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Progress In Electromagnetics Research B, Vol. 34, 247262, 2011

PRINTED TEM HORN ANTENNA FED BY BALANCEDMICROSTRIP LINE

S. Lin1,2,*, X.-Q. Zhang2, X.-Y. Zhang2, and Y. Tian2

1Electronic Science and Technology Post-doctoral Research Center,Harbin Institute of Technology, Heilongjiang 150080, China

2School of Electronics and Information Engineering, Harbin Instituteof Technology, Heilongjiang 150080, China

AbstractIn this paper, a new kind of printed TEM horn antennawith high-gain fed by balanced microstrip line is proposed. Theradiation part of the antenna (printed on the FR4 epoxy substrate)is composed of two symmetrical triangular metal foil branches fedby balanced microstrip line. The antenna has been simulated byCST MICROWAVE STUDIO software and the simulated results

show that the proposed antenna is a kind of traveling wave antenna.Besides, an equivalent adopted V-shaped antenna model is proposed todescribe the radiation characteristic of the antenna. The simulated andmeasured results indicate that in the frequency ranging from 1.64 GHzto 9 GHz, the reflection coefficient of the antenna is less than6 dB andin the work frequency band, the average gain value is over 8.2 dB. Theantenna gain will be improved greatly by extending the length of thedielectric slab appropriately (in the main radiation direction) withoutinfluencing the bandwidth. The measured and simulated results havea good consistency. This antenna will have wide application in theUWB field.

1. INTRODUCTION

Nowadays, ultra-wideband antenna research with the extension of thefrequency spectrum of wireless instruments and a higher requirementfor the bandwidth of antenna is raising more and more concerns. Theultra-wideband antenna has an extremely wide impedance bandwidth,thus several communication systems are able to share the same

antenna which will reduce the number of antennas and volumeReceived 1 August 2011, Accepted 6 September 2011, Scheduled 18 September 2011

* Corresponding author: Shu Lin ([email protected]).

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of the communication equipments. Moreover, the development ofUWB technology promotes further researches on the time-domain

characteristics of ultra-wideband antennas. Current ultra-widebandantennas mainly include log-periodic antenna [1], Archimedean spiralantenna [2], equiangular spiral antenna [3], bow-tie antenna [4], teardrop antenna [5], tapered slot antenna [6, 7], traditional TEM hornantenna [810], etc. Among them, traditional TEM horn antennafails to benefit the integration of communication system because of itsstereo-structure with relative big volume and heavy weight. The otherscould be designed as the structure of printed types to reduce the sectionand volume, and the antenna planarization can come into effect. Theradiation models of Archimedean spiral antenna and equiangular spiralantenna are bidirectional circular polarization radiation. In order toattain one-way radiation of the antenna, the radiator reflector must beincreased at the cost of heightening antenna section. Bow-tie antennaand tear drop antenna have omnidirectional liner polarization radiationstructures. In order to realize their directional radiation, theseantennas need to compose relevant antenna array or add reflectorsat the cost of introducing a complex feeding network. The log-periodic antenna is usually fed from the short dipole side, and thedirectional radiation can be realized by overlapping feeding. However,

it will introduce a length of coaxial line so that the application in themillimeter-wave and integrated circuits will be restricted. In addition,tapered slot antenna is a planar printed type of antenna, and it usuallyconsists of feeding network and radiator. Furthermore, the plane figureof the radiator is similar to the projection on the E plane of TEM hornantenna with adding ridge. Consequently, the tapered slot antennacan be classified as planar TEM horn antenna. These antennas canbe divided into Vivaldi antenna, LTSA antenna [1113] and others,according to different tapered forms of the slot lines. All of themhave relatively complex structures with ultra-wideband and balancedfeeding.

Different researchers have different opinions about the definition ofUWB antennas. It is usually considered that an antenna with a ratiobandwidth (ratio of the upper and lower limit working frequencies)over 2 : 1 can be called as a UWB antenna. The frequency rangeof the civil UWB communication with band of 3.110.6 GHz (rationbandwidth is 3.42 : 1) proposed by FCC has gained wide attention.Researchers have designed a lot of antenna working in this band, andantennas in this type all have ratio bandwidth over 3.42 : 1.

In this paper, a new kind of printed TEM horn antenna withhigh-gain fed by balanced microstrip line is proposed. The radiationpart of the antenna consists of two symmetrical triangular metal foil

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Progress In Electromagnetics Research B, Vol. 34, 2011 249

branches which are printed on the FR4 epoxy resin substrate andfed by balanced microstrip line. The antenna has been simulated

by CST MICROWAVE STUDIO

software, and the parameters aregained. The simulated and experimental results indicate that in theoperating frequency band from 1.64 GHz to 9 GHz, the ratio bandwidthis 5.5 : 1. Additionally, the antenna has stable directional radiationcharacteristics, and the feeding structure is simple. The analysis of thesimulation and experiment results are shown in the paper.

2. THE DESIGN OF THE ANTENNA

The whole antenna (shown in Fig. 1) consists of a feeding part and aradiation part, while the radiation part is composed of metal radiatorand medium focusing part. The medium focusing part has a novelstructure introduced into planar TEM horn antenna in this paper.With such a structure, the electronic field energy density will beenhanced, so that the antenna gain can be greatly increased. Themetal radiator can be regarded as a gradually opening slot line. Thedesign of the metal radiator can be calculated according to the relativeformula in [14], and the obtained value can be set as the initial valueof the simulation.

The feeding part is a balanced microstrip line, and the originalport impedance of the feed port is 50 . Moreover, the joint port

b

a

c

d

lf

Wf1

Wf2

Via-holePart A

Part B

Part Cls

2

(a) Top View (b) Bottom View

Figure 1. Sketch of the proposed antenna (Part A: feeding part, PartB: metal radiator, Part C: medium focusing part).

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Table 1. Antenna parameters.

Parameters Value

Dielectric slab length a 213mm

Dielectric slab width b 120mm

Dielectric slab height h 1.5mm

Dielectric slab reflection coefficient r 4.4

Dielectric slab Loss Tangent 0.025

Triangular radiator bottom line length c 30mm

Triangular radiator height d 73mm

Feeding length lf 35mm

Feeding port feeding length wf1 2 mm

Radiator port feeding length wf2 0.8mm

Focusing part length ls 100mm

impedance between the terminal of the feeding line and the antennashould match the imputing port. And the input impedance of antennaradiator equals the input impedance of the tapered slot line which isopen-ended (200 ). Thus the balanced microstrip line should also bedesigned as a tapered form. The major parameters depend on theratio of the width and height and reflection coefficient of the dielectricsubstrate slab. Their characteristic impedance calculation methodcan be referred by the method of calculating characteristic impedanceof the microstrip line. Thereforethe fundamental parameters of the

antenna are shown in Table 1.

3. SIMULATION ANALYSIS

3.1. Ultra-wideband Characteristics

Most antennas consist of feeding line and radiator. An antennacan achieve ultra-wideband characteristics only when the radiatorhas the ultra-wideband impedance characteristics. The balancedmicrostrip line in this paper is a kind of TEM wave transmission

line which has ultra-wideband transmission characteristics. Therefore,the ultra-wideband characteristics of the antenna can be analyzedthrough surface current distribution of the transmission line. The

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=10

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=30

Figure 2. Surface current density and reflection coefficient of the

antenna. (a) Surface current density and reflection coefficient atthe feeding point. (b) Influence of angle on antennas reflectioncoefficient.

antenna model which is set up according to the structure in Fig. 1and parameters in Table 1 has been simulated by CST software.The simulated results of the surface current density on the antennaradiation feeding line and the feeding terminal connecting with antennaradiator (shown in Fig. 2(a)) indicate that the surface current density is

a wideband traveling current so that the ultra-wideband characteristicsof the antenna are reflected. Besides, the simulated results of reflectioncoefficient shown in Fig. 2(a) indicate ultra-wideband characteristicsof the antenna (in the work frequency band from 1.64 GHz to 9 GHzreflection coefficient is less than 6 dB the standing wave coefficientis less than 3.0) because the feeding line of the antenna is TEMwave transmission line, and antenna radiator of the antenna can beregarded as the slot line gradually opening, which is also an ultra-wideband transmission line. The whole antenna can be viewed as thestructure of Tapered balance microstrip line-Tapered slot line-Free

space. Such three components of this structure are all ultra-wideband,so the antenna is with the ultra-wideband characteristics.

The value of angle 2 between two branches of the slot linechanging gradually has great effect on the reflection coefficient. Andthe final angle is determined by the simulated results. Antennareflection coefficients with different lapha values are shown in Fig. 2(b).The figure indicates that there exists a best value of to make antennareflection coefficient the lowest. In the design of antenna in this paper,the index of reflection coefficient is firstly satisfied, and it is also the

most important one. Thus, through further simulation, the final angleis = 22.

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i1

i2

i3

i4

x

y

O

2i 3i

2E

3E

r0

2ir

3ir

23

b-2c

d

Figure 3. Distribution of the antenna surface current.

3.2. Directional Radiation Characteristics

For the metal radiator, metal surface current is the main cause of theradiation. In Fig. 3, main surface currents that may generate radiation(named i1 i4) are listed. Among them, currents i1 and i4 distributeon the upper and lower branches of the transmission line, and generallythey hardly generate radiation. The radiation is mainly caused by i2

and i3. The flowing physical length of current i2 and i3 have the samelength of 78.9 mm calculated by the dimension of the mental radiator.Amplitude and phase distributions of the surface current density inthe length at frequency points of 1.7 GHz, 5.0 GHz and 8.5 GHz aresimulated separately (shown in Fig. 3). For the above three frequencies,the electrical sizes of the antenna radiators physical length are 0.45,1.32 and 2.24.

It is found in the simulation that, considering the flowing currentdirection, the amplitude and phase distributions of the two mainradiating currents i2 and i3 are the same. However, the directionsdiffer by 180. Therefore, Fig. 4 just shows the amplitude and phasedistribution ofi2. The amplitude distribution of the current shows thatthere is no wave node in length over one wavelength (at 5.0 GHz and8.5 GHz), and it indicates that the current distributions on the antennaradiator at such three frequencies are all traveling wave distribution.Besides, the current phase distribution also proves the same point.The current phase shifts at these three frequencies on the radiator are174.5, 505.9, and 923.9.

In the calculations of the currents in radiator edge and phases

in different frequencies, we should note that the phases shown inFigs. 4(a)(c) are the main values of the arguments of complex i2, andthe changing range of main values of the arguments is from 180 to

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Figure 4. Amplitude and phase distributions of i2 at three selectedfrequencies.

+180. Therefore, additional integer times of 360 are needed in thephase calculations. For example, the phase is changing consistentlywith no jump in Fig. 4(a). Thus the phase value can be gained as174.5 only by terminal phase subtracting initial phase. And Fig. 4(b)shows that the phase value has one jump and gradually decreases to180, and the changing phase value is 260.95. Then the value jumpsto +180 and gradually decreases to get a phase change of 244.9.Therefore, the total phase change is 260.95 + 244.9 = 505.9 whencurrent i2 working at 5.0 GHz. Similarly, the picture shown in Fig. 4(c)indicates that the phase value has jumped twice and that additionalphase of 360 is needed in calculation. Thus the final calculation resultis 923.9.

Accordingly, this TEM horn antenna can be equivalent to a typicaltraveling wave antenna V-shaped antenna. So the antenna radiationcan be explained by the synthetic model of the two sections of traveling-

wave single-wires which compose the V-shaped antenna (shown inFig. 3).Shown in Fig. 3, the radiation of the antenna is provided by two

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radiating arms supporting currents i2 and i3. Previous simulationresults indicate that the amplitudes and phases distribution of i2 and

i3 along the linear direction are basically the same. Thus, the sameexpression can be used to describe:

I(l2) = I0ejkl2 (1)

In the expression, I0 is the current at y = 0, and =cv

is the slow-wavecoefficient. I(l2) is the current value after the length of l2. And l2 isthe path that i2 flows (l2 the distance between i2 and O point inFig. 3).

y = l2 cos (2)

I

ycos

= I0ejky

cos (3)

According to expression (3), radiation field, generated by currents i2and i3, can be calculated separately. Then by adding them up, theradiation field of the proposed antenna in this paper can be obtained.Note that when calculating the total field generated by stacking, theeffect on the total field generated by the phase changes (caused bythe position) should be taken into full consideration. Because thedirections of the two traveling wave currents are different, instead

of the principle of pattern multiplication, only the method of vectorsuperposition could be used.Detailed solution is shown as follows:1. Radiation field of the micro-unitIn Fig. 3, the radiation field generated by the current micro-unit

i2 (length l2) is:

E2 = j60i2(l2)

ri2sin 2e

jkri2 (4)

The radiation field generated by the current micro-unit i3 (length

l3) is

E3 = j60i3(l3)

ri3sin 3e

jkri3 (5)

The relationship between the locations of the two current micro-unitsi2 and i3 can be expressed by the following formula (6)(14):

yi2 = yi3 (6)

ri2 = r0 y

cos cos 2 = r0

y

cos cos( ) (7)

ri3 = ri2 + (2y tan )sin = r0 ycos cos 2 + 2y tan sin

= r0 y

cos cos( ) + 2y tan sin (8)

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2 = (9)

3 = + (10)

l2 = dy/ cos (11)l3 = dy/ cos (12)

(Ei)x = Ei cos , i = 2, 3 (13)

(Ei)y = Ei sin , i = 2, 3 (14)

The explanations of the above variables are as follows (refer to Fig. 3):

ri2 distance between the current micro-unit i2 and far-fieldobservation point;

ri3 distance between the current micro-unit i

3and far-field

observation point;

r0 distance between O point and far-field observation point;

2 angle between traveling current i2 and i3; angle between r0 and y axis;

2 angle between ri2 and i2;

3 angle between ri3 and i3;

2. The total radiation field x direction component (Etotal)x and ydirection component (Etotal)y on the xy plane (antenna E-plane)

(Etotal)x =

d0

[(E2)x + (E3)x] (15)

(Etotal)y =

d0

[(E2)y + (E3)y] (16)

Finally, the power directivity function on the xy plane of the antenna

is:P() = Re2x + Im

2x + Re

2y + Im

2y (17)

In the expression,

Rex = A1 cos(kdB)A2 cos(kdC) (A1 A2) (18)

Imx = A1 sin(kdB) + A2 sin(kdC) (19)

Rey = A3 cos(kdB)A4 cos(kdC) (A3 A4) (20)

Imy = A3 sin(kdB) + A4 sin(kdC) (21)

A1 =cos sin( )

B (22)

A2 =cos sin( + )

C(23)

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A3 =sin sin( )

B(24)

A4 = sin sin( + )C

(25)

B = cos( )

cos (26)

C = B + 2 tan sin (27)

3. The total radiation field (with only x direction component) onthe yz plane (H-plane of the antenna)

(Ei)x = Ei(i = ), i = 2, 3 (28)

The relationship between the locations of the two micro-units can beexpressed by the following formulas (29)(30):

cos =cos icos

, i = 2, 3 (29)

r0 ri = ycos icos

, i = 2, 3 (30)

is the angle between the directions of viewing direction (r0) andmaximum radiation (+y).

Finally, the power directivity function on the yz plane of theantenna is:

P() =

sin kdBH2

kdBH2

2(31)

In the expression, BH = / cos cos .Based on the above expressions, the power pattern (or the field

strength pattern) of the antenna in xy plane and yz plane can becalculated. Then both of the calculated pattern and simulated one

are drawn in Fig. 5. The conclusions can be drawn as followed: (1)Both of the calculated and simulated results indicate that the antennais directional. The validity of proposed equivalent V-shaped antennais proved. (2) The main lobe of calculated results in the pattern islarger than that of the simulated ones. Especially, the main lobes of Hplane at 1.7 GHz and 5.0 GHz have larger errors because the radiationgenerated by the current (in X axis) on the bottom margin of thetriangular branches is considered in calculation. This current will alsoinfluence the calculated results although its amplitude is small. (3)The calculated and simulated results of the side and back lobe in thepattern differ a lot because the influences of the medium and radiationgenerated by the current at the triangular patches lateral edge are nottaken into consideration. (4) Although the calculated and simulated

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Figure 5. Comparison between the simulated and calculated resultsof antenna pattern.

results have differences, their trends are very anastomosing. Thus, themode of the equivalent V-shaped antenna can well simulate and explainthe characteristics of the antenna in this paper. This mode can alsoexplain similar antennas, for example, Vivaldi antenna, LTSA, etc.

According to the simulated and calculated results, the main reasonthat the antenna can have characteristic of directional radiation is thatthe radiation of traveling wave current could form two main lobes onthe E-plane. The two main lobes will lean to the side lagging of thecurrent phase. Therefore, the main lobe fields of the two traveling

wave currents will be added up to form a new main lobe by makingthe two traveling waves form a V-shape as well as selecting the includedangle properly. Then the directional radiation of the antenna can beachieved.

In addition, the side lobe of the E-plane pattern at higherfrequencies is greater and relatively close to the main lobe becausethere are two E-plane main lobes formed by the radiation of travelingwave current, so the higher the frequency is, the smaller the includedangle between the maximum radiation-directions of the two main lobes

will become. However, after the two traveling wave currents form theV-shaped antenna, each main lobe will form a new one in the far fieldzone, and other main lobes will form side lobes. Thus, an effect similarto the pattern splitting at high frequencies will be generated. As a

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result, the application of this antenna will be limited, and the radiationpattern bandwidth would be less than its impedance bandwidth.

3.3. High-gain Characteristics

Simulation, focusing on influence on antenna gain, is done on the lengthof the dielectric slab in radiation direction. The simulated results arepresented, and ls = 0, 50 mm, 100 mm and 150 mm is taken in Fig. 6.The results indicate that after lengthening the dielectric slab, antennagain is obviously improved. And the length of the dielectric slab andantenna gain are of positive correlation. For the proposed antennain this paper, the value of the length of the dielectric slab is 100 mm

(compared to ls = 0, the gain increases by 3.9 dB, which has the largestdifference in-band). Furthermore, in the simulation process, there isalmost no influence on the antenna wideband with such a method toimprove the gain.

freq.(GHz)

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9

Ls=0mmLs=50mmLs=100mmLs=150mm

Gain

(dB)

Figure 6. The effects of antenna gain with varying length ls of theextended dielectric slab.

(a) Front (b) Back

Figure 7. Prototype of the proposed antenna.

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4. EXPERIMENTAL RESULTS

According to the acquired antenna parameters, prototype of theproposed antenna (shown in Fig. 7) has been manufactured andmeasured in the anechoic chamber with the Agilent E8363B vectornetwork analyzer. Reflection coefficient, radiation pattern and gain areincluded in the measuring content, and the measured results are shownin Figs. 8 to 10. The experimental results show that the reflectioncoefficient of the proposed antenna is less than 6 dB (standing wavecoefficient less than 3.0) in the frequency band of 1.69 GHz, and thereflection coefficient is less than 8 dB (standing wave coefficient lessthan 2.3) in 2.76.8 GHz. This measured results basically accord with

the simulated ones. The value of the measured gain is smaller by12 dB than the simulated one. The measured and simulated resultsof the radiation pattern have slight difference in the main-lobe area,while have a relatively large difference in the side-lobe. In general, theantenna property meets the characteristics of ultra-wideband antenna.And the reasons for the difference between the simulated and measuredresults can be explained as the following: (1) The printed substrateis normal FR4 epoxy resin substrate which has large loss, and theloss tangent is about 102 order of magnitude, much larger than the

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Figure 8. Comparison between the simulated and measured resultsof antenna pattern.

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freq.(GHz)

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Figure 9. The measured resultsof reflection coefficient.

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Simulate d

Measure d

Figure 10. Antenna gain curve.

common Rogers series materials (104 order of magnitude). So theresulted energy loss will lead to the decrease of reflection coefficientand gain. (2) Antenna weld and radiation-pattern test fixture willinfluence the measured results of the side-lobe voltage level. Besides,the measured results of the antenna radiation pattern will be alsoinfluenced by the instability of FR4 substrate.

Note that the reason for the difference between the traditionalV type traveling antennas and the antenna in this paper is that theantenna is printed on the FR4 epoxy resin lamina with dielectric loss(loss tangent tan = 0.025) and that the radiation caused by currentson the radiator part will be absorbed by surrounding dielectric. Thus,the antenna reflection coefficient is decreased, and the ultra wide bandeffect can form. Compared to the matched load with loading lumpedparameters of traditional V type antenna, the antenna in this paper isequal to the matched load with loading distributed parameters. Theresults of doing so will make the proposed antenna similar to axialmode spiral antenna and utilize the radiation of antennas radiator toachieve matching. But it will lead to the decrease of antenna efficiency.

5. CONCLUSION

In this paper, a printed high-gain TEM horn antenna high-gain fedby balanced microstrip line is proposed. The proposed antenna is atraveling-wave antenna and does not introduce lossy matched load.The surface current of the metal antenna has been simulated andanalyzed, and the ultra-wideband characteristics of the antenna areexplained. Moreover, according to the simulation results of the metal

radiator surface current, the V-shaped traveling-wave antenna modelwith equal radiation characteristics to the TEM horn antenna isproposed, and the directivity-function analytical expression of theantenna is deduced to explain the directional radiation characteristics.

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The results also indicate that extending the length in the radiationdirection of the dielectric slab could enhance the antenna gain without

influencing the antenna bandwidth and that this method is a superiorway to obtain high antenna gain. The experimental results showthat the antenna can operate at 1.64 GHz to 9 GHz with the averagegain of 8.2 dB, so this antenna can be widely applied in the UWBfield. Moreover, the balanced microstrip line method can avoid theintroduction of a coaxial line. Consequently, this proposed antennacan be applied to the millimeter-wave and integrated circuit systemfield.

ACKNOWLEDGMENT

The authors would like to express their sincere gratitude to CST Ltd.,Germany, for providing the CST Training Center (Northeast ChinaRegion) at our university with a free package of CST MWS software.

The authors would also like to express their sincere gratitude tothe Fundamental Research Funds for the Central Universities (GrantNo. HIT.NSRIF.2010096) and the funds supported by Heilongjiangpost-doctorial financial assistance (LBH-Z09187).

REFERENCES

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