The radiation pattern of a first-resonance loop antenna peaks at right angles to the plane of the loop. If a vertically oriented loop is fed at the bottom, its radiation will be horizontally polarized feeding it from the side will make it vertically polarized. 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. A rectangle twice as high as its width obtains slightly increased gain and also matches 50 Ω directly if used as a single element. Triangular loops (‘ △’) have also been used for vertical loops, since they can be supported from a single mast. ![]() This design can also be turned 45 degrees to a diamond shape supported on a ‘ +’ shaped frame. There may be one or more additional loops stacked parallel to the first as 'parasitic' director and / or reflector element(s), creating an antenna array which is unidirectional with gain that increases with each additional parasitic element. ![]() 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. 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. Shape Ī quad antenna is a self-resonant loop in a square shape this one also includes a parasitic element. 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. Large loop antennas can be thought of as a folded dipole whose parallel wires have been split apart and opened out into some oval or polygonal shape. At higher frequencies their sizes become smaller, falling to a diameter of about 11 ft (3.4 m) at 30 MHz. 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 ft (53 m) at the largest, around 1.8 MHz. At that frequency, one whole wavelength is slightly smaller than the perimeter of the loop, which is the smallest that a "large" loop can be. Large, self-resonant loop antennas įor all of the large loops described in this section, the radio's operating frequency is assumed to be tuned to the loop antenna's first resonance. 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 A common example of small loop is the ferrite (loopstick) antenna used in most AM broadcast radios. They are used mainly as receiving antennas, but are sometimes used for transmission despite their reduced efficiency loops with a circumference smaller than about 1 / 10 wavelength become so inefficient they are rarely used for transmission. ![]() Small loop antennas (or magnetic loops or tuned loops) have a perimeter smaller than half the operating wavelength (typically no more than 1 / 3~ 1 / 4 wave). ![]() 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 limit for small 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. Halo antennas are shortened dipoles that have been bent into a circular loop, with the ends not quite touching.Large loop antennas have a two-lobe radiation pattern at their first, full-wave resonance, peaking in both directions perpendicular to the plane of the loop They are the most efficient of all antenna types for both transmission and reception. Large loop antennas (or self-resonant loop antennas or full-wave loops) have a perimeter close to one or more whole wavelengths at the operating frequency, which makes them self- resonant at that frequency.Within this physical description there are two (possibly three) distinct types: A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load.
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