Magnetic Loop Antenna
This article presents my magnetic loop antenna (MLA). The antenna covers 7 MHz, 10 MHz and 14 MHz HAM radio bands. Magnetic loop antennas are based on a resonation circuit made of a loop and a tunable capacitor. In my case, the diameter of the loop (coil of two turns) was 52 cm and a butterfly air variable capacitor (12 - 125 pF, 3 kV) was used. The antenna can be tuned by hand but there is high voltage at the capacitor when transmitting, so the tuning knob must be properly isolated. The resonation frequency can be shifted when the hand is placed near the antenna, also due to the high Q of the antenna the resonation is very sharp and it can be difficult to fine-tune the MLA. Therefore, I decided to build my MLA with a motor. For precise positioning, I chose a stepper motor with a gear unit (100:1). As I wanted to avoid an external cable from the antenna to the control unit, a Bluetooth connection was used. The control unit features an LCD touch screen. The firmware for it was written in AVR C and an interesting fact is that I wrote the firmware in a way that the same control board based on ATmega 2560 can be used for both the antenna and the control unit. It allowed me to debug the entire system with just one board. I have already made many contacts around Europe from my flat with this antenna. Usually, 5 - 10 W is sufficient for CW contacts. I have a lot of ideas for further improvements, so I will update this article once I have new modifications.