Wideband Microstrip Patch Antennas and their Modifications for Practical Applications
Printed antennas play an significant role in satellite, mobile, and other wireless communications networks, military systems and several more emerging applications in- cluding radar sensing and imaging. Several of these new applications not only have special demands for antenna designs but also require incorporation of smart features into it. In addition, research for high gain, wideband and compact antennas for several emerging technologies like 5G and IoT are rapidly progressing. In this work, design and analysis of several variants of wideband microstrip patch antennas are taken up. We begin with the design of a single layer suspended antenna where a small coplanar capacitive strip is used as the feed. Following a recent research from the group, di erent radiating patch shapes like rectangular, triangular and semi- elliptical patch geometries are compared with each other for various performance parameters. It is important that these wideband antennas must be able to transmit and receive short time pulses without any distortion and dispersion. Thus, parameters like group delay(GD) and fidelity factor(FF) are studied along with other parameters like re ection loss (S11), gain, polarisation, e ciency and radiation pattern. Full wave electromagnetic simulations using CST microwave studio is used to analyze these antenna characteristics. Triangular and semi-elliptical patches are fed in two di erent configurations (edge-feed and vertex-feed). Simulation studies showed that among the five configurations the vertex fed triangular antenna has the best performance in terms of at gain and low group delay variation over the frequency band of operation. Fabricated antenna has a return loss bandwidth from 3.1 GHz to 4.2 GHz at S11 better than -10 dB where the group delay variation is less than 2 ns. This antenna has a peak gain of 7 dBi and a beamwidth of 60 in both principal planes at the center frequency. Pulse propagation characteristics of this antenna is also studied using modulated sinusoid waveforms as the transmit signal. The degradation of pulse shape with angle is studied through simulations. The transmitted and received pulses are compared with two identical antennas at boresight to experimentally analyze the impact on time domain waveforms. A similar performance has been obtained for an antenna with the feedstrip embedded within the patch by providing a slot surrounding the strip. While retaining most of the performances the footprint of the antenna could be reduced by this approach. Surface wave generation is considered a common drawback in relatively thick microstrip configurations including suspended microstrip patch antennas (MPA) as this may cause less gain, asymmetry in the radiation pattern at higher frequencies within the operational band and increased mutual coupling when these antennas are used in arrays. To address these issues, conducting walls are introduced surrounding the above suspended MPA. The resulting configuration is a wideband cavity backed microstrip patch antenna (WCMPA). The proposed configuration has the widest impedance bandwidth reported for a cavity backed MPA. Measured S11 band- width at -10 dB return loss is from 2.89 GHz to 5.18 GHz (% BW > 55%) with peak gain of 7.6 dBi. The radiation pattern is symmetrical throughout the operational frequencies. The group delay variation is < 1ns and FF is above 0.9. In addition to these performance improvements, the addition of cavity walls provides physical stability to the antenna. Further, a modification to this antenna is investigated to increase its gain. By increasing the lateral dimensions of the cavity the gain is increased from 7.6 dBi to 10.2 dBi. However, this results in an overall footprint of about 1.3 for the entire antenna. The impact of having conducting sidewalls on mutual coupling is also studied for two element arrays in both E-plane and H-plane. A comparison of arrays with and without cavity showed that, the isolation between elements improves by 14 dB with cavity walls for a similar distance between elements in the E-plane. Furthermore, these two-element arrays can be designed for combined peak gain of 11 dBi by choosing appropriate inter-element spacing. Another modification to this antenna is proposed here, where a short horn is mounted on the substrate to increase the overall gain. The gain throughout the frequency range of the antenna can be improved by appropriately choosing the dimensions and are angle of the horn-like structure. Extensive parametric studies have been conducted on this wideband quasi-planar antenna (WQA) to analyze the impact of various design parameters. Measured peak gain for this antenna is 13.2 dBi. This low profile antenna o ers the best gain for similar quasi-planar configurations. The measured impedance bandwidth is 46.7% from 3.1 GHz to 4.6 GHz and the beamwidth is around 35 in both principal planes. While the calculated aperture efficiency is comparable to any other horn antenna, the proposed antenna geometry has a lower height compared to horn antennas and the coaxial transition is similar to those used in planar technologies. In these respects, the proposed quasi-planar an- tenna eliminates the complexity involved in integrating conventional horn antennas for compact wireless terminals. Furthermore, it has been shown that the design can be appropriately modified for di erent frequencies of operation and similar performance is achieved for these designs. One of these designs have been employed in a passive radar developed by the group. The wideband high gain antennas proposed in this research may find several applications in wideband RADAR and imaging. As a case study, we integrated this with a wideband frequency modulated continuous wave (FMCW) RADAR. A WCMPA is used as the transmitter and a two-element array of WCMPA is used as the receiver. A rat race hybrid circuit is integrated with the receiver antennas to obtain sum and di erence patterns in a monopulse configuration. This information can be used to measure the angular position to track a target. 2D and 3D tracking antenna integrated circuits are designed and the results are validated experimentally. Another case study dealt with application of high gain antenna WQA for passive radar since this wideband antenna with large beamwidth helps locate the target with good res- olution. Thermal radiations emitted by the human body in the microwave range are sensed using radiometric system. Experimental results demonstrated capability of the system to detect the presence of a person upto a distance of 2 m where the range improvement may be attributed to the high gain of the WQA used. In summary, the work reported in this dissertation enables the design and analysis of several variants of wideband microstrip antennas with low profile. In addition to extensive simulation studies, some of these designs are experimentally validated and employed in practical wideband radar applications.