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Radiation Characteristics of Near-Field Beam
Focusing for an Active Array Antenna
Hye Sun Ju*, Shin-Young Cho, Joonho So and Seog Bong Kim
The 3rd R&D institute – Aircraft Radar System PMO
Agency for Defense Development
Daejeon, Republic of Korea
Abstract— In order to test an active phased array antenna in
laboratory, we will develop a test environment within few meters
using near-field beam focusing(NFBF) technic. This paper
analyzes NFBF algorithm using radiation delay in each source
points on the planar active Array Antenna. Also, we analyze
radiation characteristics such as beam steering and monopulse
characteristics in near-field.
Keywords— Active Array Antenna, Beam Focusing Algorithm,
Far-Field, Near-Field;
I. INTRODUCTION
Recently, AESA(active Electronically Scanned Array)
antenna using T/R(Transfer/Receiver) module technology
becomes a remarkable solution in airborne and ground-based
radar applications due to the manifold advantages that it offers
when compared to mechanically scanned system. In the
development of airborne AESA radar system, the performance
of the radar system must be tested and verified at the subsystem
development level, system prototype level, and final system
level in the ground-test facility to reduce the cost and time of
flight before it is tested FTB(Fighting Test Bed) and deployed
in aircraft.
The ground-test facility is divided into far-field ground-test
facility and near-field ground-test facility [1]. Most radar
developers utilize a far-field ground-test facility as roof-lab(or
roof house) where the radar system is mounted atop a building
or a tower and is slid out on rails to view targets of opportunity.
As a result, with real aircrafts or simulated signals of beam
tower, they can test the performance of the radar system such
as range ambiguity resolution, pulse compression, detection
thresholds, and false alarm. For this reason, without exception,
they utilize far-field ground-test facility to test and evaluate the
radar hardware and software.
However, the airborne fighter’s radar system typically utilizes
a deployable planar phased-array antenna structure with a
nominal operating range of several hundred kilometers. Due to
this long operating distance, the radar system receives the
planar-shaped wavefronts from targets, clutter, and jamming. A
very long far-field test distance is required to approximate
plane-wavefronts conditions in ground test facility. However, it
is impossible to place many radiating test sources at a far-field
ground-test range over a wide angular coverage in order to
make smoothly maneuvering targets, side lobe jamming signals,
and various clutter. As a result, the search and track
performance cannot be fully tested and verified over the wide
angle coverage. To solve this problem, this paper describes
radiating characteristics of near-field beam focusing for an
active array antenna upto the maximum azimuth and elevation
coverage of +120°.
II. NFBF ALGORITHM
A.J. Fenn [1] introduces a focused near-field testing concept
in high-quality controlled-environment anechoic chamber.
According to this focused near-field testing concept, plane-
wavefronts are made in near-field region. It means that
smoothly maneuvering targets, side lobe jamming signals, and
various clutter are made from many radiating test sources in
small high-quality controlled-environment anechoic chamber.
This near-field concept is proved from simple analysis results
[1, 2].
Figure 1 and figure 2 show the aperture grid of array sources
of spacing about 0.5 wavelength at 10GHz and the
measurement points. This planar array has the number of
elements, 1024(32 by 32). The source elements are considered
as equivalence source [3]. The electric fields are calculated at
the measure points on the z = Focal length, F using equations
(1, 2). To get the phase delay at the source point, array delay,
(a) (b)
Fig. 1. Planar array antenna((a) source plane, (b) measurement plane)
Fig. 2. Source plane and Measurement plane at near-field region
2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea
[ThP-53]
769
𝑑𝑛 between 𝑟𝑛 and 𝑟𝑓 is calculated. The ideal source points on
the z = 0 plane are radiated with phase delay, and then the
electric field is integrated for array sources at each
measurement point.
E(𝑟𝑚𝑠) = 𝐸𝑜 ∑ 𝑎𝑛 exp(𝑗𝑘 ∙ 𝑟𝑚𝑠 − 𝑗𝑘 ∙ 𝑑𝑛) /4𝜋𝑟𝑚𝑠1024𝑛=1 (1)
𝑑𝑛 = 𝑟𝑛 − 𝑟𝑓 (2)
where k is the propagation constant, 𝑎𝑛 is amplitude weighting
of array, 𝑟𝑚𝑠 is distance to source points from measurement
points, 𝑟𝑛 is distance to source points from focal point and 𝑟𝑓 is
distance to center of source points from focal point.
Figure 3(a) shows the phase delay of array element or the array
delay, 𝑑𝑛 in case of boresight beam focusing, and figure 3(b)
shows in case of +60°elevation beam focusing.
III. MONOPULSE CHARACTERISTICS
We analyzed in detail NFBF characteristics in variety of
focal length [2]. At focal length of one to two of aperture
diameters, its results are equivalent to its far-field pattern [1].
For analyzing NFBF according to the azimuth and elevation
angle, focal length is 1.5 times of aperture length. Figure 4
shows simulation results in case of boresight and
azimuth/elevation angles are +30°/-30°. These results are similar
to the far-field array pattern. In order to analyze monopulse
characteristics in NFBF, monopulse sum and difference patterns
are calculated [4]. Figure 5 shows plots of the electrical field as
the Sum and Delta pattern of 16 by 32 sub-arrays grid in near-
field analysis. The monopulse slope in the azimuth plane, 𝑘 is
2.6 value, which is almost the same of monopulse slope ratio of
the far-field result as mentioned by A.J. Fenn [1].
IV. CONCLUSION
In this paper, the NFBF algorithm for radiation in near-field
of an active array antenna is analyzed. The Phase delay of each
antenna aperture to its beam focusing point is used as phase
delay and we calculate electric field. Performance of an active
array antenna using NFBF is verified through analysis of
radiation characteristics such as beam focusing results and
monopulse pattern plots.
REFERENCES
[1] A. J. Fenn, “The development of phased-array radar technology,” Lincoln
Laboratory Journal, vol. 3, no.1, pp. 23-40, 1990.
[2] H. S. Ju, S. Y. Cho, J. H. So and S. B. Kim, “Near-field beam focusing
test of active array radar system,” 2017 korean Institute of Militaty
Science and Technology Conference, pp.710-711, Jun. 2017.
[3] C. A. Balanis, Antenna Theory Analysis and Design. John Wiley&Sons:
New York, 1982, pp. 142-144.
[4] S. M. Sherman and D. K. Barton, Monopulse Principles and Techniques.
Artech House: 2011, pp. 77.
(a) (a) (b)
Fig. 4. NFBF beam steering results ((a) bore sight, (b) +30° azimuth, -30° elevation)
(b) (a) (b)
Fig. 3. The phase delay of antenna aperture according Fig. 5. Radiation characteristics at the boresight ((a) Sum and Delta(azimuth) pattern,
to the beam focusing ((a) boresight, (b) +60° elevation) (b) monopulse slope, k)
2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea
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