AUTs,and compared reasonably well in the far-out sidelobe region
of the AUTs. We hope you enjoy October's AMTA Corner.
Feedback and Contact Information
Any questions or feedback may be forwarded to me via e­
mail at stephen.schneider@wpafb.af.mil or to Jeff Kemp at
jeff.kemp@gtri.gatech.edu. If you wish to reference other previous
AMTA publications, AMTA members can do so through our
online archive at http://www.amta.org. If you are not a member,
$50.00 and a few mouse clicks will get you registered as a member
today! Both Jeff Kemp and I are open to feedback. Until next time!
Photonic Probes and Advanced (Also
Phaseless) Near-Field Far-Field Techniques
1
1
1
1
2
2
1
A. Capozzo/i , C. Curcio , G. D'Elia , A. Liseno , P. Vinetti , M. Ameya , M. Hirose ,
2
2
S. Kurokawa , and K. Komiyama
lUniversita di Napoli Federico II, Dipartimento di Ingegneria Biomedica, Elettronica e delle Telecomunicazioni (DIBET)
via Claudio 21, 1-80125 Napoli, Italy
Tel: +39-081-7683115; Fax: +39-081-5934448; E-mail: a.capozzoli@unina.it
2Electromagnetic Wave Division, National Institute of Advanced Industrial Science and Technology (AIST)
Tsukuba-shi, 305-8568, Japan
Tel: +81-29-861-5676; Fax.:+81-29-861-3492; E-mail: masa-hirose@aist.go.jp
Abstract
We present innovative near-field test ranges, named compact-near-field (CNF) and very-near-field (VNF). These use photonic
probes, and advanced near-field far-field (NFFF) transformations from amplitude and phase (complex) or phaseless
measurements. The photonic probe allows AUT-probe distances of less than one wavelength. This drastically reduces test­
range and scanner dimensions, improves the signal-to-clutter ratio and the signal-to-noise ratio, and reduces the scanning
area and time. In both the cases of complex and phaseless measurements, the neat-field-to-far-field transformation problem is
properly formulated to further improve the rejection of clutter, noise, and truncation error. The advantages of the compact­
near-field and very-near-field test ranges are discussed and numerically analyzed. Experimental results are presented for both
planar and cylindrical scanning geometries.
Keywords: Antenna measurements; antenna radiation patterns; near-field far-field transformations; photonic sensors; very
near field; compact range; singular value decomposition; singular value optimization; phaseless.
232
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1. Introduction
200 �m. Each pair of antenna arms was separated by a gap region
of ) 2 /.lm in width, so that the total length amounted to 2.4 mm.
hotonic probes [1-3] have paved the way to new scenarios in
near-field (NF) antenna characterization [4-6] in terms of per­
formance, cost, and speed. This has been due to their reduced RCS,
dimensions, and weight, the non-invasiveness of dielectric con­
nections, and the possibility of implementing probe arrays [7],
allowing for parallel (faster) acquisition [8].
was based on channel waveguides realized by titanium ( Ti+) in­
diffusion, working at a wavelength of 1.55 /.lm. A silica
(Si02 buffer layer was introduced onto the substrate to allow easier
P
Indeed, to reduce the interactions among the antenna under
test (AUT), the probe, and its mount, the probe and the AUT must
be sufficiently spaced from each other. However, in doing so the
field intensity decreases and its effective support widens. Accord­
ingly, the signal-to-clutter ratio (SCR: the ratio between the signal
of interest and undesired signals due to environmental reflections)
[9J deteriorates the SNR (the ratio between the signal of interest
and the measurement noise) for a fixed AUT input power. Fur­
thermore, larger anechoic environments and scanners are required.
Finally, the truncation error increases if the scanning area is held
constant.
Following [6], in this paper we deal with innovative near­
field test ranges, named the compact-near-field (CNF) and very­
near-field (VNF) ranges. These were developed in the framework
of the cooperation between the Dipartimento di Ingegneria Bio­
medica, Elettronica e delle Telecomunicazioni (DIBET) of the
Universita di Napoli Federico II, Naples, Italy, and the Electro­
magnetic Waves Division at the National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba, Japan. The
ranges use photonic probes and advanced near-field-to-far-field
(NFFF) transformations [6, 10, ))]. The probe allows distances of
less than one wavelength between the AUT and the probe. This
drastically reduces the test-range and scanner dimensions,
improves signal-to-clutter ratio and SNR, and reduces the scanning
area and time. The near-field-to-far-field transformations exploit
both amplitude and phase (complex) or phaseless measurements
(here for the very first time with photonic probes). In both the
cases, the near-field-to-far-fie1d transformation problem is properly
formulated to further improve the rejection to clutter, noise, and
truncation error.
The advantages of the compact-near-field and very-near-field
ranges are more deeply discussed here as compared to [6]. They
are thoroughly numerically analyzed by pointing out the fruitful­
ness of performing measurements as close as possible to the
radiator in terms of truncation error, of achievable SNR, and of
rms reconstruction error for both complex and phaseless data.
Experimental results are presented for both planar and cylindrical
scanning geometries, also explaining the satisfactory cross-polari­
zation performance of the probe.
The optical circuit, integrated within the probe's substrate,
deposition of the antenna electrodes. The optical circuit imple­
mented a Mach-Zender interferometer by means of channel
waveguides and a single Y-junction. The probe operated in a
reflection mode, exploiting a reflecting coating properly realized
on the terminal face of the substrate. This configuration allowed
using a unique fiber optic for both the uplink and downlink con­
nections to the optical controller. This reduced the perturbations
unavoidably introduced by the use of two connections, especially if
the sensor was integrated into a probe array.
2.2 The Measurement Setups for
Photonic Probe Measurements
For both planar and cylindrical acquisition, the measurement
chain employed can be subdivided into a radio-frequency (RF) part
and an optical part.
The RF part consisted of an Anritsu 37165A vector network
analyzer (VNA) with SM5392 S-parameter test set for planar
measurements. For the cylindrical case, an HP 8719D vector net­
work analyzer, an HP 8348A signal preamplifier, and an HP
84498 power amplifier were used.
The optical part included an NECrrokin OEFS-S I optical
controller, integrating a light source, a circulator, and a
photodetector. An NIT Electronics FAS550DCS optical amplifier
and an Agilent Technologies NIT 427 variable attenuator were
inserted to control the signal power feeding the photodetector.
Finally, the planar scanning was implemented in a semi­
anechoic, planar test facility (the Tokai Techno near-field planar
scanner NAS300), at the Application Technology Labs of Kyoto
Research Park Corp. Kyoto, Japan, kindly made available to the
research team (Figure 2). On the other side, the cylindrical scan­
ning was set up in one of the anechoic-chamber test facilities of
AIST, Tsukuba,Japan (Figure 3).
2. The Photonic Sensor and the
Measurement Setups
2.1 The Probe
The photonic probe was designed at the Electromagnetic
Wave Division of AIST. It consisted of a lithium-niobate
( LiNb03), X-cut crystal wafer, having a thickness of 0.5 mm and
dimensions of 8 mm x 3 mm, with an array antenna printed on it
(Figure I). The array was made up of seven parallel and identically
printed short dipoles, spaced I 00 �m apart from each other. Every
arm of each dipole had a total length of 1.194 mm and a width of
Figure 1. The photonic sensor in front of the tested horn
antenna.
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233
pattern (FFP) when either complex or phaseless data were
involved. The purpose of the algorithms was to provide a field rep­
resentation useful to both cases. In particular, the aim was to
improve the robustness against the truncation error, the environ­
mental clutter, and the noise, with further improvements in the
phaseless case in terms of reliability and accuracy. The algorithm
is described by referring to a generic geometry of the acquisition
surfaces, quoting the Appendix for particularizations to the planar
and cylindrical cases.
Let us assume that the AUT lies in the xz plane of a Cartesian
coordinate system Oxyz, and radiates towards the y > 0 half-space,
as shown in Figure 5. The aperture field is assumed to be vanish­
ingly small outside an "effective " aperture, A, contained within the
rectangular
domain,
smallest
(coordinate)
Da
p
Figure 2. The Tokai Techno planar test range.
=[ -aap,aap Jx[ -bap,bap ] centered at O.
For the sake of simplicity, we will refer the discussion to one
Cartesian component of the aperture field (ga
=
Ealz), so that we
can deal with a scalar problem.
The z-component E of the plane-wave spectrum (PWS) can
be expressed as
where JD
""
denotes the Fourier-transform operator limited to Dap.
When the plane-wave spectrum is an essentially finitely supported
Figure 3. The cylindrical range of AIST.
2.3 The Measurement Setup for Open­
Ended-Waveguide Probe Measurements
For cylindrical measurements exploiting (un-flanged) open­
ended waveguides, the indoor testing environment was the anech­
oic chamber designed and realized by MI-Tech [12] for the
Microwave and Millimetre-Wave Lab of DIBET (Figure 4). The
chamber was set up to work in the 900 MHz to 40 GHz frequency
band. It was equipped with both planar and cylindrical scanning
systems, driven by an external MI-4190 controller. The measure­
ments were performed by means of an Anritsu vector network
analyzer 37397C, operating in the 40 MHz to 65 GHz frequency
band.
3. Advanced Near-Field Far-Field
Techniques
3.1 Near-Field Characterization Algorithm
We will now outline the near-field characterization algo­
rithms for aperture antennas. These aimed at retrieving the far-field
234
Figure 4. The cylindrical range of DIBET.
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z
throughout the paper to a "standard" near-field-to-far-field trans­
formation that obtains the far-field pattern by avoiding the use of
Equation (2).
In the case of phaseless measurements, the anm are searched
for as those coefficients making the squared amplitudes of the cor­
responding theoretical fields on
Sl and S
on
as close as possible to
� ( ):::::;"M
Ei B {.rDup [Ea (�)]}
the measured data. Denoting by
the unknowns, and by
2
anm
=
the matrix of
the (complex) field
=
Si' 'B being the operator connecting the spectrum to the near
field, then the unknown � is obtained by minimizing the objective
functional [II]
(4)
Figure 5. The problem geometry with arbitrary scanning sur­
faces.
where
I .I �( Sj)
is the usual
£.2 norm on Si' i
=
1,2 - say
£.2 (Si) -
i:t? (r) and i:ti (r) are the squared amplitude measurements.
and
function with support n = [-u,u]x[-il, il], then the aperture field
can be expanded as [13, 14]
N-lM-l
Ea(x,z)= L L
n=O m=O
[
]
[
3.2 Advanced Field Sampling
(2)
anm<I>n cx.x <I>m cz'z ,
]
Recently, a new perspective was given to the field-sampling
[ w , w] is the prolate-spheroidal wave function (PSWF)
with
space-bandwidth product
cw'
Cx
uaap' Cz Vhap ,
N [2Cx/1e], and M
[2cz/n], with the symbol [x] denoting the
where <I>i c
=
=
=
and field-reconstruction problems [15]. It is capable of also pro­
viding a measurement strategy for the very-near-field region of the
radiator.
By exploiting the representation in Equation
=
integer part ofx [10, II, 13, 14].
The characterization problem amounts to reconstructing the
E
an from the knowledge of the complex
field acquired over the
m
single surface SI in the case of complex measurements, or of the
squared amplitude,
IEI2,
{(xp,zp )};=I
by
of the E field over two surfaces,
SI and
S2' in the case of phaseless measurements (see Figure 5). Follow­
ing the retrieval of the anm, then the plane-wave spectrum can be
determined from Equation (I), and the far-field pattern can be
{Ep=E(Xp,Zp)}:=1
.Jl. the operator connecting the measured
measurements and according to Equation (2), the unknowns
an
m
locations,
and
letting
be the very-near-field samples (see Fig­
field of the AUT and the anm is described by a proper matrix
r
[10, IS]. Accordingly, for fixed sample number P and locations,
the anm can be recovered from the
Ep
by using the singular-value
decomposition (SVD) approach [lO, IS].
The retrieval of the
field to the plane-wave spectrum, then in the case of complex
sampling
ure 6), the relationship between the sampled field in the very-near­
deduced.
On denoting by
the
(2), on denoting
vided that
a nm
can be reliable and accurate, pro­
r is well-conditioned. On the other hand, the matrix r
is not univocally defined, since it depends on the choice of P and
on the sample distribution. In other words, we have at our disposal
can be determined as
XI
_
. .
.
.
X2
y
XII
XN
• . . . . . . . • . . . . .• . . • . . . . . . . . . •. . . . . . • . . . . . • . • . . . . . . .
(3)
where E is the measured (complex) near field,
:r.-1
is the inverse
o
d
Fourier transform truncated to n, and Ak is the eigenvalue corresponding to <l> k' To emphasize the filtering power of the prolate­
spheroidaJ-wave-function representation in Equation (2), the com­
plex
near-field-to-far-field
transformation
will
be
compared
o
X
Figure 6. Sampling in the reactive region of the AUT.
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235
a family of matrices
r with different behavior of the singular val­
ues. The inversion should therefore be performed by exploiting the
element of the family with "the most convenient" singular-value
aperture plane
behavior.
From this point of view, among all the matrices
r and for a
probe
fixed P, it is convenient to choose the locations of the sampling
points providing the "flattest" singular-value behavior. In other
words, for a fixed P and on letting K = min
tional '¥ [10, 15],
{P, M x N} the func­
,
AUT
(5)
evaluating the "area" subtended by the normalized singular values
compact near field
erd erl , is maximized.
(cylindrical or spherical)
measurement surface
The choice of the "optimal" number of samples P can be per­
formed by observing that '¥ admits an interpretation in terms of a
generalized Shannon number [16]. On denoting by '¥ opt
(p) the
optimum of '¥ for a given P, adding further sampling points will
increase '¥ opt
(p) until the maximum amount of information that
Figure 7b. Compact-near-field cylindrical or spherical meas­
urements.
can be gathered from them is reached. Beyond this, no further
information can be acquired by any newly added field sample. This
corresponds to the appearance of very small singular values, and
thus to a "saturation" behavior of '¥ opt
(p) as a function of P [10,
aperture plane
15, 16]. The number, P, at the saturation knee represents the mini­
mum number of samples needed to achieve the information avail­
able on the anm.
probe
AUT aperture
compact near field
cylindrical or spherical
measurement plane
measurement surface
Figure 8. Compact-near-field cylindrical or spherical meas­
urements.
aperture plane
probe
4. Compact Near Field
We now discuss how photonic sensors and proper near-field
processing algorithms enable the implementation of accurate and
reliable compact-near-field setups, thanks also to the possibility of
AUT aperture
performing the measurements under unconventional geometries.
very near field
Indeed, employing such kinds of probes allows longitudinal driv­
ing and the straightforward generalization of the geometries of the
scanning surfaces, trying to minimize the range dimensions and the
truncation error.
To begin with, some examples of unconventional compact­
near-field measurements are illustrated in Figures 7 and 8.
Under planar scanning, the compact-near-field range can be
implemented by drawing the measurement plane in the very close
proximity of
Figure 7a. Compact-near-field planar measurements.
236
a (planar)
aperture antenna (Figure 7a).
When
required, cylindrical or spherical compact-near-field environments
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can be also used to test the AUT by means of "planar-cylindrical "
or "planar-spherical " approaches,in order to:
a.
b.
.0
,.-,.
f{3
'"-"'
Limit the data truncation, by encircling the radiator
and/or feeding structures as in the case of retlectors or
retlectarrays, thus avoiding the need for using large
scanning areas (Figure 7b).
Reduce the error due to truncation on the far field (Fig­
ures 7b and 8), by profiting from a proper representa­
tion of equivalent planar-aperture fields.
�
0
.....
ro
u
§
cz:::
.l:l
Z 27
§
r/J
It should be remarked that in the presence of feeding struc­
tures (Figure 7b), the measurement radius of the enclosing surface
can be several wavelengths. On the other hand (Figure 8), the
measurement radius can be of the order of the wavelength or less,
depending on the antenna's size.
The advantages of a compact-near-field test range are first
enlightened numerically. To this end, we here address the case of
planar acquisitions when the AUT simulates the hom antenna con­
sidered also for the experimental test case. The hom had an aper­
ture of 193 mm x 144 mm, a phase center located 290 mm behind
the aperture, and worked at 9 GHz (Scientific Atlanta 12-8-2 stan­
dard-gain hom). For the sake of brevity, the numerical investiga­
tions will be led only in the case of complex acquisitions, since
similar considerations can be extended to the phaseless case.
4.1 Robustness as a Function of
Data Truncation
For a measurement surface fixed in size and moving away
from the AUT, the truncation of the data obviously increases. Fig­
ure 9 reports the data truncation undergone by a planar surface,
260 mm x 180 mm,centered on the AUT's aperture and scanned at
different distances, d, from the AUT ranging over (0.2..1,,20..1,).
Figure 10 shows the percentage rms error as a function of d, fol­
lowing the reconstruction of the far-field pattern by the standard
near-field-to-far-field transformation. On the same figure, the per­
formance achieved by using the filtering power of the prolate­
spheroidal-wave-function representation is also depicted. By com­
paring the two graphs, how a proper expansion of the unknown also in the case of a complex near-field-to-far-field transformation
·
- can be of significant help against data truncation clearly
appeared. In other words, the use of the representation provided a
filtering of the truncation error in the reconstructed far-field pat­
tern.
In order to provide a benchmark of the convenience of com­
pact-near-field systems against truncation, we stress that for the
case above, when the surface was set at d 6..1, (a typical distance
for standard probes), the rms error was 21.3%. To reach the 3%
error corresponding to a distance of 0.4..1" the scanning surface,
still at d 6..1" would have to be extended to 1222 mm x 846 mm,
i.e.,to an overall area that is approximately 22 times larger.
=
=
4.2 Robustness as a Function of
Disturbances
In order to analyze the case above to enlighten the effects of
near-field disturbances on the quality of the far-field pattern, the
,
,
10
dJA
12
,.
16
"
'-50
20
a
.·a
.....
:;E
Figure 9. The SNR (solid) and minimum truncation level (cir­
cles).
30
�
15
dJA
Figure 10. The rms error in the estimation of the far-field pat­
tern amplitude under a standard (solid) and prolate-spher­
oidal-wave-function-based
(circles)
near-field-to-far-field
transformation.
analysis was performed by keeping the scanned area fixed. Fig­
ure II reports a cut along the z direction of the numerically simu­
lated amplitude of the field radiated by the hom at different values
of d. As expected, as long as d increased, the support of the field
broadened and the intensity weakened. Hence, for a measurement
surface fixed in size, moving away from the AUT, both the SNR
and the signal-to-c1utter ratio decreased. In real terms, Figure 9
reports the numerical evaluations of the disturbance (SNR and sig­
nal-to-c1utter ratio): for the sake of simplicity,modeled as additive,
Gaussian, and spatially uncorrelated. Such statistics could be con­
sidered not very realistic to model the signal-to-c1utter ratio, but
this gap will be filled in the subsection devoted to the experimental
results, wherein real cases are considered. (A more-detailed analy­
sis of the signal-to-c1utter-ratio effect in the case of phaseless
acquisitions was reported in the very recent contribution [11].)
By referring to the same case in the previous subsection,Fig­
ure 12 shows the percentage rms error following the reconstruction
of the far field by the standard near-field-to-far-field transforma­
tion, and that achieved by using the prolate-spheroidal-wave-func­
tion representation of the aperture field. Again, the utility of a
proper expansion of the unknown was confirmed.
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237
collected over the first of the mentioned surfaces. Good agreement
could be seen between the far-field pattern retrieved from sur­
face #1 and the simulated pattern. Analysis of the algorithm's per­
formance as long as the scanning plane was moved away from the
AUT was reported in [6]. Surfaces #2 and #3 were employed for
the phaseless analysis in the following subsection.
4.4 Experimental Phaseless
Compact Near Field
·��.-----.,�o----��----�----��----,�o-----7,.·
y/'A.
Figure 1 1. The field amplitude radiated by a horn antenna at
different distances.
d. from the aperture: (black) d
=
O.3A;
(blue) d = 4.2A ; (red) d = 8.lA; (green) d = 12A.
In the phaseless case, bounding the test-range dimensions
was even more relevant, since these techniques require larger
measurement volumes, needing two sufficiently spaced scanning
surfaces to provide reliable results. We now show the possibility
and the advantage of implementing compact-near-field phaseless
techniques, again numerically and experimentally.
Resuming the numerical analysis with two planar surfaces,
260 mm x 180 mm in size and located 0.2A and 3A apart from the
aperture, respectively, the rms error equaled 7.1%. On the other
side, when the first surface was at a standard near-field distance
and, in particular, d = 6A, reaching the same rms error was possi­
ble only by considering two planar surfaces 520 mm x 360 mm in
size, and with the second one at d 12A
r--
�
22
'--"
1-0
o
C20
=
(1)
en
.
Figure 13 illustrates a comparison between the cuts along the
E ••
�
Concerning amplitude and phase measurements, a thorough
analysis was already presented in [6].
v axis of the retrieved far-field patterns under complex and phase­
' 6�
,.10L
�=:::;:I=--=::!:::;:'
12
14
Ie
18
�
n
SNR (dB)
�
�
==::-:�o
»
u
Figure 12. The rms error in the estimation of the far-field pat­
tern amplitude under a standard (solid) and prolate-spher­
oidal-wave-function-based
(circles)
near-field-to-far-field
transformation for d = 3A •
less reconstructions, when one of the two surfaces was in the very
near field. The complex near-field-to-far-field transformation was
the same as before when applied to surface # I, while the phaseless
near-field-to-far-field transformation was applied to data from sur­
faces #1 and #2, which were spaced approximately 3.57A apart.
Good agreement could be appreciated between the complex and
phaseless techniques.
5. Very-Near-Field (VNF ) Measurements
We now further enlighten the possibilities of very near field.
By this term, we mean those configurations for which the probe
4.3 Experimental Complex
Compact Near Field
To test the above arguments, we tum attention now to the
experimental analysis. In this, three planar surfaces were scanned
at different distances from the AUT, namely
,.--..., ·10
�.,.
Planar surface # 1, spaced 129 mm ( O.3A) apart from
the AUT and 300 mm x 240 mm in size;
Planar surface #2, spaced 129 mm ( 3.n) apart from
the AUT and 400 mm x 400 mm in size;
Planar surface #3, spaced 295 mm ( 8.9A) apart from
the AUT and 640 mm x 640 mm in size.
Figure 13 shows a comparison among the cuts along the v axis of
the far fields, as evaluated by the prolate-spheroidal-wave-func­
tion-based near-field-to-far-field procedure applied to the data
238
' --�
�2
O'��
�,�-�
�.'�������
��
��--�O�2��O�
�--�
O.�
' --�
v/6
Figure 13. The far-field pattern retrieved from surfaces # 1 and
#2: (black) numerical reference; (red) complex near-field-to­
far field; (blue) phaseless near-field-to-far field.
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employed allows performing the acquisitions so close to the AUT
as to acquire information of better quality, or even more informa­
tion than that usually available in standard near-field ranges.
..
·10
First, in phaseless near-field-to-far-field, direct access to the
quantity of interest during the AUT's characterization - namely,
the amplitude of the aperture field - could significantly help the
reconstructions. Indeed, on resuming the last experimental test of
the foregoing section, Figure 14 illustrates the same comparison as
for Figure 13. However, now the situation was when both the pla­
nar surfaces were in the near-field range (surfaces
and #3) with
a spacing of approximately 52, that is, larger than the reciprocal
distance between surfaces I and
As could be seen, the best
agreement between the complex and phaseless near-field-to-far­
field transformations was obtained when one of the two surfaces
was very close to the antenna's aperture.
�.15
'-"
#2
#
·M
�2�� �� �
0 2��0�
�,�-7
A--�
.' --�
O�
. --�
O.��
�.•�-��.•����
viP
Figure 14. The far-field pattern retrieved from surfaces #2 and
#3. (black) numerical reference; (red) complex near-field-to­
far field; (blue) phaseless near-field-to-far field.
#2.
Second, the very-near-field region of the AUT can also con­
vey information about the evanescent (invisible) part of the plane­
wave spectrum, which is to the contrary unavailable under standard
near-field scanning.
6. Experimental Results on a Slot-Array
Antenna and on a Patch Antenna
In this section, we finally present the results obtained in a
cylindrical compact-near-field range.
As a first test case, an array of 13 slots realized in a rectangu­
lar waveguide was considered. The slots were located in alternat­
ing positions with respect to the waveguide's centerline, spaced
2g
from each other ( 2g being the wavelength in the
j2
waveguide). The work was done at X band (in particular, 9.4 GHz)
(Figure 15). To characterize the mentioned antenna, two cylindri­
cal surfaces were acquired by the photonic probe at AIST, with
point coordinates denoted by ( r cos cp, r sin cp, z), radii r = r] = I2
and r = r2 = 5.62, and -
son, scans with a standard open-ended waveguide over two cylin­
drical surfaces with radii r = 1j = I I2, r = r2 = 02 and
Figure 15. The tested slot-array antenna.
-,,/2
viP
Figure 16. The slot array far-field patterns: (black) complex
near-field-to-far field by open-ended-waveguide probe; (red)
near-field-to-far
field
phaseless near-field-to-far field.
by
photonic
probe;
,,12
2
-:;, cp -:;,
were also performed at DIBET. Figure 16 shows
cuts along the v axis of the far-field patterns as retrieved from
complex near-field-to-far-field transformations by the photonic and
the open-ended-waveguide probes, compared to the results
obtained by a phaseless near-field-to-far-field transformation.
Furthermore, Figure 17 depicts the results obtained by complex
near-field-to-far-field transformations from cross-polarized data by
the photonic and the open-ended-waveguide probes. As could be
seen, the cross-polarized results for the two probes were approxi­
mately at the same level (-40 dB) below the retrieved co-polarized
far-field pattern.
..
complex
2.
"/2 -:;, cp -:;, ,,/2. For the sake of compari­
(blue)
Figure 18 shows cuts along the v axis of the far-field pattern
retrieved from a cylindrical scan when the AUT was an array of
four patches, of which only one patch was radiating and the others
were parasitic. A radius of I2 was used for the measurement sur­
face. In more detail, Figure 18 compares the far-field pattern
retrieved by the considered approach with that determined when a
standard near-field-to-far-field transformation from very-near-field
data was considered, and with that obtained when a standard open­
ended-waveguide (open-ended waveguide G) was used, with a
measurement radius of 6.52. The improvement produced by the
adopted prolate-spheroidal-wave-function representation could be
clearly observed.
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239
Future developments branch out in three directions_ First, we
mention the possibility of setting up very fast systems based on
probe arrays. Second, proper very-near-field sampling techniques
should be developed to also profit from the information arising
from the evanescent part of the plane-wave spectrum. Third, the
employed field representation by equivalent planar apertures is not
a limitation, as different (e.g., ellipsoidal) surfaces matching the
antenna's geometry can be employed.
-10
Finally, the system can be extended to spherical scanning
[17]. It can find applications in many other fields, such as security
(intelligence) [18], protection (safety of people and structures),
electromagnetic compatibility [19], and diagnostics (inverse elec­
tromagnetic scattering) [20].
.7�1�--o-:':.•:---:
-o�
. --:!'
-o ..,---JL-o
•
:":
.2-�L-O=-=
. 2'----:
- 0""
A --:':
O .•:--�--':""..J
v/j3
8. Appendix
Figure 17. The slot array far-field patterns. (black) complex
near-field-to-far field by open-ended-waveguide probe and co­
polarized
data;
(red):
complex
near-field-to-far
field
by
8.1 Planar geometry
photonic probe and cross-polarized data; (blue) complex near­
field-to-far field by open-ended-waveguide probe and cross­
The generic surface, Sj, is a portion of a plane y =Yo . The
polarized data.
operator .Jl. particularizes as
E( u, v) = If E( x,Yo,z)e}(ux+vz+wYo)dxdz
.
(6)
s,
On the other side, the operator 'B is
)
(
E x,yo,z = 'B
=
(27r)
n
8.2 Cylindrical Geometry
..
.10 '------'
v/j3
Figure 18. The patch array far-field patterns: (black) standard
near-field-to-far-field
( E) _1-2 If E(u, v)e}(ux+vz+wYo)dudv. (7)
transformation;
(red)
proposed
approach with photonic probe; (blue) open-ended waveguide G
reference.
The generic surface, Sj, is a portion of a cylinder with radius
rj,
(
that
is,
the
) (
scanning
)
points
ture antennas, Ij should be chosen larger than max
ip is strictly limited to
)
•
We discussed the performance of a measurement system
employing some of the recent innovations introduced in near-field
antenna-testing facilities, allowing non-conventional scanning
geometries and accurate data processing. The system is based on a
noninvasive photonic probe, performing acquisitions in the very­
near-field or compact-near-field regions, without significant probe­
AUT coupling. It adopts reliable and accurate algorithms for
antenna characterization from amplitude and phase or phaseless
data. The improvements over standard near-field acqujsitions have
been numerically and experimentally discussed. The setup allows
significant reductions of the dimensions (and costs) of usual near­
field test ranges. The potentials of very-near-field measurements
have been pointed out, along with the satisfactory cross-polarized
isolation of the probe.
240
coordi­
{Gap' hap} , and
(-"/2,,,/2). The operator .Jl. particular­
izes as
( )
sin ()
00
E( u, v = .Jl. E = 4A-.- L fbn
7. Conclusions and Future Developments
have
nates x,y,z = ljcosip,rjsinip,z . As we are dealing with aper-
sm rp n=-oo
(!3 cos(} ) eJn¢, (8)
.
where u = -/3 sin () cos rp, v =/3 cos () , and
h=
�p2 - y2 , and H�2) ( ) is the Hankel function of nth order
.
and second kind.
On the other side, the operator 'B is
(
)
E Ij,z,rp = 'B
( E) If E(u, v)e-}[WiCOS¢+vz+w'isin¢]dudv.
=
n
(10)
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�
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241