New Concept Design of Magnetic Loop Antennas
- New Concept Design of Magnetic Loop Antennas
Oldřich Burger-OK2ER, Lev Kohút - OK2PLL
The new MLA-SMART design of magnetic loop antennas is a leap forward. The paper presents a new design concept of magnetic loop antenna which can function over an incredibly wide frequency range, and with a high usable input RF power, unusual for such small-size antenna.
Both features of a MLA in this price category have no comparison in the world. What is particularly interesting is that MLA-SMART can be manufactured at home without workshop tooling. We can even say that it could be made in a small living room in an apartment block.
The new design is no compromise but it generates a well-functioning antenna covering nine shortwave amateur bands. All costs of the needed material is close to the cost of a two-person lunch in an average restaurant.
The key idea which allowed a significant simplification of MLA design, was the concept of MLA- ER (1,2). Its principle and design is so bizarre, simple and clever that even experienced antenna experts missed a robust high-voltage capacitor for MLA-ER. It is, however, important to note that MLA-ER is not a QRP-type device but that it can handle RF input power exceeding 100 Watts.
The bizarre design of MLA-ER caused a long delay in acceptance among the radio-amateur community, although it was presented in English in a monograph (2). A paradoxic fact is that MLA-ER was mentioned in important radio-ham magazines (3,4) at the same time when the new concept of magnetic.loop antennas is being finalized as MLA-SMART, and here it is presented.
A happy coincidence occurred in 2019 when MLA-ER was tested by Jindra Macoun, OK1VR, who indicated that MLA-ER could be made more practical (5). The original concept of MLA-ER was, even having undisputed advantages, not too user-friendly, required to use an antenna analyzer and experienced users for a good performance. Improvements by OK1VR have pushed MLA-ER to a better antenna but it mainly inspired OK2ER to design the new MLA-SMART. New ideas create more new ideas in life.
Jindra's improvement of MLA-ER (5) stimulated OK2ER in testing several models of magnetic loop antennas (see TESTED VERSIONS). The new principle is described below. The new loop design I named MLA-SMART. It comprises not only one model of a MLA but a new loop design. The new solution was registered as EUIPO new protected design (6.7,8).
The basic principle of magnetic loop antennas is identical in all types: The radio-frequency current in a resonating loop generates an electromagnetic field in its vicinity, with a phase-shifted E/H components that can, according to Maxewll's theory, propagate from the source by the speed of light. The base of any MLA is the L/C circuit that can be shaped in an „mfmite“ number of shapes and sizes. The inductance of this resonant L/C circuit is made as one turn (or more) of a thick conductor, and the capacitor C is a sum of all capacitances in this L/C circuit. The capacitance C may be created not only as a single component like shown in Fig.1, but it is also the inter-tun capacitance if the loop has more than on turn. This feature is utilized in MLA-ER/II and MLA-W (2)s with two-turn loop, which are tuned by varying the inter-turn capacitance.
In MLA-ER with one turn loop, the capacitor is shaped as an inserted pad, Fig.2, which was almost invisible. In the MLA using OK1VR's concept, this „capacitive pad“ is formed as the center conductor of a coaxial cable the length of which is defined by the user (defining the loop resonance) in a bit complicated way (5). With MLA-SMART the solution is much simpler. Here all involved capacitances, series and parallel, are utilized in the L/C circuit. In practice it is a simple combination of two connected loop ends of the coaxial cable (6,7) which take part in the resonant capacitance of the L/C circuit. For the user, both versions of the circuit (6,7) are defined by one external jumper. This defines the top and bottom section of a short-wave band, Fig.3.
The advantage of MLA-SMART design is that with a suitable loop length, using a simple tuning capacitor, the full short-wave band coverage is achieved. Another advantage of this design is that the resonant RF voltage in the L/C circuit is divided among the circuit capacitances, so even a small-size air-gap variable capacitor (a type used in receivers) which in other MLA designs allowed to use only up to 10-20 W of RF power input, with MLA-SMART the RF input can be higher by 23 times.
Coupling RF power from a cable to a MLA is done by a variety of methods (2). The most often used is the induction by Faraday Coupling Loop, FCL. There are numerous other ways, for instance, galvanic or capacitive coupling (2), which offer a simpler mechanical design in a MLA. With those methods, the FCL requiring mechanical movement can be avoided.
The typical capacitive coupling is used in MLA-M (2). One of the mentioned prototypes of MLA- SMART uses FCL induction coupling, Fig.6. Another model used the „triangle match“, Fig.11. Another prototype of MLA-SMART uses a special capacitive coupling employing the center conductor of a coaxial cable section (8), Fig. 12, or an added external conductor, Fig.13 which is only another form of (8).
To seriously evaluate which of the mentioned coupling methods is best we have so far not enough data. Always user's opinion would matter.
MLA-SMART PRACTICAL DESIGN
Over three months, around ten varieties of MLA-SMART were made and evaluated. They differed in design details as well as by the preferred way of using the antenna, see below in Tested Versions. The design principle of MLA-SMART is utilized in other models, MLA-DT, MLA-DUO, MLA- DIGI, see Figs. 6,7,8. For a majority of MLA-SMART, as the loop material a coaxial cable was used. When larger-diameter cables were used, like Nordic 10/50, Celflex SCFL 1.2-50J, LRDK- 50A, TXM SF-1/2 and other types, it is preferred to use matching connectors to make rigid loops with an ideal diameter of 80-100 cm. With lower-cost cables like RG213, RG214 with suitable RF parameters, some rigid support structure for the loop is needed.
Some examples can be seen in Figs. 9,10. Detailed descriptions of various mechanical designs are out of scope for the PE-AR magazine. I would admit that the concept of MLA-SMART may need to separately describe individual designs as the prototypes had to be modified according to various user requireents. The best approach is to use the internet and author's website, www.loop2er.eu. Let us notice that the common denominator of all new MLA-SMART types is the use of 3D printing. This is also the novelty by which the new generation of OK2ER products differs from the earlier MLA generations, MLA-M, MLA-T and others in which the author utilized low- cost components available within the technology of water piping, heating and gas devices.
As two real examples of practical versions of MLA-SMART, for this paper two models were selected, MLA-DT (desk-top), and MLA-TR (tripod mounted) which are also the simplest, see Figs. 6A, 6b. Those antennas operate from 3.5 to 30 Mhz, are manually tuned and positioned, see in the following.
CAUTION, however: I must point out that even with the QRP input power, less than 10 W, it is required that any person handling the MLA must keep a distance longer than one meter or more. It is not clever to operate with a human head in the loop. Theoretical study of the safety problem can be found in (2,9).
Technical parameters of MLA-DT (desk top)
Frequency bands: 3.5, 5.3, 7, 10, 14, 18, 21, 24, 28 Mhz
RF Input power: 30 W
Output impedance (adjustable): 50 Ohm
Achievable SWR: 1:1
Loop diameter: 80 cm
Weight: 1 kg
This new all-band MLA replaces the widely used multiband MLA-M, the advantages of which were: frequency range, quality of manufacture, and the small loop diameter, 60 cm. The new design allows for a higher input power, mainly at 3.5 Mhz band. This makes MLA-DT the best over all competing MLAs on commercial market. One loop design improves the performance at higher bands, without short-circuiting the loops like in MLA-M which degraded the Q factor. MLA-DT can also be turned by hand in azimuth while it is laid e.g. on a desk.
Technical parameters of MLA-TR (tripod-mounted)
Frequency bands: 3.5,5.3,7, 10, 14, 18, 21, 24, 28 Mhz
RF Input power: 30 W
Output impedance (adjustable): 50 Ohms
Achievable SWR: 1:1
Loop diameter: 80 cm
Weight: 1 kg
It can be seen that in technical parameters, MLA-TR is equal to MLA-DT. Only the design concept is different, a special arrangement allows to attach the antenna to a tripod for an easy turning. The mount device utilizes UNC 1/4 thread common in cameras, so any camera tripod can be used.
Atenna pointing is also manual.
TESTED VERSIONS OF MLA-SMART
- Desk-top, manually tuned
- Tripod-mounted, manually tuned
- Remotely tuned by a DC motor
- Remotely tuned with an integrated rotator, DC motors
- Remotely tuned, stepping motor
- Remotely tuned with an integrated rotator, stepping motors
- Remotely tuned, with a memory to sture variable capacitor position (frequency), and position of the coupling loop (impedance)
Including sub-versions there are more than ten versions. It was a „design storm“, HI.
EVALUATION OF MLA-SMART BY OK2PLL
I received gradually four versions of MLA-SMART to assess the new concept of magnetic loop antennas:
- A loop made of a coaxial cable with a remote control by a DC motor,
- Loop made of Cuprotherm, with manual tuning,
- Loop made of a coaxial cable, with a remote tuning and rotating, both by DC motors,
- Loop made of PEXAL, with an integrated tuner (capacitive coupling), and manual tuning.
All above versions had a 80 cm diameter loop, with the L/C circuit designed by the new unique
design by OK2ER. As the modification of a classical MLA into MLA-SMART can be easily done, I modified accordingly two of my older MLAs, so I could compare features of six MLAs.
For my older MLAs I used the Cellflex cable 7/8, with the loop diameter of 90 cm, another my MLA has 1 m diameter and was made of Nordinx coaxial cable 10/50.
All tested MLAs are fully operational, tuning range from 3.5 to 30 Mhz, one older MLA with a larger diameter could operate only to 25 MHZ. Switching between up and low bands is done by a jumper, see Fig.3. The version No.4 can be used with a tuner without a coupling loop in lower bands, 3.5, 5.3 and 7 Mhz. For higher bands it is required to use the FCL coupling loop.
About all six versions tested MLA connected as MLA-SMART can be said that they function equally well. Minor differences can be seen over loop diameter and conductor diameter. With one version of my own design I tested the remote sensing by a stepping motor with an encoder. This method is similar to manual tuning but the remote tuning can operate well over 10 meter distance (verified), possibly even more.
The best advantage of MLA-SMART design is the wide frequency tunability, and the substantial input power capability with a common RX-type variable capacitor, without the earlier requirement for a high-voltage variable capacitor with wide gaps. The MLA model with remote tuning and the integrated rotator, Fig.7, is a very user-friendly design a it allows antenna pointing to the direction of an opposite station. All MLA-SMART antennas I have tested also in a horizontal loop position in which the antenna becomes omni-directional, and allows to receive signals from any direction. With that position I did not see any degradation of its practical use. Only the radiation effect changed. As the test TRX I used SDR TRX mcHF set to to 1 W output power, and programmed by WSJT-X. As I noticed with the practical tests, with MLA-SMART I could comuni- cate within Europe but with good conditions, also farther destinations. Based on the mapped response of WSPR beacons, only one Watt can achieve communication beyond Europe, also to more distant regions, depending upon the frequency and propagation conditions.
Using CW and one-Watt input power, I could achieve DX QSOs over 8000 km, also from a MLA located indoors, although the MLA is affected by surrounding objects and building materials.
Some examples I included in photos from WSPR.net. The MLA-SMART concept is well usable as an indoor QRP antenna, ideal for digital operation modes.
The concept of MLA-SMART is a novel design of a well-known magnetic loop antenna which probably would make this antenna available to more radio amateurs than before. One reason why the oldest antenna design reappeared after more than 100 years, is its minimal occupied area. Seeking antennas requiring less space and being invisible is linked to official restrictions (local codes) which tend to limit using of long-wire antennas. I do not include only „monsters“ like multielement directional rotary antennas. Even using a simple wire antenna became quite limited.
Using MLAs was so far limited by their lower efficiency at lower bands, and cost. The important ratio, D/λ, is a quite small number in practical MLA, with loop diameter 1 m approx. This ratio significantly degrade antenna ERP. The rapidly growing number of short-wave stations using digital modes (mostly FT8), will apparently push the MLA use in lower SW bands, 1.8 Mhz, 3.5 Mhz and 5.3 Mhz, where the poorer efficiency and higher cost caused so far their lesser use.
Modern digital operation modes allow to use a fraction of RF power to make DX QSOs, than was possible with SSB. For the „classical“ phone operation, however, the long-wire antenna (if it can be used) the best solution.
The commercially available MLAs exhibit a less versions capable to operate also at 1.8 and 3.5 Mhz. The reasons are not only the lower efficiency, others are loop size, mass, and generally more difficult mechanical design. Above all, product cost is one of the main reasons for those deciding on what antenna to choose.
The new low-cost MLA-SMART concept probably will allow to use the magnetic loop antenna, with under 1 m diameter, also on lower bands. One only needs to be cautious about antenna circumstances: a small magnetic-loop antenna laid on an interior desk cannot really function well compared to a half-wave wire dipole in a good height above ground. One should avoid situations in which the waves behave „strangely“ and the laws of physics seem invalid: such situations are rarer than those where physics laws hold.
I invite you to try these „toys“ that can give you a joy of nice QSOs.
- MLA-ER PE-AR 2019
- MLA Slightly Different Each Time, IVEdition
- CQ HAM RADIO JA1 2020
- Funkamateur 2, 2020
- PE-AR OK1VR 10, 2020
- Master's Thesis, Petr Jaburek, Near Field of an electrically small loop antenna
- Capacitive inserts MLA-ER
- Capacitive coupling
- Desk-top Design Example
- DUO Design Example
- Tripod-Mount Design
- Rigid Frame
- Triangle Match coupling
- Capacitive coupling
- Added conductor