An Experimental Study of the Printed-Circuit Elliptic Dipole Antenna with 1.5-16 GHz Bandwidth

doi:10.4236/ijcns.2008.14036

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I. J. Communications, Network and System Sciences, 2008, 4, 285-385

Published Online November 2008 in SciRes (http://www.SciRP.org/journal/ijcns/).

An Experimental Study of the Printed-Circuit Elliptic

Dipole Antenna with 1.5-16 GHz Bandwidth

Chunchi LEE

, Hsinsheng HUANG, Chengda YANG, Chiawei WANG

Department of Computer and Communication, Shu-Te University, Kaohsiung, Taiwan, R.O.C

E-mail:

homer1@mail.stu.edu.tw

Received August 1, 2008; revised September 22; accepted September 29, 2008

Abstract

Printed-circuit board (PCB) elliptic antennas with useful bandwidth exceeding 10:1 ratio are suitable for

wideband radar, wireless ultra wideband (UWB) and other wireless communication applications. We present

wideband PCB elliptic dipole antennas which are capable of achieving the bandwidth requirements for all

the applications. A set of elliptic dipole antennas with varying eccentricities have been fabricated for

demonstration. We find one specific size (specific eccentricity) dipole that can yield an impressive 1.5-16

bandwidth exceeding the currently available. A couple of elliptic dipole antennas suitable for UWB

application have been presented. We have measured swept frequency response, impedance and radiation

patterns of all dipoles. An empirical formula is given for calculating the starting resonant frequency within

the operating band. The calculated values are found in good agreement with measured results.

Keywords: Elliptic Dipole Antenna, Wideband Radar Antenna, Printed-Circuit Board (PCB), Ultra

Wideband (UWB)

1. Introduction

The printed-circuit elliptical antenna has proven to be an

efficient and effective radiator with broadband per-

formance. In its simplest configuration, circular or

elliptical antenna can be designed to produce a broad

beamwidth as well as broadband with linear polarization

and a radiation pattern having a broadside maximum.

The most direct approach to provide a broad beamwidth

and broadband performance from such an antenna is to

use a printed-circuit dipole with the upper and lower

radiation elements having a circular or elliptical shape.

The antenna dipole could be fed with a 50 ohm micro-

stripe line, extending into the dipole center (the point

where the two adjacent circular or elliptical radiators

join). It was found that the current on the radiator at all

frequencies is largely concentrated on the peripheral

edge with very low current density approaching inward

towards the center. For elliptic (circular) dipoles, one can

picture that numerous semi-elliptic thin-line dipoles of

varying lengths are effectively formed to excite multi-

linear modes hence resulting in a wide bandwidth with

linear polarization.

Several methods have been proposed to study the

(a) (b)

Figure 1. (a) Geometry of an elliptical printed-circuit dipole

antenna. (b) An elliptical dipole antenna etched on PCB

with dielectric constant

=4.2.

impedance of elliptical printed-circuit antennas [1–3]. A

printed crescent patch antenna [4] and a bottom fed

elliptical antenna [5] were investigated experimentally to

provide broadband performance with linear polarization

without added complexities inherent in the feed circuit.

296 C. C. LEE ET AL.

Broadband printed-circuit elliptical dipole antennas

covering 750 MHz to 6.0 GHz for WLAN and WiMax

applications have been fabricated and tested [6].

Recently, Powell [7] has shown that broadband linear

polarization could also be achieved by two differential

crescent patches fed at the dipole center of the two

adjacent elliptic elements. They achieve a broadband

performance covering the 3.1-10.6 GHz ultra wideband

(UWB) spectrum with a swept frequency return loss of

about 11dB.

In this paper, experiments are carried out to

investigate the impedance bandwidth and swept

frequency measurement for several elliptic dipole

radiators using various eccentricities (equivalently,

various b/a ratios, see Figure 1). The radiation patterns

are also measured.

The rest of the paper is organized as follows. Section

2 presents experimental results of the bandwidth

performance for a set of elliptic dipoles of various sizes

(various eccentricities) by measuring the frequency

return loss. An optimum dipole size is found to yield

widest bandwidth among all. Section 3 discusses the

starting resonant frequency. Section 4 discusses the

antenna products for UWB application. Then, Section 5

measures the radiation patterns of the optimum dipole.

Finally, Section 6 concludes the paper.

2. Experimental Results of Bandwidth

Performance

The geometry of a printed-circuit elliptic antenna dipole

is shown in Figure 1(a). A photograph of an elliptical

dipole antenna etched on PCB with dielectric constant of

4.2 is shown in Figure 1(b). In the lower elliptic radiator,

portion of the area is cut off in the shape of an ellipse to

accommodate the 50 ohm micro stripe feed line. The first

part of our experiment study is to investigate the

broadband properties. A set of dipoles with varying b/a

ratios were etched on the printed-circuit board (PCB).

We varied the b/a ratio progressively from 1.00 (a circle),

0.945, 0.897, 0.852 to 0.813. The minor diameter 2b was

held constant at 26 mm, while the major diameter 2a was

progressively increased from 26 mm to 32 mm. Thus,

five different size dipoles are etched on microwave

printed-circuit boards (PCBs) using 41.2 mm

88.1 mm

to 38.1 mm

53.4 mm FR4 with thickness d = 0.762 mm

and dielectric constant 4.2.

The swept frequency return loss for an elliptic dipole

with 2a=32 mm and 2b=26 mm (b/a = 0.813) is first

measured using a network analyzer. We can see from

Figure 2 that, in the 1.5 GHz to 16 GHz range

(bandwidth ratio 10.66:1), the return losses are all better

than -10 dB. This is an impressive broadband result as

the bandwidth has exceeded the 10:1 ratio. Next, Figure

3 presents the swept frequency measurement of this

particular dipole in Smith chart format. Then, a plot of

the real and imaginary parts of the input impedance

against frequency is given in Figure 4. From this figure,

we can see multiple resonance peaks indicating that the

radiator effectively consists of multi-elliptically-shaped

thin-line dipoles of various lengths exciting many linear

modes thus resulting in a broad bandwidth with linear

polarization.

Next, we vary 2a from 26.0, 27.5, 29.0, and 30.5 mm

while holding 2b constant at 26 mm (b/a

ratios of 1.000,

0.945, 0.897, and 0.852) and repeat the experiment. For

these b/a

ratios, the measured swept frequency return

losses are compared and results are presented in Figure 5.

Typical swept frequency input impedance derived from

corresponding Smith chart measurements are given in

Figure 6 and Figure 7. The first dipole with 2a=32mm

outperforms all. Then, we have also fabricated elliptic

dipoles with reducing b/a ratios of 0.800 and 0.750

(increasing 2a beyond 32mm) and found that the

performance also starts to fall.

The bandwidth performance (bandwidth is defined

here as the frequency range with return loss better than

Figure 2. Measured return loss for elliptic dipole with 32

mm × 26 mm, b/a = 0.813.

Figure 3. Smith chart display for elliptic dipole with 32 mm

× 26 mm, b/a = 0.813.

Return Loss (dB)

-5

-10

-15

-20

-25

-30

-35

b/a=0.813

1 2.5

4 5.5

8.5

11.5

14.5

Frequency (GHz)

AN EXPERIMENTAL STUDY OF THE PRINTED-CIRCUIT ELLIPTIC DIPOLE ANTENNA 297

WITH 1.5-16 GHZ BANDWIDTH

Figure 4. Impedance vs. frequency for elliptic dipole with

32 mm × 26 mm, b/a = 0.813.

Figure 5. Comparison of measured return loss between

elliptic dipoles (b/a = 0.852, 0.897, 0.945, and 1.000).

Figure 6. Impedance vs. frequency for elliptic dipole with

30.5 mm ×

26 mm, b/a = 0.852.

-10 dB) versus b/a ratio for the above five dipoles is

given in Figure 8. It is observed that as the radiator shape

becomes less elliptical, the number of effective semi-

elliptical-shaped thin-line dipoles of various lengths

appears to decrease resulting in a narrower bandwidth.

Figure 7. Impedance vs. frequency for elliptic dipole with

26 mm ×

26 mm, b/a = 1.000.

Figure 8. Bandwidth vs. b/a ratio for elliptic dipoles with

minor diameter 26mm.

3. The Starting Frequency

The second part of our study is to investigate the starting

frequency in the operating band of the elliptic dipole. It

is well-known that the minor diameter 2b of a dipole

radiator determines the resonant frequency. To

investigate the resonant frequency of the elliptic dipole

experimentally, we vary 2b from 27.0, 28.0, 29.0, and

31.0 mm while holding 2a constant at 32 mm (b/a ratios

of 0.844, 0.875, 0.906, and 0.969) and repeat the

experiment. For these b/a

ratios, the swept frequency

Smith charts are measured and the typical input

impedances derived from the corresponding Smith charts

are given in Figures 9 through 12.

For a PCB material of dielectric constant

and a

given 2b (mm), we found an empirical formula for

calculating the starting resonant frequency (defined for

return loss < -10 dB) in GHz as

8.40

(1)

1 4 7 10 13 16

Frequency (GHz)

160

(

OHM)

b/a=0.813

(32mm×26mm)

X (OHM)

b/a=0.852

b/a=0.897

b/a=0.945

b/a=1.00

Return Loss (dB)

1 2.5 4 5.5 7 8.5 1 0 1 1 .5 1 3 1 4 .5 1 6

Frequency (GHz)

X (OHM)

160

(

OHM)

1 4 7 10 13 16

Frequency (GHz)

b/a=0.852

(30.5mm×26mm)

160

R (OHM)

X (OHM)

1 4 7 10 13 16

Frequency (GHz)

b/a=1.00

(26mm×26mm)

10.

9.5

Bandwidth

0.8 0.84 0.88 0 .9 2 0 .9 6 1

b/a Ratio

8.5

298 C. C. LEE ET AL.

For elliptic antenna dipoles with 2b varied from 26.0,

27.0, 28.0, 29.0, and 31.0 mm and with 2a held constant

at 32 mm, using (1), the calculated values of

corresponding starting resonant frequencies are

respectively about 1.53, 1.47, 1.42, 1.37, and 1.28 GHz.

These calculated frequencies are found in good

agreement with the measured frequencies (at frequencies,

the impedance values correspond to better than 10 dB

return loss) as shown in Figure 4 and Figures 9 through

12. Thus, (1) proves to be quite useful and handy.

4. Products for UWB Application

Utilizing Equation (1) in Section 3, printed-circuit

elliptical dipole antennas for 3.1-10.6 GHz UWB

applications have been fabricated and tested [8]. Figure

13 shows a photograph of elliptical dipole antennas

etched on PCB with dielectric constant of 4.2 and 10.2,

respectively.

Figure 9. Impedance vs. frequency for elliptic dipole with

32 mm ×

27 mm, b/a = 0.844.

Figure 10. Impedance vs. frequency for elliptic dipole with

32 mm ×

28 mm, b/a = 0.875.

To achieve the required 3.42:1.00 UWB impedance

bandwidth properties, low eccentricity elliptic dipole

radiators were first etched on FR4 PCB with an overall

size of 24 mm

46 mm. The measured return loss, as

shown in Figure 14, in the 3.1 GHz to 10.6 GHz range is

generally better than -12.6 dB. Figure 15 presents the

swept frequency measurement in Smith chart format.

Next, to reduce he size of the product, an elliptical dipole

antenna of the same design is etched on a flexible

laminate PCB with a thickness d=0.635 mm and

=10.2.

With an overall PCB size of 15 mm × 28 mm, this

elliptic dipole provides suitable impedance properties

across major portions of the frequency spectrum. The

swept frequency return loss of this elliptic dipole

fabricated on the flexible laminate PCB is presented in

Figure 16. Due to the non-uniform properties of the

flexible laminate PCB, this antenna can only achieve

close to -10 dB return loss performance in the specified

3.1 GHz to 10.6 GHz frequency band.

Figure 11. Impedance vs. frequency for elliptic dipole with

32 mm × 29 mm, b/a = 0.906.

Figure 12. Impedance vs. frequency for elliptic dipole with

32 mm ×

31 mm, b/a = 0.969.

160

(

OHM)

1 4 7 10 13 16

Frequency (GHz)

X (OHM)

b/a=0.844

(32mm×27mm)

1 4 7 10 13 16

Frequency (GHz)

160

(

OHM)

X (OHM)

b/a=0.906

(32mm×29mm)

160

R (OHM)

1 4 7 10 13 16

Frequency (GHz)

X (OHM)

b/a=0.875

(32mm×28mm)

X (OHM)

160

(

OHM)

1 4 7 10 13 16

Frequency (GHz)

b/a=0.969

(32mm×31mm)

AN EXPERIMENTAL STUDY OF THE PRINTED-CIRCUIT ELLIPTIC DIPOLE ANTENNA 299

WITH 1.5-16 GHZ BANDWIDTH

Different from in wideband radar applications, for

UWB impulse radio application, antenna requires

sufficient impedance matching, linear ungroup phase

response or near constant group delay throughout the

entire 3.1 to 10.6 GHz band. As shown in Figure 13, the

presented antennas are small, compact, and should

exhibit fixed phase center property. Owing to that, this

antenna tends to radiate a mostly non-dispersive

waveform which cause less pulse shape distortion to the

transmitted waveform and provides suitable frequency

domain characteristics and performance. For UWB

applications, we are preparing equipment to perform the

time domain transmission tests required to assess the

impulse response and fidelity characteristics of these

antennas.

5. Radiation Patterns

The third part of our experiment is to investigate the

radiation patterns of the 1.5-16 GHz elliptic dipole. For

demonstration purpose, we only present the

measurements for our optimum dipole (32 mm × 26 mm,

b/a = 0.813). The radiation patterns on the x-z, x-y, and

y-z planes of the optimum elliptic dipole at 2.0, 4.0, 10.0,

and 14.0 GHz are measured in an anechoic chamber and

shown in Figure 17, Figure 18, and Figure 19

respectively. Results indicate reasonable omnidirectional

radiation patterns on all the three planes. Consistency of

the patterns, similar to a typical dipole radiation pattern,

can be observed across major portions of the frequency

band. The measured gain is about 2 dBi.

6. Conclusions

For elliptic dipole antennas with 2b varied from 26.0,

27.0, 28.0, 29.0, and 31.0 mm and with 2a held constant

at 32 mm, we found an optimum size 32 mm × 26 mm,

b/a = 0.813 can yield a maximum bandwidth

performance in the 1.5-16 GHz range. Thus, we have

Figure 13. Elliptical dipole antennas etched on PCB with

dielectric constant

= 4.2 (Left) and 10.2 (Right).

Figure 14. Measured return loss of the (24 mm × 46 mm)

elliptic dipole etched on FR4 with

= 4.2.

Figure 15. Smith chart display of the (24 mm × 46 mm)

elliptic dipole etched on FR4 with

= 4.2.

Figure 16. Measured return loss of the (15 mm × 28 mm)

elliptic dipole etched on a flexible laminate PCB with

10.2.

shown that, with a proper choice of minor to major axis

ratio, a printed-circuit elliptic dipole antenna using a

-5

-10

-15

-20

-25

-30

-35

-40

1 2 3 4 5 6 7 8 9 1 0 1 1

Frequency (GHz)

Return Loss (dB)

300 C. C. LEE ET AL.

simple single-feed network can provide a useful

operating bandwidth exceeding the 10:1 ratio. Aside

from swept frequency and impedance measurements for

elliptic dipoles of various b/a ratios, the radiation

patterns for the optimum dipole are measured and found

to be in consistency with typical dipole radiation patterns.

By properly choosing the minor diameter for the dipole

made of PCB of known dielectric constant, the starting

operating frequency can be easily calculated using an

empirical formula for system design. A couple of elliptic

dipole antennas for 3.1-10.6 GHz (3.42:1 bandwidth

ratio) UWB application have been presented to

demonstrate that these dipole antennas can be designed

for other fixed operating frequency bandwidth ratio.

Figure 17. Measured radiation pattern on x-z plane of

antenna (32 mm ×

26 mm, b/a = 0.813).

Figure 18. Measured radiation pattern on x-y plane of

antenna (32 mm ×

26 mm, b/a = 0.813).

Figure 19. Measured radiation pattern on y-z plane of

antenna (32 mm × 26 mm, b/a = 0.813).

7. References

[1] L. C. Shen, “The elliptical microstrip antenna with

circular polarization,” IEEE Transactions on Antenna

and Propagation, Vol. AP–29, No. 1, January 1981.

[2] S. A. Long and M. W. McAllister, “The impedance of an

elliptical printed-circuit antenna,” IEEE Transactions on

Antenna and Propagation, Vol. AP–30, No. 6, November 1982.

[3] S. A. Long, L. C. Shen, D. H. Schaubert, and F. G. Farrar

“An experimental study of the circular-polarized

elliptical printed-circuit antenna,” IEEE Transactions on

Antenna and Propagation, Vol. AP–29, No. 1, January 1981.

[4] N. C. Azenui and H. Y. D.Yang, “A printed crescent

patch antenna for ultrawideband applications,” IEEE

Antennas and Wireless Propagation Letters, Vol. 6, 2007.

[5] H. Schantz, “ Apparatus for establishing signal coupling

between a signal line and an antenna structure,” U. S.

Patent 6, 512, 488, January 28, 2003.

[6] C. C. Lee, C. W. Wang, R. Y. Yen, and H. S. Huang,

“Broadband printed-circuit elliptical dipole antenna

covering 750 MHz-6.0 GHz,” International Conference

on Microwave and Millimeter Wave Technology

Proceedings, Nanjing, China, Vol. 3, pp. 1207–1209,

April 2008.

[7] J. Powell and A. Chandrakasan, “Differential and single

ended elliptical antennas for 3.1-10.6 GHz ultra

wideband communication,” 2004 IEEE Antennas and

Propagation Society Student Paper Competition, 1st

Place.

[8] C. C. Lee, H. S. Huang, R. Y. Yen, C. D. Yang, and S. C.

Nan, “Printed-circuit elliptical dipole antenna for 3.1-

10.6 GHz UWB application,” The 4th IEEE

International Conference on Wireless Communications,

Networking and Mobile Computering, October 2008,

Dalian, China.