AN IMPEDENCE MATCHING TECHNIQUE FOR INCREASING THE BANDWIDTH OF MICROSTRIP ANTENNAS

ABSTRACT

Broad-band impedance- matching is proposed as a natural solution to increase the bandwidth. The maximum obtainable bandwidth is calculated using Fano’s broad- band matching theory. It is found that by using an optimally designed impedance-matching network, the bandwidth can be increased by a factor of at least 3.9, the exact value depending OD the degree of matching required.

INTRODUCTION

WHAT IS ANTENNA

Antenna is a transmitter or receiver used to send or receive the signals (used in communication: be it audio or visual). It either releases the electromagnetic signals having high power and moving with the speed of 3 lakh km per second or receive the signals spread all around it in the atmosphere of the frequency respective and conjugated with the length.

Type of Antenna depends on distances, Time of signal transmission, frequency. Antenna length or height should be equal to either half of wavelength or one- fourth of the wavelength of respective signals.

WHAT IS MICRO STRIP ANTENNA

A metallic strip or patch mounted on the dielectric surface (substrate) which is supported by a ground plane.

Side view of micro strip

Different shapes of patches

VARIOUS METHOD OF INCREASING THE BANDWIDTH OF MICROSTRIP ANTENNA

There are various methods to increase the bandwidth of micro strip patch antennas. For example, by decreasing the Q-factor of the patch and by increasing the substrate height and lowering the dielectric constant the bandwidth of micro strip patch antennas can be increased.

Another way could be use of multiple resonators located in one plane. We can also use multilayer configurations with multiple resonators stacked vertically. And lastly very effective way of increasing bandwidth of micro strip antenna is use of impedance matching networks.

IMPEDEN MATCHING TCHNIQUE FOR INCREASING THE BANDWITH OF MICROSTRIP ANTENNA

This method is unique as it does not alter the radiating element itself

Instead, a reactive matching network is added to compensate for the rapid frequency variations of the input impedance.

The validity of the technique is based upon the relative frequency insensitivity of the radiation pattern and gain characteristics as compared to the resonant behaviour of the input impedance. Dividing the bandwidth after enlargement using the impedance matching technique quantity by the bandwidth in general one, a bandwidth-enlargement factor is found which depends only on the bandwidth criterion expressed as a maximum allowable voltage standing-wave ratio (VSWR).

THEORY

We know that that the impedance variations are the dominant bandwidth-limiting factor, whereas the gain (=directivity x radiation efficiency) and radiation pattern variations are almost negligible over a moderate 10 to 20 percent bandwidth. According to the theory of model expansion in cavities the total input impedance can be written as a sum of modal impedances where each modal impedance behaves as a parallel-resonant circuit. In the same way, the total radiation field can be written as a vector sum of modal radiation fields where each modal field is given as the product of a nearly frequency independent normalized pattern and a frequency dependent scalar excitation coefficient. Thus, it follows that in all cases where only one dominant mode is ex- cited, the input impedance will behave as a parallel-resonant circuit, whereas the (relative) radiation pattern will show al- most no frequency variation.

CALCULATIONS AND FORMULATION

Bandwidth enlargement factor

In the vicinity of its fundamental resonant frequency, the input impedance of a micro strip antenna can be modelled by either a series-resonant or a parallel-resonant RLC circuit.

In series resonant circuit

In parallel resonant circuit

Where Q is the quality factor.

And,

where f is the frequency variable and fr the resonant fre- quency.

If the bandwidth criterion is taken to be VSWR 5 S, and f, and f2 are the lower and upper band edge frequencies, respec- tively, so that VSWRCfl) = VSWR(j.2) = S, the bandwidth is given by

where T = ZO/RO in the series-resonant case, and T = Ro/Zo in the parallel-resonant case. T normally equals to 1.

This shows the principle of broad band matching

Note, however, that, in order to maximize B, it would be best to take T = T opt # 1 where

lefttop

General form

It is evident that the above-calculated bandwidth of the above equation can be increased, at least in principle, by using an impedance matching network, as shown in above picture. Ideally, this network would transform the frequency- dependent complex antenna impedance Zi, to a pure real resistance ZO over as large a bandwidth as required. Indeed, it is impossible to realize a perfect match over a continuous band of frequencies by means of a purely reactive (i.e., linear, passive and lossless) network. The best one can do is to realize a constant (but not perfect) match within the band of operation and a total mismatch out- side this band. In that way, one can either optimize the degree of matching if the bandwidth is given a priori, or maximize the bandwidth if the degree of matching (e.g., VSWR 5 S) is given. The maximum VSWR = S bandwidth obtainable for a series- or parallel-resonant circuit, can be calculated in a straightforward manner using Fano’s theory. The result is given by

lefttopmodified form

This equation expresses that the maximum realizable band- width is inversely proportional to both the element quality factor and the specified return loss (expressed in dB). Because above represents the optimum that is theoretically achievable using broad-band matching and BT gives the normally obtained bandwidth, the maximum bandwidth- enlargement factor is found by dividing both quantities:

shows this factor which only depends on S and has a minimum value of 3.90 for S = 2.64.

RESULT