Download
1 / 13
240 likes | 501 Views
Antenna Design. EKT 341 By Dr. Soh Ping Jack Muhammad Ezanuddin Cheor Wai Loon. Radiator This is the basic element of an antenna (and is often called "the element".) An antenna can be made up of multiple radiators. Range
E N D
Antenna Design EKT 341 By Dr. Soh Ping Jack Muhammad Ezanuddin CheorWai Loon
Radiator This is the basic element of an antenna (and is often called "the element".) An antenna can be made up of multiple radiators. Range The radial distance from an antenna to an object, particularly in radar. Along with azimuth and elevation, it forms the spherical coordinate system that is used in antenna analysis.. Boresight Error (BSE) The maximum radiation intensity is supposed to occur at boresight, but nothing works perfectly in the analog world, and often it is slightly skewed. The angle that the physical or optical boresight differs from the electromagnetic boresight is the boresight error. This is most important when your antenna is used in a tracking radar. Boresight The direction in which you are physically pointing the antenna, with the intention of maximum electromagnetic illumination. The word comes from the early military applications of microwaves, when radar was perfected to help shoot stuff down.
Azimuth • The angle from left to right from a reference point, from 0 to 360 degrees, or from -180 to +180 degrees. Antenna pattern An antenna pattern, or radiation pattern, is a 2D (or 3D contour) plot which shows the angular variation in an antenna parameter such as the relative field strength in the far-field. The pattern is usually presented in polar coordinates and with a dB scale. Elevation The angle the horizontal (xy plane), from -90 degrees (down) to +90 degrees (up). Related to the spherical coordinate ( ),which is measured from the vertical (z-axis). Often abbreviated "EL". Reciprocity Reciprocity means that an antenna performs exactly the same way in transmitting as it does in receiving a signal.
Isotropic radiator • A theoretical radiator that emits (or receives) electromagnetic radiation equally in all directions (both AZ and EL), with zero loss. There is no isotropic radiator in practice. Beamwidth The width of the main lobe (or main beam) spanning a -3dB difference in gain. In the above example, the beamwidth of the antenna pattern is about 60 degree. Dipole A common antenna type which, in its simplest form, consists of a straight wire cut in the middle so that each half may be connected to one of the two conductors of a transmission line Omnidirectional radiator An antenna that radiates equally in all azimuthal directions.
Directivity • The ratio of electromagnetic radiation of a real antenna at an AZ/EL angle (typically specified at boresight) to its radiation in all directions averaged over a sphere. Measured in the far field • Directivity of an isotropic radiator which radiates equal intensity in all directions, is 0 dB. Directional antennas are measured against the isotropic radiator, because they favor one direction, their directivity is positive when expressed in dB. Directivity can be calculated as a function of the "effective area (aperture)" and wavelength: Gain The maximum signal intensity of an antenna at a specified AZ/EL angle, typically at boresight, with respect to (usually) an isotropic radiator, expressed as dBi (decibels from isotropic). The narrower the beam width, the higher the gain. Gain is equal to directivity times efficiency, or directivity plus efficiency when expressed as dB: Efficiency The efficiency of an antenna takes into account resistive losses, and is equal to the total radiated power divided by the radiated power of an ideal lossless antenna (or subtract them in decibels). Efficiency is not a function of AZ/EL angles Near field The region close to an antenna where the electromagnetic fields do not follow a simple 1/R relationship with the range R
Far field • The region far from an antenna where the electromagnetic power densiity (power per unit area) falls off as 1/R^2, proportional to the area of the ever-widening sphere. Peak sidelobe ratio Ratio of the highest sidelobe to the central beam intensity of an antenna. Antenna ranges Ranges are chambers that are used to study the behavior of antennas, which is a huge topic in itself. Any range vendors out there want to help us out? Sidelobes Unwanted gain response of an antenna, in a direction other than the main beam.
MSA • The microstrip antenna was first proposed by G.A. Deschamps in 1953, but didn't become practical until the 1970s when it was developed further by researchers such as Robert E. Munson and others using low-loss soft substrate materials that were just becoming available. • Advantages of microstrip antennas include: • Low cost to fabricate • Conformal structures are possible (it's easy to form curved surfaces, as long as the curve is in one direction only) • Easy to form a large array, spaced at half-wavelength or less • Light weight • Disadvantages include: • Limited bandwidth (usually 1 to 5%, but much more is possible with increased complexity • Low power handling
MSA • The size of a microstrip antenna is inversely proportional to its frequency. At frequencies lower than microwave, microstrip patches don't make sense because of the sizes required. At X-band a microstrip antenna is on the order of 1 centimeter long (easy to realize on soft-board technology). If you wanted to make a microstrip antenna to receive FM radio at 100 MHz it would be on the order of 1 meter long (which is a very large circuit for any type of substrate!) For AM radio at 1000 KHz, the microstrip patch would be the size of a football field, utterly impractical.
Rectangular Single Polarization MSA • This is by far the most popular type of MSA. The figure on the right shows the geometry of the rectangular microstrip antenna, not including the ground plane and dielectric which would be underneath. The dimension L is universally taken to mean the long dimension, which causes resonance at its half-wavelength frequency. The radiating edges are at the ends of the L-dimension of the rectangle, which sets up the single polarization. Radiation that occurs at the ends of the W-dimension is far less and is referred to as the cross-polarization
Rectangular Single Polarization MSA • The image on the right is a side view which attempts to show a snapshot of the E-field under the patch. Note that the fields under the L-edges are of opposite polarity (due to the half-wave nature of the patch). When the field lines curve out and finally propagate out into the direction normal to the substrate they are now in the same direction (both facing left). In the far-field perpendicular to the substrate, the radiation from the two sides adds up because the fields are in phase and voila you have a an antenna! • As you look out in directions off of boresight, the intensity drops off as the fields of the two edges become farther and farther out of phase. At two angles the fields exactly cancel. Thus the microstrip patch radiation intensity depends on what direction you are facing it from (it has gain and directivity).
Rectangular Single Polarization MSA • The ratio of E to H field is proportional to the impedance that you see when you feed the patch. If you adjust the location of the feed point between the center and the edge, you can get any impedance you'd like, including fifty ohms! • Perhaps another intuitive way to look at the input impedance to a microstrip patch is to think about how far you are from an open circuit. If you feed it at the center, you are looking at a short circuit in both directions, because you are a quarter-wave from a short circuit. If you feed it at the edge you see an open circuit, because you are a half-wave from another open. • The image on the right shows two ways to feed the microstrip patch, on the left is a microstrip feed and on the right is a coax feed.
Rectangular Single Polarization MSA • What Dielectric Constant Defines The Half Wavelength? The dielectric constant that controls the resonance of the antenna is the effective dielectric constant of the microstrip line. You can use our microstrip calculator to come up with the value! • What Is The Best Choice For The Dimension W? The dimension helps maximize efficiency. You need to pick W so that: W=c/(2f0xSQRT(εr+1)/2))) In other words, use the average of the value for εr of the substrate and εr of air(=1) to obtain a half-wavelength. • What Controls The Bandwidth? Bandwidth is proportional to h/SQRT(εr)
