DSpace at VNU: Fast solution for main beam pattern measurement using Digital Beam Forming technique

Fast Solution for Main Beam Pattern Measurement Using Digital Beam Forming Technique Le Quang Thao, Dam Trung Thong Radio Physics Department, Physics Faculty Hanoi University of Science

Fast Solution for Main Beam Pattern Measurement

Using Digital Beam Forming Technique

Le Quang Thao, Dam Trung Thong

Radio Physics Department, Physics Faculty

Hanoi University of Science, VNU

Hanoi, Vietnam thaolq@vnu.edu.vn Mobile: 84.983.71.29.41

Nguyen Thi Ngoc Minh Scientific and Technological Institutes of Military

Radar Institute

17 Hoang Sam, Hanoi, Vietnam minh_viet08447@yahoo.com

Abstract—In this paper, we suggest a fast method to measure the

main beam pattern of phased array antenna This method is done

by electrical steering which using Digital Beam Forming (DBF)

technique to steer the main beam to designed direction with little

change to the main beam and the sidelobe instead of using

mechanical steering The result tested on our 4x8 array antenna

supports that this method has acceptable accuracy for the main

beam pattern

Keywords: digital beam-forming; electrical steering; phase

array antenna; far-field measurement; main beam; side lobe

I INTRODUCTION

Phased array antenna which is known as smart antenna has

been used more and more in wireless communication, medical,

radar, etc… with the developing of DBF technique Because

using DBF, we can easily change the phased shifting or

amplitude scaling of each element in phased array antenna in

order to change its main beam direction as its pattern to

designed direction

Far-field pattern is one of the most important properties of

an antenna Therefore, as the developed of antenna technique,

we had many methods to measure it [1], [2] But for most of

methods, it took a long time [3] to measure because we had to

sample data from every direction In order to reduce time of

measuring the antenna pattern while testing quality of antenna,

we should care much more about the main beam and apart of

sidelobe near the main beam than other directions

Furthermore, to phased array antenna, we suggest that we

could use DBF technique to steer the main beam instead of

sampling data by used mechanical steering method Normally,

steering beam will change the pattern of antenna, but theory

and simulation has shown that there is algorithm [4] could steer

the beam with least change in main beam pattern Therefore

this algorithm could be used for a quick main beam pattern

measurement

Our method has been tested by measuring the main beam

pattern from -30 to 30 degree in azimuth plane of our 4

channels, 4x8 elements antenna (Figure 1) It operates at

X-band, and is used in radar systems

Figure 1 4 channels, 4x8 elements antenna

II DIGITAL BEAM FORMING TECHNIQUE

Phased array antenna [5] is known as active antenna or smart antenna It means we can easily change it pattern by changed some antenna’s parameter such as element’s distance, permittivity ε, etc… to change phased shifting and amplitude scaling for all elements in array

In DBF, the phased shifting and amplitude scaling for each antenna element, and summation for receiving or transmitting are done digitally (figure 2) Therefore it make phased array antenna more flexible, now we could change antenna pattern without change to antenna system Everything can be done by general - purpose DSP or dedicated chip

For simple, suppose that we have a linear array antenna with n elements and the data received from each channel is:

1 2

s [ , , , ] = s s sn (1)

To steer the beam to designed angle, we use a correlation

matrix w with each element is phased shifting and amplitude

scaling for each antenna channel:

T

w = [ , w w , , wn] (2) (Transpose is convenient for matrix product)

Figure 2 Digital beam forming system for receiver

Forget s, w and DBF for instant, we come back to normal

beam forming method for linear array antenna For the signal

comes from ϑ in the azimuth plane (figure 3) We have:

k k

Δ = Δ (3) WithΔ φ is the different phase between 2 consecutive

elements, andΔ φk is the different in phase of kth from 1st

0

2

λ

Δ = Δ = (4)

Therefore:

0

2 sin

k

kd π

λ

Δ = (5)

From (5) we see that signal come from different angle have

different phased shifting Therefore pattern of array antenna is

not the same in all direction, there will be some directions with

higher amplitude than the others

Figure 3 Signal come from ϑ

Now we consider the case when we add more phased shifting Δ φklike in (3), (5) to kth element in compare with 1st

elements Suppose that at initial the highest direction or main beam is at 0 degree

From (5) we have:

0

arcsin

2

d

φλ ϑ

π

Δ

© ¹ (6)

It means that if we shift the phase of kth element to k Δ φ, almost antenna pattern is steered ϑ degrees This is the basic idea of beam forming

As above, if we use DBF technique, we need build matrix

w with each element of w is the phased shifting like in (6) and

equalized amplitude scaling for each antenna element Because

ϑ is related to arcsin function, we choose w in this form:

w = [1, ejV φ, , ej n− V φ]

(7)

At here, we equalized amplitude scaling because if we change this parameter it’ll change radiated energy Apply this

to the signal, the pattern will be steered:

0

arcsin

2d

φλ π

Δ

© ¹ (8) Base on this equation, by changing Δ φ we can steer the pattern to any arbitrary angle

We have simulated this algorithm in matlab (figure 4 and 5)

by steering main beam from 0 to ± 15 degrees From the simulation, although there is a little changed in the pattern but the ratio of S/N and the main beam didn’t change so much This result shows that our idea in using electrical steering in measuring main beam pattern is possible, but we still need experimental result to support it

Figure 4 Simulation result of steering beam

Figure 5 Simulation result of steering beam in polar

III MAIN BEAM PATTERN MEASUREMENT

Antenna far-field pattern refer to the performance of

antenna Normally, people want their antenna has narrowed

main beam with low sidelobe level But the matter of fact is

that we couldn’t reach both of them [5], we have to choose

either main beam or sidelobe

Improving measurement method is as important as

improving antenna performance In present, there are 2 main

methods [2] to measure far-field pattern of antenna: directly

sample data in far-field, indirectly by sample data in near-field

with data was transmitted from a standard antenna For both

methods, it’s needed to steer whole antenna to every direction

to sample data Although we have automated system done it,

it’s stilled cost a long time to sample all data In industrial

environment where there are large a mount of antennas were

produce, it’s difficult to examine the quality of product To

reduce time cost, we could sample less data, for example we

could measure only the main beam because if the antenna has

the same design and main beam pattern normally has the same

quality But it’s still cost a lot of time

As mention above, if we use DBF technique with only

changing phased difference, the pattern’s appearance near the

main beam is almost the same Base on it, we suggest that we

could use DBF technique to steer the antenna’s pattern about ±

30 degrees

IV MEASUREMENT SCHEDULE

In order to perform our measurement, we need a standard

antenna which can operate at X-band, a display device to show

the result in a chamber room We also need a DSP board to

perform DBF technique and send result to display device

Our measurement contains 2 steps: Firstly, we need to fix

transmitted antenna, then tested antenna was put far enough

from transmitted antenna and looked directly to transmitted

antenna (figure 6) for calibration Secondly, we sample data

and using DSP to calculate the main beam pattern of tested

antenna Because using DBF to steer received data can perform

simultaneously for many directions, we need sample only once

Therefore, using this method could reduce a lot of time for measuring

Figure 6 Far-field measurement’s apparatus

V MEASUREMENT RESULT

With our schedule measurement, we have finished measuring for our 4 channels, 4x8 elements array antenna The result reached less than one minute since begun sample data is shown in figure 7

From the result we can see the main beam pattern from -30

to 30 degrees in the bottom left corner At any time, in the bottom-right corner is phase different of 4 channels and in the top is the phase of each channel

Our measurement method need sample only once, in comparison with other methods [3] which need sample at a lot

of direction Therefore, it’s obviously that our method could reduce a lot of time in measuring

Figure 7 Experimental result

VI CONCLUSION

This paper discusses about an antenna measurement

method using DBF technique which can reduce a lot of time in

measuring antenna main beam pattern Our experimental result

supports that this method is possible and could be used widely,

especially in testing quality of antenna

ACKNOWLEDGMENT

We would like to thank Vietnamese Scientific and

Technological Institutes of Military for their supported

equipments in measuring experimental result

REFERENCES

[1] Myron D Fanton “Array antenna pattern measurement techniques” ERI Technical Series, Vol 6, April 2006

[2] “Antenna measurement theory” reprinted with the permission of ORBIT/FR Inc

[3] Mathew D Valerio and Robert R Romanofsky “Implementing an automated antenna measurement system” NASA/TM-2003-212337 [4] Németh Z., Imre S., Balázs F., "Adaptív antennarendszerek", Híradástechnika, LVII évfolyam, május, 2002

[5] Hubregt J Visser “Array and phased array antenna basics” - John Wiley

& Sons, Ltd (2005)