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.
Large, self-resonant loop antennas For the description of large loops in this section, the radio's operating frequency is assumed to be tuned to the loop antenna's first resonance. At that frequency, one whole free-space wavelength is slightly smaller than the perimeter of the loop, which is the smallest that a "large" loop can be. Self-resonant loop antennas for so-called "short" wave frequencies are relatively large, with a perimeter just greater than the intended wavelength of operation, hence for circular loops diameters between roughly 175 feet (53 m) at the largest, around 1.8 MHz. At higher frequencies their sizes become smaller, falling to a diameter of about 11 feet (3.4 m) at 30 MHz. Large loop antennas can be thought of as folded dipoles whose parallel wires have been split apart and opened out into some oval or polygonal shape. The loop's shape can be a circle, triangle, square, rectangle, or in fact any closed polygon, but for resonance, the loop perimeter must be slightly larger than a wavelength.
Shape
Loop antennas may be in the shape of a circle, a square, or any other closed geometric shape that allows the total perimeter to be slightly more than one wavelength. The most popular shape in amateur radio is the quad antenna or "quad", a self-resonant loop in a square shape so that it can be constructed of wire strung across a supporting ×-shaped frame. There may be one or more additional loops stacked parallel to the first as "parasitic" director or reflector element(s), creating an antenna array which is unidirectional with gain that increases with each additional parasitic element. This design can also be turned 45 degrees to a diamond shape supported on a +-shaped frame. Triangular loops (△-shaped) have also been used for vertical loops, since they can be supported from a single mast. A rectangle twice as high as its width obtains slightly increased gain and also matches 50 Ω directly if used as a single element. Unlike a dipole antenna, the polarization of a resonant loop antenna is not obvious from the orientation of the loop itself, but depends on the placement of its feedpoint. If a vertically oriented loop is fed at the bottom, then its radiation will be horizontally polarized; feeding it from the side will make it vertically polarized.
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