Loop antenna
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| Antennas |
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A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Loop antennas can be divided into three categories:
Large loop antennas: Also called self-resonant loop antennas or full-wave loops; they have a perimeter close to one or more whole wavelengths at the operating frequency, which makes them self-resonant at that frequency. Large loop antennas have a two-lobe dipole like radiation pattern at their first, full-wave resonance, peaking in both directions perpendicular to the plane of the loop.
Halo antennas: Halos are often described as shortened dipoles that have been bent into a circular loop, with the ends not quite touching. Some writers prefer to exclude them from loop antennas, since they can be well-understood as bent dipoles, others make halos an intermediate category between large and small loops, or the extreme upper size limit for small transmitting loops: In shape and performance halo antennas are very similar to small loops, only distinguished by being self resonant and having much higher radiation resistance. (See discussion below)
Small loop antennas: Also called magnetic loops or tuned loops; they have a perimeter smaller than half the operating wavelength (typically no more than 1 /3 to 1 /4 wave). They are used mainly as receiving antennas because of low efficiency, but are sometimes used for transmission; loops with a circumference smaller than about 1/ 10 wavelength become so inefficient they are rarely used for transmission. A common example of small loop is the ferrite (loopstick) antenna used in most AM broadcast radios. The radiation pattern of small loop antennas is maximum at directions within the plane of the loop, so perpendicular to the maxima of large loops.