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Multiprobe Planar Near-field Range
Antenna Measurement System
with Improved Performance
Samvel Antonyan1and Hovhannes Gomtsyan1,2
1National Polytechnic University of Armenia, Yerevan, Armenia,
2European University of Armenia, Yerevan, Armenia
https://doi.org/10.26636/jtit.2024.3.1624
Abstract This article presents a novel multiprobe planar
near-field range (PNFR) measurement system. The said sys-
tem simplifies the overall mechanical design, making it simpler
than the existing scanning probe PNFR measurements, and also
significantly reduces testing time. A dielectric-based probe is
introduced to reduce the antenna size, thereby improving reso-
lution. The probe under consideration is an antipodal Vivaldi
antenna offering broadband support and ensuring wideband
characteristics of aerials. Numerical results for representative
X-band antenna models, presented in the Matlab environment,
demonstrate robust performance of the developed measurement
system.
Keywords microstrip patch antenna characteristics, multiprobe
PNFR measurements, planar near-field antenna range
1. Introduction
The high degree of complexity associated with antennas op-
erating at high frequencies, resulting from the proportional
increase in their far-field, makes standard far-field measure-
ments challenging in open space environments. To avoid
these testing difficulties, a near-field antenna measurement
technique has been introduced. This technique has gained
popularity due to its reduced size and lab footprint.
Despite the fact that near-field measurements are typical-
ly performed on a planar, spherical, or cylindrical surface,
planar or plane-rectangular near-field measurements are par-
ticularly popular due to the simplicity of processing and data
acquisition [
1
]. Assuming that the number of near-field data
points is
2n+ 1
, where
n
is a positive integer, the full plane
transformation of the far-field can be computed in a time
proportional to:
n·a2·log2(n·a),
where
a
is the radius of the smallest circle in which the test
antenna is inscribed [
2
]. Earlier, other methods, such as bipo-
lar or plane polar near-field measurements, were introduced.
Nonetheless, these techniques offer limited benefits when con-
sidering the computational cost of the more math-intensive
transformation procedure involved [3].
This article proposes a novel technique that leverages mul-
tiprobe plane rectangular near-field antenna measurements
y
x
z
0
Ey
Ex
Antenna under test
Sampling plane Measurement point
Fig. 1. Planar near-field scanning surface.
to improve antenna measurement test time performance and
capabilities.
2. Problem Statement
In all of the aforementioned cases, acquisition of near field
EM vectors is realized by placing the probe at a particular
position, pointing it at the direction of the antenna under test
(AUT) and allowing the electric field surrounding the probe
to generate current. The same procedure should be repeated
in two polarizations in order to be able to construct the far-
field of the AUT [
2
]. This technique also refers to “probe
scanning”, where the probe needs to be moved physically
within the specified surface of interest.
Acquisition of planar near-field data is usually conducted over
a rectangular
x−y
grid, as shown in Fig.
1
, with a maximum
near-field sample spacing of:
∆xmax = ∆ymax =λ
2.
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Samvel Antonyan and Hovhannes Gomtsyan
To ensure near-plane wave measurement conditions at the
AUT aperture, the phase taper across the AUT should vary
maximum by
∆ϕ=π/8
or
22
.
5
° based on its optical ana-
logue, which was proposed in [
4
]. In practice, this condition
can be expressed as:
∆ϕ(<π
8)≈π D2
4λ R ,(1)
where
D
is the aperture of the AUT,
λ
is the operating
wavelength, and Ris far-field distance.
2.1. Plane Wave (Modal) Expansion
Mathematical formulations of the planar near-field to far-
field (NF/FF) system rely on the utilization of plane wave
(modal) expansions through Fourier transform techniques. In
theory, any form of a monochromatic wave, regardless of its
arbitrary nature, can be expressed as a superposition of plane
waves propagating in diverse directions, each with distinct
amplitudes, but sharing a common frequency.
The primary objective of this plane wave expansion lies in the
determination of the unknown amplitudes and propagation
directions associated with these constituent plane waves [
5
].
The relationships between the near-zone
E
fields and far zone
fields for planar systems can be represented by [4]:
Ex(x, y, z = 0) = 1
4π2
∞
ZZ
−∞
fx(kx, ky)e−j(kxx+kyy)dkxdky,
(2)
Ey(x, y, z = 0) = 1
4π2
∞
ZZ
−∞
fy(kx, ky)e−j(kxx+kyy)dkxdky,
(3)
where
fx(kx, ky)
and
fx(kx, ky)
represent the plane wave
spectrum of the field,
x
and
y
are the components of the
electric field measured over a plane surface, and
k
is the wave
number. The far field pattern of the antenna, in terms of plane
wave spectrum function f, can be found by:
Eθ(r, θ, ϕ) = jke−jkr
2πr (fxcos ϕ+fysin ϕ),(4)
Eϕ(r, θ, ϕ) = jke−jkr
2πr (−fxsin ϕ+fycos ϕ),(5)
The generalized methodology for identifying the far field re-
gion of an AUT involves several steps. Initially, electric field
components are measured in the near field region. Subse-
quently, the plane wave spectrum functions, denoted as
fx
and
fy
, are derived by performing a direct inverse fast Fourier
transform (FFT) on Eqs.
(2)
and
(3)
representing the electric
field components. Finally, the far field region is computed
by using Eqs.
(4)
and
(5)
for the electric field components in
spherical coordinates [2].
Although near-field methods are more complicated physi-
cally and mathematically, the ability to use small distances
means that it is possible to make measurements in a climate-
controlled and electromagnetic-controlled environment of an
antenna measurement facility, which is not possible in the
case of regular far-field measurements. Potentially, this fea-
ture can also result in improvements in security, measurement
accuracy, and test throughput [6].
In this paper, a multiprobe-based rectangular planar near-
field range (PNFR) measurement system, together with an
antipodal Vivaldi antenna (AVA)-based near-field probe, is
presented. Simulation of the measurement system has been
performed with the use of Altair FEKO software, and the
near-field to far-field reconstruction has been competed using
Matlab.
3. Probe Design
In general, the standard approach to performing a PNFR
measurement involves using open-ended waveguides (OEW),
as they are quite homogenous in terms of gain across wide
frequency ranges. Nevertheless, designing OEWs for higher
frequencies can be challenging due to the need to maintain
a small footprint to satisfy the
λ/2
sampling point criteria.
One of the possible solutions could consists in implementing
antennas on dielectrics, which are gaining in popularity these
days. The benefit of this approach is that the size of the antenna
can be reduced by the
εr
, i.e. the dielectric constant of the
substrate [7].
It is also worth noting that one of the other requirements of
the PNFR measurement system is that its bandwidth must
be sufficient to cover wide frequency ranges. To satisfy all
these scenarios, AVA-based near-field probes are designed to
be linearly polarized and to operate over a wide bandwidth
with high gain [
8
]. The size of these antennas is quite small
due to their dielectric-based substrate. Therefore, they can
be easily used in a PNFR lattice. In addition, AVA antennas
offer a good return loss and are characterized by minimum
signal distortion.
A basic Vivaldi antenna consists of a feed line, usually of the
microstrip or stripline type, and the radiating structure. Many
Vivaldi antenna designs with different radiating structures
have been described in the literature. The exponentially shaped
type is the most widely used, as it can provide the broadest
band solution [
9
]. The structure of an AVA antenna is shown
in Fig.
2
. It includes two main parts, namely the feed line and
the radiation flares.
The flares are designed to have the shape of electrical curves,
as these configurations provide wide broadband characteris-
tics. The equation for this tapered slot is [9]:
y(x) = ±(C1ex+C2),(6)
where C1and C2are given by:
C1=y2(x)−y1(x)
eax2−eax1,(7)
C2=y1(x)eax2−y2(x)eax1
eax2−eax1,(8)
18
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Multiprobe Planar Near-field Range Antenna Measurement System with Improved Performance
Top layer Bottom layer
W
Wf
L
y(x)
4
y(x)
3
y(x)
1y(x)
2
L2
L1
x
y
Fig. 2. AVA antenna design.
In these equations,
C1
and
C2
are constants,
a
is the expo-
nential curve rate,
x1
,
y1
,
x2
, and
y2
are the start and end
points of the exponential curve [10] and are defined by:
y1(x)→x1= 0, x2=L, y1=−Wf
2, y2=W
2,
y2(x)→x1= 0, x2=L, y1=Wf
2, y2=−W
2,
y3(x)→x1= 0, x2=L1, y1=Wf
2, y2=W
2,
y4(x)→x1= 0, x2=L1, y1=−Wf
2, y2=W
2.
(9)
The minimum operating frequency range
fmin
, the thickness
of substrate
h
, and its dielectric
εr
constant, as well as length
L
of the antenna structure are calculated using the following
equation [8]:
L=c
2fmin√εef f
,(10)
where:
εeff =εr+ 1
2+εr−1
2·1 + 12 h
W,(11)
The AVA layout is designed at
fmin = 10
GHz on a substrate
with the dielectric constant
εr= 4.6
and height
h= 10−3
m.
From Eq.
(9)
, the aerial’s parameters are:
W= 0.0125
m,
‒2
‒4
‒6
0
0
30
60
90
120
150
180
210
240
270
300
330 4
2
o
Max. (0 , 4.1 dBi)
SLL:1.96 dB
o
104.23
Fig. 3. AVA radiation pattern at 10 GHz in θplane.
L= 0.024
m,
Wf= 2.3·10−3
m for such the line with
50 Ω impedance is achieved.
The coefficients of Eq.
(9)
are obtained during the simula-
tion by optimizing reflection and radiation characteristics.
Consequently, the end points of curves
y3(x)
and
y4(x)
are
determined at
L1= 9.3·10−3
m, and the equations for top
and bottom tapered slots are:
y1(x) = 17 ·10−5e150x−17 ·10−5,
y2(x) = −17 ·10−5e150x−17 ·10−5,
y3(x) = −2·10−3e200x+ 0.3·10−3,
y4(x) = 2 ·10−3e200x+ 0.3·10−3.
(12)
As shown in Fig.
3
, the beamwidth in
θ
plane is:
2θ0.5=104.23° with a gain of 4.104 dBi.
4. Results and Discussions
The PNFR measurement setup for the operating frequency
fop =10
GHz (X-band) was developed with the use of the
designed AVA-based near-field probe. A
1
×
4
phased array
antenna is tested as AUT and it is defined in the FEKO
environment, for far-field analysis. Results of the design’s
analysis are then compared with measurements concerning
the NF-FF conversion.
4.1. AUT NF Results with PNFR System
As shown in Fig.
4
,
17
near field AVA probes have been
used in order to create the PNFR measurement setup. The
distance between each probe is
3
.
75
mm, which is sufficient
to meet sampling point requirement, as the AUT’s operating
wavelength is
λop/2 =14
.
98
mm >
3
.
75
mm [
2
]. The AUT is
placed at the center of the setup to capture
E
field data at the
X= 0 position, where the field strength is assumed to be at
its highest value.
JOURNAL OF TELECOMMUNICATIONS
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Samvel Antonyan and Hovhannes Gomtsyan
Multiprobe
PNFR system
Antenna under test
1×4 array
Fig. 4. PNFR measurement system using AVA probes.
‒30
‒19
‒20
‒21
‒22
‒23
‒24
‒50
‒52
‒54
‒56
‒58
‒60
‒30
‒20
‒20
‒10
‒10
0
0
10
10
20
20
30
30
Probe position [mm]
Probe position [mm]
NF E(mag) [dB]
Y
NF E(mag) [dB]
X
Fig. 5. Measurement results of: a) Ey field and b) Ex field.
The distance between the AUT and the farthest probe is
87
.
47
mm, which satisfies the radiative near-field distance
criteria [
6
]. To obtain both
X
and
Y
polarized electric field
components, the AUT was rotated by
90
°. The collected
E
field data is shown in Fig. 5.
4.2. NF–FF Conversion
Resolution of the far-field pattern can be increased by adding
artificial data sampling points (with zero value) at the outer
extremities of the near-field distribution. This method in-
‒1000
‒1000
‒1000
‒1000
‒800
‒800
‒800
‒800
‒600
‒600
‒600
‒600
‒400
‒400
‒400
‒400
‒200
‒200
‒200
‒200
0
0
0
0
200
200
200
200
400
400
400
400
600
600
600
600
800
800
800
800
1000
1000
1000
1000
–1
k [m ]
X
–1
k [m ]
X
–1
k [m ]
Y
–1
k [m ]
Y
–4
×10
–4
×10
0
0
2
2
4
4
6
6
a)
b)
| f | [V/m]
X
| f | [V/m]
X
| f | [V/m]
Y
| f | [V/m]
Y
0.025
0.025
0.02
0.02
0.015
0.015
0.01
0.01
0.005
0.005
0
0
Fig.
6
.Interpolated plane wave spectrum for a)
x
axis and b)
y
axis.
creases the number of sample points without changing spac-
ing between measurement sample, thus resulting in fixed
wavenumber limits. The additional wavenumber spectrum
points are within the original wavenumber limits. This leads
to increased resolution in the computed far-field patterns [
2
].
Using the Matlab environment, the plane wave spectrum of
both fields is solved by adding artificial data sampling points
and applying spline interpolation, as shown in Fig.
6
a-b, by
using Eqs. (2) and (3).
As mentioned in subsection
2
.
1
, the far-field pattern of the
antenna can be determined in terms of plane wave spectrum
functions
fx
and
fy
by using Eqs.
(4)
and
(5)
. The reconstruct-
ed far-field for a
1
×
4
phased array AUT using such functions
is illustrated in Fig.
7
. As one may notice, beamwidth in
θ
plane is 2θ0.5≈25.55◦.
4.3. AUT Design Calculation Results with Ideal Scanning
Probe Approach
The procedure discussed in subsection
4
.
1
has been repeated
for
51
ideal data sampled results analyzed in the FEKO
environment. Based on the gathered dataset, a
2
D plot of
the analyzed ideal Efield is shown in Fig. 8.
20
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Multiprobe Planar Near-field Range Antenna Measurement System with Improved Performance
‒10
‒15
‒20
‒25
‒100 ‒80 ‒60 ‒40 ‒20 0 20 40 60 80 100
‒5
0
Directivity [dBi]
o
θ [ ]
X= –12.65, Y=–2.94 X= 13.13, Y=–3.16
Fig. 7. Reconstructed electric field in θplane.
0
0
–5–10
–10
–20
–30
–40 –15–20 5 10
10
20
30
40
15 20
Y position [mm]
X position [mm]
4
8
12
16
20
24
28
32
36
40
44
Fig.
8
.
2
D plot of ideal
E
in [V/m] in near-field of antenna under
test.
‒10
‒15
‒20
‒25
‒100 ‒80 ‒60 ‒40 ‒20 0 20 40 60 80 100
‒5
0
Directivity [dBi]
o
θ [ ]
X= –12.65, Y=–2.996 X= 13.13, Y=–3.203
Fig.
9
.Reconstructed far-field based on ideal continuous near-field
Edata in θplane.
As a result of plane modal expansion and after applying Eqs.
(4)
and
(5)
, the reconstructed far-field of AUT with the ideal
data is shown in Fig. 9.
As one may see, the beamwidth in
θ
plane equals
2θ′
0.5≈
25.5◦
and is similar to the obtained results, i.e. those that
have been reconstructed based on data captured by near-field
probes: ∆2 θ0.5≈2θ0.5−2θ′
0.5≈0.05◦.
5. Conclusion
This paper presents the design of a novel multiprobe planar
near-field range (PNFR) measurement system, coupled with
an antipodal Vivaldi antenna (AVA)-based near-field probe.
The multiprobe PNFR system offers a simplified mechanical
structure compared to traditional scanning probe methods,
leading to significant improvements in terms of testing time
and efficiency.
The probe design outlines the process of designing AVA
antennas, considering such factors as dielectric substrate
properties and geometric configurations to optimize perfor-
mance. Through simulation and optimization, the AVA an-
tennas demonstrate decent characteristics, including compact
size, wide bandwidth, and minimum signal distortion, making
them suitable for integration with the PNFR system.
17
dielectric-based AVA probes have been used to character-
ize a
1
×
4
phased array as an antenna under test. Near-field
probe data of the AUT and ideal near-field data from the
FEKO environment were captured for a comparative anal-
ysis of AUT’s directivity. The captured near-field data was
converted to far-field information using an NF-FF recon-
struction algorithm implemented in the Matlab environment.
Far-field representation of the captured data shows that the
difference between the ideal and probe-captured methods is
∆2 θ0.5≈ ±0.05◦
. This small difference proves the validity
of the measurement system.
Results from the multiprobe PNFR measurements prove
the effectiveness of the system in capturing near-field data
with a high degree of precision. Comparison with the ideal
scanning probe approach further validates the accuracy and
reliability of the multiprobe PNFR method in reconstructing
far-field patterns of antennas.
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Samvel Antonyan, M.Sc.
Institute of Information and Telecommunication Technolo-
gies and Electronics
https://orcid.org/0009-0007-4523-2090
E-mail: samwellantonyan@gmail.com
National Polytechnic University of Armenia, Yerevan,
Armenia
https://polytech.am/en/home/
Hovhannes Gomtsyan, Ph.D.
Institute of Information and Telecommunication Technologies
and Electronics
https://orcid.org/0009-0003-2479-7923
E-mail: hovhannes.gomcyan@polytechnic.am
National Polytechnic University of Armenia, Yerevan,
Armenia
https://polytech.am/en/home/
European University of Armenia, Yerevan, Armenia
https://www.eua.am
