The field of filter engineering has a history of 80-90 years. Filter is the most important part of all transmitter and receiver section, where selection and rejection of a particular frequency or band of frequency is a major concern. We cannot assume any frequency operated circuit without filter. Different research and development in the field of filter started from 1937. Apart from this, most of the modern concept and theory of filter came into existence during the period of World War II. In the designing of low pass filter the conventional technique is stepped impedance and open stub [1-3]. But the major drawback is that the roll off rate is not very sharp in conventional design of filter. It falls gradually instead of sharply. So the major concern is the rejection characteristics of such filter. To overcome this problem we can sacrifice the pass band characteristics in traditional filter by increasing the number of sections. But due to this method physical size also increases which is not good in PSB and VLSI technology. It is also combatable to Printed Circuit Board (PCB) and MMIC technology [4-7].

One of the many technologies in metamaterial, is the use of Hex-Omega structure [8] in the base of microstrip line to enhance the stop band characteristics and selectivity of microstrip LPF. In the environs of response, the use of metamaterial in the shape of hexagonal structure produce a sharp cutoff. By implementing the hex omega and other different predefined structure at the ground of basic microstrip low pass filter, it is possible to implement and realize a filter which has better pass band characteristics that does not contain much ripples, as well as carry a sharp and steep roll-off rate without compromising on the size of the circuit.

Metamaterial is a man made substance, which has some special characteristics which is not found in other available natural substances. Vassalage explained the theory of metamaterial and said that in metamaterial the direction of phase variation and actual direction of wave (pointing vector) is mutually opposite to each other [9]. He called metamaterial a left hand substance. From last few decades, metamaterial in microstrip technology act as an integral part of many designs.

In the designing of low pass filter, reduced size of low pass filter with better performance is the major concern in the area of microstrip technology. Low profile with improved filter characteristics is intensively investigated in the field of low pass filter with different alternative [10]. Metamaterial (MTM) technology was introduced by a Russian scientist Victor Veselago in 1967. Low pass filter designed and implemented on microstrip transmission line is compact in nature, and accept all the advantage of microstrip technology. Structure form on the patch of microstrip is designed on the basis of several important laws and procedures like transformation from prototype, Richards's transformation, and Kurda's identities. The pass band and stop band characteristics depend upon the mathematical calculation which is done during the design procedure. Length and width of microstrip line are also calculated by some standard formulas and procedures. The physical length actually reflects the characteristics of prototype component parameter value in a LC low pass network. The general structure of microstrip filter is shown in Figure 1. Distance of ports from the main structure boundary play an important role on the performance of low pass filter. Pass band gain, insertion loss, ripples are the main parameters of filter, which can be adjusted by varying the physical size of patch and operating frequency range or both simultaneously, according to application.

Figure 1. General structure of a microstrip filter

1.1 Lumped Element LC Filter

In the frequency range of MHz to GHz LC tank resonator circuit provides proper frequency selection. The disadvantage of such lumped filter is low quality factor and large size. Also, as frequency decreases from KHz range to few Hz size inductor becomes bulky. So, low frequency filter is designed using crystal to overcome this problem, which has great frequency stability and exhibit both series and parallel resonance.

1.2 Planar Filter

In comparison of size and performance, microstrip transmission line and coplanar waveguide (CPG) are also good resonators than lumped element filters. The steps and procedure to make a microstrip circuit is quite similar to manufacturing of PCB. Low pass filter designed using planar technology consist of high quality factor and low loss tangent of dielectric material for substrate.

1.3 Bandwidth

Bandwidth is basically defined as the range of frequency over which magnitude falls 0.707 times of the maximum magnitude. In this range, signal is approximately the exact replica of input in terms of magnitude. In typical low pass filter, lower 3db cutoff frequency is approximately near to zero.

Using metamaterial it is possible to achieve the negative permeability, negative permittivity and negative refractive index at the same time [11]. Victor Veselago is known as the father of this special material. Characteristics of magnetic and electric field is different in metamaterial as compared to other known conventional substances. Veselago first found that in metamaterial, the value of electric permittivity and magnetic permeability is less than zero (negative) [13]. This field is characterized by the left hand rule. This is the reason that metamaterial is known as left (LH) Metamaterial [12]. Four different states of this is shown in Figure 2. From Figure 2. it is clear that, for the reflection of wave, it is necessary that wave must fall into the plasma from the air. So in metamaterial negative refraction occurs [14]. Veselago explained about the occurrence of metamaterial in his paper work “The electrodynamics of substance with simultaneously negative permittivity and negative permeability”. So now a-days left/right hand material is widely used in transmission line to improve the filter performance [15]. Metamaterial structure having Split Ring Resonators (SRRs) are used to produce negative permeability and thin wire elements are used to to generate negative permittivity. SRR is a novel design consisting of two concentric rings with a split on each ring. The structure is called resonator since it possess a certain magnetic resonance at a particular frequency. Split ring resonators can result in an effective negative permeability over a particular frequency region. The SRR structure is made by two metallic rings having a common center with a split on opposite sides. This acts as an LC resonator with distributed inductance and capacitance that can be energized by a alternating external magnetic field component of the normal direction of resonator.

Figure 2. ε-μ Variation

2.1 Metamaterial having Negative Refractive Index

Till now, it is clear that metamaterial contain negative refractive index. It means that it does not follow snell's law, and actually its characteristics is just opposite to snell's law, so it is called material having negative index, which is shown in Figure 3.

Figure 3. Normal and negative refraction

The effect of negative refractive index is that the direction of group velocity is exactly opposite to that of phase velocity which is also the actual direction of flow of energy [16].

2.2 Developments in Metamaterials in term of filter designing

• Negative permittivity can also be obtained using an array of metallic wire. This theory was explained by Pendry in the year 1996-1999 [17]. Also negative permeability can be obtained by metallization of split ring [18]. Later Smith invented that both can be simultaneously generated in 2000 [19].
• Ground can be defected in the shape of flower, in which ground plane is etched off on the backside of metallic plate in microstrip line. This shape has 5 different Defected Ground Structure (DGS) units.
• With the help of metamaterial we can also implement tunable filter having dual negative refraction, composed of ferrite slab and metallic resonators including SRR. These are use to provide two different resonant frequency. So by substituting ferrite based material like Yttrium Iron Garnet (YIG), the performance of filter is improved.
• Meta material in shape of hex-omega structure is widely used now-a-days to improve the roll off rate at cutoff frequency. In low pass filter, characteristics of group delay also vary in hex omega structure; in pass band it is constant. Because off this arrangement, zero delay filters is obtained which has enhanced signal integrity. C and X band filter implemented using hex-omega structure possess sharpest roll off rate among available filter at, same frequency range.
• Another classical advantage of metamaterial technique is compatibility with left and right handed transmission line (CR/LH TL). CR/LH transmission line contains two desired and two predefined impedance at two particular frequencies, it also has nonlinear phase characteristics. Due to this special property the physical size of structure is reduced as compared to conventional transmission line. Apart from filter CR/LH transmission lines, SRRs can be used for design of both active and passive device like phase shifter, microwave amplifier, couplers, and antennas. The CR/LH transmission line has wide range of application and low loss [20].
• To reduce the size of resonator (hence filter) one of the most popular method is of Stepped Impedance Resonator (SIR), it reduces size without reducing the quality factor of unloaded Q and also controls the spurious noise at different frequency [21] .

Progressive development in the field of metamaterial can be summarized as-

1967- Theoretically proposed by Vesalogo.

1999- First negative Mu material by Pendry

2000- First Metamaterial by Smith

2003- Transmission approach by Caloz

2009- Miniaturized structures for optical frequency

2.3 Important Application of Metamaterial in term of Filter

• Periodically loaded SRRs was first implemented using left hand microstrip line in 2005 by Gill [22]. Planner filter with controlled bandwidth was proposed by J.bonache [23] in 2006. These types can easily correlate with LC limped element.
• For great level of miniaturization, P.K. Singhal build filter based on metamaterial [24] which also has low cast and easy to fabricate.
• On the basis of left and right hand transmission line S.Kahng designed a new ultrawide band (UWB) filter in 2008. It has strong capacitive coupling in interdigital coupled line. Structure of interdigital line is shown in Figure 4.
• Zachary M. Thomas et.al. proposed Symmetric Split- Ring Resonator (SSRR) which have repetition of metamaterial. For realizing the substrate of metamaterial, the metallic SSRR is printed sixteen times on a dielectric strip. In this case the region of negative permeability of substrate is analogous to stop band characteristics in microstrip.

Figure 4. Structure of Interdigital line

Microstrip filter based on composite right/left-handed metamaterial transmission lines have been showed in this work. Also microstrip filter which is based on resonant principle (left handed) is also discussed here. It is demonstrated that metamaterial structure behaves as an effective (continuous) medium with left handed wave propagation in the pass band. It is possible to design a compact planar microstrip filter with the help of metamaterial because of small electrical length of the resonator. At last it is observed that filter designed with metamaerial has significant advantages over conventional microwave filter in terms of size and performance.