MICROSTRIP LINES

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

INTRODUCTION

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.

STRIP LINES

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

or

The Characteristic Impedance of a narrow Micro-

Strip line is expressed as

The velocity of the Propagation is given by

Characteristic Impedance (Z0)

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

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

Radiation Losses

Losses in Micro-Strip Lines

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;

Radiation Losses

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.