Microwave Horn Antenna Design and Test System EE198B: Senior Design Project II San Jose State University Fall 2003 Presented by: Vishal Ohri Ozair Amin Hiruy Gebremariam Benjamin Dubois1 Table of Contents Abstract ………………………………………………………………… 2 Introduction ………………………………………………………………… 3 Background and Theory ………………………………………………………… 4 Basics of Antenna Patterns Measurements ………………………………………… 5 Results and Analysis ………………………………………………………… 8 I Waveguide design and tuning ………………………………………… 8 II Horn Antenna design ………………………………………… 10 III Test Setup and Measurements ………………………………………… 11 Discussion …………………………………………………………………. 14 Concluding Remarks …………………………………………………………………. 14 Reference ………………………………………………………………….. 15 Appendix ………………………………………………………………….. 162 Abstract This paper is our report for our senior design project on Microwave Antenna Design and Test System. This project requires two primary areas of concern; a pyramidal horn antenna design and a test system that will determine the performance of our antenna. A brief theory on microwave horn antennas will be discussed along with the results of our design. Our results and analysis show that the project was within the scope of our ability to design and test the horn antennas.3 Introduction In today’s technological society, wireless communication has become an increasingly important part of daily life. We have come to depend on our pagers, cellular phones, satellite dishes, radios, etc., usually without understanding how they work. The common element to all of these wireless systems, whether they transmit or receive, is the antenna. The antenna is responsible for coupling the RF energy from the transmission-line feed (guided) to free space (unguided), and vice versa. Antennas are characterized using several parameters, such as geometry, gain, beamwidth, side-lobe level, frequency of operation, efficiency, and polarization. Keeping this in mind for this senior design project we designed two microwave horn antennas and implement a test system that will test the performance of our antenna and the efficiency of our test system. This paper will address the theoretical and practical construction of a 2.4GHz horn antenna and methodology used in testing the antennas. The pyramidal horn antenna is part of the aperture antennas family that has a conical radiation pattern, linearly polarized and is ideal in high gain transmission and receiving, peer to peer communications, and as a dish feed.4 Background and theory Currently there are many companies developing microwave antennas and highly sophisticated test systems that range in the millions of dollars. Our aim is to build an affordable horn antenna, less than $20, and an inexpensive antenna test system setup. Horn antennas are extremely popular in the microwave region (above 1 GHz). Horns provide high gain, low VSWR (with waveguide feeds), relatively wide bandwidth, and they are not difficult to make. There are three basic types of rectangular horns: Figure 1: Basic types of horn antennas We are concerned with the pyramidal horn antenna shown in Figure 1(c). The horns can be flared exponentially, too. This provides better matching in a broad frequency band, but is technologically more difficult and expensive. The rectangular horns are ideally suited for rectangular waveguide feeders. The horn acts as a gradual transition from a waveguide mode to a free-space mode of the EM wave. The open-ended waveguide will radiate, but not as effectively as the waveguide terminated by the horn antenna. The wave impedance inside the waveguide does not match that of the surrounding medium creating a mismatch at the open end of the waveguide. Thus, a portion of the outgoing wave is reflected back into the waveguide. The horn antenna acts as a matching network, with a gradual transition in the wave impedance from that of the waveguide to that of the surrounding medium. With a matched termination, the reflected wave is minimized and the radiated field is maximized. Designing the horn antenna is easy once we determine the dimensions of our horn antenna. There are many software programs available for download that can calculate the E-field and H-field dimensions of our horn for a given frequency (2.4GHz) and gain of about 19dB.5 The horn antenna we design fit the following specification: • 2.4 GHz (S-band) • Beam width ~17o • ~ 19 dB gain • Linearly polarized • Return loss of > -10dB (SWR < 1.2:1) • WR430 Waveguide standard (4.30”X2.15”) • Waveguide to Coax adapter N-type connectors Basics of Antenna Pattern Measurements A general system designed for antenna measurements uses the following algorithm for performing a far-field antenna pattern measurement. An antenna under test (AUT) goes through all the desired angular configurations, while the AUT’s response to RF stimulus (illuminated by a still source antenna) is being recorded. The plot of magnitude of the received signal versus angle displays the pattern directivity. As the process of measurement (rotating the AUT and recording the pattern) is usually done by the system, an operator has to set up (install/mount) the AUT and source antenna correctly. There are two major requirements to be satisfied: Realizing which plane (E or H) is to be used. This defines the placement of the AUT on the rotating table; Matching the polarization of AUT with the polarization of source antenna (if not measuring cross-polarization).The radiation pattern of an antenna describes its far field directional characteristics. When the antenna is transmitting the pattern indicates the relative power density radiated in different directions in the plane relative to the antenna principal direction of radiation (“bore-sight”). When receiving the pattern indicates the variation in the received signal level relative to bore-sight signal level as the antenna orientation is changed. Figure 2 shows a typical antenna