An Antenna Measurement System Based on Optical Feeding

Hindawi Publishing Corporation
International Journal of Antennas and Propagation
Volume 2013, Article ID 528950, 9 pages
http://dx.doi.org/10.1155/2013/528950
Research Article
An Antenna Measurement System Based on Optical Feeding
Ryohei Hosono and Ning Guan
Optics and Electronics Laboratory, Fujikura Ltd., Sakura-shi 285-8550, Japan
Correspondence should be addressed to Ryohei Hosono; ryohei.hosono@jp.fujikura.com
Received 30 January 2013; Revised 29 March 2013; Accepted 31 March 2013
Academic Editor: Sara Burgos
Copyright © 2013 R. Hosono and N. Guan. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
A radiation measurement system by using optical feeding is proposed. The system replaces conventional electrical feeding to antennas by the optical feeding which is composed of an electrical/optical (E/O) converter, a graded-index (GI) optical fiber, and an optical/electrical (O/E) converter. The GI fiber is used so as the O/E converter becomes very compact by using a simple means of coupling between the fiber and the photo-diode in the converter. A vertical surface emitting laser (VCSEL) is used in the E/O converter
to make the system available till 6 GHz. This combination also makes the system cost-effective. The validity as well as the advantage
of the system is demonstrated by measuring an ultra-wideband (UWB) antenna both by the optical and electrical feeding systems
and comparing with a calculated result. Ripples in radiation pattern due to the electrical feeding are successfully suppressed by the
optical feeding. For example, in a radiation measurement on the azimuth plane at 3 GHz, ripple amplitude of 1.0 dB that appeared in
the electrical feeding is reduced to 0.3 dB. In addition, a circularly polarized (CP) antenna is successfully measured by the proposed
system to show that the system is available not only for amplitude but also phase measurements.
1. Introduction
Recently, as wireless mobile devices become smaller and
smaller, antennas used in the devices are required to be very
compact. It becomes even more important to measure precisely such compact antennas for antenna development.
However, an antenna must be fed when the antenna is to be
measured and the feeding is generally made electrically by
microstrip lines, coaxial cables, and so forth. Electrical feeding lines disturb the radiation of the antenna under test
(AUT) due to their metallic bodies. In addition, there causes
unbalanced currents on the feeding lines to excite unwanted
radiation which interferes the original one. The influence
becomes significant when the AUT is small. As a consequence, ripples or unwanted peaks appear in the radiation
patterns of the AUT. Furthermore, the influence arises not
only on the measurement of amplitude but also phase which
is a very important parameter in some antennas such as a circularly polarized (CP) antenna.
Using smaller feeding lines and baluns is conventional
technique to reduce the influence of feeding lines on antenna
measurement. Ferrite chokes and cylindrical conductive caps
set on coaxial cables are known to work as balun [1–6].
However, the balun or chokes are only available at narrow and
low frequency band, for example, sleeve balun is generally
designed near quarter wave length so that the bandwidth is
limited [1–5]. On the other hand, replacing electrical feeding
by optical one is an effective solution to this problem. For
example, optical feeding is introduced to shut off unbalanced
currents on electrical feeding lines and avoid influence on
radiation of antenna under test (AUT), for wireless handheld
terminals [7–9], a log-periodic antenna [10], a biconical
antenna [11], a monopole antenna and a log-periodic antenna
[12], a planar inverted F antenna (PIFA) [13], and a dipole
antenna [14]. Both amplitude and phase of radiation are also
evaluated by optical fiber measurement systems [8–10]. In
these examples, a edge-emitting lasers such as distributed
feed-back (DFB) laser and a single-mode fibers have been
implemented. Although the implementations provide wide
band measurement, they need external modulators, precise
alignment, and relatively large spaces [11, 14]. This makes the
system expensive and limits the system miniaturization.
In this paper, we propose a measurement system for
radiation patterns by using optical feeding. We use direct
2
International Journal of Antennas and Propagation
Transmitting
antenna
Receiving
antenna
Coaxial
cable
O/E
Anechoic chamber
Network analyzer
Optical
fiber
E/O
Coaxial
cable
Figure 1: Measurement setup of optical feeding.
30
RF
output
Optical output
10
DC source
RF input
30 VCSEL
15
PD + amplifier
DC source
Unit: mm
(a)
(b)
Figure 2: Details of E/O and O/E converters.
6.5 mm
0
|21 | (dB)
−10

33 mm
−30
−40

33 mm
−20
22 mm
1
2
3
4
5
Frequency (GHz)
6
7
8
Figure 3: Frequency characteristics of transmission for directly connected E/O and O/E converters.
modulation on a vertical surface emitting laser (VCSEL) and
a graded-index (GI) optical fiber as transmission line so
as we can realize a compact and cost-effective system and
extend the measuring frequency range up to 6 GHz easily.
The VCSEL allows high speed modulation and low driving
current thanks to its surface emission structure [15, 16]. Its



Figure 4: Rolled UWB antenna for measurement.
emission profile matches very well with a multimode fiber so
that cost-effective and compact butt-joint is available. In addition, the wafer-level testing makes the VCSEL chips costeffective themselves.
To show the validity and the advantage of the system, a
small antenna operating at an ultra-wideband (UWB) is measured by this system. We investigate the influence of the feeding line by changing its wiring with calculated and measured
results. The measured results are compared with those measured by a conventional electrical feeding and those obtained
International Journal of Antennas and Propagation
3

AUT

Microcoaxial
cable
200 mm


50 mm
150 mm
O/E converter
or
coaxial cable
O/E converter
or
coaxial cable
Wiring B
Wiring A
Figure 5: Antennas with two different wirings of micro-coaxial cable.
Antenna
Delta gap
source
Micro-coaxial cable
Figure 6: Antenna calculation model.
0
|11 | (dB)
−5
−10
−15
−20
−25
−30
1
2
3
4
5
6
7
Frequency (GHz)
8
9
10
Calculation
Measurement
Figure 7: Input characteristics for rolled UWB antenna.
11
4
International Journal of Antennas and Propagation
Unit: dBi
90 (+)
10
120
Unit: dBi
90 (+)
10
60  (deg)
0
0
−10
150
30
−20
−30
0 (+)
330
210
240
−30
(−) 180
330
240
300
300
270 (−)
Wiring A/optical feeding
(a)
120
(b)
Unit: dBi
90 (+)
10
60  (deg)
120
−10
−10
150
30
−30
0 (+)
210
330
240
30
−20
−20
(−) 180
60  (deg)
0
0
150
0 (+)
210
270 (−)
Wiring A/electrical feeding
Unit: dBi
90 (+)
10
30
−10
150
−20
(−) 180
60  (deg)
120
300
−30
(−) 180
210
330
240
300
270 (−)
Wiring B/electrical feeding


0 (+)
270 (−)
Wiring B/optical feeding


(c)
(d)
Figure 8: Measured radiation patterns in -plane at 5 GHz for optical and electrical feedings.
theoretically. A CP patch antenna for the global positioning
system (GPS) application is also measured for investigating
the accuracy on phase measurement. A comparison between
the optical and electrical measurements and the calculation is
also carried out. It is demonstrated that the proposed system
can be used for precise measurement for not only amplitude
but also phase characteristics of small antennas.
2. Measurement Setup
The proposed measurement setup is schematically shown in
Figure 1. It replaces conventional electrical feeding line by
electrical/optical (E/O) and optical/electrical (O/E) converters with a GI-fiber. The replacement is done only for the
coaxial cable connecting to transmitting antenna. Details of
International Journal of Antennas and Propagation
5
3
2.67
2.33
 (A/m)
2
1.67
1.33
1
0.67
0.33
0
Wiring A
Wiring B
Figure 9: Current distributions on antenna and coaxial cable.
the E/O and O/E converters are shown in Figure 2. The E/O
converter is composed of a VCSEL which is directly modulated by RF signal. A dynamic resistance of the VCSEL has
a characteristic impedance of 35 Ohm at operating point so
that there is slight impedance mismatch. The E/O converter
has a size of 30 × 30 × 5 mm3 which does not include the size
of DC supply such as battery. The O/E converter is composed
of a GaAs PIN photo-diode (PD) and an amplifier where the
peak-to-peak output voltage is 250 mV. The PD is spatially
coupled with the optical fiber by resin which acts as mirror
so as the O/E converter including a tiny RF connector has a
size of 10 × 15 × 5 mm3 which does not include the size of DC
supply. These E/O and O/E converters give much more compact and simpler configuration than previous reports in [10–
14].
A micro-coaxial cable which has the characteristic impedance of 50 Ohm is used to connect the O/E converter to
the AUT. The length of the coaxial cable is optimized to minimize metallic influence of the O/E converter and the radiation
from the unbalanced currents on the coaxial cable. Figure 3
shows frequency characteristics of the transmission for
directly connected O/E and E/O converters where the input
power from a network analyzer is 0 dBm. The curve keeps
flatness up to 6 GHz and shows a gradual decrease for higher
frequencies. Loss emerged in the output power at whole frequencies is due to the conversion loss in the E/O and O/E converters.
3. Experimental Results
3.1. Amplitude Measurement. In order to verify the validity
of the proposed system, an UWB antenna shown in Figure 4
is measured, where the antenna is rolled from a flexible flat
film so as it has omni radiation patterns in its azimuth plane
even at high frequencies [17]. The micro-coaxial cable with a
diameter of 0.8 mm and a length of 200 mm is used to connect
the antenna and the O/E converter. This long cable is used
because we can separate the O/E converter from the antenna
and try to investigate the influence of the cable by changing its
wiring, as shown in Figure 5. Wiring A sets the coaxial cable
parallel to the antenna axis while Wiring B sets a part of the
coaxial cable perpendicular to the antenna axis. The antenna
is also measured by the electrical feeding where micro-coaxial
cable is used. Relative gain measurement [18] is used so that
there is no influence from receiving antenna.
To theoretically analyze the antenna, an antenna model is
constructed by using the commercial available software
WIPL-D as shown in Figure 6, where the feeding cable fed by
a delta-gap source [19] is taken into account.
Figure 7 compares the input characteristics of the antenna
for the measurement and calculation. Reasonable agreement
is obtained between them.
Figure 8 shows the measured radiation patterns in plane at 5 GHz for the antenna with two different wirings
shown in Figure 5, for the optical and electrical feedings. In
Wiring A, omni-directional radiation patterns are observed
in  both for the electrical and optical feedings but fewer
ripples occur in the latter one. The radiation of  remains
low level compared with that of  for both the feedings. In
Wiring B, The level of the radiation of  reaches to 0 dBi and
becomes compared to that of  in the electrical feeding. In
contrast, the level of the radiation of  keeps relatively low
with the maximum value less than −5 dBi, in the optical one.
This phenomenon can be explained as follows: unbalanced
currents existing on the coaxial cable perpendicular to the
antenna axis couple strongly with the radiation of the antenna
in the electrical feeding but they are successfully suppressed
in the optical one.
Figure 9 shows the calculated current distributions on the
antenna and coaxial cable at 5 GHz. It is demonstrated that
currents flow on the antenna itself and through the outer conductor of the cable. It suggests that the unbalanced currents
flow not only along the direction parallel to the antenna axis
but also currents flow along the direction perpendicular to
the antenna axis. Many ripples appear in the case of Wiring B
because the radiation due to the unbalanced currents couples
strongly with that from the antenna itself. Figure 10 shows
the radiation patterns for total field at 3 GHz for both the
optical and electrical feedings where Wiring A is applied.
The calculated radiation patterns are also shown in Figure 10
where the micro-coaxial cable is modeled. The radiation patterns obtained by the optical feeding are better fit to the calculated ones. Null points can be recognized clearly and ripples
appearing in the patterns of the electrical feeding are almost
suppressed for the optical feeding. For example, ripple amplitude, which is defined as a RMS difference between the measured gain and its 10-point-centered moving average, is 1.0 dB
in the electrical feeding and 0.3 dB in the optical one, for
the measurement in -plane. Measured radiation patterns
in -plane of the electrical and optical feedings at 3, 4 and
5 GHz are shown in Figure 11. It is also shown that the optical
feeding can effectively reduce ripples and obtain omni-directional characteristics in a wide band.
3.2. Phase Measurement. Phase characteristics are also
measured for a CP patch antenna shown in Figure 12.
6
International Journal of Antennas and Propagation
Unit: dBi
Unit: dBi
0 (+)
10
0 (+)
10
30  (deg)
30
30
0
0
−10
60
60
60
−10
60
−20
−20
−30
(−) 90
30  (deg)
90 (+)
120
120
120
120
150
150
150
90 (+)
−30
(−) 90
150
180 (−)
180 (−)
-plane
-plane
(a)
(b)
Unit: dBi
120
90 (+)
10
60
 (deg)
0
150
−10
30
−20
(−) 180
0 (+)
−30
210
330
240
300
270 (−)
-plane
Calculation
Electrical feeding
Optical feeding
(c)
Figure 10: Measured and calculated radiation patterns for total field at 3 GHz.
The dimensions of SMA connector are precisely considered
in this investigation. The antenna consists of a patch where
two corners of the patch are truncated and the feeding point
is shifted from the center to generate a right-handed circularly
polarized (RHCP) electric field on a dielectric substrate
backed by ground plane [20, 21]. This antenna is designed for
the frequency range of the GPS applications from 1573.42 to
1577.42 MHz. The CP patch antenna is evaluated by using the
phase-amplitude method [22, 23] in this investigation.
Figure 13 shows the calculated and measured input characteristics for the CP patch antenna. Reasonable agreement
is obtained between the calculated and measured results. A
difference less than 1.6 dB is observed from 1573 to 1577 MHz.
It is shown that the antenna has a sufficient bandwidth at the
GPS band.
Figure 14 shows the frequency characteristics of the axial
ratio defined as the ratio between the amplitudes of the major
and minor polarizations of the antenna at the direction of
International Journal of Antennas and Propagation
7
Unit: dBi
90 (+)
10
Unit: dBi
90 (+)
10
120
120
 (deg)
60
0
0
−10
150
150
30
−10
−20
30
−20
−30
(−) 180
60  (deg)
0 (+)
210
(−) 180
330
240
0 (+)
−30
330
210
300
240
300
270 (−)
270 (−)
Electrical feeding
Optical feeding
3 GHz
4 GHz
5 GHz
3 GHz
4 GHz
5 GHz
(a)
(b)
Figure 11: Measured radiation patterns for total field at -plane at several frequencies for Wirng A.
0
95


46


5
95
−5
46


1550
1600
Frequency (MHz)
1650
1700
Calculation
Measurement

1.6
−20
−30
1500
5
 = 4.1
−15
−25
5
11.5
−10
|11 | (dB)
5

Figure 13: Input characteristics of CP patch antenna.
Unit: mm
Figure 12: Geometry of CP patch antenna.
 = 0 and  = 0 for both the measurements of the electrical
and optical feedings as well as the calculation. Reasonable
agreements are also obtained between calculated and measured results especially in those of optical feeding.
Figures 15 and 16 show the frequency characteristics of
the electric field amplitude ratio of  to  polarizations and
the phase difference between  and  polarizations of electric
fields at the point of  = 0 and  = 0, respectively. Reasonable agreements are also obtained between calculated and
measured results.
4. Conclusion
We have developed a compact and cost-effective radiation
measurement system by using the optical feeding. The system
8
International Journal of Antennas and Propagation
2
is composed of a VCSEL, a PD and a GI-fiber and provides
available measurement up to 6 GHz. It has a compact O/E
converter to feed AUT by a micro-coaxial cable. It is demonstrated that the system successfully suppresses unbalanced
currents on the feeding line and can be used for precise
measurement not only for amplitude but also phase measurements.
1
References
5
Axial ratio (dB)
4
3
0
1540
1550
1560
1570
1580
1590
1600
Frequency (MHz)
Calculation
Electrical feeding
Optical feeding
Figure 14: Frequency characteristics of axial ratio.
Amplitude ratio | |/| | (dB)
10
5
0
−5
−10
1540
1550
1560
1570
1580
Frequency (MHz)
1590
1600
Calculation
Electrical feeding
Optical feeding
Figure 15: Amplitude ratio for CP patch antenna.
Phase of  / (deg)
180
135
90
45
0
1540
1550
1560
1570
1580
Frequency (MHz)
1590
Calculation
Electrical feeding
Optical feeding
Figure 16: Phase difference for CP patch antenna.
1600
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