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A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; [1] these elements are most often metal rods (or discs) acting as half-wave dipoles. [2]
Similar to how a vertical monopole uses its ground system to produce a "phantom" image of the rest of a dipole, the missing lower half of the half-loop is replaced by its image in the ground-plane. If shaped like a half of a square, a half-loop can operate either as a loop antenna or on its first harmonic as a dipole antenna whose ends have ...
Due to the size required to create a far-field range for large antennas, near-field techniques were developed, which allow the measurement of the field on a distance close to the antenna (typically 3 to 10 times its wavelength). This measurement is then predicted to be the same at infinity.
Patch antenna gain pattern. A directional antenna or beam antenna is an antenna which radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only.
The gain and input impedance of the antenna is dependent on the length of the whip element, compared to a wavelength, but also on the size and shape of the ground plane used (if any). A quarter wave vertical antenna working against a perfectly conducting, infinite ground will have a gain of 5.19 dBi and a radiation resistance of about 36.8 ohms.
Yagi_uda_antenna.jpg (299 × 500 pixels, file size: 108 KB, MIME type: image/jpeg) This is a file from the Wikimedia Commons . Information from its description page there is shown below.
Moxon antenna for the 20-meter band.The antenna is the faint rectangle of wires held in tension by the bent X-shaped support frame. Moxon antenna for the 2-meter band. The Moxon antenna or Moxon rectangle is a simple and mechanically rugged two-element parasitic array, single-frequency antenna. [1]
The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a shorted quarter-wave parallel transmission line stub. [5] [1] [6] For a transmitting antenna to operate efficiently, absorbing all the power provided by its feedline, the antenna must be impedance matched to the line; it must have a resistance equal to the feedline's characteristic impedance.