Magnetic loop antenna MLA-S (RT)

Magnetic loop antennas have ceased to be ostracized, especially in the last ten to fifteen years, and have become a fairly common indication of station technical equipment. This is due, among other things, to sophisticated digital modes of operation, especially FT8, where very weak signals are sufficient to ensure reliable establishment of links even between continents. These types of antennas do not mind being placed indoors and we commonly encounter the "indoor MLA" indication. The advantage is their transportability and small size, which does not arouse the curiosity and dislike of neighbours. They are also advantageous for holiday activities, where the complicated justification and approval from the owners of the rented property is eliminated. Especially in recent years, the range of commercial MLA types has increased by an order of magnitude and it is difficult to navigate and decide which type of MLA and which manufacturer to favour. As long as the basic physical and design rules are followed in the design of the MLA, the "walkability" of magnetic loop antennas should be very similar and to some extent price may be the main criterion in the selection process.

In what other ways can MLAs differ?

• Maximum usable power
• the number of HAM bands where a given type of MLA can be used
• the diameter of the loop, where, in simple terms, the larger the diameter, the greater the gain.
• disassemblability - transportability
• tuning method (manual/remote)
• for remote tuning (RT), common RF excitation and single coax tuning can be an advantage
• CB (control box) solution, the idea is to control the direction and speed of tuning
• CB power supply

The MLA-S (RT) from OK2ER belongs to the third generation of magnetic loop antennas that can be purchased on a turnkey basis.

The MLA-S (RT) operates from the HAM band 60 m up to 10 m. However, it requires manual switching of the band segments using JUMPers. MLA-S (RT) is remotely tuned, control of tuning and RF excitation up to 40 W is solved by Bias-T over a single coaxial cable. The original CB4M with PWM is replaced by an "electronic selsyn", where the speed of the knob rotation on the CB left/right is transferred to the motor rotation on the MLA tuning capacitor. Power for the CB is provided by integrated Li-Ion batteries, which can be charged using a standard USB-C cable. The advantage of this solution is that it eliminates the RF QRM from the switching power supply. The current state of the art is such that for most commercial applications and uses (except for radio communication purposes), 50 Hz linear power supplies have been successfully replaced by lighter and smaller switching power supplies operating typically at 200 kHz, but which cause an increase in noise levels on the QRM bands of up to several S. The new CB solves this problem elegantly with an integrated rechargeable battery.

The description of MLA-S (RT) in its original version described and labeled as MLA-S (light QRP RT) and the experience with its operation was described in the Czech electro magazine PE-AR by Lada OK1LO, see the attached translation of the original article. His description was complemented by two other OKs, namely Jirka OK1CJN and Tomas OK3TV.

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