Resonant Antennas: Wires and Patches Dipole Antennas Antenna 48 Current distribution approximation Un-normalized pattern: and
Antenna 49 Radiating power: For half-wave dipole and,, or at exact resonance. And
Antenna 50
Antenna 51 Effect of dipole thickness 1. Resonant length: decreases as thickness increases 2. Resonant resistance: decreases as thickness increases 3. Bandwidth: increases as thickness increases
Antenna 52 Flat Dipole Vee Dipole Formula for maximum directivities: and the directivity Properties: 1. Greater directivity than dipole 2. Smaller input impedance than dipole 3. Lower sidelobe and one of the main beams.
Antenna 53
Antenna 54 Folded Dipole Antennas Input impedance for the transmission line mode: Thus, Let be the input impedance of a dipole of length The input impedance will be
Antenna 55 Note: Properties: 1. Impedance can be made to be, the same as the common two wire transmission line. 2. First and second resonances have similar impedances. 3. Wider bandwidth than dipoles For different wire sizes: where
Antenna 56
Antenna 57 Microstrip Patch Antennas Resonant half wave patch: 2:1 VSWR Bandwidth:
Antenna 58 Input resistance adjustment factor, where is the distance from the radiation edges. Patterns:, E-plane,, H-plane, Properties: 1. Low bandwidth. A few percent for single patch. 2. Low profile, light weight. 3. Easy to construct. 4. Easy to integrate to microwave circuits. 5. Low gain. 6. Increase substrate electric thickness, increase bandwidth. But, also increase surface wave, reducing efficiency. Shorted quarter wavelength patch Example 5-2
Antenna 59
Antenna 60 Gain calculation: where is the spacing, is number of elements.
Antenna 61
Antenna 62 Yagi-Uda Antennas Sumarry: 1. Reflector a. Longer than driver. b. Effect input impedance and the back lobe. c. Optimum spacing: 0.15 to 0.25 wavelength. d. Usually only one element. More elements only increases gain a little bit. e. Exact optimum length is primarily a function of the tube diameter and the boom diameter. 2. Driver: usually a half-wavelength dipole.
Antenna 63 3. Director a. Shorter than driver typically by 10 to 20%. b. Optimum spacing: 0.2 to 0.35 wavelength. c. Effect pattern shape and gain. d. Gain increases as number of directors increases. e. Exact optimum length is a function of the tube diameter, the boom length and the diameter of the boom. 4. Design procedure: a. Look up table 5-4 to get the lengths of the directors and reflector. b. Adjust the length by the tube diameter from Figure 5-37 c. Compensate for the changes in the lengths of parasitic elements due to the tube diameter by Fig. 5-37. d. Compensate for the changes in the lengths of parasitic elements due to the boom diameter by Fig. 5-38.
Example 5-1 Parameters 1. Number of element:12. 2. Wavelength: 1.46m. 3. Tube diameter: 1cm. Antenna 64
Antenna 65
Antenna 66 Corner Reflector Antennas Summary: 1. Simple and achieve gain of 10 to 12 db over a halfwave dipole. 2. Good directivity for. 3. Maximum directivity at with input impedance. 4. At, input impedance is, negligible decrease in gain. 5. Effect of finite extent of the plates a. Broaden the pattern, but have little effect on the input impedance.
Antenna 67 b. Usually choose such that the main beam is not degraded. c. Usually choose to be 1.2 to 1.5 times of the length of the feed to minimize the direct radiation by the dipole feed into the back region. Large Loop Antennas
Summary 1. Resonate at integer multiple of wavelength. 2. Input impedance at one wavelength. 3. Gain 3.09 db at one wavelength. Antenna 68
Antenna 69 Wire Antennas Above an Imperfect Ground Plane Vertical oriented dipole Horizontal oriented dipole xz-plane: yz-plane:
Antenna 70 In the united states, typically and. Summary: 1. Plane wave reflection is only an approximation. 2. Surface wave propagates along the ground plane surface. For HF and VHF frequencies, the surface
Antenna 71 attenuates very rapidly. 3. For near,. Vertical antennas close to a real earth have zero radiation. Surface wave accounts for all propagation as in AM broadcasting. 4. For vertical antennas, surface wave can be neglected and plane wave reflection can be used.. 5. For horizontal antennas, the antenna should be above the earth for the plane wave reflection coefficient method to be valid.
Antenna 72 Feeding Wire Antennas Impedance matching 1. Matching network 2. Adjust feed point. For half-wave dipole
Balun (Balance-unbalance) Antenna 73
Antenna 74