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Encyclopedia > Magnetic loop

Magnetic loop antennas (also known as Small Transmitting/Receiving Loops) have a small antenna size compared to other antennas for the same wavelength. The antenna is typically smaller than 1/4 wavelength of the intended frequency of operation. Antennas for shortwave communication are normally very large sometimes covering hundreds of feet or meters in length. The advantage of the magnetic loop is that with its small size it maintains very high efficiency levels. A Yagi-Uda antenna An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. ... The wavelength is the distance between repeating units of a wave pattern. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... A solid-state, analog shortwave receiver Shortwave radio operates between the frequencies of 2,310 kHz and 30 MHz (30,000 kHz) [1] and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly...

The technical mechanism is to use a capacitor to "enlarge" the antenna and bring it to resonance. The disadvantage of this method is the low bandwidth of the antenna, also known as "high Q". However, a "high Q" antenna also has advantages as well. In reception: Since Magnetic Loop antennas only function within a narrow range of frequency when tuned, they reject harmonic noise from other radio sources. This rejection of interfering noise from other harmonically related frequencies keeps the noise level down compared to other antennas like the common 1/4 wave vertical antenna. Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. ... The Tacoma Narrows Bridge (shown twisting) in Washington collapsed spectacularly, under moderate wind, in part because of resonance. ... Bandwidth is a measure of frequency range and is typically measured in hertz. ... Radio noise If there was no noise picked up with radio signals, then tiny transmitters could be received at any distance just by making a radio receiver that was sensitive enough. ...

As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna needs to be retuned by changing the capacitive value of the antenna. Bandwidth is the usable frequency range of an antenna in relation to the area of desired operation. When the antenna is operated outside of its bandwidth, the energy from the transmitter is reflected back from the antenna, down through the feedline back to the transmitter. The term bandwidth relates to the concept of Standing Wave Ratio or SWR. When the reflected power exceeds a 2.5:1 power reflection ratio (too much energy being reflected from the antenna back into the feedline) the antenna will not maintain its performance characteristics. This type of condition relates specifically to the antenna's ability to transmit radio energy from the transmitter to the antenna. In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum). ...

The magnetic loop antenna is an old antenna, however, many military, commercial, and amateur radio operators still use them today. The Magnetic Loop was widely used in the Vietnam War due to its high portability. Commercial broadcasting - Wikipedia /**/ @import /skins-1. ... An amateur radio operator An amateur radio operator is an individual who has been assigned an amateur radio license by a governmental regulatory authority. ... Combatants Republic of Vietnam United States Republic of Korea Thailand Australia New Zealand The Philippines National Front for the Liberation of South Vietnam Democratic Republic of Vietnam People’s Republic of China Democratic Peoples Republic of Korea Strength US 1,000,000 South Korea 300,000 Australia 48,000...

External articles and further reading

General websites
  • "magnetic loop antenna 10-30 mhz", mashoo.funpic.org, personal website of a radio enthusiast
  • "Theory of operation of Mobile HF NVIS Magnetic Loop Antenna"

  Results from FactBites:
What is a magnetic loop antenna (1029 words)
A magnetic loop antenna is so called because unlike a conventional wire dipole the small transmitting loop aims to generate the magnetic field rather than the electrical one.
A magnetic loop antenna is comprised of a (usually but not necessarily) round conductor of a total length perhaps one quarter (Roberto I1ARZ Radio Communication March 1992 p66-68 and April 1992 p30-31) or more commonly for short wave one fifth or one eight of a wavelength.
Such loops are simple to build, and are regularly a solution when a transmitting antenna for shortwave frequencies is needed indoors, in a very small garden, to be hidden, or where there is local electrical interference, or a high density of broadcast television receivers.
Intermediate-sized loop antennas. (4546 words)
Since the current is assumed constant around the loop circumference, the lengths of the phasors involved in this calculation are identical, and, were it not for the phase delay for the radiation to cross the diameter of the loop, the phasor contributions would be exactly opposite in direction, resulting in complete cancellation.
Loop radiation is often (usually) measured with the loop mounted in a vertical plane, and one goes away a significant distance on a flat ground so that the radiation is measured on a horizontal (level) path in the plane of the loop.
In the case of the 500mm circumference loop with an antiresonance at 171MHz, we may deduce that the inductance of the loop, regarded as a single turn of non-radiating wire, is tuned or balanced by the capacitance of a folded dipole mode, for a total dipole length of lambda/4.
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