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

*hyesunju@add.re.kr

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

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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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