many applications it is necessary to design antennas with very directive characteristics to meet the demands of long distance communication. This is accomplished by increasing the electrical size of the antenna.
Another way to enlarge the dimensions of the antenna is to form an assembly of radiating elements in an electrical and geometrical configurations. This new antenna formed by multi elements is referred to as an array. The total field of the array is determined by the vector addition of the fields radiated by the individual elements3.
To provide very directive patterns, it is necessary that the fields from the elements of the array interfere constructively (add) in the desired directions and interfere destructively (cancel each other) in the remaining space. In an array of identical elements, there are at least five controls that can be used to shape the overall pattern of the antenna.1. The geometrical configuration of the overall array2. The relative displacement between the elements.3.
The excitation amplitude of the individual elements.4. The excitation phase of the individual elements.
5. The relative pattern of the individual elements.6. 2.Array Antennas : Linear array Some antenna sources can be seen as isotropic elements, which mean that they radiate equally well in all directions 18. An array of identical elements all of identical magnitude and each with progressive phase is referred to as a uniform array3.
Design Methodology Antenna is an interface between the signal source and the air for any wireless application. The efficiency of an antenna array is a prominent figure of merit of any radar system. Antenna efficiency can be in the terms of gain, directivity, return loss 3. Port impedance is normally chosen as 50 ?.
To design an antenna array at a desired frequency , the first step is to design single antenna. FR-4 material can be used which is cheap and readily available.Design of microstrip array is divided into four steps(i) Defining specifications(ii) Design of single patch(iii) Design of an array(iv) Simulation of the arrayFeeding technique is another important aspect in the design of patch antennas. Some feeding techniques are given below.
(i) Microstrip Line Feed(ii) Co-axial Probe Feed(iii) Aperture Coupling(iv) Proximity Coupling Microstrip patch antennas can be fed by a variety of methods.Microstrip Line Feed In this type of feeding technique, a conducting strip is connected directly to the edge of the microstrip patch. The conducting strip is smaller in width as compared to the patch and this kind of feed arrangement has the advantage that the feed can be etched on the same substrate to provide a planar structure.Co-axial Probe FeedThe coaxial feed is one of the very common technique used for feeding Microstrip patch antennas. The inner conductor of the coaxial connector extends through the dielectric and is soldered to the radiating patch, while the outer conductor is connected to the ground plane. Aperture Coupled FeedIn this technique, the radiating patch and the microstrip feed line are separated by the ground plane.
Coupling between the patch and the feed line is made through a slot in the ground plane and variations in the coupling depends upon the size. Aperture couple feed has advantage such as no physical contact between the feed and the radiator, wider bandwidths, better isolation between antenna and feed network.Proximity Coupled FeedThis type of feed technique is also called as the electromagnetic coupling. Two dielectric substrates are used such that the feed line is between the two substrate and the radiating patch is on top of the upper substrate. The main advantage of this feed techniques is that it eliminates spurious feed radiation and provides very high bandwidth.
Microstrip patch antennas are extensively applied in radar systems & wireless communication systems which are high gain, low cost, light weight & low profile & can accurately control radiation patterns. Two types of array feeding structures are commonly used named parallel fed & series fed structures. Regarding the parallel fed scheme, numerous power dividers containing many discontinuities & long transmission lines are required, the presence of which causes spurious radiation & substantial dielectric loss to occur. Conversely, the series fed structure uses short transmission lines & enhances antenna efficiency. In the series fed structure, resonant & travelling wave feeds are commonly used.
The bandwidth of a travelling wave feed is wider than that of a resonant feed. Antenna Array Design Antenna array design can be classified into series feed array and corporate feed array. In this literature, different corporate feed antenna array designs were presented. In 5.1 corporate feed structure is presented. The performance of an antenna is also discussed. 4. Corporate Feed Design Corporate feed antenna array is presented.
For first design we used two antenna elements. But for this design return loss is not less than -10 dB. Upto -8dB it is achieved.
For four array design return loss is improved at its greater value.