Question

what is different between the Boe tie antenna and Rounded bowtie antenna?

Answer 1

Abstract—In this paper, three types of novel bowtie antennas with

round corners are presented and studied carefully, including quadrate,

rounded-edge and triangular shapes, which have better return loss,

flatter input impedance, more stable radiation patterns and smaller

area at the same time. The effect of round corners is attached

importance to due to their novelty. The conclusion is drawn that

adding the round corners at the sharp vertexes of radiation surfaces

can have positive effects on performances of not only bowtie antennas

but also the others.

1. INTRODUCTION

Much attention has been paid to ultra wide band (UWB) systems

in which UWB antennas are the important components. As one of

UWB antennas, bowtie antennas fed by coaxial line [1, 2], coplanar

waveguide [3, 12] and stripline [4], have many advantages, such as

low profile, ultra wide impedance band, high radiation efficiency and

easy to manufacture etc. They are used in many domains due to the

advantages mentioned above, such as ground penetrating radar [6–8]

and pulse antennas [1, 2, 9]. However, the patterns of bowtie antenna

are not ideal in full impedance band for pulse exciting, because they

are distorted greatly in high frequency, and its scale is large at the same

time. In paper [1], the stable patterns from 1.2 GHz to 3 GHz and wide

impedance bandwidth are obtained by RC-loaded but the efficiency is

reduced greatly. Bowtie antenna with a circle cap is designed and

fabricated in paper [5], but its reflection coefficient in higher frequency

is poor because of the matching network.

In this paper, the performances of three types of bowtie antennas,

quadrate, rounded-edge and triangular shapes, are improved by adding

round corners on their radiation surfaces. The better return loss, flatter

input impedance and more stable radiation patterns are obtained at

the same time. All results for comparison are obtained numerically and

the validity of simulations is proved by experiment. All these works

are used to show that putting round corners at the sharp vertexes of

radiation surfaces can bring the positive effects on performances of not

only bowtie antennas but also the others.

2. COMPARISONS AND ANALYSIS

In this section, three types of bowtie antennas, quadrate, rounded-

edge and triangular shapes, are studied carefully, including input

impedance, return loss and gain in normal direction. The stability of

radiation patterns can be shown by gain in normal direction indirectly,

which is chosen for studying the former conveniently. So the radiation

patterns of every antenna are not given in detail. The effects of round

corners are analyzed by comparing bowtie antennas with round corners

(BARC) with ones without them (BA).

In all simulations, the substrate is not considered due to two

reasons. One is for simulating simply and the other is that the

emphasis of our works is researching the effect of round corners on

bowtie antennas, which cannot be influenced by substrate.

To prove the validity of our numerical analysis, six antennas must

be fabricated on the substrate with dielectric constant 4.4 and thickness

0.8 mm. The experimental and numerical results are compared with

each other. The good agreement can prove it.

2.1. Quadrate Bowtie Antenna

First of all, the familiar quadrate bowtie antenna (QBA) is exampled

Fig. 1 shows the geometry of quadrate bowtie antenna with round

corners (QBARC) and the general QBA, whose sizes are listed in the

caption of the figure. Fig. 1(a) and (b) have the same height H and

flare angle α for easy comparison. The gap distance between the upper

radiation surface and below of two antennas in Fig. 1 is set to be

0.4 mm for simulation and experiment. It is obvious the former has

smaller area than the latter, but it has flatter real and imaginary parts

of input impedance shown by better return loss indirectly from the

following analysis due to the round corners with radius R1.

In this section, parameters H and α are fixed because they have

been discussed carefully in [5]. R1 is attached importance to mainly

due to its novelty. Fig. 2 shows the input impedance and return loss of QBA and QBARC with different R1.

All numerically studied antennas

in this section are matched to 188.5 Ω.

In Fig. 2, the imaginary and real parts of input impedance of

QBARC are flatter than QBA. However, the antenna area decreases

with R1 increasing, which causes the first resonance rise slightly but

does not influence the lowest frequency for S11 ≤ −10 dB. The best

return loss occurs when R1 = 12 mm.

Gain in Z-axis direction can show the stability of radiation

patterns. As far as the bowtie antennas are concerned, the flatter

the gain in Z-axis direction is, the more stable the radiation patterns

are. The existence of round corners influences the radiation patterns

strongly, especially in high frequency. In Fig. 3, gain in Z-axis direction

of QBARC with R1 = 12 mm is enhanced over 10 dB than QBA except

a small band near 8 GHz.

The existence of round corners does not change the average input

impedance of QBARC, but makes it flatter and then the better return

loss is obtained. The existence of round corners decreases the area of

QBARC, but its impedance bandwidth is not changed obviously.

These improvements can be explained as follows. Round corners

at the sharp vertexes of QBARC decreases the reflection of incident

current near the edges and changes the current distribution on

radiation surfaces compared with QBA, for example, in Fig. 4 at

3 GHz, and the radiation patterns, return loss and input impedance

are improved subsequently. This method is also used in [11] to obtain

wider impedance band.

The mirror method is used to measure the QBARC and QBA

fed by 50 Ω coaxial transmission line. The antennas are fabricated

on the substrate with dielectric constant 4.4 and thickness 0.8 mm and

their parameters are shown in Fig. 5, and measured by 10 MHz–20 GHz

ZVM of R&S. The numerical and experimental results are given in

Fig. 6, and the finite ground plane instead of the infinite and inaccurate

dielectric constant of substrate cause the disagreement. The return loss

of QBARC has 2 dB improvement than QBA on average.

2.2. Rounded-edge Bowtie Antenna

Secondly, the rounded-edge bowtie antenna (REBA) is exampled. It

is used extensively in many domains, such as pulse-exciting antennas

[1, 2, 9], ground-penetrating radar [8]. Fig. 7 shows the rounded-edge

bowtie antenna with round corners (REARC) and REBA. Cutting

away two round corners with the same radius R2 at the top sharp

vertexes of REBA forms the REARC. The two antennas has the

same height L and flare angle β for comparing the effect of round

corners easily, which are fixed that L = 70.7 mm and β = 90◦ because

they have been studied carefully in [8]. The substrate supporting the

radiation surfaces is not considered due to the reason at the beginning

of Section 2. The gap distance between the upper radiation surface

and below is set to be 0.4 mm for simulation and experiment.

Fig. 8 shows the input impedance and return loss of QBA and

QBARC with different R2. All numerically studied antennas in this

section are matched to 188.5 Ω.

In Fig. 8, the input impedance of REARC becomes flatter and

then steeper with increase of R2, and at the same time the first

resonance rises due to the area decrease of REARC. However, the input

impedance on average is not changed, which causes the better return

loss. It is exciting that the lowest frequency for S11 ≤ −10 dB does not

rise obviously with increase of R2 and area decrease. The optimum R2

is equal to 12 mm.

The best gain in Z-axis direction, which shows the most stable

radiation patterns, is obtained when R2 = 15 mm. It is improved close

to 10 dB near 8.5 GHz. It is good for pulse-exciting antennas, which

need the flat and high gain in the special direction and wide impedance

band. These improvements can be explained from current distribution

as in Section 2.1, which at 3 GHz is shown in Fig. 10.

To prove the validity of numerical results, the mirror method

is used to measure the REARC and REBA fed by 50 Ω coaxial

transmission line. The antennas are fabricated on the substrate with

dielectric constant 4.4 and thickness 0.8 mm and their parameters are

shown in Fig. 11, and measured by 10 MHz–20 GHz ZVM of R&S.

The numerical and experimental results shown in Fig. 12 agree well

with each other, and the finite ground plane instead of infinite in

simulation and inaccurate dielectric constant of substrate cause the

disagreement. The return loss of REARC has 3 dB improvement than

REBA on average.

2.3. Triangular Bowtie Antenna

Third, the triangular bowtie antenna (TBA) is exampled. This type

of antenna is also used extensively in many domains, such as ground

penetrating radar [6–8], mobile station [4] and Ultra wide band (UWB)

communication (3.1–10.6 GHz) [10]. The geometries of triangular

bowtie antenna with round corners (TBARC) and TBA are shown in

Fig. 13, whose sizes are listed in the caption of the figure. Fig. 13 (a)

and (b) have the same height D and flare angle θ for easy comparison.

The gap distance between the upper radiation surface and below of two

antennas in Fig. 13 is set to be 0.4 mm for simulation and experiment.

Parameters D and θ are fixed, and R3 is studied carefully for the

reasons mentioned above in Section 2.2. All numerical return losses are

matched to 188.5 Ω. The numerical input impedance and return loss

are shown in Fig. 14. It is obvious that the existence of round corners

does not influence the input impedance on average, and the flatness of

input impedance becomes better and then worse with increase of R3

from 0 to 14 mm. The best return loss is obtained when R3 = 12 mm.

However, the area of TBARC is smaller than TBA, which causes the

lowest frequency for S11 ≤ −10 dB rises slightly.

Gain in Z-axis direction indicating the stability of radiation

patterns is shown in Fig. 15, denoting by G(0, 0). The below and the

upper frequency band edges for G(0, 0) ≥ 0 dB rises at the same time

and the total is not improved obviously between TBA and TBARC.

When R3 = 8 mm, the gain in Z-axis direction of TBARC is better

slightly. The change of current distribution brings the improvement of

performances of TBARC as shown in Fig. 16.

To prove the validity of numerical results, the measurements are

done using the mirror method to feed the antennas by 50 Ω coaxial

transmission line, which is not restricted by frequency bandwidth.

The antennas are fabricated on the substrate with dielectric constant

4.4 and thickness 0.8 mm and their parameters are shown in Fig. 17,

and measured by 10 MHz–20 GHz ZVM of R&S. The numerical and

experimental results shown in Fig. 18 agree well with each other, and

the finite ground plane instead of infinite in simulation and inaccurate

dielectric constant of substrate cause the disagreement. The return

loss of TBARC has 5 dB improvement than TBA on average.

Three types of bowtie antennas are exampled to prove the positive

effect of round corners by numerical and measured results. The return

loss and stability of radiation patterns are improved simultaneously

by changing the current distribution on radiation surface due to

the existence of round corners. For widening impedance band,

resistance-loading, RC-loading and different feeding technologies are

used [1, 2, 4, 6–10]. However, the efficiency reduces greatly due to

loading and the volume is enlarged greatly at the same time. Different

feeding technologies cannot exhibit the best performance of bowtie

antennas. The efficiency of bowtie antennas with round corners does

not decrease due to none loading, and their areas are reduced and the

better return loss and more stable patterns are obtained at the same

time.

The authors think round corners at the sharp vertexes of radiation

surface have positive effect not only on bowtie antennas but also the

others. However, the paper about it is not seen infrequently. In [11],

the impedance bandwidth is enhanced greatly by adding four round

corners at the four vertexes of rectangular slot on the ground plane,

which gives a good proof of this idea.

3. CONCLUSION

Three types of novel bowtie antennas with round corners are proposed

in the paper. The effects of round corners on the input impedance

and return loss have studied carefully. The existence of round corners

improves the return loss and flatness of input impedance and can

enhance the stability of radiation patterns more or less at the same

time. The conclusion is drawn that adding the round corners at

the sharp vertexes of radiation surfaces can have positive effects on

performances of not only bowtie antennas but also the others.