Functional principle and properties of magnetic loop antennas
The electromagnetic field consists of two components, the magnetic and the electric field. So-called "magnetic antennas" respond predominantly to the magnetic component of the electromagnetic field. Other common names are "Magnetic Loop", "Magnetic Loop Antenna", or more generally just "Loop" or "Loop Antenna" - hereafter referred to as MagLoop for short.
A MagLoop forms a parallel resonant circuit together with the tuning capacitor. Due to the high current within the resonant circuit in the resonance case, the MagLoop radiates a strong, almost exclusive magnetic field in the near field. At a greater distance from the antenna, an electric field is again formed, so that the wavefront no longer differs from the radiation field of a conventional electric antenna (such as long wire, dipole, beams and vertical antennas, etc.).
In practice, MagLoops are almost always mounted vertically for vertical polarization. Occasionally one sees a horizontal arrangement resulting in horizontal omnidirectional radiation. This would even be advantageous for DX with an elevation angle of only about 14°. Unfortunately, this advantage occurs only at a setup height of one wavelength above a largely optimal earth.
In the vertical setup, the vertical diagram shows a uniform omnidirectional radiation over all elevation angles, which is advantageous for DX as well as for the near range. The horizontal diagram has a figure-of-eight characteristic with two broad maxima along the loop plane and two pronounced, narrow and deep minima, transverse to the loop plane. Since the magnetic field in the immediate vicinity of the antenna is less subject to environmental influences, MagLoops can also be operated effectively close to the earth. In addition to other advantageous properties, this fact in particular makes MagLoops ideal for this area of application. By means of a motor drive of the tuning capacitor remotely controlled via an operating device or manually operated, it is possible to switch between the amateur radio bands and to adjust the working and resonance frequency.
At a loop circumference of a quarter wavelength of the operating frequency, MagLoops achieve their maximum efficiency. At a circumference above this, the electric field component increases at the expense of the magnetic field and the loop increasingly becomes an electric antenna again. Below a loop circumference of lambda/4, the efficiency also decreases again, but the preference for the purely magnetic field remains. Unfortunately, a MagLoop with the circumference of a quarter wavelength would become much too large on the lower and middle shortwave bands. In practice, loop circumferences down to 1/20 or 1/32 wavelength are therefore more feasible. On the upper bands from 20 m to 10 m, these sizes are quite comparable in radio practice with a low-suspended half-wave dipole. On the lower bands one has to compromise and accept 2 to 3 S-stages less.
S-stages
What is that?
S-stages
S stands for signal strength. It is an indication of how loud, how strong or how well a distant station is transmitting signals. There is a scale from 1 - 9, 1 stands for 'very poor' and 9 for 'very strong'.
Due to the narrowband nature of the tuned parallel circuit, you get a "large portion" of preselection "for free" on the receiving side. It is always impressive, if you have for example a dipole or a vertical radiator as a comparison antenna. After switching, the signals are clear and distinct against a much quieter "background". The decisive factor for intelligibility is ultimately not the signal strength, but the signal-to-noise ratio (SNR). In the waterfall diagram of an SDR-based transceiver, however, not much can be seen with a tuned MagLoop above and below the receive frequency.
Advantages of magnetic loop antennas: Efficiency and flexibility
- symmetrical, no counterweight required
- high efficiency relative to the small size
- tuning to resonance, low standing wave ratio (SWV) adjustable
- no additional matching device required
- omnidirectional radiation and radiation at all vertical elevation angles
- Quiet reception and additional high preselection
- Suppression of QRM and man-made-noise with the nulls
- Still works well at low elevation and indoor operation
Above all, it is also interesting to see what a MagLoop is still capable of doing on the lowest usable frequency. This is certainly of interest for many, because with little space for antennas a MagLoop comes into question above all, in order to be QRV also still on the deepest band, because there the antenna problem is largest.
Designs and applications of magnetic loop antennas
MagLoops are available for permanent, weatherproof outdoor installation, as well as for temporary portable use or exclusive indoor operation. Stationary operation requires a sturdy, mechanical design with larger aluminum tubular diameters and a weatherproof, virtually waterproof protective housing for the tuning unit. The tuning unit is inserted into the top of the loop and is stabilized at the bottom with a support tube for larger diameters. At the lower end of the loop the coupling loop is placed.
Of course, these designs are electrically suitable for indoor use. However, due to the high mechanical effort of the weatherproof constructions, this would be "casting pearls before swine" and uneconomical, because in this design MagLoops are not exactly a cheap product. In this case, there are cheaper versions that fully satisfy the lower mechanical requirements of a below-roof installation. Those who ultimately operate their MagLoop in the shack in front of the window also no longer necessarily need remote tuning, but can operate it directly manually. These indoor versions usually have the tuning condenser and the coupler installed together, on a stand plate at the bottom.
Frequency ranges and tuning options for magnetic loop antennas
Unfortunately, it is not possible to cover the entire shortwave range from 1.8 MHz to 30 MHz with a single antenna. With the principle of the MagLoop, a frequency range of at least two octaves, depending on the coupling also somewhat more, can be swept from the lowest, usable lower frequency. Thus one would need at least two or further optimized, three MagLoops for the complete short wave range. However, these antennas are often used only for the upper bands. Here one must decide, in order to remain "on the carpet" willy-nilly, on which bands one puts its priorities.
Coupling: Effective signal transmission with magnetic loop antennas
Among the various possibilities for coupling into the loop, symmetrical inductive coupling has become widely accepted. It does not require any further matching measure, such as an antenna matching device. The loop can be grounded directly at the lower mounting point for lightning protection. The coupling can only be adjusted to an optimum, low SWV by deforming the coupling coil. Due to its ease of use, the inductive coupling loop has become widely accepted. Feeding via a gamma match is mechanically somewhat more complex, but can also be set without problems in a solid design. The purely capacitive coupling is more critical in terms of weather protection, which is why this method is only found in some MagLoops for portable operation or indoor use.
Maximum transmit power and manufacturers of magnetic loop antennas
Apart from the portable and QRP versions, the outdoor stationary versions are usually rated for 100 W transmit power. In addition, there are MagLoops for higher output powers of maximum 250 W or 500 W, all of which use contactless split-stator rotary capacitors (AMA) or special designs with retractable and extendable plate packs (Mazzoni). Especially these last two brands are manufacturers of mag-loops also for higher transmit power.
The Tuning Capacitor: Essential Element for Magnetic Loop Antennas
Next to the loop, the tuning capacitor is the essential and critical component of the antenna. High voltages and currents place high demands on this component. To withstand the high RF voltages of up to several kV without flashovers, a plate spacing of several millimeters is the minimum requirement. To avoid the corona effect, unwanted discharges and flashovers, the edges of all plates on the rotor and stator must be rounded. For good contact pickup and to avoid corrosion, the components capacitor plates, support bars and spacers should be made of the same material and all insulating materials should be RF compatible. Above all, good contact pickup at the rotor is a neuralgic point. It is optimal, especially for transmitting powers of more than 100 W, to use a contactless split-stator variable capacitor or butterfly variable capacitor from the outset. For exact tuning, the capacitor axis must run free of play in ball bearings. For UV resistance of the protective pot of the tuning unit, GRP is the material of choice.
Magnetic loop antennas compared to other short-range antennas
There is little mutual interference between a MagLoop and other near-field electrical antennas, especially horizontally polarized ones. Due to the vertically polarized magnetic field of the MagLoop and the horizontally polarized electric field of electric antennas (wire antennas such as dipoles and the like), the coupling is relatively small.
Gain and Efficiency of Magnetic Loop Antennas:
An Assessment
Gain and efficiency are difficult or impossible to measure with amateur measuring equipment, as with all other antennas. From the table "Theoretical gain and efficiency", only approximate values can be estimated. The values refer to the extensive series of AMA antennas, but are exemplary for the order of magnitude of these data of other magnetic antennas. It can be assumed, however, that the theoretical values cannot be achieved in practice.
Theoretical gain and efficiency
-
- Band
- Gain [dB]
- Efficiency [%]
- 40 m
- -4,9
- 21
- 30 m
- -1,3
- 49
- 20 m
- 0,6
- 76
- 17 m
- 1,2
- 88
- 15 m
- 1,4
- 92
- 12 m
- 1,6
- 95
- 10 m
- 1,7
- 97
Theoretical values according to Christian Käferlein, DK5CZ, based on the mathematics of the magnetic antenna by Hans Würtz, DL2FA.
SWV values for outdoor operation
-
- Frequency [MHz]
- SWV
- 7,100
- 1,2
- 10,125
- 1,3
- 14,175
- 1,4
- 18,112
- 1,0
- 21,225
- 1,0
- 24,940
- 1,5
- 28,0–28,5
- 1,7–1,8
SWV values for indoor operation
-
- Frequency [MHz]
- SWV
- 7,100
- 1,0
- 10,125
- 1,5
- 14,175
- 1,7
- 18,112
- 1,0
- 21,225
- 3,0
- 24,940
- 2,5
- 28,0–28,5
- 3,0
An effect, which is caused by the small bandwidths and appears particularly clearly on the lowest band of a MagLoop: The MagLoop tuned to 7 MHz during the day was exposed to solar radiation and thus temperature in the housing of the tuning unit increased. Cooling overnight caused the resonant frequency with the SWV minimum to be 7 kHz lower at 7.093 MHz in the early morning of the next day. This temperature effect is predominantly caused by the variable capacitor, which has a larger plate spacing when heated and a smaller plate spacing when cooled. Because of the larger bandwidths this effect is not noticeable on the higher bands.
Special effect with MagLoops
Directivity
One should not expect too much here, because the maxima are not particularly pronounced but fall out quite broadly. On the other hand, the narrow and deep nulls transverse to the loop plane can be used very well for fading out interfering signals (whether QRM or man-made-noise), if the MagLoop can be rotated. This bearing effect is particularly pronounced in the shortwave band, on groundwave signals and on the higher shortwave bands, on shallowly incident DX signals.
Does the use of a rotor make sense?
In the 1980s, when AMA antennas came on the market, rotatable arrays of AMA antennas on a rotor were often seen. Since the maxima are very wide and thus maximum pointing on the higher bands is only possible in individual cases, a rotor would be uneconomical, at least from a financial point of view. With the additional operation of the rotor for tuning the antenna, not much joy will arise in radio practice.
It is recommended to align the MagLoop for a stationary installation, with its narrow side, i.e. the loop plane, in east/west direction. Then most continents will be well covered for DX. In addition, rotation is not necessary for DL and European traffic on the upper bands, because there is hardly any directivity to be achieved via the steep beam. In contrast to 80 m and 160 m, with restriction still in the 40 m band, there a rotatable mounting would be in the double sense already rather a profit. There, interfering signals, whether local QRM or interfering signals from other stations, can be attenuated or faded out. Of course, a rotatable magnetic antenna would be the optimal case. However, the rotation speed of a conventional antenna rotor is much too slow to find the right setting quickly. This is much faster if you can rotate the MagLoop by hand, which unfortunately is only possible for indoor or portable use.
MagLoops for stationary use
AMA Beetle
Due to their high mechanical complexity and weatherproof construction, AMA antennas are primarily designed for vertical outdoor mounting. The smallest MagLoop AMA 21 has a diameter of 0.8 m and is designed for the range from 20 m to 10 m at 150 W transmitting power. The largest version, AMA 164, is the ultimate MagLoop for the 160 m, 80 m, 60 m and 40 m lowbands. In between are 10 other versions, sensibly graded in diameter, frequency range and power handling, whose ranges overlap and can each be tuned without gaps. Thus, the entire shortwave range from 1.8 MHz to 30 MHz can be served with just two AMA loops. All AMA-MagLoops are supplied with the remote control unit AMA Control-Unit.
Chameleon
Even if a tuned MagLoop for transmit and receive already has very good reception characteristics - there is one thing you can't do with it: Get a quick overview of what's going on on the band or the other bands. A resonant MagLoop is too narrowband and selective for this. It must be continuously retuned for reception even with small frequency changes.
In this case, only a switchable, separate receiving antenna provides a quick overview. In the simplest case this can be a short auxiliary wire antenna on a 1:9-Unun. At best, the Chameleon RX Loop pro would be suitable, but it must be spatially decoupled from the MagLoop used for transmission.
Chameleon offers a loop antenna for reception under the name RX Loop Pro. It is the ideal complement to the operation of a MagLoop as a transmitting antenna. The RX Loop PRO from Chameleon Antenna is a broadband, untuned MagLoop, exclusively for continuous reception from 50 kHz to 30 MHz. As a loop in untuned short-circuit mode, it responds broadband, also almost exclusively to the magnetic component of the field. Thus it also has the DF characteristics already mentioned, in particular the possibility of fading out interfering signals by the minimum bearing. In the end, SDR transceiver operators are also well served by this, which allows them to use their wide waterfall display again without restrictions.
Ciro Mazzoni
The Baby-Loop is the smallest MagLoop from the "antenna forge of Ciro Mazzoni". With a diameter of 1 m, the antenna covers all bands between 40 m and 10 m without any gaps. The loop tube is 50 mm thick and welded directly to the connection ends of the plate condenser without any screw connections. The plate packets are not rotated, but retracted and extended by a stepper motor. The feed is provided by a gamma match. The automatic control unit automatically tunes the loop after startup, without a cumbersome search for the SWV minimum, and is included in the scope of delivery. Already a mounting height on a standpipe, 3 m above ground, is sufficient.
The Stealth-Loop, also from Mazzoni, has been further optimized in its dimensions and in its optical appearance bears little resemblance to the common conception of an antenna. This means that it is well camouflaged against overly curious neighbors in particularly difficult space conditions. The technology with the stepper motor controlled plate capacitor is identical to the BabyLoop. The frequency range is from 6.6 MHz to 29.8 MHz, the maximum RF power is 125 W. The loop is tuned via an ATU 2.0 automatic tuning unit and can accept frequency information from the transceiver for "concurrent tuning" via CAT control. In any case, the Stealth Loop should be set up freely outdoors, with a minimum distance of two meters upwards and to the sides to the surroundings.
With a diameter of 2 m and a maximum permissible power of 250 W, the Midi-Loop is Mazzoni's largest MagLoop. The usable frequency range thus also includes the 80 m band, but the range ends at 14.5 MHz at the top. The design allows a maximum RF power of 250 W. The proven technology with stepper motor control and a control unit with automatic tuning is installed around the plate capacitor. As with the BabyLoop, the feed-in is via a gamma match.
MagLoops for portable and indoor use
Sometimes with indoor operation, the achievable SWV values can be somewhat worse than with outdoor mounting. With indoor operation, unfortunately, the various influences from the immediate environment do come into play, even though MagLoops are generally less sensitive in this respect. The deviations from free outdoor mounting can be compensated by deforming the input loop and tuning it to a low SWV.
Alexloop
Designed specifically for portable use is the Hampack from Alexloop. It is small and lightweight, can be assembled and disassembled without tools, transported in its own backpack, and weighs only 1.3 kg. The Hampack's frequency range extends from 7 MHz to 30 MHz. With a maximum RF power of 10 W AM, FM, CW or 25 W SSB, this MagLoop is ideal for QRP operation on vacation. For setup a short standpipe or a tripod is sufficient. The tuning unit is installed at the base of the loop, reaching the resonance point and the SWV minimum is indicated by an illuminated LED. The exact SWV can be read on a separate SWV meter or on the SVW display on the transceiver. A 2.6 m coaxial connection cable is included.
BTV Loops
Primarily, the MagLoop MLA-M V.9 is designed for stationary indoor operation. In dry weather, it is of course also suitable for portable outdoor use, but not for permanent outdoor installation. The loop with a diameter of 60 cm can be loaded up to a maximum of 10 W and is therefore only suitable for QRP. The tuning unit is located in a larger, stable plastic housing. With two jumpers on the housing it is possible to switch between the three ranges 80 m or 20 m to 10 m, or 60 m to 30 m, respectively.
MLA-T-Pro
With the MLA-T-Pro, BTV-Loops offers a particularly sophisticated MagLoop, which could also be used as a portable unit, but due to the complex remote control with a stepper motor and a remote controller, as well as a weight of 12 kg, it is more suitable for stationary use, but protected from the weather. Control and power supply are via the coaxial cable. With a total of 4 turns of the loop with a diameter of 80 cm, the MLA-T-Pro is a low-band version among the portable MagLoops, which in addition to the bands from 80 m to 40 m also has the 160 m band "on board". The tuning is done by a stepper motor, so that the MLA-T-Pro, which can be loaded with up to 100 W, can also be operated remotely via a controller.
Chameleon
The counterpart to the RX-Loop, which is suitable for transmitting and receiving, is the F-Loop, which can be used for 80 m to 10 m. The F-Loop is a lightweight but at the same time robust construction. Due to the light but at the same time robust construction this MagLoop is portable without restrictions and with a maximum power handling of 25/10 W SSB/CW generously designed for QRP operation. The F-Loop is made of lightweight, high quality coaxial cable. With an optimal cable extension, the loop circumference can be doubled, extending the frequency range down to the 80 m band. In the F-Loop Plus design, which is also available, the loop consists of an aluminum ring 91.5 cm in diameter composed of individual segments. Together with the larger conductor cross-section, this version is significantly more efficient. However, there is no 80 m extension available for this version.
Icom
With the MagLoop AL-705 there is also a "quasi type-bound" MagLoop from Icom for the brand-new QRP transceiver IC-705. Although this MagLoop works of course also at every other transceiver, it is at least in the design for the IC-705. As a lightweight among the MagLoops, it is with a maximum of 10 W CW, 20 W SSB, the ideal companion to this transceiver, to be on the road with a few handles and without tools, quickly QRV. A 3 m long connection cable, with BNC connector suitable for the IC-705, is included.
Alternative solutions for confined spaces
MagLoops already work with low installation height above the ground. Of course, as with any other antenna, there is no harm in mounting a MagLoop as high as possible on a tubular or lattice mast. But given the options, other antennas would probably be considered as well. In most cases, a MagLoop will be the first choice in the absence of an antenna permit, small lot on the carport, balcony or roof terrace of a rental or condominium. Among all the other alternatives available for confined spaces, certainly the best! Not only that one is relative to the small antenna size still with an amazingly good signal. The next problem, if you squat close to your neighbors, is also well under control with a MagLoop - the local QRM. In the close range, you can align a MagLoop in such a way that a permanent source of interference is placed in the null and largely faded out. In addition, a MagLoop is much less responsive in general, compared to the predominantly electrical interference of man-made-noise. To deepen the theoretical and practical basics of magnetic antennas, we finally have a Wimo book recommendation: "Magnetic Loop Antennas
A MagLoop should be the ideal antenna especially for the camper and camping fan. After arriving at the campsite the MagLoop is simply folded up and locked - and you are QRV. You don't have to ask your neighbors first and hang obscure wire structures over their motorhomes and tents.
S-stages
S stands for signal strength. It is an indication of how loud, how strong or how well a distant station is transmitting signals. There is a scale from 1 - 9, 1 stands for 'very poor' and 9 for 'very strong'.
Due to the narrowband nature of the tuned parallel circuit, you get a "large portion" of preselection "for free" on the receiving side. It is always impressive, if you have for example a dipole or a vertical radiator as a comparison antenna. After switching, the signals are clear and distinct against a much quieter "background". The decisive factor for intelligibility is ultimately not the signal strength, but the signal-to-noise ratio (SNR). In the waterfall diagram of an SDR-based transceiver, however, not much can be seen with a tuned MagLoop above and below the receive frequency.
