ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program

(973) under Grant 2009CB320204 and the National Natural Sci-

ence Foundation under Grant 60821062 of China.

REFERENCES

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notch for ultrawideband filter application, IEEE Antennas Wireless

Propag Lett 7 (2008), 210–212.

VC 2009 Wiley Periodicals, Inc.

GAIN-ASSISTED METAMATERIALEMBEDDED WITH GAIN ELEMENTS

Tao Jiang,1,2 Liang Peng,1 Jiangtao Huangfu,1 Zhiyu Wang,1,2

Yu Luo,1,2 Hongsheng Chen,1,2 and Lixin Ran11 Department of Information and Electronic Engineering, ZhejiangUniversity, Hangzhou 310027, China; Corresponding author:ranlx@zju.edu.cn2 The Electromagnetic Academy at Zhejiang University, ZhejiangUniversity, Hangzhou 310027, China

Received 20 April 2009

ABSTRACT: In this letter, an active metamaterial sample embeddedwith miniature monolithic microwave amplifiers is designed,

experimentally realized, and measured. Experiments show that by properdesign and direct current (DC) bias, the metamaterial sample providesalmost linear amplification to an electromagnetic incidence, which can

be used to compensate the loss inherent in traditional passivemetamaterial and magnify the signal entered the metamaterial. This

property would have important potentials in promoting the furtherresearches of metamaterial-based applications. VC 2009 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 52: 92–95, 2010;

Published online in Wiley InterScience (www.interscience.wiley.com).

DOI 10.1002/mop.24871

Key words: metamaterial; lossless; gain-assistance; nonlinearity

1. INTRODUCTION

Metamaterial is a sort of artificial structural composite possess-

ing extraordinary electromagnetic (EM) properties. By appropri-

ate design, metamaterial can exhibit permittivity and/or perme-

ability with arbitrary value of positive and negative [1–3]. Such

property is difficult to found in normal, naturally occurring ma-

terial and has fascinating applications like perfect lens [4],

highly directive antennas [5], and cloaks of invisibility [6, 7],

which have attracted much attentions in past few years. Never-

theless, the actual applications of metamaterial deeply rely on

its EM performance, in which loss is one of the most important

factors influencing the future applications of metamaterial. As

we have known that even with small loss, the performance of

the perfect lens and cloaks will be greatly degraded.

Upto now, the fabrication of traditional metamaterial is based

on the combination of sub-wavelength electric and/or magnetic

resonant structures, yielding effective constitutive parameters

with Drude or Lorentz model dispersion characteristics. The

strong dispersive nature of metamaterial theoretically determines

that the loss is inherent and cannot be eliminated completely. In

the past years, efforts have been made to decrease the loss of

metamaterials. For example, in Ref. [8], the EM-induced trans-

parency is used in fabricating a low-loss metamaterial, in Ref.

[9], displacement current resonance are used to avoid the metal

loss in high-frequency range, and in Ref. [10], hot-press tech-

nique is used to decrease the internal scattering inside and

between the unit cells to obtain lower loss. However, till now, it

is still difficult to find a metamaterial with satisfactory loss per-

formance to realize perfect metamaterial-based components.

Figure 5 Measured radiation patterns on E-plane and H-plane for pro-

posed antenna (a) E-plane (yz-plane) and (b) H-plane (xz-plane)

92 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 DOI 10.1002/mop

To solve the loss issue, the idea of introducing gain into

metamterial has appeared. In a pioneering work reported in Ref.

[11], radio frequency (RF) amplifiers are inserted in a pair of

sensing/driven elements (loops or dipoles), and by controlling

the gain and phase of the amplifiers, the loss cancellation and

dispersion reduction are observed. However, in Ref. [11], the

amplifiers are either a desktop one or in a module shape, and

cables have to be used to connect the elements and amplifiers,

which make it difficult to be used in the construction of a real

metamaterial. In this letter, we realize a real effective metamate-

rial slab by directly embedding miniature monolithic microwave

amplifiers into the sub-wavelength unit cells, measure the EM

properties and directly present the amplificatory effect. We

show that by proper design and bias, such active metamaterial

slab behaves a nearly linear power gain in a specific frequency

band to a small EM incidence. Self-oscillation issue is first dis-

cussed, to which attentions should be paid in the design of the

active metamaterial with gain. This work implies that the real-

ization of loss free and gain-enhanced metamateiral is possible.

2. EXPERIMENTAL RESULTS AND ANALYZATION

Figure 1(a) shows the structure and a realization of the unit cell

used to fabricate the metamaterial sample in this letter. In the

realization, four metal strips, marked A, B, C, and D, are printed

on a 1-mm-thick F4 type substrate (the dielectric constant is

2.55), and a compact monolithic wideband microwave amplifier,

Avago Technologies’ MGA-53543 with a working band upto

6 GHz, is mounted in the center of the four metal branches. The

dimensions of the unit cells are l ¼ 18.6 mm, h ¼ 6.8 mm, g ¼0.8 mm, w ¼ 1.2 mm, respectively. The connections of the metal

branches and the amplifier are carefully considered to avoid the

self-oscillation caused by the strong in-phase coupling: branch A

is connected to the input pin of the amplifier, branch D is con-

nected to the output one, and branches B and C are attached to

the ground pin and ground plane simultaneously, and by supply-

ing a direct current (DC) power, the amplifier is biased through a

choke inductor with 3.9-nH inductance to branch D, as shown in

Figure 1(a). For an EM incidence, branch A collects the power,

amplified by the amplifier, and radiated to the outer space again

by branch D. We choose branch D, other than branch B, to be the

radiating branch is to minimize the coupling between the input

and output of the amplifier to decrease the possibility of genera-

tion of self-oscillation. We will discuss this issue later.

Figure 1(b) shows the configuration of the experimental

setup. A slab-like metamaterial sample consists of five pieces of

the unit cells, spaced by 18 mm, and aligned along the y direc-

tion, is placed in the central segment of self-made rectangular

waveguide with the dimensions denoted in Figure 1(b). Small

holes are drilled on the top and bottom covers of the waveguide,

through which the ground reference and power supply from a

DC source are attached to branches B, C, and D by thin wires.

Basically, the unit cell shown in Figure 1 is an active oscilla-

tor composed of a gain element (the amplifier) and a passive

resonator (the split-wire dipoles), therefore, self-oscillation eas-

ily occurs if the gain and the feedback phase between the

branches satisfy the oscillation condition. In our case, we

attempt to introduce gain to the metamaterial, but do not hope

this is due to the local self-oscillation. Therefore, before meas-

uring the EM properties of the metamaterial, a calibration to the

DC bias voltage of the amplifier is carried out first, to make

sure that self-oscillation does not exist at a specific bias, such

that the possible gain measured in the later measurement is due

to the amplificatory effect we expect. To do this, a monochro-

matic wave of 2.5 GHz with 0-dBm power generated from a

Vector Signal Generator (Agilent E8267C) is fed into the wave-

guide via a coaxial-waveguide adapter, whereas at the other end,

a Spectrum Analyzer (HP 8350B) is attached to a waveguide-

coaxial adapter to measure the spectrum of the wave coming

through the metamaterial sample for different bias voltages. We

find that when the bias voltage is higher than 0.9 volts, there is

an erratic spectrum even when the incidence is turned off,

implying that the self-oscillation has been excited. When the

bias is 0.9 volts or lower, a regular spectrum with multiple har-

monics appears, and the harmonics disappear with the shut-

down of the bias. Figure 2 shows the spectrum when bias is 0.9

volts and 0 volt, respectively. We see that without the bias, the

spectrum shows only a 2.5-GHz monochromatic component,

whereas with the bias, in addition to a 2-dB larger fundamental

component, there appear the second to sixth order harmonic

components. We see that when the DC bias is added, the

Figure 1 Schematic picture of experimental setup. (a) Schematic pic-

ture of the unit cell and practical structure. (b) Experimental setup. A

rectangular waveguide with five pieces of the unit cells aligned in y

direction. [Color figure can be viewed in the online issue, which is avail-

able at www.interscience.wiley.com]

Figure 2 Spectrum of harmonic generation measured in the rectangu-

lar waveguide for the passive and active metamterial, with 2.5 GHz,

0 dBm monochromatic input wave. [Color figure can be viewed in the

online issue, which is available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 93

spectrum changes from a linear one to a nonlinear one. How-

ever, since the second-harmonic suppression referring to the fun-

damental component in the nonlinear spectrum is lower than

�30 dB, we may regard that the amplifiers primarily work in a

linear amplifying status without self-oscillation. We will use this

bias in the following experiments.

As illustrated in Figure 1(b), in the transmission experiment,

a wide-band swept signal from DC to 3.5 GHz generated from

one port of an Agilent vector network analyzer (VNA, Agilent

8722ES) is imported into the waveguide, and the transmitted

power is received by the other port, to measure the reflection

and transmission simultaneously. In the experiment, we select

even a small power of �18 dBm (other than the 0 dBm in pre-

vious measurement) to further decrease the influence of the non-

linear harmonics; therefore, in later calculations, only fundamen-

tal component is concerned without bringing too many errors.

From figure 12 of the MGA-53543’s datasheet, we estimate that

at 0.9-volt bias, MGA-53543 provides a 1-dB compression point

(P1dB) bellow �10 dBm; therefore, the �18-dBm incidence can

ensure a linear amplification. The experiment is performed in

two steps: first, we measure the reflection and transmission with-

out the bias, where the metamateiral behaves as a passive one.

Then, we turn on the bias, where the metamateiral behaves as

an active one, measure the reflection and transmission again.

The results are shown in Figure 3.

From the reflection coefficients in Figure 3(a), we see that

the working band moves to the lower frequencies in the active

case, which is because the existence of the biased amplifier

introduces additional impedance to the unit cell and hence influ-

ences the overall resonance. From the transmission coefficients

in Figure 3(b), we see that in a band just higher than the cut-off

frequency of the waveguide, i.e., from 1.5 to 2.5 GHz, the trans-

mission in active case is obviously higher than that in passive

case. To evaluate if there is a gain between the input and the

output, we define a relative power gain Gp as follows:

Gp ¼ Pout=Pin ¼ ðS212Þ=ð1� S112Þ (1)

where S21, S11 are linearized S parameters measured by the VNA.

As mentioned earlier, since the power of harmonics is much less

than the fundamental component’s with this specific bias and small

incidence, Eq. (1) approximately calculates the ratio of transmitted

power over the power actually entering the metamateial slab. The

Gp is calculated from 1.5 to 3.5 GHz and is displayed in Figure 4.

We see that from 1.5 to 2.5 GHz, the Gp varies mainly between 5

and 10 dB, which is in accordance with the 8-dB gain described in

the MGA-53543’s datasheet at 0.9-volt bias.

Although Figure 4 shows the existence of the gain provided by

the active metamaterial, there is still an issue of loss. It is seen that

the loss of this metamateiral is large, such that even with the gain,

the S21 þ S11 þ Gp is still less than 0 dB. Further improvements

can be done through two possible approaches: one is to optimize

the passive unit and the impedance matching between the passive

pattern and the amplifier, to enhance the overall quality factor of

the active cell; another is to eliminate the self-oscillation to supply

higher bias voltage, and consequently obtain higher gain provided

by the amplifier. For traditional materials, the constitutive rela-

tions are usually D ¼ (er þ iei)E, B ¼ (lr þ ili)H, with positive

imaginary parts of the permittivity and permeability due to the

loss. Once we have obtained a power gain, or negative loss, such

that S21 þ S11 þ Gp > 0 dB, we may expect that negative imagi-

nary parts of ei and/or li can be retrieved, just as illustrated in Ref.

[11]. Till then, we can have loss free or gain-enhanced metamate-

rial, which would be helpful to realize important applications,

such as perfect lens, cloaks, and so on.

3. CONCLUSIONS

In summary, an active metamaterial sample embedded with min-

iature monolithic microwave amplifiers are designed, experimen-

tally realized, and measured. Experiments show that by proper

design and bias, the metamaterial sample provides almost linear

amplification to an EM incidence, which can be used to com-

pensate the loss inherent in traditional passive metamaterial and

magnify the signal entered the metamaterial. This property

would have important potentials in promoting the further

researches of metamaterial-based applications.

Figure 3 The experimental results of reflection and transmission for

the passive and active metamterial with no bias and 0.9 V bias. [Color

figure can be viewed in the online issue, which is available at

www.interscience.wiley.com]

Figure 4 Power gain of the passive and active metamateiral with no

bias and 0.9 V bias. [Color figure can be viewed in the online issue,

which is available at www.interscience.wiley.com]

94 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 DOI 10.1002/mop

ACKNOWLEDGMENTS

This work is sponsored by the NSFC (No. 60531020, 60671003,

60701007, and 60801005), 863 Project (No. 2009AA01Z227), the

NCET-07-0750, the ZJNSF (No. R1080320 and Y1080715), and

the Ph.D. Programs Foundation of MEC (No. 20070335120 and

200803351025).

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6. D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry,

A.F. Starr, and D.R. Smith, Metamaterial electromagnetic cloak at

microwave frequencies, Science 314 (2006), 977.

7. S.A. Cummer, B.-I. Popa, D. Schurig, D.R. Smith, and J.B. Pendry,

Full-wave simulations of electromagnetic cloaking structures, Phys

Rev E 74 (2006), 036621.

8. P. Tassin, L. Zhang, Th. Koschny, E. Economou, and C. Soukoulis,

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induced transparency, Phys Rev Lett 102 (2009), 053901.

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of standard dielectric resonators, Phys Rev Lett 98 (2007), 157403.

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VC 2009 Wiley Periodicals, Inc.

A NOVEL MONOPOLE ANTENNA WITH ASELF-SIMILAR SLOT FORWIDEBAND APPLICATIONS

Kun Song, Ying-Zeng Yin, and Li ZhangNational Key Laboratory of Antennas and Microwave Technology,Xidian University, Xi’an, Shaanxi, 710071, People’s Republic ofChina; Corresponding author: songkun@mail.xidian.edu.cn

Received 21 April 2009

ABSTRACT: In this article, a printed flame-shaped monopole antenna

with a self-similar slot for wideband application has been presented andinvestigated. The wideband characteristic is easily achieved by inserting

a self-similar slot in the flame-shaped radiator. The effect of the slot isinvestigated. We discover that the slot can be effectively used forbandwidth enhancement, while the position of the slot has a negligible

effect on the performance of the proposed antenna. Experimental resultsshow that the proposed antenna obtains a bandwidth from 2.65 to over

12 GHz with VSWR < 2. VC 2009 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 52: 95–97, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.24873

Key words: monopole antenna; self-similar slot; wideband

1. INTRODUCTION

In recent years, the increasing demand for wireless communication

services stimulates the need for antennas capable of operating at a

wide frequency range. To satisfy such a requirement, various

printed monopole antennas have been studied and many antenna

configurations have been presented with broadband characteristic.

It has been regarded as mostly suitable for these applications due

to their simple structures, low cost, and ease of construction. The

impedance bandwidth is continuously widened by using various

techniques and configurations [1–8]. Using a beveling technique

[1], multifeeds [2–3], and tapered CPW-fed [4] can get to control

and enhance the impedance bandwidth. Many configurations, such

Figure 1 Geometry of the proposed monopole antenna (Units: mm).

[Color figure can be viewed in the online issue, which is available at

www.interscience.wiley.com]

Figure 2 A photograph of the proposed monopole antenna. [Color fig-

ure can be viewed in the online issue, which is available at

www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 95