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. Within this physical description there are two (possibly three) distinct types:
Building A Magnetic Loop Antenna
For 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. 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.[2]
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. 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.[2]
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.[e]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.
Low frequency one wavelength loops "lying down" are sometimes used for NVIS operation. This is sometimes called a "lazy quad". Its radiation pattern consists of a single lobe straight up (radiation toward the ground which isn't absorbed is reflected upward). The radiation pattern and especially the input impedance is affected by its proximity to the ground. If fed with higher frequencies the antenna input impedance will generally include a reactive part (and a different resistive component) requiring use of an antenna tuner. As the frequency increases, the radiation pattern breaks up into multiple lobes which peak at lower angles relative to the horizon which is advantageous especially at higher frequencies.
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). Like a short dipole antenna, the radiation resistance is small. The radiation resistance R r a d \displaystyle R_\mathsf rad is proportional to the square of the area:
Small loops have advantages as receiving antennas at frequencies below 10 MHz.[7] Although a small loop's losses can be high, the same loss applies to both the signal and the noise, so the receiving signal-to-noise ratio of a small loop may not suffer at these lower frequencies, where received noise is dominated by atmospheric noise and static rather than receiver-internal noise. The ability to more manageably rotate a smaller antenna may help to maximize the signal and reject interference.
A typical diameter of receiving loops with "air centers" is between 30 and 100 cm (1 and 3.5 feet). To increase the magnetic field in the loop and thus its efficiency, while greatly reducing size, the coil of wire is often wound around a ferrite rod magnetic core; this is called a ferrite loop antenna. Such ferrite loop antennas are used in almost all AM broadcast receivers with the notable exception of car radios,[citation needed] since the antenna for the AM band needs to be outside the obstructing metal car chassis.
Small loop antennas are also popular for radio direction finding, in part due to their exceedingly sharp, clear "null" along the loop axis: When the loop axis is aimed directly at the transmitter, the target signal abruptly vanishes.[9]
Wasted power is undesirable for a transmitting antenna, however for a receiving antenna, the inefficiency is not important at frequencies below about 15 MHz. At these lower frequencies, atmospheric noise (static) and man-made noise (radio frequency interference) even a weak signal from an inefficient antenna is far stronger than the internal thermal or Johnson noise generated in the radio receiver's own circuitry, so the weak signal from a loop antenna can be amplified without degrading the signal-to-noise ratio.[10]
For example, at 1 MHz the man-made noise might be 55 dB above the thermal noise floor. If a small loop antenna's loss is 50 dB (as if the antenna included a 50 dB attenuator) the electrical inefficiency of that antenna will have little influence on the receiving system's signal-to-noise ratio.
Surprisingly, the radiation and receiving pattern of a small loop is perpendicular to that of a large self resonant loop (whose perimeter is close to one wavelength). Since the loop is much smaller than a wavelength, the current at any one moment is nearly constant round the circumference. By symmetry it can be seen that the voltages induced in the loop windings on opposite sides of the loop, will cancel each other when a perpendicular signal arrives on the loop axis. Therefore, there is a null in that direction.[11] Instead, the radiation pattern peaks in directions lying in the plane of the loop, because signals received from sources in that plane do not quite cancel owing to the phase difference between the arrival of the wave at the near side and far side of the loop. Increasing that phase difference by increasing the size of the loop has a large impact in increasing the radiation resistance and the resulting antenna efficiency.
Another way of looking at a small loop as an antenna is to consider it simply as an inductive coil coupling to the magnetic field in the direction perpendicular to plane of the coil, according to Ampère's law. Then consider a propagating radio wave also perpendicular to that plane. Since the magnetic (and electric) fields of an electromagnetic wave in free space are transverse (no component in the direction of propagation), it can be seen that this magnetic field and that of a small loop antenna will be at right angles, and thus not coupled. For the same reason, an electromagnetic wave propagating within the plane of the loop, with its magnetic field perpendicular to that plane, is coupled to the magnetic field of the coil. Since the transverse magnetic and electric fields of a propagating electromagnetic wave are at right angles, the electric field of such a wave is also in the plane of the loop, and thus the antenna's polarization (which is always specified as being the orientation of the electric, not the magnetic field) is said to be in that plane.
Thus mounting the loop in a horizontal plane will produce an omnidirectional antenna which is horizontally polarized; mounting the loop vertically yields a weakly directional antenna with vertical polarization and sharp nulls along the axis of the loop.[i]
Since a small loop antenna is essentially a coil, its electrical impedance is inductive, with an inductive reactance much greater than its radiation resistance. In order to couple to a transmitter or receiver, the inductive reactance is normally canceled with a parallel capacitance.[j]Since a good loop antenna will have a high Q factor (narrow bandwidth), the capacitor must be variable and is adjusted to match the receiver's tuning.
Small loop receiving antennas are also almost always resonated using a parallel plate capacitor, which makes their reception narrow-band, sensitive only to a very specific frequency. This allows the antenna, in conjunction with a (variable) tuning capacitor, to act as a tuned input stage to the receiver's front-end, in lieu of a preselector.
Instead of triangulation, a second dipole or vertical antenna can be electrically combined with a loop or a loopstick antenna. Called a sense antenna, connecting and matching the second antenna changes the combined radiation pattern to a cardioid, with a null in only one (less precise) direction. The general direction of the transmitter can be determined using the sense antenna, and then disconnecting the sense antenna returns the sharp nulls in the loop antenna pattern, allowing a precise bearing to be determined.
AM broadcast receivers (and other low frequency radios for the consumer market) typically use small loop antennas, even when a telescoping antenna may be attached for FM reception.[12] A variable capacitor connected across the loop forms a resonant circuit that also tunes the receiver's input stage as that capacitor tracks the main tuning. A multiband receiver may contain tap points along the loop winding in order to tune the loop antenna at widely different frequencies.
Inclusion of a magnetically permeable core increases the radiation resistance of a small loop,[1] mitigating the inefficiency due to ohmic losses. Like all small antennas, such antennas are tiny compared to their effective area. A typical AM broadcast radio loop antenna wound on ferrite may have a cross sectional area of only 1 cm2 (0.16 sq in) at a frequency at which an ideal (lossless) antenna would have an effective area some hundred million times larger. Even accounting for the resistive losses in a ferrite rod antenna, its effective receiving area may exceed the loop's physical area by a factor of 100.[14]
A small transmitting loop antenna with a perimeter of 10% or less of the wavelength will have a relatively constant current distribution along the conductor,[1] and the main lobe will be in the plane of the loop. Loops of any size between 10% and 30% of a wavelength in perimeter, up to almost exactly 50% in circumference, can be built and tuned with series capacitor to resonance. A capacitor is required for a circumference less than a half wave, an inductor for loops more than a half wave and less than a full wave. Loops in this size range may have neither the uniform current of the small loop, nor the double peaked current of the full sized loop and thus cannot be analyzed using the concepts developed for the small receiving loops nor full-wave loop antennas. Performance is best determined with NEC analysis. Antennas within this size range include the halo (see above) and the G0CWT (Edginton) loop. 2ff7e9595c
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