Loop antenna

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor usually fed by a balanced source or feeding a balanced load. Within this physical description there are two distinct antenna types. The large self-resonant loop antenna has a circumference close to one wavelength of the operating frequency and so is resonant at that frequency. This category also includes smaller loops 5% to 30% of a wavelength in circumference, which use a capacitor to make them resonant. These antennas are used for both transmission and reception. In contrast, small loop antennas less than 1% of a wavelength in size are very inefficient radiators, and so are only used for reception. An example is the ferrite (loopstick) antenna used in most AM broadcast radios. Loop antennas have a dipole radiation pattern; they are most sensitive to radio waves in two broad lobes in opposite directions, 180° apart. Due to this directional pattern they are used for radio direction finding (RDF), to locate the position of a transmitter. Self resonant loop antennas are relatively large, governed by the intended wavelength of operation. They are mainly used at frequencies above 3.5 MHz where their size is feasible. They can be viewed as a folded dipole split into an open shape, just as a folded dipole is a full-sized loop, bent at two ends and squashed into a line. The loop's shape can be a circle, triangle, square, rectangle, or in fact any closed polygon; the only strict requirement is that its perimeter must be (slightly over) one full-wavelength. The maximum radiation for a 1 wavelength loop is at right angles to the plane of the loop (See pattern below). At the lower shortwave frequencies a full loop is physically quite large, and for practical reasons must be installed 'lying flat', that is the plane of the loop horizontal to the ground, its wires supported at the same height by masts at its several corners. The main beam is upwards. Above 10 MHz, the loop is more frequently 'standing up', that is with the plane of the loop vertical, in order to direct its main beam towards the horizon. If feasible, a vertical loop may be rotatable, in order to control the direction of the strongest signal. Compared to a dipole or folded dipole, it transmits slightly less toward the sky or ground, giving it about 1.5 dB higher gain in the two favored horizontal directions. Additional gain (and a uni-directional radiation pattern) is usually obtained with an array of such elements either as a driven endfire array or in a Yagi configuration (with all but one loop being parasitic elements). The latter is widely used in amateur radio where it is referred to as a quad antenna (see photo). 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 one wavelength. The most popular shape in amateur radio is the quad antenna or 'quad' (quadrilateral) consists of a resonant loop in a square shape so that it can be constructed of wire strung across a supporting ‘X’ frame. Usually there are other, additional loops stacked parallel to the first as parasitic elements, that make the composite antenna directional. Other 'quads' rotate this 45 degrees to a diamond shape supported on a ‘+’ frame. Triangular loops have also been used. A rectangle twice as high as its width gives a bit more gain than the square loop and also matches 50 ohms directly if used without a reflector. The polarization of such an antenna is not obvious by looking at the loop itself, but depends on the feed point (where the transmission line is connected), and whether it is being operated as a 1, 2, or 3 wavelength loop. If a vertically oriented loop is fed at the bottom at its 1 wavelength frequency, it will be horizontally polarized; feeding it from the side will make it vertically polarized.