# Dual Feed, Single Element Antenna for

Authors: Frank M. Caimi; Mark Mongomery
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

A novel u-shaped single element antenna having two feed ports is compared with two equal length monopoles separated by a distance equivalent to the width. A discussion of relative performance metrics is provided for MIMO applications, and measured data is given for comparison. Good impedance match and isolation of greater than−10 dB are observed over the operating bandwidth from 2.3 to 2.39 GHz. The antenna patterns are highly uncorrelated, as illustrated by computation of the antenna pattern correlation coefficient for the two comparison monopoles.


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## Body

## 1. Introduction

With
the continued proliferation of wireless devices comes the increasing need for
data services for e-mail, world wide web access, video on demand, and streaming video. For those in the industry, this
means providing reliable services without interruption, delay, and data corruption. Wireless carriers are forced to maintain an
infrastructure to meet increasing needs for this reliability and higher data
rates, creating the need for communications systems with higher capacity. Meeting these needs has been a challenge, but
developments in technology continue to provide solutions or alternatives.Multiple input,
multiple output (MIMO) communications protocols are one such development along
the path, and have demonstrated significant data communications capacity
improvements by exploiting multiple independent spatio-temporal channels. These
channels are found in signal environments where scattering and rotation of the
electric vector of the signal are common—buildings,
offices, homes, and so forth. Perhaps
surprisingly, these techniques were used for military acoustic communication in
the oceans prior to the coining of the term MIMO, since signals travel by
discernibly different paths and often many paths are needed to ensure
communications reliability. In order to
exploit these channels, the antenna system should meet certain requirements.
This paper reviews metrics used for characterization of antennas used in MIMO
systems and covers advances in antenna technology that allow a single antenna to
be used having multiple feed ports.


## 2. Antenna Metrics for MIMO Systems

In
order to fully exploit independence of signal channels for the communications system, it is commonly believed that the transmitter and receiver must utilize multiple
transducers (antennas) having certain characteristics. Since the signals may arrive from different
directions, the antennas should exhibit characteristics that allow for highest
independence of the received signals. One such characteristic is antenna
pattern independence—measured by the pattern
correlation coefficient [1]
(although that alone  is not sufficient, as it does
not include the effect of the communications channel). The pattern correlation coefficient can be calculated from full sphere
antenna pattern measurements and is given in terms of the electrical field
components by [1] (1)ρp=∫02π∫0πA12dΩ∫02π∫0πA11dΩ∫02π∫0πA22dΩ,whereAmn(θ,φ)=XEmv(θ,φ)Env*(θ,φ)+Emh(θ,φ)Enh*(θ,φ),andthecross‐polarizationpowerratioX=SvSh. The
subscript indices (m,n) refer to either antenna port 1 or 2, depending on their
integer value; Ω(θ,ϕ) is the spatial angle in steradian as
commonly depicted; and E1,v,E1,h,E2,v, and E2,h are the complex envelopes of either the vertical or horizontal field components
resulting from excitation of either antenna port 1 or 2, respectively. The cross polarization ratio is defined as
the ratio of the mean received power in the vertical polarization to the mean
received power in the horizontal polarization, as shown.Correlation
coefficients less than a prescribed threshold are desirable, and envelope
correlations(ρ2) less than 0.5 are preferred for acceptable data capacity
according to many studies.Antennas
should not only exhibit low port-to-port signal correlation, but should also exhibit
isolation from one another so that signals being broadcast or received from one
feed port do not appear at the opposite port or feed. The isolation is typically measured at the
antenna terminals by using the commonly deriveds-parameters. Isolation figures are typically given in
decibels and are designated as S12 or S21 depending on
whether the measurement is from antenna 1 to 2 or vice versa. If the isolation is poor, the source antenna
will deliver substantial power into the adjacent antenna’s termination
impedance (typically 50-ohms resistive), with a corresponding reduction of the
overall radiation efficiency. S21 values
of −10 dB are regularly deemed acceptable and  resulting in power loss of less than
10%.Since
the antenna near-field coupling is related to the far-field pattern, it is
possible to approximately express the correlation coefficient in terms of thes-parameters measured at each antenna terminal.
This is given by [2] (2)|ρs|=|S11*S12+S21*S22|(1−(|S11|2+|S21|2)1/2(1−(|S22|2+|S12|2)1/2.Comparison
of measurements taken by this approach can approximate those computed using the
pattern method, and are much simpler. Most
studies and specifications for correlation coefficient refer to the envelope
correlation which is approximately equal to the magnitude squared ofρ as given in (1) and (2). From this equation,
however, it is clear that small values for S12 and S21 can result in small values for ρs. There are, however, cases where the correlation coefficient might be
small even if S12 and S21 are larger, depending on the
input match reflected in the S11 and S22 values.In
spite of the ease by which the correlation coefficient can be computed using
the previous equations, prediction of the communications system performance also
relies on some measure of the signal independence resulting from the
environment. This factor has been
investigated and can be included in the antenna pattern correlation coefficient
calculation to produce a more complete measure of the received signal
correlation or independence in nonlaboratory conditions. The method involves the use of a simple
environmental model that has been experimentally verified [3]. In this model, the signal power is described
as a function of angle of arrival by probability functions Pv and Ph, which can be included in (1) as follows: (3)Amn(θ,φ)=XEmv(θ,φ)Env*(θ,φ)Pv(θ,φ)+Emh(θ,φ)Enh*(θ,φ)Ph(θ,φ).Measurement
data have been taken to define parameters for Gaussian forms ofPv and Ph in various environments [4]. This data
supports the general methodology for computing ρ based on a field model (versus that of pattern
alone) with good correlation between the two computational approaches. Perhaps most interesting is that the
correlation coefficient depends primarily on the phase difference between the
two antenna patterns—indicating that
good correlation performance requires careful consideration of the antenna
placement and design in the case, where multiple single antennas are used in
close proximity.Briefly
then, the antenna system, its associated pattern, and the environment are all clearly
important in establishing independence between the signals received at each
antenna, and therefore the overall system data transfer rate.


## 3. Multiple versus Single Antenna Design Approaches

A
standard approach to achieve MIMO operation is to develop multiple antennas
that are sufficiently separated to achieve a desired level of signal
independence and port-to-port isolation. In small handheld terminals, this approach
taxes the industrial design process to accommodate double or triple the space
allocation commonly reserved for the antenna, as well as requiring additional
feedline length. Perhaps the most
stressing geometric configurations are for USB form factors requiring HSPA or
WiMAX operational capability, since space allocation is extremely small—nominally 20 mm
wide by 10 mm long.Conversely,
if it were possible to develop a single antenna having multiple feed ports, with
equivalent or better performance characteristics than the separate antenna approach;
it would represent a significant advantage by reducing the cost of assembly and
manufacture while providing equivalent performance in an equal or perhaps
smaller volume. Several studies have been reported in the literature describing
potential antenna designs for MIMO applications in small handheld terminals. Generally,
these make use of external lumped element decoupling networks between the feed
ports to allow matching of even and odd modes to a common impedance, thereby
producing small cross correlation and maximum gain over a limited frequency
range [5]. In one recent publication, a
ground wall and connecting line were used in conjunction with shorting pins to
develop a high isolation two port MIMO antenna
consisting of 2 folded  monopoles
for WiBro service in Korea [6]. Here, we report on a single antenna with 2
feeds and two unique resonance modes that occur at the same frequency. Operation
of the antenna can be described by an electromagnetic model that illustrates
the self-induced and conducted currents in relation to an active reference feedpoint. The opposite, symmetrically located feedpoint
of the antenna is isolated from the reference feedpoint by 10 dB or more versus
that achievable from two separate single antennas occupying the same or larger
physical volume, and polarized equally. This
method will be referred to as an isolated mode antenna technology or “iMAT” in
this paper. A novel feature of the
method is that the isolation improves as the source is swept through the
resonance frequency, as compared with 2 adjacent monopole antennas.The
basic advantages of the single element MIMO antenna using iMAT versus the use
of multiple antennas are illustrated in Figure1. The coupling for the conventional antennas increases
in the band region of interest, while for the isolated mode antenna, it
decreases, as shown. In the conventional
paired antenna arrangement, the current in the antenna increases toward
resonance, which due to coupling to the
adjacent antenna results in decreased efficiency. In contrast to the conventional antenna case,
the iMAT approach relies on the electromagnetic modal distribution to achieve
isolation, and therefore the frequency at which the modes are supported results
in improved isolation as well as high-radiation efficiency as resonance is
approached.Illustration of a conventional antenna versus a two port iMAT antenna for MIMO application.
(a)(b)In
the following example, the antenna pattern for the iMAT antenna is also
substantially different from the independent antenna case, as in Figure2. The
antenna pattern exhibits independence depending on which feed or port is
excited, and therefore the correlation coefficient computed from the pattern is
generally smaller than with independent separately located antennas in close
proximity. An example of the pattern
differences as shown in Figure 2. In
Figure 2, the meanderline-loaded monopoles are separated by an average 10 mm or
approximately 1/12 wavelength in vacuum,
and consequently are highly coupled. The
antenna pattern is tilted away from azimuth due effects from the counterpoise,
which accounts for the smaller gain, as compared to a free space dipole
configuration. In addition, the
coupling into the adjacent load resistance accounts for some additional gain
reduction. The far-field pattern
differences due to separate port excitation are generally expected to provide
an improvement in signal independence by sampling different spatial channels.Example of antenna azimuthal gain pattern
produced by: (a) two closely spaced monopoles and (b) single element iMAT antenna
of similar size showing substantially different far-field pattern compared to
(a).
(a)(b)The
performance of an antenna using iMAT is best illustrated by example against
other designs using the same space allocation. We have purposely chosen a monopole type of design, although it is
possible to use a balanced antenna with similar results.


## 4. iMAT Antenna Design Example

An
example of a USB WiMAX antenna design suited for the 2.3-2.4 GHz band is
illustrated in Figure3. A USB device
typically consists of a printed circuit board (PCB) assembly, enclosed in a
plastic housing, with USB connector at one end.
In this case, the space available for the antennas is assumed to be at
the opposite end of the PCB from the USB connector. The width of the device is 20 mm and the
length and height available for the antennas are 10 mm and 2.5 mm,
respectively. The majority of the PCB,
between the antennas and the USB connector, is taken to be a continuous RF
ground.Figure 3
Example of WiMAX USB antenna illustrating small antenna
volume of 19 mm wide by 10 mm high and extending away from PCB distal end by 10 mm, [7].


## 5. Comparison Study

A similar antenna designed for the 2.3–2.4 GHz
WiMAX/WiBRO  band is illustrated in
Figure4. Two different antenna
solutions are taken for comparison. The
first consists of two meanderline monopoles in the same form factor as
illustrated in Figure 3. The distance from
the ground available for the radiator is 10 mm, or λ/12 in free-space; however,
the monopoles are made an effective λ/4 length by using a meanderline conductor.
The second case considers the iMAT solution that includes a similar meanderline
structure for compactness, but one that maintains a half-wave equivalent
electrical length as a single resonator. The iMAT solution in this case uses a half-wave mode of a u-shaped
element, where the location of each feed port is symmetrically positioned and
selected for the minimum coupling to the opposite feed port. A simulation analysis of the current
developed by excitation at a single port shows that two fundamental competing
modes contribute to a null condition at the opposing port, which is also
supported by an analytic approach using transmission line theory of the
u-shaped structure.Comparison of (a) VSWR, (b)S21, (c) radiation efficiency, and
(d) envelope correlation coefficient for 2 cases: two monopoles and iMAT antenna
over the frequency range from 2.3 to 2.4 GHz.
(a)(b)(c)(d)A
comparison of measured performance for the two solutions is shown in Figure4. Each
approach yields good impedance match with a VSWR of 2 or better over the band (Figure 4(a)). However, the coupling (S21) for
the monopoles is poor—about −4 dB as
shown in Figure 4(b). In as similar
application, the addition of the ground extension between the antennas provides
marginal improvement of S21 to about −6 dB [7]. The iMAT solution has considerably better isolation, with S21 values between −10 and −15 dB in the designated band as shown in Figure 4(b).TheS21 value has a direct impact on efficiency due to signal lost to
the neighboring antenna and its associated load; and so the iMAT solution shows
better efficiency than the other solution as shown in Figure 4(c). Similarly, the iMAT solution produces more
diverse antenna patterns as indicated by the graph of correlation coefficients
calculated using (1) as shown in Figure 4(d).


## 6. Summary

Results
have been presented for a novel antenna that uses a higher-order modal
excitation from 2 separate feedpoints to achieve isolation of 10 dB or more
over a limited bandwidth. The antenna
size is similar to that of two identical monopole or dipole antennas that are
closely coupled at less than 0.15 wavelength spacing. In addition, the antenna correlation
coefficient for this approach is superior to the case of two separate closely
spaced monopoles.



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*Source: 219838-2008-05-11.xml*