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As with all antennas that are physically much smaller than the operating wavelength, small loop antennas have small radiation resistance which is dwarfed by ohmic losses, resulting in a poor antenna efficiency. They are thus mainly used as receiving antennas at lower frequencies (wavelengths of tens to hundreds of meters).
When the antenna is fed at a point of maximum current, as in the common center-fed half-wave dipole or base-fed quarter-wave monopole, that value is mostly the radiation resistance. However, if the antenna is fed at some other point, the equivalent radiation resistance at that point can easily be calculated from the ratio of antenna currents ...
The Foltz drawing pin like antenna from 1998 size 0.62 and 22% bandwidth. The Rogers cone from 2001 is size 0.65 and right on the limit. Lina and Choo planar spirals in size ratios range from 0.2 to 0.5; The fractal Koch curve antenna approaches the limit. [5] A meander line antenna optimizes the size for narrower bandwidths of the order 10%. [11]
Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical. Log-periodic and frequency-independent antennas employ self-similarity in order to be operational over a wide range of bandwidths.
The nulls in the radiation pattern of small receiving loops and ferrite core antennas are bi-directional, and are much sharper than the directions of maximum power of either loop or of linear antennas, and even most beam antennas; the null directionality of small loops is comparable to the maximal directionality of large dish antennas (aperture ...
It is a fundamental property of antennas that the receiving pattern (sensitivity as a function of direction) of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is a consequence of the reciprocity theorem of electromagnetics and is proved below. Therefore, in ...
For antennas shorter than half of the wavelength of the radiation they emit (i.e., electromagnetically "short" antennas), the far and near regional boundaries are measured in terms of a simple ratio of the distance r from the radiating source to the wavelength λ of the radiation. For such an antenna, the near field is the region within a ...
A halo antenna is distinct from the small-loop antenna in size, [a] radiation resistance, and efficiency, but their radiation patterns are nearly the same. A halo antenna is a self-resonant antenna: Its feedpoint impedance is reactance-free / purely resistive at the design frequency.
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