Hanyang University MODERN ANTENNA HANDBOOK by CONSTANTINE A
Hanyang University MODERN ANTENNA HANDBOOK by CONSTANTINE A. BALANIS ch. 3. 2. 5 - 3. 3. 2 Kim Sung Peel 1/22 Antennas & RF Devices Lab.
Hanyang University Contents 3. 2 COMMON APPLICATION OF APERTURE ANTENNAS - 3. 2. 5 Array Elements 3. 2. 5. 1 Slot Antennas : General Comments 3. 2. 5. 2 Slotted Waveguide Arrays - 3. 2. 6 Less Common Applications of Aperture Antennas 3. 3 SPECIFIC ENVIRONMENTS - 3. 3. 1 Space Applications - 3. 3. 2 Harsh Environments 2/22
Hanyang University 3. 2. 5 Array Elements • While any of the aperture antennas discussed here can be used as an array element, there are restrictions in practice. One of these is the physical size of the aperture. • There applications where the main interest is radiation near boresite. • array factor, mutual coupling between neighboring elements, and other aspects of the array environment need to be addressed 3/22
Hanyang University 3. 2. 5. 1 Slot Antennas : General Comments • A slot antenna can be defined as any narrow aperture in a metal surface that is excited to radiate • Most common type of slot used is the simple narrow slot, where the slot width (the narrow dimension) is small compared to the wavelength. • A slot in an infinite ground plane is readily characterized by the equivalent flat wire antenna. • A slot in an infinite ground plane radiates on both sides of the plane. 4/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 1 Slot Antennas : General Comments • In many applications radiation is required on one side only and one way to achieve this is to back the slot by a cavity. • When a slot is cavity backed in a practical application, the solution is no longer a simple one. <cavity-backed slot in a finite ground plane> 5/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 1 Slot Antennas : General Comments • A slot antenna on the body of an aircraft or missile, the slot will reside on a closed structure approximated by a cone or cylinder. • A widespread application of a radiating slot is where it is cut into the wall of a waveguide. • While this can be any type of waveguide, the most common use is with a rectangular waveguide. <slot cut in a cylindrical structure> <Various types of waveguide slots> 6/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 1 Slot Antennas : General Comments • Unlike the other aperture antenna types discussed in this chapter, slot antennas do not easily lend themselves to meaningful simple design formula • When this antenna is embedded in a finite structure, edge and curvature effects need to be addressed • While these and other applications of slot antennas have been developed extensively over many years, deducing simple design formulas from this body of work has remained elusive. 7/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays (a) Displaced-longitudinal slot array (b) inclined-center slot array (c) edge-slot array • The array can operate as either a resonant array or a traveling-wave array • To shape the radiated power distribution, one can vary the position of the slots in order to change the value of the radiation conductance in line with the required power distribution. • Longitudinal slots are used to produce the radiation polarized perpendicular to the array axis. • Inclined series slot is not popular because of its high cross-polarization level, which could be a serious drawback in some applications • Edge-wall slots are used to produce the radiation polarized parallel to the array axis. 8/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • Example of slotted waveguide arrays radiation pattern →Radiation patterns of linear resonant edge-wall slotted guide arrays with 6, 20 and 50 elements respectively. 9/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • In the case of the longitudinal broad-wall slot, the radiation conductance is where δ(D) is the slot displacement from the guide center. 10/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • In the case of the inclined-center slot, the radiation resistance is 11/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • In the case of the edge slot, the radiation conductance is • edge-slot arrays are commonly used given that the element spacing, , can be kept within a half-wavelength to avoid grating lobes in wide-angle scanning applications 12/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays 13/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays →Main beam radiates at a frequency-dependent angle: 14/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • Traveling-wave array should be well matched otherwise the reflected power from a termination will radiate an unwanted sidelobe at an angle of −θ. • The reflected power from the termination must be kept sufficiently small so that the spurious beam is below the required sidelobe level. 15/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 5. 2 Slotted Waveguide Arrays • Ridge waveguide : ridge waveguide presents a solution to the frequency bandwidth enhancement of a waveguide slot array antenna. A design based on normal ridge waveguide can yield 7% or even more frequency bandwidth. • Radial line slot antennas for both circular and linear polarization. <Radial line slot antenna> <Ridge waveguide> 16/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 6 Less Common Applications of Aperture Antennas • We mention briefly here other applications where aperture antennas(horns) Tracking Feeds These feeds are designed to enable a target to be tracked. An example is an Earth-station antenna that is required to maintain a link to a given satellite. Probes Simple circular or rectangular waveguide working in the dominant mode used mainly for antenna measurements in near-field scanners. Be aware that the wall thickness at the aperture of the probe has a significant effect on the radiation pattern. <probe antennas> 17/22 Antennas & RF Devices Lab.
Hanyang University 3. 2. 6 Less Common Applications of Aperture Antennas Monopulse feed systems are often used in radar applications. Prime-Focus Feeds for Large f/D Paraboloid Reflectors These reflectors are sometimes used for very small aperture terminal (VSAT) antennas or for applications where very low antenna sidelobes are required. <Monopulse feed system> 18/22 Antennas & RF Devices Lab.
Hanyang University 3. 3 SPECIFIC ENVIRONMENTS 3. 3. 1 Space Applications • An important aspect to consider for space applications is the premium on space on a spacecraft. • Trade-off between compactness and performance as it is not always possible to design a compact antenna with the required performance. →Much effort is needed to reduce the size and weight of L-band (1 -GHz) or S-band (2 -GHz) antenna. • Satellite antennas tend to have low sidelobes to limit these interactions. (Risks of interference and obstruction from surrounding structures) • Another point to keep in mind is the choice of materials →A number of dielectric materials are “space qualified” in that they have been designed and approved for space applications • They also need to withstand the harsh changes in temperature. 19/22 Antennas & RF Devices Lab.
Hanyang University 3. 3 SPECIFIC ENVIRONMENTS 3. 3. 2 Harsh Environments • Definition: conditions that could be harmful to the antenna itself to the extent that it adversely affects its performance. Water Where heavy rainfalls are the norm, high frequency operations (e. g. , Ka-band(20~30 GHz)) can suffer and become almost completely ineffective during heavy rainfall. Ice and Snow The formation of ice and snow on antennas will severely degrade the performance. →Some kind of anti-icing system is required. Maritime Applications As for satellite applications, the available space on the deck of a ship is limited and there are numerous possible scatterers near the antennas. For these applications, the same global approach as for satellite antennas is necessary. 20/22 Antennas & RF Devices Lab.
Hanyang University 3. 3 SPECIFIC ENVIRONMENTS 3. 3. 2 Harsh Environments Wind The mechanical design has to be such that the antenna is given a “wind-factor” rating. It is related to the survivability and performance of the antenna itself. There may be a necessity to keep the antenna steady under heavy wind loading and to prevent damage to the antenna. (e. g. , Earth station antenna) Vibrations and High Shocks This is especially important for flyaway-type antenna systems and military applications. <flyaway-type antenna> 21/22 Antennas & RF Devices Lab.
Hanyang University Thank you for your attention 22/22 Antennas & RF Devices Lab.
- Slides: 22