What is microstrip line? What are the advantages and disadvantages of microstrip line? Show the electric and magnetic field lines of microstrip line explain the working principle of a microstrip antenna.

Microstrip line:

Microstrip line is an unsymmetrical strip line that is nothing but a parallel plate transmission line having dielectric substrate, the one face of which is metalized ground and the other face has a thin conducting strip of certain with 'w' and thickness 't'. This is shown in Fig.1 (a). The top ground plane is not present in a Microstrip as compared to a strip line. Sometimes a covered plate is used for shielding purposes but it is kept much further away than the ground plane so as not to affect the micristrip field lines as shown in Fig.1 (b).

Fig.1. (a) Microstrip line an (b) Microstrip line with cover plate

There are certain advantages of Microstrip lines over strip lines, coaxial line and waveguides

1. Complete conductor pattern may be deposited and processed on a single dielectric substrate which is supported by a single metal ground plane. Thus fabrication costs would be substantially lower than strip line, coaxial or waveguide circuits

2. Due to the planar nature of the Microstrip structure, both package and un package semiconductor chips can be conveniently attached to the microstrip element

3. Also there is an easy access to the top surface making it easy to mount passive or active discrete devises and also for making minor adjustment after the circuit has been fabricated. This also allows access for probing and measurement purposes.

What are the advantages and disadvantages of microstrip line?

The Microstrip line can be used at microwave particularly in those applications where in the more bulky and expensive to manufacture conventional plumbing is at a disadvantage.

However there are a number of limitations to Microstrip

1. Open structure of Microstrip leads to a somewhat greater coupling between sid by side configurations as compared to waveguide or coaxial system. The absolute value of coupling is however small.

2. Higher attenuation compared to waveguide structure. Hence cannot be used in systems where extremely low loss is the requirement. For example, in microwave receivers were line length are smaller, the insertion loss can be made negligible compare to other losses.

3. Low resonant impedance is inherent in Microstrip structure is limiting the magnitude of the obtainable Q.

The electric and magnetic field lines of microstrip line.

The Electric field (E- field) and Magnetic field (H- field) distribution of the Microstrip line is given in Figure 2(a) and (b).

Fig.2 Electric and Magnetic field distribution of Microstrip line

Working principle of a microstrip Antenna:

Microstrip antennas are essentially suitably shaped discontinuities that are designed to radiate. The discontinuities represent abrupt changes in the Microstrip line geometry e.g., a step change in width, and open end or a microstip bend. Discontinuities alter the electric and magnetic field distribution. These result in energy storage and sometimes radiation at the discontinuity. As long s the physical dimensions and relative dielectric constant of the line remains constant, virtually no radiation occurs. However, the discontinuity introduce by the rapid change in line with at junction between the feed line and patch radiate. The other end of the patch where the metallization abruptly ends also radiates. When the fields on a Microstrip lines encounter and abrupt change in with at the input to the patch, electric fields spread out. It creates fringing fields at this edge, as indicated. After this transition the patch looks likes another Microstrip line. The fields propagate down this transmission line until the other edge is reached. Here the abrupt ending of the line again creates fringing fields as for the open end discontinuity. The fringing fields store energy. The edges appear as capacitor to ground since the changes in the electric field are greater than that for the magnetic field. Because the patch is much wider than a typical Microstrip line, the fringing field also radiate, this is represented by a conductance in shunt with the edge capacitance, which accounts for power loss due to radiation as shown in Fig. 4.

Fig. 3(a) Microstrip antenna

Fig.3(b) Different shape of Microstrip antenna

Fig. 4(a) and (b) respectively shows the Fringing field distribution and equivalent circuit of the Microstrip patch.

Some limitation of Microstrip antenna

1. Narrow bandwidth near about 2-4%

2. Low antenna gain

3. Radiate into a half plane

4. Poor end fire radiation performance

5. Low power handling capability

6. Possibility of excitation of surface wave.

There are various models are available to explain the Microstrip antenna.

1. Transmission line model: Two opposite sides of a rectangular patch are joined together by a low impedance strip line

2. Cavity model: Field between patch and ground plane is expended in terms of a series of cavity resonant mode

3. Integral and differential equation model: Methods of moment. Here integral equation is formulated by imposing the boundary condition

                 a)  Finite element method

                 b)  Finite difference method etc.