dual frequency planar microstrip antenna

7/28/2019 Dual Frequency Planar Microstrip Antenna

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Dual Frequency Planar Microstrip Antenna

Satyajit Chakrabarti

SAMEER Kolkata Centre, Plot-L2, Block-GP, Sector-V, Salt Lake Electronics Complex,

Kolkata 700091, India,

(R&D Laboratory under Dept. of Information Technology,Ministry of Communication and Information Technology, Govt. of India)

Ph: 91-33-2357-4875, Fax: 91-33-2357-4875

Email : [email protected]

Abstract- In this paper a simple technique to develop a two layer

dual frequency antenna with identical polarization at two far

apart frequencies has been presented. Dual rectangular

resonator proximity coupled planar microstrip antennas has

been designed, fabricated and tested. The design optimisation

was carried out using commercial MoM based simulation

software. The patch width, intermediate spacing between the

coplanar patches are optimized by successive simulations usingthe commercial software. Measured gain at both the frequencies

is over 6dBi. Measured results are also presented.

I. INTRODUCTION

Microstrip patch antenna are popularly used in

communication systems for their well known attractive

features, such as low cost, light weight, compact and can be

integrated with planar microwave circuits. Besides these

microstrip antennas can be designed to yield required

radiation characteristics, such as gain, beamwidth, side lobe

level, boresight or squinted beam etc. But the major

disadvantage of the microstrip antenna is the inherentlynarrow bandwidth (1.5-2 %). Nevertheless the compactness

of such antenna has made it an indispensable part of modern

communication circuits. Frequently the communication links

use transmission and reception frequencies which are farapart. Apparently this needs broadband antenna. Use of

thicker dielectric substrate can increase the bandwidth (3-4

%) at the cost of modal purity. Electromagnetically coupled

patch (EMCP) can yield wider bandwidth (8-10 %).

Aperture coupled patch antennas are popularly used to

generate even higher bandwidth (20-50 %). In applicationswhere the transmission and receiving frequencies are too far

apart, none of the above techniques are suitable. A valid

alternative to the total broadening of the bandwidth is the use

of dual frequency antenna. The two antennas can be

circularly polarized or linearly polarized. Dual band

microstrip antenna which allow independent frequencyselection, have the most design utility.

Dual frequency antennas exhibit a dual resonant behaviour.

In applications where the transmission and receiving

frequencies too far apart, dual frequency patch antenna can

avoid the use of two different antennas. Dual frequency

patch antenna is very convenient in terms of space and

overall cost. The antenna should offer good impedance

matching at the two frequencies individually. Once the

impedance matching is achieved the antenna exhibits almost

similar radiation characteristics. Despite of the interesting

features, dual frequency antenna has got limited attention

probably due to the complexity in the feed structures.

Dual frequency antenna can be realized in number oftechniques [1-9]. Good overview are given by Maci and

Gentili [1]. The first group of the dual frequency antenna is

categorized as orthogonal mode dual frequency patch

antenna. This type of antenna uses first resonance of two

orthogonal dimensions of the patch i.e. TM100 and TM010

modes. One can choose the patch dimensions a and b to

produce the desired frequencies. The position of the feed

point is optimally decided which simultaneously matches the

two modes. These antennas can utilize single or dual feeds

[2-3]. This technique generates two cross-polarized

frequencies. The second group of the dual frequency antenna

is categorized as multi-patch dual frequency antenna. This

type of antenna consists of two radiating patches of different

dimensions in multi-layer or single layer. This technique

generates two frequencies of identical polarizations [4 -7].

The third group of the dual frequency antenna is reactively

loaded dual frequency patch antenna. This technique was

first introduced by Richards et. al. [8], where an adjustable

co-axial stub was used. In the last category of dual band

microstrip antenna, a broad side radiation pattern with

identical polarization is realized where a single patch is

driven in TM100 and TM300 modes. In this category the upper

frequency should be approximately 3 times the lower

resonant frequencies.

In this present paper, design optimisation technique torealise single layer dual radiator dual frequency microstrip

patch antenna has been described. The design was optimised

by successive simulations using commercial MoM based

simulation software, IE3D. The antenna has been fabricated

and tested. Measured results are also presented.

II. DESIGN OF THE ANTENNA

A two layer proximity coupled dual resonator dual

978-1-4244-4819-7/09/$25.00 2009 IEEE


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frequency microstrip rectangular patch antenna is our

consideration. The top layer consists of two coplanar

rectangular microstrip patches of different dimensions with

intermediate spacing (XL+XU) between them. The patches

are excited by a microstrip feed line as shown in fig-1. These

two rectangular patch resonators are in the same plane. This

design technique provides better control of the desired upper

and lower resonant frequencies and also provide goodimpedance matching. The width of the individual rectangular

patches helps to achieve the desired impedance matching.

The lower layer consists of microstrip feed line and the feed

line extends approximately upto the centre of the microstrip

patches. The rectangular patches are thus edge fed. The two

layers are kept one-atop-another without any air spacing. The

matching depends on the patch offsets from the centre of the

feed line, width of the individual patches, depths of the feed

line beneath the patch. Judicial selection of these parameters

decides the optimum impedance matching.

Fig. 1(a). Top view of two-layers dual frequency microstrip antenna.

Fig. 1(b). Side view of two-layers dual frequency microstrip antenna.

III. SIMULATIONS RESULTS

The initial lengths of the rectangular patches are estimated

from the design equation, based on cavity model analysis.

First we have studied the matching [9] of larger patch with

patch offset (XL) from the centre of the feed line in absence

of the other patch. The variation of the return loss is shown

in fig-2. As the slot offset is increased, the matching

improves monotonically upto certain patch offset at which

best matching is achieved. Beyond this particular patch

offset, the matching degrades monotonically. This gives the

near optimum offset for the first patch. The resonance

frequency remains fixed at 3.93 GHz for different slot

offsets.

Now we have studied the matching of smaller patch with

patch offset (XU) from the centre of the feed line in absence

of the larger patch. The variation of patch offsets on thematching is similar to that of first patch. This determines the

near optimum offset for the second patch. But unlike, the

resonance frequency shifts downward by about 10MHz at

higher slot offset. The variations of the patch offsets on

matching are shown in fig-2.

Next we have decided the patch offsets in presence of

both the patches. The moment the second patch is

introduced, the resonance frequency shifts downward by

about 5 MHz. From this study the optimum patch offsets are

achieved.

-40

-35

-30

-25

-20

-15

-10

-5

0 0.005 0.01 0.015 0.02 0.025

Normalised Slot Offset

S11

(dB)

XUXL

Fig. 2. Effect of patch offsets on matching of the antenna.

TABLE 1

SIMULATED RETURN LOSS FOR THE ANTENNA IN PRESENCE OF BOTHPATCHES FOR DIFFERENT PATCH OFFSETS

For Lower Resonance

Frequency

For Higher Resonance

Frequency

XL FL(GHz)

S11 (dB) XU FU(GHz)

S11 (dB)

0.0117

3.925 -21.7 0.0024 4.86 -24.5

3.930 -22.6 0.0048 4.85 -18.4

3.930 -23.3 0.0072 4.85 -15.6

0.0097 3.930 -16.6

0.0024

4.86 -21.3

0.0117 3.930 -21.7 4.86 -24.5

0.0136 3.930 -17.8 4.86 -26.3

.0107 3.930 -24.1 0.0024 4.86 -22.9

Keeping the patch positions fixed with optimum offsets,depth of the feed line beneath the patch was varied. The

study shows that matching has significant dependencies on

the depth (D0) of the feed line beneath the patch. This

dependency is plotted in fig-3. For optimizing performance

of the antenna feed-line depth beneath the patches should

also be chosen judiciously. It suggests that for the larger

patch the feed line extending little more than 50% of the

patch length gives good matching. For the smaller patch, the

feed line depth of 50% of the patch length yields reasonable


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matching. The length of the feed line outside the patch does

not significantly affect the matching of the antenna.

-27

-25

-23

-21

-19

-17

-15

-0.06 -0.04 -0.02 0 0.02 0.04 0.06

Normalised Feed Depth

S11

(d

B)

FL

FU

Fig. 3. Effect of feed line depth beneath the patches on matching of the

antenna (XL= 0.0107L, XU = 0.0024U) .

Variation of the simulated boresight gain with frequency isshown in fig-4. The simulated gain at 3.93 GHz is around 6.9

dBi and at 3.86 GHz is around 6.2 dBi. The width of patcheshave significant role on determining the resonance frequency

as well as on the matching of the antenna. As the widths of

the patches are increased (one at a time), both the resonance

frequency shifts downward, as shown in fig-5. When the

larger patch width is increased from 0.19 to 0.35 the, thelower resonance frequency decreases about 100 MHz.

Likewise, the lower resonance frequency decreases around

115 MHz, when the smaller patch width is increased from

0.24to 0.385.

Fig. 4. Simulated gain characteristic of the dual frequency

antenna

(XL = 0.0107L, XU = 0.0024U).

When the width of the one patch is increased, the

corresponding resonance frequency shifts downward, while

the other resonance frequency remains unchanged. The

variation of return loss with patch width is given in fig-6. As

the patch width is increased, initially the return loss improves

upto certain patch width and then degrades monotonically.

These behaviour implies the existence of optimum patch

width. While varying the width, the offset of the patch edge,

are kept unchanged. Interestingly, variation of the width of

the any particular patch neither affects the other resonance

frequency nor its impedance matching.

3.8

4

4.2

4.4

4.6

4.8

5

0. 175 0. 2 0. 225 0. 25 0. 275 0. 3 0. 325 0. 35 0. 375 0. 4

Normalised Patch Widt h

Frequency

(GHz)

FU

FL

Fig. 5. Simulated variation of resonance frequencies with normalised

patch width.

-35

-30

-25

-20

-15

-10

0 .1 75 0 .2 0. 22 5 0 . 25 0. 275 0 .3 0. 325 0. 35 0 .37 5 0 .4

Normalised Patch Width

S11

(dB)

FU

FL

Fig. 6. Simulated matching dependency of the antenna with

normalised patch width.

IV. MEASURED RESULTS

The antenna was fabricated on 31mil RT Duroid substrate

of relative dielectric constant 2.2 with the optimised

dimensions. The two layers of the antenna were stacked one-

atop-another without any air gap. The integrated antenna is

tested. Measured return loss characteristic is shown in fig-7,

which is in very close agreement with the simulated return

loss.

Fig. 7. Measured return loss characteristic of the dual frequencyantenna.


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Measured radiation pattern is shown in fig-9 at 4.082 GHz

and beamwidth is found to be 72 in H plane and be 76 inE plane. The measured power levels at E and H plane differs

less than 0.1dB.

Fig. 8. Measured radiation pattern of the antenna at lower

resonance frequency.Fig-10 shows the measured radiation pattern at 5 GHz.

Measured beamwidth is found to be 76 in H plane and 86

in E plane. The measured peak power level difference

between E and H plane is less than 0.1dB. The beamwidth in

H plane is narrower due to the presence of other patch in

that plane. Slight asymmetry of the main beam pattern at

both frequencies is attributed to fabrication error in the

antenna and improper assembly.

Fig. 9. Measured radiation pattern of the antenna at upperresonance frequency.

Measured gain at 4.082GHz is approximately 7.3 dBi and

at 5 GHz is 6.5 dBi. Thus, the measured gains are very close

to the simulated values. Measured main beam in H-plane is

tilted by nearly 2-3 because of the presence of the other

patch.

V. CONCLUSIONS

Design optimisation technique to realise dual frequency

microstrip patch antenna has been described. The technique

gives better and easier control of the patch parameters to

achieve the desired upper and lower resonance frequency.

This technique is particularly suitable where two discrete

frequency with arbitrary band separation is desired.

Measured frequencies are slightly higher than the simulated

resonant frequencies. This is attributed to the inaccuracies in

fabrications and assembling of the antenna. Due to overremoval of copper material, the patch length and width have

decreased which has shifted the resonance frequencies. Drill-

bit with thinner diameter could have been used for

fabrication (etching of the patches) with better accuracies,

which was not available at the time fabrication. The main

beam peak is slightly squinted ( 3 ) from boresight due to

the presence of the other patch in proximity. Since the

antenna resonates at two narrow band frequencies which are

far apart, the electromagnetic spectrum at the undesired

frequencies is protected. The experimental results are in very

close agreement with the theoretically predicted gain,

bandwidth. These prove the validity of the design technique.

Thus this antenna can function as an alternative to large

bandwidth planar antenna.

ACKNOWLEDGMENT

The author is indebted to Dr. A. L. Das & Mr. D. Dan for

their valuable suggestions during the development. The

author is grateful to Mr. P. P. Sardar and Mr. S. Basu fortheir measurement support during the development and

performance evaluation of the antenna. The author is also

grateful to Mr. S. Mondal and Mr. I. Paul for their

mechanical Support.

REFERENCES

[1] S. Maci, and Biffi gentili, "Dual Frequency Patch Antennas,"IEEE

AP Magazine, Vol. 39, No. 6, 1997, pp.13-20.

[2] J. S. Chen, and K. L. Wong, "A Single Layer Dual- FrequencyRectangular Microstrip Patch Antennas," Microwave and optical

Technology Letters, Vol. 11, No. 2, 1996, pp.38-84.

[3] Y. M. Antar, A. I. Ittipiboon, and A. K. Bhattacharyya, "A DualFrequency Antenna using a Single Patch and an Inclines Slot,"

Microwave and optical Technology Letters, Vol. 8, No. 6, 1995,

pp.309-310.[4] S. A. Long, and M. D. Walton, "A Dual Frequency Stacked Circular

Disc Antenna,"IEEE AP-S, Int. Symp. Digest,, 1978, pp.260-263.

[5] Q. R. Lee, and K. F. Lee, " Experimental study of the two layerelectromagnetically coupled rectangular patch antenna,"IEEETtran.

AP, Vol. 38, 1990, pp.1298-1302.

[6] A. Sabbin, " A new broadband stacked two layer microstripantenna,"IEEE. AP-S,, Int. Symp. Digest,, 1964, pp. 63-66..

[7] C. H. Chen, A. Tulintseff, R. M. Sorbello, Broadband two layer

microstrip antenna, IEEE AP-S, Int. Symp. Digest, 1984, pp 251 -254.

[8] W. F. Richards, S. E. Davidson, S. A. Long, Dual Band Reactively

Loaded Microstrip Antenna, IEEE Transaction on Antennas andPropagation, Vol 33, No. -5, 1985, pp 556-560.

[9] C. S. Lee, V. Nalbandian, Impedance Matching for a Dual-

Frequency Microstrip Antenna with an Air Gap, IEEETransaction on Antennas and Propagation, Vol 41, No. -5, 1993,

pp 680-682.

dual frequency planar microstrip antenna

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