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Small rhombic UHF television antenna from 1952. Its broad bandwidth allowed it to cover the 470 to 890 MHz UHF television band. A rhombic antenna is made of four sections of wire suspended parallel to the ground in a diamond or "rhombus" shape. Each of the four sides is the same length – about a quarter-wavelength to one wavelength per ...
The rhombic is a travelling wave antenna, Each segment of the rhombus has a radiation pattern as shown (grey) having two lobes pointed forward at a certain angle. By making the corner angle equal to twice the tilt angle, the main lobes of each of the 4 sides point in the same direction and reinforce each other, increasing the gain.
In most antennas, the radiation from the different parts of the antenna interferes at some angles; the radiation pattern of the antenna can be considered an interference pattern. This results in minimum or zero radiation at certain angles where the radio waves from the different parts arrive out of phase , and local maxima of radiation at other ...
The rhombic is a nonresonant traveling wave antenna so the length of the elements is not critical, each of the 4 sides is several UHF wavelengths long. The antenna is composed of two wires which are connected at the near end (upper left) with a resistor equal to the characteristic impedance of 470 - 500 ohms, and at the far end to the 300 ohm ...
Antenna directivity is the ratio of maximum radiation intensity (power per unit surface) radiated by the antenna in the maximum direction divided by the intensity radiated by a hypothetical isotropic antenna radiating the same total power as that antenna. For example, a hypothetical antenna which had a radiated pattern of a hemisphere (1/2 ...
The radiation pattern of a thin wire antenna is easily predictable using antenna modeling.For a straight wire, the radiation pattern can be described by axially symmetric multipole moments with no component along the wire direction; as the length of the wire is increased, higher multipole contributions become more prominent and multiple lobes (maxima) at angles to the antenna axis develop. [4]
Radiation resistance is instead calculated by computing the far-field radiation pattern of the antenna, the power flux (Poynting vector) at each angle, for a given antenna current. [23] This is integrated over a sphere enclosing the antenna to give the total power P r a d {\displaystyle \ P_{\mathsf {rad}}\ } radiated by the antenna.
However, that radiation is unpolarized, whereas the antenna is only sensitive to one polarization, reducing it by a factor of 2. To find the total power from black-body radiation accepted by the antenna, we must integrate that quantity times the assumed cross-sectional area A eff of the antenna over all solid angles Ω and over all frequencies f: