al.: THIN-FILM HTS PLANAR ANTENNAS 171
(a)
(b)
Fig. 5. Radiation patterns of an “H” antenna using (3) and (5)–(7) compared
with the radiation pattern calculated using full wave analysis and experimental
results. (a) jEj in E plane ( = 0): (b) jEj in H plane ( = 90).
mately described in a similar manner, but with dimension
(in Fig. 1) being the length of transmission line. However,
the large changes in width of this transmission line affect
the effective overall length of the line, as does the field
overlap at both of the ends and edges. Both these effects
can be taken into account using simple expressions derived
in the literature [16]. In equivalent circuit terms, the “H”
shape is sometimes called a “stepped impedance resonator.”
The results of this model are also shown in Fig. 3. Standard
microwave design packages use a similar technique and can be
used to predict the frequency of operation of the antenna. An
alternative method for calculating the resonance frequency of
the antenna is to consider it as a short transmission line loaded
with two short stubs at each end. This method is described
further in [15].
Finally, full wave analysis software can be used to describe
the antenna, the aperture, and the feed network. Using Sonnet
Software em, an accurate prediction of the frequency is obtained
as shown in Fig. 3. It is interesting to look at the current
and charge distributions on the “H” antenna at resonance and
these are shown in Fig. 4.
C. Radiation Pattern
The radiation pattern of a microstrip patch antenna can be
calculated by using equivalent sources of magnetic current on
the region of dielectric which bounds the antenna structure
[17]. This magnetic current, can be calculated from the
Fig. 6. Power losses in the “H” antenna using (8) and (9) for the dielectric
and conductor losses, respectively. The losses are shown as a percentage of
the total loss. The loss tangent for the dielectric used is 105.
Fig. 7. Efficiency of copper antennas at 300 K and 77 K compared with a
superconducting antenna at 77 K using (11), (10), (6), and (7). The loss tangent
for the substrate is 105.
Experimental results for copper and superconducting
antennas are also shown.
electric field in the dielectric on the antenna boundary and its
image in a ground plane, which is assumed to be of infinite
extent
(1)
172 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 8, NO. 4, DECEMBER 1998
(a)
(b)
Fig. 8. Comparison of antenna efficiency against substrate thickness for (a)
different conducting materials and (b) different loss tangents. Experimental
results and full wave analysis using the method of moments are included
from [9] and compared with (11) using (6) and (7).
Here is the unit normal to the boundary and is the
directed electric field on the boundary. The equivalent
magnetic current is therefore directed around the boundary
of the antenna patch.
(a)
(b)
(c)
Fig. 9. The quality factors associated with the “H” antenna. Conductor,
dielectric, and radiation quality factors are shown for (a) copper antenna at
300 K, (b) copper antenna at 77 K, and (c) YBCO antennas. The loss tangent
for the substrate is 105.
Surface resistance are as given in Fig. 6.
In general, the electric potential at a point is given by
(2)
This integration is around the perimeter of the patch and
inside the dielectric. Electric and magnetic fields in the farfield
LANCASTER et al.: THIN-FILM HTS PLANAR ANTENNAS 173
Fig. 10. Three functions of the HTS antenna array.
zone are then given by
(3)
This method of calculating the farfield radiation pattern
requires the assumption of the current distribution. As a
first estimate, assume that the electric field has only a
linear variation on each side of the “H” of the antenna. This
assumption is later justified by the good agreement of this
method with other results. An expression can thus be written
down as
(4)
and are the width and length of the antenna defined in
Fig. 1, and and are fields which define the rate of
increase of the assumed linear increase of electric field of the
patch. Using this value of electric field an analytical solution
for the electric potential of a “H” antenna can be calculated
(5)
where
(6)
Fig. 11. Four-element antenna array.
and
(7)
with
174 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 8, NO. 4, DECEMBER 1998
Fig. 5 shows the radiation pattern of the “H” antenna using
the above expressions. This radiation pattern is compared with
full-wave analysis and experimental measurements.
D. Efficiency
The efficiency of the antenna can be calculated by considering
losses. The input power to the antenna is dissipated in:
1) the conductor making up the antenna ; 2) the power
dissipated in the dielectric ; and 3) power radiated .
These can be calculated from the electromagnetic fields by
(8)
(9)
(10)
The losses in both copper and superconducting antennas are
shown in Fig. 6 as a function of frequency. All dimensions,
including the substrate thickness, are assumed to scale with
frequency and normalized to the antenna dimensions given
above at a frequency of 4.5 GHz. A loss tangent of the
substrate material of 10 is used in the calculation. Values
of and are chosen in the example
below. These particular values are deduced from the field
distribution shown in Fig. 4(b). As can be observed, the
conductor losses increase with frequency but are much less
for the superconducting antenna.
Surface wave loss is not important if the dielectric thickness
is less than about one-tenth of a free space wavelength [17].
The efficiency of the antenna is simply given by
(11)
The efficiency as a function of frequency is shown in Fig. 7,
and the same parameters are used as those in Fig. 6. The
improvement in efficiency, especially at higher frequencies,
for the YBCO antenna is self-evident. The main losses contributing
to the efficiency of the HTS antenna in Fig. 7 are the
losses associated with the dielectric. Experimental results for
both copper and superconducting antennas are also shown in
the figure for comparison.
A further comparison of the above expression can be
done by looking at the data obtained by Chaloupka in [9].
A method of moments analysis was done on the antenna
structure to compute the efficiency, and experimental results
were also included in the reference and are shown in Fig. 8.
The efficiency increases as a function of the substrate thickness
because the radiation increases. However, as seen in Fig. 8,
there is a considerable difference between the efficiency of the
copper and superconducting antennas. The values of
and were deduced from a full wave analysis of the
antenna structure with a thickness of 1.27 mm. These values
do not change appreciably for different “H” antennas.
Fig. 12. Measured return loss and isolation of the dual-band self-diplexing
four-element “H” antenna array.
Fig. 13. Four-frequency operation of the four element two port “H” antenna
array.
E. Quality Factor
Finally, the quality factors of the antenna can be deduced.
The dielectric, conductor, and radiation quality factors are
given by
(12)
where is the total stored energy at resonance given by
(13)
Fig. 9 shows the quality factors associated with the various
antennas under discussion. The YBCO antenna has a much
higher conductor quality factor then the copper antennas, as
expected.
III. ANTENNA ARRAYS
In order to fully exploit the distinct advantages of small
planar superconducting antennas they need to be included into
antenna arrays. Fig. 10 shows three separate possible functions
of an HTS antenna array. Fig. 10(a) shows diagrammatically
how the array can produce a number of separate radiation
patterns, the radiation pattern produced being dependent upon
the frequency of the input signal. The same type of array
can act as a channelizer as depicted in Fig. 10(b). Here the
LANCASTER et al.: THIN-FILM HTS PLANAR ANTENNAS 175
Fig. 14. Series fed “H” antenna array.
output of the antenna array has a number of ports, and the
port that a signal emerges from depends upon the frequency of
the received signal. The construction of separate channelizers
as post processors has been the topic of many research
investigations, and many different types of technology have
been studied for this application in the past. The third use of the
array is shown in Fig. 10(c); it can be configured as a front-end
filter. Here filtering of an incoming signal can be accomplished
in the antenna array itself. This can be simply used to increase
the bandwidth of the array or perform a more complicated
filtering action. In addition, the polarization of any of the
elements in the array can be controlled by the feed network
and element orientation. With correct design, any of these three
functions can be combined to form a multifunction array. An
additional advantage is that the whole multifrequency antenna
array is of a similar size to a conventionally designed antenna
array operating at only a single frequency.
In the proposed arrays, the antennas are close together,
and if they operate at the same frequency could interact
strongly. However, because they are at different frequencies,
the interaction becomes less of a problem and this will aid
the design considerably. Alternatively, if the antennas are at
the same frequency the orientation of the individual antennas
can be changed to reduce mutual coupling. Below we discuss
three practical examples that relate to each of the concepts
shown in Fig. 10.
A. Dual-Band Self-Diplexing Four-Element
“H” Antenna Array
Fig. 11 shows the arrangement of a four-element antenna
array; the size of this array is about the same as a single
square microstrip patch antenna. By using an appropriate feed
network, this array can be configured for a number of functions
[15]. By using two of the antennas connected in series at
one frequency and two at another frequency, the array can
be used to produce an array that acts as a diplexer. Additional
isolation between the ports is obtained because the polarization
is mutually orthogonal as the “H” antennas are normal to each
other.
Fig. 12 shows the return loss for each of the two inputs and
the isolation between the two channels. The isolation is good
and is dominated by cross coupling in the feed network. It can
be improved by using thinner substrates. The 10-dB bandwidth
is 0.6%, small as expected for this antenna. In addition, the
same feed network can provide a four-frequency operation; the
return loss of an example network is shown in Fig. 13. In this
case, there are two ports, but each of the four “H” antennas
operates at a different frequency.
This four-element array can also be configured as two
circular polarized antennas by changing the feed network if
two of the orthogonal antennas are fed by a signal 90 out of
phase circular polarization results.
B. Four-Element Series Array
A series array is shown in Fig. 14. In this case, there is a
single feed point, and with the appropriate “H” antenna configuration
a number of interesting functions can be produced.
Fig. 15 shows the return loss for three examples of how this
type of antenna array may be used. Fig. 15(a) shows how the
array may be used for four separate frequencies, the bandwidth
being dependent upon the antenna element itself. Fig. 15(b)
shows how the bandwidth of the antenna may be increased,
and this time the array has two frequencies of operation; each
operating frequency has an increased bandwidth due to the
use of two “H” antennas. If a single frequency of operation is
required with extended bandwidth, this is obtained by using
a larger number of antennas of close operating frequency.
Fig. 15(c) shows the return loss of a wider band antenna array.
C. Six-Element Array for Dual-Beam
Dual-Frequency Operation
Fig. 16 shows the feed network of dual-beam dualfrequency
antenna array, with the position of the “H” antenna
shown as dotted lines. The array produced two beams at 30
from the normal. One beam operates at 8 GHz and the other 12
GHz. The feed network was designed using Hewlett Packard
MDS. The 50- input transmission line is first split and radial
stubs are used to block the appropriate frequency. The feeds
for both 8 and 12 GHz are then split using the appropriate
impedance transmission lines, resulting in 75- transmission
lines to feed the “H” antenna.
176 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 8, NO. 4, DECEMBER 1998
(a)
(b)
(c)
Fig. 15. Return loss of a four-element series-fed antenna array. (a) Multifrequency
operation, (b) two-frequency operation with extended bandwidth,
and (c) Wider band operation.
The radiation patterns of the arrays are shown in Fig. 17.
The beam directions are about 30 to the normal in both cases
and have low directivity. The two main beams have about a
10-dB larger level than the background; this is expected from
Fig. 16. The feed network for a dual-beam dual-frequency antenna array.
The “H” elements are shown as dotted lines.
(a)
(b)
Fig. 17. Radiation patterns from the dual-beam, dual-frequency, six-element
antenna array. (a) 8 GHz. (b) 12 GHz.
an array with such a small number of elements. The efficiency
of both arrays has been measured at about 10% when they are
made in copper.
LANCASTER et al.: THIN-FILM HTS PLANAR ANTENNAS 177
IV. CONCLUSION
In almost all practical applications of superconducting antenna,
arrays will be used. The “H” antenna is a suitable
candidate for inclusion in multifunction arrays. The “H”
element is about the size of a standard square patch antenna
and its efficiency can be considerably improved when used in
its superconducting form. Arrays with multifunction capability
are considerably easier to design if several antennas can be
enclosed in an area of a subwavelength square. Such arrays
can have functions of channelization, front end filtering, and
multibeam and diverse polarization capability.
ACKNOWLEDGMENT
The authors would like to thank K. Y. Liew, N. A. Abdual
Karim, and S. Ansvananda for help with the experimental
investigations on antenna arrays. They would also like to
thank Dr. F. Wellhofer and Dr. P. Woodall for producing the
superconducting thin films by laser ablation.
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