- 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. A magnetic loop is one in which the current amplitude is constant round the circumference, and it is therefore small enough to avoid a significant standing wavein the current distribution. The antenna is typically smaller than 1/4 wavelength in circumference at the intended frequencyof operation. Antennas for shortwavecommunication are typically very large, sometimes several hundred meters. The advantage of the magnetic loop is its small size, making it suitable for mobile use. Analysis of the design by antenna professionals, confirmed by controlled experiments, has shown however that high radiation efficiency is not obtained and the main advantage of the antenna is its high-angle radiation and compact size. Since radio links with a range of 200-400 km can be maintained with around 1 Watt of radiated power, it is found that small loop antennas when used with transmitters of around 100 Watts are effective in military vehicles.
The technical mechanism uses a
capacitorto "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, which limits efficient operation to a narrow frequency range. A high-Q can be advantageous, however. Since well-tuned magnetic loops function best within a narrow frequency range when tuned, they tend to reject harmonic noise from other RF sources. This keeps the level of unwanted noise down as compared with wider-bandwidth antennas. However all transmitted signals require a finite bandwidth, which can be compromised if the Q factor of the antenna is too high.
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 addition to its narrow bandwidth, a magnetic loop has a radiation resistance one or two orders of magnitude less than a full size antenna such as a dipole. Efficiency thus depends on low loss construction, typically use of thick conductors, and low loss air, mica, or vacuum dielectric capacitors.
In addition to the high currents resulting from the low radiation resistance, high voltages appear across the capacitor when the loop is used for transmission; a kilowatt transmitting loop will have currents of the order of 100 Amperes and voltages at the capacitor of several tens of kV. Conceptually, the magnetic loop can be thought of as a high Q tank circuit having very large diameter to length ratio to facilitate the magnetic "leakage" that makes it effective as an antenna.
Feeding the loop is typically done with 50 ohm coax connected across a smaller coupling loop 1/5 to 1/8 the size of the antenna. This feed loop will provide a match to the loop's low feed resistance over the widest frequency range if it is located on the side of the antenna opposite the capacitor. A less common feed system breaks the tuning capacitor into a two series capacitors with the feed across one of them.
The magnetic loop antenna is an old antenna which is not in universal use in view of its low radiation efficiency. However many
military, commercial, and amateur radio operators still use them today. The Magnetic Loop was widely used in the Vietnam Wardue to its high portability.
External articles and further reading
* " [http://www.aa5tb.com/loop.html Small Transmitting Loop Antennas - AA5TB] "
* " [http://www.stealth.ae/plugins/custompages/articles.php?id=10 Theory of operation of Mobile HF NVIS Magnetic Loop Antenna] "
* " [http://www.standpipe.com/w2bri/ W2BRI's Magnetic Loop Site] "
* " [http://www.g4fon.net/MagLoop.htm Portable magnetic loop Construction] "
* " [http://www.antennex.com/hws/ws0201/nonmag.htm Small non magnetic loop construction] "
* " [http://www.standpipe.com/w2bri/software.htm Antenna design software] "
* " [http://dxradio.50webs.com/ SWDXER.CO.NR] " Receive only loops for shortwave that work.
* " [http://bulldogtrust.com/index12.htm] " Elevated Magnetic Loop for 160 & 75 Meters
* " [http://bulldogtrust.com/index11.htm] " Magnetic Loop Antenna Experiments"Performance of a small loop antenna in the 3-10 MHz band", Boswell, Tyler and White, IEEE Antennas and Propagation Magazine, Vol. 47, No. 2, April 2005.
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