Passive Transformer Hurdles
Impedance Matching for Antenna
D
esigning mobile communication antennas installed in
latest mobile communication terminals has been progressively becoming more difficult.
The reason being the wide frequency
bands being used by these mobile communication terminals coupled with the
addition of new communication systems, such as Long Term Evolution
(LTE). Meanwhile, the available space
for antennas in mobile communication
terminals is becoming smaller because
of the increasing volume of rechargeable batteries. Therefore, reducing the
size of the antennas is considered to
be an urgent issue. When the size of
an antenna is reduced, however, the
antenna’s impedance tends to become
lower in comparison with the input/output impedance (50-Ω transmission line)
of a radio frequency (RF) circuit. Under these conditions, manufacturers are
facing a dilemma where it is difficult to
Fig. 2: Equivalent circuit of transformer that contains parasitic element
match the impedance of an RF circuit
with the impedance of an antenna in all
communication bands.
LC Component Issues
To achieve impedance matching, it is
a common practice at present to use LC
components, such as inductors (L) and
capacitors (C). The Antenna Q (quality
factor), however, is known to degrade
after impedance matching is performed,
and the bandwidth declines as LC components generate frequency characteris-
Fig. 1: Reversing the ground and antenna connection ports
tics in the reactance component.
Transformers, which are mainly used
in the low frequency range, are an example of components that are not easily affected by frequency characteristics
during impedance conversion. A transformer converts impedance using a percentage of the inductance (L value) of
two coils (transformer coils) in the state
of magnetic-field coupling. As a result,
the transformer does not have frequency
characteristics in an ideal state. Murata
Manufacturing Co., Ltd. has come up
with an idea of using this method for impedance matching.
Results of Transformer Adoption
There are three problems that arise in
the use of a transformer for mobile communication antennas: (1) Difficulty to
achieve a high coupling coefficient since
the magnetic permeability of the magnetic material becomes approximately
one in the microwave band; (2) Significant effect of transformer loss because
an antenna has low input impedance; (3)
Varying input impedance value of an antenna depending on the frequency band.
Because of these problems, transformers have not been used for impedance
matching of mobile communication antennas until now. Murata has resolved
these problems using the company’s
unique approach.
The coupling coefficient changes according to the distance between two
transformer coils, which configure a
transformer, and the relativity of the
shape of magnetic flux produced by the
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Copyright©2014 Dempa Publications, Inc.
19
Fig. 3: Prototype high-frequency transformer
coils. Therefore, Murata has developed
an architecture that freely controls the
inductance value of each coil in a state
where the shapes of transformer coils
are identical. This is done in order to
control the transformer ratio while
maintaining a high coupling coefficient.
By configuring this architecture in low
temperature co-fired ceramics (LTCC),
it is possible to make the distance between transformer coils by several tens
of micrometers. The company was able
to achieve a transformer coupling coefficient of 0.7 or higher even in the microwave band.
Reversing Connecting Port
Because a high-frequency transformer
is connected to an antenna that has an
impedance of about 10Ω, the insertion
loss (I.L.), which occurs depending on
the characteristics of the transformer materials, seems larger in comparison with
a high-frequency device connected to a
50-Ω transmission line. For that reason,
it has become difficult to use conven-
tional transformers with a high inductance value and a resistance component
(used in the low-frequency range) in the
high-frequency range.
Therefore, Murata has adopted the
configuration of the high-frequency
transformer in order to reduce this resistance component and maintain the
transformer ratio. This transformer has
a structure that connects the ground
and antenna connection ports of a standard transformer structure in reverse.
Using this structure makes it possible
to obtain the transformer ratio (Fig.
1.) When this structure is applied, it is
possible to reduce the inductance value
of the coil used in a transformer and
maintain the insertion loss, which is
caused by the resistance component of
a high-frequency transformer, to a low
level because the mutual induction (M)
that occurs due to coupling is reflected
on the transformer.
Compliance of Transformer Ratio
The communication bands that are
used for mobile communication anten-
Fig. 4: S22 of prototype transformer and conceptual image of its operation
20
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Copyright©2014 Dempa Publications, Inc.
nas are classified based on the 1GHz
boundary into two ranges, namely the
low band (low range) and the high
band (high range). For open antennas,
normally the fundamental wave of the
antenna is allocated to the low band,
whereas the higher harmonic wave
of the antenna is allocated to the high
band. When the antenna is not provided
with an impedance matching function
such as a short pin, the real part of impedance in the low band will often become about 10Ω, whereas the real part
of impedance in the high band will often become about 19Ω.
When a transformer with a constant
transformer ratio is installed in such an
antenna, impedance matching can be
performed for either one of the bands
only. Therefore, it is necessary to design
the antenna transformer in such a way
that the transformer ratio complies with
the impedance frequency characteristic
of the antenna. This compliance method
using an equivalent circuit, which was
disassembled into an ideal transformer
block and a parasitic element block, is
explained in Fig. 2.
The parasitic element of this newly
developed transformer structure can be
expressed by the two parasitic elements
of “inductor in series” and “inductor in
parallel.” Among these parasitic elements, the influence of the parasitic element connected in series can be reduced
by increasing the coupling coefficient.
The parasitic element connected in parallel, however, occurs even if the coupling coefficient (k) is 1. It is impossible
to eliminate the influence of the parasitic
Fig. 5: Impedance matching using high-frequency
transformer
element in parallel in a high-frequency
transformer, which must be designed
with a small inductance value. It is possible, however, to make the transformer
ratio comply with the impedance frequency characteristic of an antenna by
controlling the value of this parasitic
element in parallel. The value of the
parasitic element in parallel can be controlled by changing the inductance value
of transformer coils. In this development
effort, the company has discovered a
combination of the inductance (L) value
and the coupling coefficient that can optimally control the parasitic element in
parallel.
impedance of the antenna connection
side and the conceptual diagram of operation when the RF circuit side of this
prototype product is connected to a 50-Ω
transmission line measuring instrument
(Fig. 4). This prototype transformer converts the low band (892MHz) from 12 to
50Ω and the high band (1940MHz) from
19 to 50Ω.
When a standard LC circuit is con-
Fig. 6 (a): Antenna shape of LC circuit
Original
[dB]
Fig. 6 (b): Shape of antenna mounted
with high-frequency transformer
High-frequency transformer
0
1710-2210MHz
Original
[dB]
High-frequency transformer
56787
0
-1
-2
-2
-3
-4
-4
-5
-6
-6
-7
-8
-8
-9
-10
-10
-12
700
900
1100
1300
1500
1700
1900
Fig. 6 (c): S11 characteristic
2100
2300
824
836.5
849
869
880
881.5
894
897.5
915
925
942.5
960
1710
1747.5
1785
1805
1842.5
1850
1880
1910
1920
1930
1950
1960
1980
1990
2110
2140
2170
Prototype Transformer
Murata has developed and evaluated
a prototype high-frequency transformer
with the abovementioned structure in
order to simplify impedance matching
of an antenna. The dimensions of this
prototype surface-mount device product are 2.0 × 1.25 × 0.6mm (Fig. 3). The
Fig. 6 (d): Total efficiency comparison
Fig. 6: Comparison of antenna shapes and their characteristics
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Copyright©2014 Dempa Publications, Inc.
21
Fig.7: Efficiency test of the transformer relative to the reduction in antenna area
nected to an antenna, impedance matching in wide bands is difficult due to
frequency characteristics. On the other
hand, when this new prototype highfrequency transformer is connected
to the antenna, both the low band and
the high band are converted to optimal
impedance values. In other words, the
impedance traces can be changed into
a shape that makes it easy to match the
impedance on the Smith chart. After
that, the impedance can be easily integrated closer to 50Ω by simply performing finer adjustment of the phase
of impedance traces using an external
adjustment device (Fig. 5).
Evaluating Prototype
To find out the advantages of this
prototype high-frequency transformer,
Murata evaluated it by comparing the
antenna characteristics of actual mobile
communication terminals with and without this transformer.
The company used the commercially available ISW16SH smartphone
by Sharp to compare and evaluate the
characteristics when impedance matching was performed on an LC circuit and
those when using a high-frequency prototype transformer (Fig. 6).
The ISW16SH smartphone has a Universal Serial Bus (USB) connector immediately below the antenna portion.
This structure made the evaluation difficult when the electric field of the antenna and this connector are coupled.
The results of this evaluation test in the
low band indicated that both the LC circuit and the high-frequency transformer
were unable to achieve “S11 < -6dB” (reflection loss of 1.2dB). In the high band,
however, the portion that complied with
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Copyright©2014 Dempa Publications, Inc.
“S11 < -6dB” was only 300MHz when the
LC circuit was used, but it was 500MHz
when the prototype high-frequency transformer was used. The results show an
improvement effect of 66 percent when
the prototype high-frequency transformer
is used (Fig. 6-(c)). The company also
achieved an improvement of 0.6dB (Fig.
6-(d)) at the high-frequency side in the
low band for the “total efficiency,” which
indicates the overall characteristics of an
antenna with the LC circuit and the highfrequency transformer. The implementation of a broader bandwidth and the reduction of insertion loss have made this
improvement possible.
Next, the company verified the effectiveness of using its prototype transformer in terms of reducing the antenna
size. Normally, antenna characteristics
improve when the antenna is farther
away from the ground (GND). However, when a large space (area excluding GND) is provided for the antenna
in a mobile communication device, it
constrains the mounting space of components besides the antenna. Therefore,
size reduction of the antenna amounts
to reducing the antenna area. To achieve
this goal, the company evaluated the
change of antenna characteristics by
allocating a part of the antenna area
to the GND space (Fig. 7-(a)). When
a 13mm-wide portion of a 52mm-wide
area in the horizontal direction of the
entire antenna area was allocated to the
GND space, the antenna characteristics
in the low band degraded and the total
efficiency became equivalent to that of
the LC circuit (Fig. 7-(b)). This result
indicates that the antenna area can be
reduced by approximately 25 percent in
real terms.
As mentioned above, the company
has verified that its newly developed
prototype high-frequency transformer
can improve the antenna characteristics
more than the conventional LC circuit.
In other words, it has become possible
to achieve a stable impedance matching characteristic in the entire frequency
band by making the transformer ratio of
a high-frequency transformer comply
with the real part of impedance in both
the low band and the high band of an
antenna.
Murata has introduced the newly
developed device into the market. The
company expects this device to be used
effectively as a third impedance matching device following inductors and capacitors. This device is a passive component that uses a transformer. Therefore,
it does not conflict with products with
active devices such as switches, which
are moving ahead in other fields, as
in the case with inductors and capacitors. On the contrary, this product has
a rather high affinity with products that
use active devices. Antennas must continue to evolve in the future in order to
cope with the implementation of more
multi-functionality in mobile terminals.
Murata strongly believes that this device will contribute to the evolution of
antennas.
About This Article:
The author, Kenichi Ishizuka, is
from the Product Development Section
II, Technology Integrated Products
Department, New Products & Business
Division, Murata Manufacuturing Co.,
Ltd.