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Unformatted text preview: YEDİTEPE UNIVERSITY DEPARTMENT OF ELECTRICAL and ELECTRONICS ENGINEERING ANTENNAS and MICROWAVE LABORATORY EE 421 – ANTENNAS AND PROPAGATION EXPERIMENT 3: Array Antennas and Beam Scanning Application Objective: Measuring fundamental antenna parameters of slotted waveguide and microstrip array antennas, and observing effects of number of array elements on antenna parameters, and also observing frequency-dependent beam rotation of slotted waveguide antenna array. Equipments: 73701 73705 73706 737 390 737 405 Gunn Oscillator PIN Modulator Isolator Set of Microwave Absorbers Rotating Antenna Platform with: Stand Rod (345 mm.) BNC cable with length = 2/(3.5) m. Plug in power supply RS 232 cable 31177 Steel tape measure Stand Base MF PC with Windows 95/98/NT or higher version 737424 737427 73721 73710 50102 73703 737035 73714 73715 737399 Slot Antenna Microstrip Antenna Large Horn Antenna Moveable Short BNC cable with length = 1 m. Coax Detector Transition Waveguide/Coax Waveguide Termination Support for Waveguide Components Set 10 Thumb Screws Aluminum Foil or Aluminum Adhesive Tape 10 / 1 1 1 1 1 1 2 2 1 2/(3) 1 2 1 1 1 1 1 1 1 1 1 2 1 1 General Information: 1. ARRAY ANTENNAS The radiation characteristic can be considerably improved when an array is used instead of a single radiator. In contrast to the parasitic elements of the Yagi-Uda antenna, the radiators of an array are combined together by a feeder system. 1.1 Slotted Waveguide Array Antenna In practice the slot antenna is very frequently used as a linear array. Due to its fan-formed beam characteristic and high side-lobe attenuation it is preferred in surveillance radar equipment for ship navigation. Slot antennas consist of several coupled single slots. When several slots are arranged in a waveguide, a linear array antenna is born. The slots are fed by a guided wave and radiate a portion of the power supplied into free space. The arrangement, size and alignment of the slots determine the directional characteristic of the complete antenna. Necessary for radiating power out of a slot is that the slot interrupts the surface currents of the wave traveling along the waveguide. If the waveguide is fed with the fundamental mode according to Figure. 1.1.1, maximum radiation results for slots (2) and (3), while the longitudinal slot (1) running symmetrically and located on the broad side of the waveguide (theoretically) will not radiate. Thus slots in this position (2) or (3) are suitable for the assembly of slotted lines, as they are commonly used in waveguide measurement techniques. Figure. 1.1.1 The construction of the slot antenna for a rectangular waveguide carrying the fundamental mode is depicted in Figure. 1.1.2. 10 / 2 Figure. 1.1.2 The slots are alternately arranged around the broad side of the waveguide, showing a slot offset x from the symmetry axis. The slot distance d0 amounts to approx. λG/2. The alternation from slot to slot around the symmetry axis compensates for the polarity reversal of the wall currents. If the slot length amounts to somewhat less than half a free-space wavelength λ0/2 at operating frequency, then the slot is in resonance. The antenna is fed at one end (e.g. by a waveguide). The opposite end is terminated by a short-circuit plate located at a distance d1 of λG/4 from the last slot. Thus, the waveguide transforms the short into an open circuit at the location of the last slot. The parallel connection of no-load with the slot impedance leads to no change in impedance. Because the slot spacing d0 amounts to λG/2, the waveguide functions like a λ/2 transformer and reproduces the slot impedances unchanged at the location of the adjacent slot. Thus, the sum total of all N slots has the same effect at the feeding point of the antenna as the parallel connection of N impedances of equal magnitude! The construction of a slot antenna is carried out in the following steps: • The operating frequency f0 and guided wavelength λG determine the length and spacing of the radiating slots = λ0/2, d0 = λG/2. • The number of slots N is a consequence of the desired directivity or the mechanical construction requirements. • The matching requirements at the feeding point give the necessary transverse impedance ZS for each slot. • From this the required slot offset x can be determined. The feeding of a resonant operating slot antenna is carried out with standing waves resulting in all slots being fed with waves of equal phase. However, if the "output" of the slot antenna is terminated reflection-free with a waveguide termination, then excitation of the slots is carried out with propagating waves. A change in the frequency leads to excitation of the slots with waves of different phase due to a change in the wavelength λG of the guided wave. In the directional diagram of the entire antenna array this leads to a squinting effect of the major lobe. If the frequency of the exciting wave is varied periodically, then the direction of the major lobe varies 10 / 3 periodically too and beam scanning results. Phase-controlled antennas permit the electronic control of the beam direction and are thus superior to the naturally sluggish mechanical systems in detecting and tracing fast radar targets. 1.2 Microstrip Array Antenna 1.2.1 Microstrip Antennas: Microstrip antennas received considerable attention starting in the 1970s, although the idea of a microstrip antenna can be traced to 1953 and a patent in 1955. Microstrip antennas, consist of a very thin (t << λ0 where λ0 is the free–space wavelength) metallic strip (patch) placed a small fraction of a wavelength (h << λ, usually 0.003 λ ≤ h ≤ 0,005 λ) above a ground plane. The microstrip patch is designed so its pattern maximum is normal to the patch (broadside radiator). This is accomplished by properly choosing the mode (field configuration) of excitation beneath the patch. End–fire radiation can also be accomplished by judicious mode selection. For a rectangular patch, the length L of the element is usually λ0/3 < L < λ0/2. The strip (patch) and the ground plane are separated by a dielectric sheet (referred to as substrate). (Figure 1.2.1.1) Figure 1.2.1.1 Microstrip antenna with rectangular patch. 10 / 4 In high–performance aircraft, satellite and missile and applications, where size weight, cost, performance, ease of installation, and aerodynamic profile are constraints, low profile antennas may be required. Presently there are many other government and commercial applications, such as mobile radio and wireless communications that have similar specifications. To meet these requirements, microstrip antennas can be used. These antennas are low–profile, conformable to planar and non–planar surfaces, simple and inexpensive to manufacture using modern printedcircuit technology mechanically robust when mounted on rigid surface, compatible with MMIC designs, and when the particular patch shape and mode are selected they are very versatile in terms of resonant frequency, polarization, pattern and impedance. In addition, by adding loads between the ground plane and the patch, such as pins and varactor diodes, adaptive elements with variable resonant frequency, impedance, polarization and pattern can be designed. Major operational disadvantages of microstrip antennas are their low efficiency, low power, high Q (sometimes in excess of 100), poor polarization purity, poor scan performance, spurious feed radiation and very narrow frequency bandwidth, which is typically only a fraction of a percent or at most a few percent. In some applications, such as in government security systems, narrow bandwidth is desirable. However, there are methods, such as by increasing the height of the substrate, that can be used to extend the efficiency (as large as 90 percent if surface waves are not included) and bandwidth (up to about 35 percent). However, as the height increases, surface waves are introduced which usually are not desirable because they extract power from the total available for direct radiation (space waves). The surface waves travel within the substrate and they are scattered at bends and surface discontinuities, such as the truncation of the dielectric and ground plane, and degrade the antenna pattern and polarization characteristics. Surface waves can be eliminated, while maintaining large bandwidths, by using cavities. Stacking, as well as other methods, of microstrip elements can also be used to increase the bandwidth. In addition, microstrip antennas also exhibit large electromagnetic signature at certain frequencies outside the operating band, are rather large physically at VHF and possibly UHF frequencies, and in large arrays there is a tradeoff between bandwidth and scan volume. There are numerous substrates that can be used for the design of microstrip antennas, and their dielectric constants are usually in the range of 2.2 ≤ ε r ≤ 12. The ones most desirable for antenna performance are thick substrates whose dielectric constant is in the lower end of the range. Because they 10 / 5 provide better efficiency, larger bandwidth, loosely bound fields for radiation into space, but at the expense of larger element size. Thin substrates with higher dielectric constants are desirable for microwave circuitry because they require tightly bound fields to minimize undesired radiation and coupling, and lead to smaller element sizes; however, because of their greater losses, they are less efficient and have relatively smaller bandwidths. Since microstrip antennas are often integrated with other microwave circuitry, a compromise has to be reached between good antenna performance and circuit design. Often microstrip antennas are also referred to as patch antennas. The radiating elements and the feed lines are usually photoetched on the dielectric substrate. The radiating patch may be square, rectangular, thin strip (dipole), circular elliptical, triangular or any other configuration. Square, rectangular, dipole (strip), and circular are the most common because of ease of analysis and fabrication, and their attractive radiation characteristics, especially low cross-polarization radiation. Microstrip dipoles are attractive because they inherently possess a large bandwidth and occupy less space, which makes them attractive for arrays. Linear and circular polarizations can be achieved with either single elements or arrays of microstrip antennas. Arrays of microstrip elements with single or multiple feeds may also be used to introduce scanning capabilities and achieve greater directivities. 1.2.2 Microstrip Antenna Arrays: One of the best features of microstrip antennas is the ease with which they can be formed into arrays. Arrays are very versatile and are used, among other things, to synthesize a required pattern that cannot be achieved with a single element. They are used to scan the beam of an antenna system, increase directivity, and perform various other functions which would be difficult with any one single element. The array element can be fed as a series-feed network. Both the series-feed network and the radiating elements can be made photolithographically, without any need for soldering to the elements. Because of the fact that a change in the excitation of one element affects the excitations of all other elements in a series-fed array, it is very helpful to have an accurate CAD capability for the design of such arrays. 10 / 6 Figure 1.2.2.1 Microstrip Antenna Array Configurations with different feeding networks. Corporate-fed arrays are general and versatile. With this method the designer has more control of the feed of each element (amplitude and phase) and it is ideal for scanning phased arrays, multibeam arrays, or shaped-beam arrays. The phase of each element can be controlled using phase shifters while the amplitude can be adjusted using either amplifiers or attenuators. Radiation from the feed line, using either a series or corporate feed network, is a serious problem that limits the cross-polarization and side lobe level of the arrays. Both cross-polarization and side lobe levels can be improved by isolating the feed network from the radiating face of the array. This can be accomplished using either probe feeds or aperture coupling. Arrays using proximity coupled elements have the advantages of improved bandwidth and reduced spurious radiation over microstrip line-fed elements, but feed radiation is still high enough so that cross-polarization or side lobe levels better than about 20-25 dB are unlikely to be attained. This is not a problem with aperture coupled patch arrays, since the feed lines are shielded by the ground plane [1], [2]. A planar antenna array is build up, when several linear arrays are combined in a parallel configuration. Planar arrays in microstrip line technology are of particular interest. Here it is possible to integrate microwave components directly into the antenna structure (PCB-technology). Thus a more economical production of compact systems can be realized. Unfortunately the radiating patches of the array often interact with each other in an uncontrollable manner. The patch radiator of a microstrip antenna can be considered as a stripline resonator operating at no-load at both ends. Frequently, rectangular or disk shaped elements are used as 10 / 7 radiators. Figure 1.2.2.2 and 1.2.2.3 show the cross section of a rectangular resonator and its related field distribution. Figure 1.2.2.2 Microstrip line and its electric field lines, and effective dielectric constant [2]. Figure 1.2.2.3 Physical and effective lengths of rectangular microstrip patch [2]. The radiation from a single rectangular-shaped patch has almost dipole character. The field distribution at the edges of the resonator demonstrates a "spilling out" of electrical field lines from the resonator. Thus, the electrical length is greater than the mechanical length. The dimensions of the patch along its length has to be extended on each end by a distance ΔL . Here, the following holds true: Leff = L + 2 ΔL 10 / 8 ⎛W ⎞ ⎛ ε reff + 0.3 ⎞ ⎜ h + 0.264 ⎟ ΔL = 0.412 h ⎜ ⎟ ⎜ ε − 0.258 ⎟ ⎜ W ⎟ ⎝ reff ⎠⎜ + 0.8 ⎟ ⎝h ⎠ ε reff = ε r +1 ε r −1 ⎡ 2 + h⎤ ⎢1 + 12 W ⎥ 2⎣ ⎦ −1 2 Preliminary Study: 1. Draw the radiation diagrams in E–Plane ( φ =0) of the following antenna array expression “ Earray ” which simulates the radiation field expression of the array antenna configuration with reflector plate as shown below in the figure. ⎛ ψx ⎞ ⎛ ψz ⎞ sin ⎜ N z ⎟ sin ⎜ N x ⎟ 2⎠ 2⎠ ⎝ Earray = f (θ ) ⋅ , ⋅⎝ ⎛ψ z ⎞ ⎛ψ x ⎞ radiation sin ⎜ ⎟ sin ⎜ ⎟ field ⎝2⎠ ⎝2⎠ expression of HWDA Array factor where ψ x = α x + β d x sin (θ ) cos (φ ) ψ z = α z + β d z cos (θ ) N x , N z correspond to numbers of array elements in x and z directions, respectively α x , α z correspond to phase differences between array elements in x and z directions, respectively d x , d z correspond to distances between array elements in x and z directions, respectively ⎛β ⎞ ⎛β ⎞ ⎛π ⎞ cos ⎜ cos (θ ) ⎟ − cos ⎜ ⎟ cos ⎜ cos (θ ) ⎟ ⎝2 ⎠ ⎝ 2 ⎠= ⎝2 ⎠ for = λ / 2 f (θ ) = sin (θ ) sin (θ ) β= 2π λ0 , λ0 ≅ 3 ⋅108 f0 a) Operating frequency f 0 : 9.40 GHz αx = π , αz = 0 N x = 2, N z = 7 or 5 or 3 d x = 2.10 cm., d z = 2.25 cm. , d x 2 is reflector − antenna distance . Trials for N z = 7 ,5,3 , compare the results. Is HPBW affected by the number of array elements? 10 / 9 b) Operating frequency f 0 : 9.40 GHz α x = π , α z = − β d z cos (θ desired ) N x = 2, N z = 7 d x = 2.10 cm., d z = 2.25 cm. , d x 2 is reflector − antenna distance . Trials for θ desired = 80o , 65o , 45o , 20o ; compare the results. Do the lobes of different phase differences ( α z ) lead to beam rotation at the desired θ desired angles? Array elements are half wave dipole antennas ( = λ / 2 ) aligned along the z axis, and with a reflector plate. 2. Draw the radiation diagrams in E–Plane (θ=π/2) and in H-Plane ( φ =0) (in 3-D if possible) of the following antenna array expression “ Earray ” which simulates the microstrip array antenna radiation field expression, to be experimented in the laboratory, as shown below. ⎡ sin ( X ) sin ( Z ) ⎤ ⎛ β Leff ⎞ j β h W E0 e− j β R sin (θ ) ⎢ sin (θ ) sin (φ ) ⎟ ⎥ cos ⎜ Z⎦ πR ⎝2 ⎠ ⎣X where Eφ = X= βh sin (θ ) cos (φ ) , Z = βW 2 2 and also array factor can be defined as; cos (θ ) , β = 10 / 10 2π λ0 , λ0 ≅ 3 ⋅108 f0 ⎛ ψ⎞ sin ⎜ N y y ⎟ sin ⎛ N z ψ z ⎞ ⎜ ⎟ 2⎠ 2⎠ ⎝ Karray = , ⋅⎝ ⎛ψ y ⎞ ⎛ψ z ⎞ sin ⎜ ⎟ sin ⎜ ⎟ ⎝2⎠ 2⎠ ⎝ where ψ y = α y + β d y sin (θ ) sin (φ ) ψ z = α z + β d z cos (θ ) N y , N z correspond to numbers of array elements in y and z directions, respectively α y , α z correspond to phase differences between array elements in y and z directions, respectively d y , d z correspond to distances between array elements in y and z directions, respectively Earray = Eφ ⋅ Karray Eφ corresponds to radiation field expression of only one rectangular patch array element. Karray is the array factor which simulates the 4x4 planar array configuration. (Thus in fact, this multiplication Earray , is the total field contribution 16 of all 16 array elements→ Eφ ⋅ Karray ≡ ∑ Eφi ) i =1 Note: You can take E0 as an arbitraray value such as 1, and the radial distance away from the antenna R λ0 should be selected at far field region! Operating frequency f 0 : 9.40 GHz Substrate data: RT duroid, with εr= 2.2 and h= 1.57 mm. Feed network: Corporated/Series. Reflector plate: 148 mm. x 130 mm. Array Configuration: Array elements are rectangular patches, 4 × 4 uniform planar array. 10 / 11 Experiment Set-Up Views: Set-up of Horizontal Diagram Measurement of Slotted Waveguide Array Antenna Measurement Configuration Test Antenna (Receiver) Set-up of Vertical Diagram Measurement of Slotted Waveguide Array Antenna Measurement Configuration Test Antenna (Receiver) 10 / 12 BEAM SCANNING BY MEANS OF FREQUENCY CONTROL: Set-up of Horizontal Diagram Measurement of Slotted Waveguide Array Antenna at Different Frequencies Test Antenna (Receiver) Source Antenna (Transmitter) Slotted Waveguide Array Antenna with Matched Load (Waveguide) Termination Vertically Polarized at different frequencies (Moveable Short Position: 21.70 mm – 8.9 GHz) (Moveable Short Position: 16.30 mm – 9.9 GHz) (Moveable Short Position: 12.50 mm – 10.9 GHz) Set-up of Vertical Diagram Measurement of Microstrip Array Antenna Test Antenna (Receiver) Source Antenna (Transmitter) 10 / 13 Set-up of Horizontal Diagram Measurement of Microstrip Array Antenna Test Antenna (Receiver) Source Antenna (Transmitter) Procedure: Notes: If free rotation of the rotary plate is hindered during the measurement, this leads to faulty position entries in the measuring table. In this case, repeat the measurement or enable the menu item go to reference point. Please, place the stand rods into the rotating antenna platform, SLOWLY! Sometimes, on the radiation pattern graph, one can observe that the 360o cycle is not completed, therefore resulting an incomplete radiation pattern graph. You can follow one of the ways: – Click “Approach Reference Point” on the CASSY–LAB software, and then restart the measurement again by pressing F9. – Turn the main–beam direction point of the receiving antenna on the rotating antenna platform into the 180° direction (opposite of the 0° direction). An then measure again. Since it starts and finishes at the back lobe position (if there exists), one can not easily become aware of the incomplete radiation diagram. You can only apply this procedure for directional antennas with small back lobe! 1. Horizontal Diagrams of the slotted waveguide array antenna with number of array elements N=7, 5 and 3: Assemble the experiment set-up as shown in Experiment Set-Up Views, above. Set up the slotted waveguide array antenna so that the slots are located on the side facing away from the source antenna. Thus you manage to get the major lobe to appear on the cartesian plot at the center! The source antenna radiates vertically polarized waves. 10 / 14 i) Record the horizontal radiation diagram of the slotted waveguide array antenna with N=7. Save the measurement and record on the result sheet. (Click on the “Append New Measurement” on CASSY-LAB software) ii) Cover the external slots (left and right) with aluminum foil or aluminum adhesive tape. Record the horizontal radiation diagram of the slotted waveguide array antenna with N=5. iii) Cover each of the outer 2 slots with aluminum foil or aluminum adhesive tape. Record the horizontal radiation diagram of the slotted waveguide array antenna with N=3. Compare the results. One can cover the outer slots with aluminum foils or aluminum adhesive tapes. 2. Vertical Diagrams of the slotted waveguide array antenna with number of array elements N=3 and 7: Assemble the experiment set-up as shown in Experiment Set-Up Views, above. The source antenna radiates horizontally polarized waves. Replace the short plate by the short with the holder, in order to set up a vertical assembly. Insert the slot antenna into the central bore for stand rods in the rotating base. i) Firstly, each of the 2 outer slots of the antenna remain covered with aluminum foil. Record the vertical radiation diagram of the slotted waveguide array antenna with N=3. Save the measurement and record on the result sheet. (Click on the “Append New Measurement” on CASSYLAB software) ii) Remove the aluminum foils or aluminum adhesive tapes from the slots. Be careful not to accidentally rotate the antenna. Record the vertical radiation diagram of the slotted waveguide array antenna with N=7. Compare the results. 10 / 15 3. Beam Scanning by Means of Frequency Control: Assemble the experiment setup as shown in Experiment Set-Up Views, for beam scanning by frequency change. Remove the coupling diaphragm between the Gunn oscillator and the isolator. Replace the simple short plate with the moveable short. The oscillator frequency can be determined using the frequency meter (if possible). The setting of the frequency can be carried out approximately using the following table: Moveable Short Position (x/mm) 21.70 16.30 12.50 Gunn Oscillator Frequency (f/GHz) 8.9 9.9 10.9 4. Vertical and Horizontal Diagrams of the microstrip array antenna: Reassemble the Gunn oscillator back into its basic form with a fixed frequency of f = 9.40 GHz (removing the moveable short configuration). i) Vertical Diagram: Assemble the experiment set-up as shown in Experiment Set-Up Views, above. Excitation is carried out with a vertically polarized E-field of the source antenna. Save the measurement and record on the result sheet. (Click on the “Append New Measurement” on CASSY-LAB software) ii) Horizontal Diagram: Excitation is carried out with a horizontally polarized E-field of the source antenna. Save the measurement and record on the result sheet. Compare the results. 10 / 16 Reference List: 1. Pozar, D. M., “Microstrip Antennas”, Proc. of the IEEE, Vol. 80, pp. 79-91, 1992. 2. Balanis, C. A., Antenna Theory, Analysis and Design, John Wiley & Sons Ltd., New York, 1997. 3. Canbay, C., Anten ve Propagasyon I, Yeditepe University Press, Istanbul, 1997. 4. T 7.6 Antenna Technology Book, Third Ed., LD Didactic Company. 5. Kraus, J. D. and Marhefka, R. J., Antennas for All Applications, New York: McGrawHill, 2002. 10 / 17 ...
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