MICROSTRIP LINES INTRODUCTION A planar transmission line is transmission line with conducting metal strips that lie entirely in parallel planes. Planar Transmission Line is the first step to fabrication of Microwave Integrated circuit Planar Transmission Lines are available in various configurations such as Strip Lines, Micro-Strip Lines, Slot Lines & Co-Planar Lines, fin-lines. The Dielectric Substrate should have the following General Properties Low dissipation factor Polished Surface Minimum variation of Dielectric constant, High thermal conductivity Uniformity of thickness& dimensional stability STRIP LINES The earliest form of planar transmission lines was stripline. Striplines are essentially modifications of the two wire lines and coaxial lines. It consists of a strip conductor entered between two parallel ground planes with two equal slabs of a dielectric, ferrite, or semiconductor medium separating the center conductor from the ground planes. Usually, the medium is a solid material, but in some applications air is the actual dielectric used. The advantages of striplines are good electromagnetic shielding and low attenuation losses, which make them suitable for high-quality factor (Q) and low-interference applications. Transverse electric and magnetic (TEM) waves propagate within the stripline. MICROSTRIP LINES The microstrip line is transmission line geometry with a single conductor trace on one side of a dielectric substrate and a single ground plane on the other side. Since it is an open structure, microstrip line has a major fabrication advantage over the stripline. It also features ease of interconnection and adjustments. For microwave device applications, microstrip generally offers the smallest sizes and the easiest fabrication. MIC using microstrip can be designed for frequencies ranging from a few gigahertz, or even lower, up to at least many tens of gigahertz. However, it does not offer the highest electrical performance. Attenuation losses and power handling are compromised. In the microstrip line, the electromagnetic fields exist partly in the air above the dielectric substrate and partly within the substrate itself. For most practical purposes, microstrip can be treated as a TEM transmission line with an effective relative permittivity that is a weighted average between air and the substrate material. But, the actual propagation of electromagnetic waves in microstrip is not purely TEM due to the combination of an open air space and a dielectric medium.(some electric field component in Z Direction(Ez)) Thus, it is usually assumed that the electromagnetic field in the microstrip line is quasi-TEM. Characteristic Impedance (Z0)of Micro-Strip Lines The Characteristic Impedance of Micro-Strip Line is a function of Strip line width (w), Strip line thickness (t), the distance between the line & ground plane (h) and the dielectric constant of the material. The Characteristic Impedance of a Micro-Strip Line was found by several techniques. One of the methods was comparative (or) indirect technique. In this technique, the Characteristic Impedance of a wire over ground transmission line is given by; Relative Effective Dielectric Constant (εre) The effective dielectric constant of a Micro-Strip line is related to the relative dielectric constant of the board material. The board materials such as Fiber-glass epoxy & Nylon Phenolic are used. Therefore, the empirical equation is expressed as; Characteristic Impedance (Z0) The cross section of Micro-Strip line is rectangular. Hence the rectangular conductor must be transformed into circular conductor. Therefore, the empirical equation for the transformation is The Characteristic Impedance of a narrow Micro- Strip line is expressed as or Characteristic Impedance (Z0) The velocity of the Propagation is given by The Characteristic Impedance of a wide Micro-Strip line is expressed as Losses in Micro-Strip Lines There are two types of losses occur in the Micro-Strip lines: 1. Attenuation Loss 2. Radiation Loss The Attenuation Loss can be further divided into; (a). Dielectric Losses (b). Ohmic Losses Dielectric Losses The conductivity of the dielectric cannot be neglected, and therefore the electric and magnetic fields in the dielectric are no longer in the time phase. In this case, the dielectric attenuation constant is given by The dielectric constant can be expressed in terms of dielectric loss tangent. i.e Losses in Micro-Strip Lines By substituting conductivity in the dielectric attenuation constant can be expressed as The Micro-Strip line is a non-Magnetic mixed dielectric System. In the upper dielectric no loss occurs. Therefore, equation can be modified as; Losses in Micro-Strip Lines Generally the attenuation can be called as, “attenuation constant per wavelength”. Hence, Ohmic Losses Ohmic losses are due to non perfect conductors. In Micro-Strip line the current density is concentrated at a skin depth in the conductor which is thick inside the surface and is exposed to the electric field. Losses in Micro-Strip Lines The conducting attenuation of a Micro-Strip line is given by Losses in Micro-Strip Lines Radiation Losses Micro-Strip lines are having the Radiation losses. The Radiation loss depends on the dielectric substrate’s thickness, dielectric constant & its geometry. The following approximation can be taken for the calculation of Radiation loss: TEM transmission Uniform dielectric in the neighborhood of the strip Neglect the radiation from the TE field component Dielectric thickness is much less than the free space wavelength Losses in Micro-Strip Lines Radiation Losses By considering the above factors, the ratio of radiated power to the total dissipated power for an open circuited Micro-Strip line is given by; The radiation factor decreases with the increasing Dielectric substrate constant. Hence, above equation can also be expressed as Quality Factor (Q) of a Micro-Strip Lines Most Microwave Integrated circuits require very high quality resonant circuits. Hence the quality factor (Q) of a Micro-Strip line is very high, but it is limited by radiation losses & low dielectric substrate constant. We know that, the Ohmic attenuation constant as The wavelength of the Micro-Strip line Quality Factor (Q) of a Micro-Strip Lines Advantages of a Micro-Strip Lines An easy accesses to top surface makes it convenient to mount discrete devices and also minor adjustments are possible after the fabrication of the circuit. Fabrication costs are lower Better interconnection features Small size and weight Increased reliability and low cost The structure is rugged (rocky & tough) & can withstand larger voltages & Power disadvantages of a Micro-Strip Lines They have greater radiation losses due to open conductor above the dielectric substrate. The are more effective to near by conductors due to the interference created by the open ended conductor. A discontinuity in the electric and magnetic fields is generated, due to nearness of the air dielectric interface with the microstrip conductor..