INTRODUCTIONIn todays 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, beam width, side-lobe level, and 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.

BACKGROUND AND THEORYCurrently 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, butis technologically more difficult and expensive. The rectangular horns are ideally suited for rectangular waveguide feeders. The horn acts as a gradual transition from 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 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 agradual 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. HOW IT WORKS?A horn antenna serves the same function for electromagnetic waves that an acoustical horn does for sound waves in a musical instrument such as a trumpet; it provides a gradual transition structure to match the impedance of a tube to the impedance of free space, enabling the waves from the tube to radiate efficiently into space.If a simple open-ended waveguide were to be used as an antenna, without the horn, the sudden end of the conductive walls causes an abrupt impedance change at the aperture, from the characteristic impedance of the waveguide to the impedance of free space, 377 ohms.[8][2] When radio waves travelling through the waveguide hit the opening, it acts as a bottleneck, reflecting most of the wave energy back down the guide toward the source, so only part of the power is radiated.[9] It acts similarly to an open-circuited transmission line, or to a boundary between optical mediums with a high and low index of refraction, like a glass surface. The reflected waves cause standing waves in the waveguide, increasing the VSWR, wasting energy and possibly overheating the transmitter. In addition, the small aperture of the waveguide (around one wavelength) causes severe diffraction of the waves issuing from it, resulting in a wide radiation pattern without much directivity.To improve these poor characteristics, the ends of the waveguide are flared out to form a horn. The taper of the horn changes the impedance gradually along the horn's length.[8] This acts like an impedance matching transformer, allowing most of the wave energy to radiate out the end of the horn into space, with minimal reflection. The taper functions similarly to a tapered transmission line, or an optical medium with a smoothly-varying refractive index. In addition, the wide aperture of the horn projects the waves in a narrow beamThe horn shape that gives minimum reflected power is an exponential taper.[8] Exponential horns are used in special applications that require minimum signal loss, such as satellite antennas and radio telescopes. However conical and pyramidal horns are most widely used, because they have straight sides and are easier to fabricate.BASIC OFANTENNA PATTERN MEASURMENTA 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 AUTs 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 indifferent 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 2shows a typical antenna radiation pattern.

EXAMPLEAs the definition says, the E-plane is determined by the direction of the electrical field. In Figure 4, a horn antenna is fed by a rectangular waveguide, where the electrical field has only components parallel to the narrower sides. Thus, the field distribution in the horn Antenna should be similar parallel to the Y-axis. Under normal circumstances, the horn Antenna has its maximum radiation in the Z-axis. These two directions (the direction of the electrical field and the direction of the maximum radiation) define the E-plane by they and Z axes (or the plane x=0). Thus, if the antenna must, can rotate an AUT in Horizontal plane, then for measuring in the E-plane, the above-mentioned horn antenna must be mounted with its E-plane parallel to the plane of rotation (horizontal plane). This Antenna mast (antenna tower) is placed inside an anechoic chamber. Through a windowing one of the walls, a source antenna illuminates the AUT. The E-plane is parallel to the floor (horizontal plane), the source antenna must be polarized horizontally. For example: if a similar horn antenna is used for the source antenna, it must be placed the same way as the AUT (here with the wider sides vertically).

OPTIMUM HORNFor a given frequency and horn length, there is some flare angle that gives minimum reflection and maximum gain. The reflections in straight-sided horns come from the two locations along the wave path where the impedance changes abruptly; the mouth or aperture of the horn, and the throat where the sides begin to flare out. The amount of reflection at these two sites varies with the flare angle of the horn (the angle the sides make with the axis). In narrow horns with small flare angles most of the reflection occurs at the mouth of the horn. The gain of the antenna is low because the small mouth approximates an open-ended waveguide. As the angle is increased, the reflection at the mouth decreases rapidly and the antenna's gain increases. In contrast, in wide horns with flare angles approaching 90 most of the reflection is at the throat. The horn's gain is again low because the throat approximates an open-ended waveguide. As the angle is decreased, the amount of reflection at this site drops, and the horn's gain again increases.This discussion shows that there is some flare angle which gives maximum gain and minimum reflection. This is called the optimum horn. Most practical horn antennas are designed as optimum horns. In a pyramidal horn, the dimensions that give an optimum horn are

For a conical horn, the dimensions that give an optimum horn are

WhereaE is the width of the aperture in the E-field directionaH is the width of the aperture in the H-field directionLE is the slant length of the side in the E-field directionLH is the slant length of the side in the H-field direction.d is the diameter of the cylindrical horn apertureL is the slant length of the cone from the apex. is the wavelengthAn optimum horn does not give maximum gain for a given aperture size; this is achieved by a very long horn. It gives the maximum gain for a given horn length. Tables showing dimensions for optimum horns for various frequencies are given in microwave handbooks.GAINHorns have very little loss, so the directivity of a horn is roughly equal to its gain. The gain G of a pyramidal horn antenna (the ratio of the radiated power intensity along its beam axis to the intensity of an isotropic antenna with the same input power) is

For conical horns, the gain is:

whereA is the area of the aperture,d is the aperture diameter of a conical horn is the wavelength,eA is a dimensionless parameter called the aperture efficiency,The aperture efficiency ranges from 0.4 to 0.8 in practical horn antennas. For optimum pyramidal horns, eA = 0.511. While for optimum conical horns eA = 0.522.So an approximate figure of 0.5 is often used. The aperture efficiency increases with the length of the horn, and for aperture-limited horns is approximately unity.

TYPES OF ANTENNAThese are the common types of horn antenna. Horns can have different flare angles as well as different expansion curves (elliptic, hyperbolic, etc.) in the E-field and H-field directions, makingpossible a wide variety of different beam profiles.Pyramidal horn - a horn antenna with the horn in the shape of a four-sided pyramid, with a rectangular cross section. They are the most widely used type, used with rectangular waveguides, and radiate linearly polarized radio waves.

Sectoral horn - A pyramidal horn with only one pair of sides flared and the other pair parallel. It produces a fan-shaped beam, which is narrow in the plane of the flared sides, but wide in the plane of the narrow sides.

E-plane horn - A sectoral horn flared in the direction of the electric or E-field in the waveguide.

H-plane horn - A sectoral horn flared in the direction of the magnetic or H-field in the waveguide.

Conical horn - A horn in the shape of a cone, with a circular cross section. They are used with cylindrical waveguides.

Corrugated horn - A horn with parallel slots or grooves, small compared with a wavelength, covering the inside surface of the horn, transverse to the axis. Corrugated horns have wider bandwidth and smaller sidelobes and cross-polarization, and are widely used as feed horns for satellite dishes and radio telescopes.

Ridged horn - A pyramidal horn with ridges or fins attached to the inside of the horn, extending down the center of the sides. The fins lower the cutoff frequency, increasing the antenna's bandwidth.Septum horn - A horn which is divided into several subhorns by metal partitions (septums) inside, attached to opposite walls.Aperture-limited horn A long narrow horn, long enough so the phase error is a fraction of a wavelength, so it essentially radiates a plane wave. It has an aperture efficiency of 1.0 so it gives the maximum gain and minimum beam width for a given aperture size. The gain is not affected by the length but only limited by diffraction at the aperture. Used as feed horns in radio telescopes and other high-resolution antennas.