Tuesday, June 10, 2008

Transmission Line Model

Transmission line method is the easiest method as compared to the rest of the methods. This method represents the rectangular microstrip antenna as an array of two radiating slots, separated by a low impedance transmission line of certain length.

The following effects are taken into account for this model:

Fringing Effects: As the dimensions of the patch are finite along the length and the width, the fields at the edges of the patch undergo fringing i.e. the field exists outside the dielectric thus causing a change in the effective dielectric constant. It is a function of the dimensions of the patch and the height of the substrate.




The above diagram shows a patch antenna from the Transmission Line Model perspective. We can observe the fringing at the edges increasing the effective length.



Effective length and Width: Due to fringing effect, electrically the patch dimensions will be bigger than its physical dimensions. A formula to calculate the effective length Leff is shown below.

Where, an approximate relation for the normalized extension of length ΔL is given below:



For an efficient radiator, a practical width that leads to good radiation efficiencies is,


Conductance: Each radiating slot is represented by a parallel equivalent admittance Y(with conductance G and susceptance B). The slots are labeled as Θ1 and Θ2. The equivalent admittance of a slot is given by,

Y1 = G1 + B1

Where for a slot of finite width (W)

For,

Since both the slots are identical, its equivalent admittance is;

Y2= Y1, G2=G1, B2=B1

The conductance of each slot can be obtained by using field expression from cavity model. In general, the conductance is defined as


The above list of equations obtained from the transmission line model can be used to calculate parameters for analysis and synthesis of antenna. For these purposes, number of models may be used in conjunction with each other.

2 comments:

aztec said...

Not enough to read about method of analysis. Iam doing a project on transmission line fed Patch antennas working on ISM frequency

Unknown said...

Where are the images ?