Patch Antenna

Innovation Report of

Study of the Effect of Substrate Height on the Performance of Patch Antenna

By: Abhishek Aggarwal (881152804) Prashant Bist (891152804) Kushal Gaba (901152804) Deepak Mittal (921152804)

Introduction to Microstrip Antennas A very popular antenna for many applications is the ‘’patch’’ or ‘’panel’’ antenna. A patch antenna gains its name from the fact that it basically consists of a metal patch suspended over a ground plane. The assembly is usually contained in a plastic radome, which protects the structure from damage (as well as concealing its essential simplicity). Patch antennas are simple to fabricate and easy to modify and customize. They are closely related to microstrip antennas, which are just patch antennas constructed on a dielectric substrate, usually employing the same sort of lithographic patterning used to fabricate printed circuit boards. A microstrip or patch antenna has a number of advantages over other antennas -- it is lightweight, inexpensive, and easy to integrate with accompanying electronics. While the antenna can be 3-D in structure (wrapped around an object, for example), the elements are usually flat; hence their other name, planar antennas. Note that a planar antenna is not always a patch antenna.

The following drawing shows a patch antenna in its basic form: a flat plate over a ground plane (usually a PC board). The center conductor of a coax serves as the feed probe to couple electromagnetic energy in and/or out of the patch.

Despite of many advantages of patch antennas, they do have some considerable drawbacks. One of main limitation with patch antenna is there inherently narrowband performance due to its resonant nature. With bandwidth as low as a few percent [2], broadband applications using conventional patch design are limited. Other characteristics of patch antenna include low efficiencies, limited power capacity, spurious fed radiation, poor polarization purity, and manufacturing tolerance.

EFFECT OF h: The most direct approach to increase the bandwidth of patch antenna is to increase the thickness of the substrate, while using a low dielectric substrate [3]. This can extend efficiency (as much as 90% if surface waves are not included) and bandwidth (up to 35%). However, surface waves must be included, since surface waves extract power from the direct radiation pattern, resulting in increased side lobe levels, antenna loss, and a decrease in efficiency. Moreover, the probability of surface wave formation increases as the thickness of the substrate increases. As patch antenna radiates, a portion of the total available power for direct radiation becomes trapped along the surface of the substrate. This trapped electromagnetic energy leads of the development of surface waves. In fact, the Ratio of power that radiates into the substrate compared to the power that radiates into air is approximately (03/2:1). Is this governed by the rules of total internal reflection, which states that any field radiated into the substrate at angles greater than the critical angle are totally internally reflected at the top and bottom surfaces. This is illustrated I figure [1]. Therefore, for a substrate with dielectric constant Er=10.2, nearly 3.1 of total radiated power is trapped in substrate with a critical angle of roughly 18.2 degrees. Surface waves effect can be eliminated by using cavities or stacked substrate techniques. The excitation of surface wave is a function of εr and h. However, this has the fundamental drawbacks of increasing the weight, Thickness, and complexity of the microstrip antennas. These complications and others prevent microstrip antennas from becoming the standard in the microwave communication community.

Experimental Setup and Observations The following values were taken as constant in the experimental patch antenna we designed to study the effect of height of substrate: Dielectric constant of substrate, εr =2.22 Loss tangent=0.0001 Length of patch=9.06mm Width of patch=11.8mm Distance of circular probe from center, Y=7mm Radius of probe=0.6mm Case1: height=1.59 mm ; Bandwidth=1.6 GHz Case 2: height= 2 mm ; Bandwidth = 1.8 GHz Case 3: height= 3 mm ; Bandwidth = Undeterminable as the VSWR never falls below 2 dB

Conclusion: As the height of the substrate increases , the performance of the antenna steadily improves.However, after a certain height , the surface waves take their toll and the performance of the antenna declines.