My 80-15m MLA antenna with “virtual” capacitor

MLA

MLA base

Introduction

This MLA uses both conductor of coaxial cable. Wiring inner and outer conductor of main loop coaxial cable in series helped me to get on lower bands. It adds “virtual” capacitor and inductance to L/C antenna circuit. I got inspiration from OK2ER and his design of MLA-S antenna that uses this kind of great idea. (The idea is also patented by him as well).

Construction

Main loop is made of 3 m long 7/8″ Cellflex cable, so overall diameter is about 1 m. Outer conductor diameter of 28 mm gives advantage in efficiency of the loop and holds shape very well without any supports. I bought the cellflex cable along with connectors for very pleasent price.

Variable capacitor is butterfly type with capacity of approx. 12-125 pF, rated 3kV. It was bought on lc-variable.eu. Capacitor is rotated by Nema 11 stepper motor with 100:1 gearbox. Maybe it is quite overkill ratio, i would be happy with 1:51, 1:25 or just with cheap 28BYJ-48 because capacitor doesn’t need much torque to rotate. Gearbox with motor makes an audible anoying noise (especially for my YL HI) but not big issue. Stepper motor is wired via ethernet cable and remotely tuned. As the variable capacitor can turn 360 degrees, i don’t use limit switches. But you can see an unconnected cable for this purpose ready for future use. I will describe my remote tuner on another post. But briefly, i simply turn the motor for highest amount of noise on my waterfall and than for best SWR.

For holding MLA on its place i use speaker stand (Adam Hall SPS023) with mounting bracket (Adam Hall SPS57) screwed to MLA base. It provides very good stability needed for this quite heavy MLA and it is cheaper than photo tripod suitable for this weight.

 

Mounting bracket and choke

Coupling loop is formed by RG58 cable. I use shielded coupling loop as you can see on picture. Shape of the loop must be adjusted on particular bands for best SWR. It is done by moving plastic bands. It is possible to get close to 1:1.01 SWR with some effort. I have several winds of coax on Amidon FT140-43 ferite core just for sure.

Faraday coupling loop [1]

Coupling loop 40-20M

Coupling loop for 80, 17 and 15M

Band selector

Tunable range of MLA with variable capacitor itself is 20-15M. To change bands it is necessary to add some fixed or virtual capacitor. This is done by banana plugs. “Virtual” capacitor is not virtual in the meaning of being non-existent, it is in fact formed by coaxial cable of the main loop. Also MLA’s inductance will be increased by using this kind of feature. Following wiring was found by experimentally.

Band selector schematic

Banana sockets

80M band

Fixed capacitor for 40M band

20M band

30M band

40M band

80M band

Performance

Maximum usable power should be around 100 watts but i am limited with my 20 watts rig so i didn’t test it. I use it mostly for CW mode and do QSOs around Europe. On my QTH surrounded by buildings, my favorite bands are 80 an 40 meters. It is an antenna of compromise locked in Faraday’s cage but still able to do QSOs. On picture bellow you can see WSPR running with 5 watts and 20M and 40M band:

WSPR 5W, 40 and 20M band

Conclusion

MLA using “virtual” capacity is an alternative to multiturn loop. Adding more turns to your MLA will lower needed capacitance but increase voltage on the variable capacitor. Here the maximum voltage on variable capacitor is actually lowered. This MLA could be able to get on wider range of bands (80-10M) by reducing diameter of the main loop. Based on OK2ER construction of MLA-S, ideal diameter should be around 80 cm. This concept of using capacitance of main’s loop coaxial cable was described by OK1VR [2] [3] and transformed into easy usable MLA by OK2ER [2] (MLA-S meaning MLA SMART [2]).

References

[1] 80-20m Mag Loop. Nonstop Systems: Consulting Services for the Development of Fault-Tolerant Systems. URL: https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

[2] Loop2ER – Magnetic Loop Antennas. URL: https://www.loop2er.cz/magnetic-loop-antennas

[3] OK1VR Macoun, J. Magneticka smyckova antena prakticky (3). Prakticka elektronika 2019-10, URL: http://www.crk.cz/FILES/VR-ANT/85.%20Mag9.pdf

[4] OK1VR Macoun, J. Magneticka smyckova antena prakticky (5). Prakticka elektronika 2020-07, URL: http://www.crk.cz/FILES/VR-ANT/88.%20Mag11.pdf

 

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