When you don’t have much real estate to put up big antennas, or when local regulations prohibit the installation of an antenna tower, a magnetic loop antenna can be a solution. Magnetic loop antennas are relatively small, their circumference is usually between 1/10th and 1/4th of a wavelength. They can be installed close to the ground whilst still having an efficiency close to that of an open dipole antenna.
A magnetic loop antenna is basically an oversized inductor, brought into resonance with an (adjustable) capacitor. They pick up the magnetic component of the electromagnetic radiation generated by other transmitting stations and are less sensitive to QRM. Typically they have a high Q-factor, which makes them very small banded. Because of this, they need to be re-tuned every few kHz the VFO changes. This need for constant tuning can be a real annoyance when it has to be done manually. You easily miss the spot with the lowest SWR and might find yourself in a situation you don’t know anymore whether you need to tune up or down.
Therefore an automatic magnetic loop tuner can become very handy. This tuner will read the frequency your VFO is tuned to and will turn the capacitor, with the aid of a stepper motor, to the exact spot, which you programmed earlier using the auto SWR tune function.
Loftur E. Jónasson – TF3LJ / VE2LJX, designed an automatic magnetic loop tuner which does exactly what I described above. His project page can be found here: https://sites.google.com/site/lofturj/to-automatically-tune-a-magnetic-loop-antenna.
Click on this link to find all related blog post about this magnificent project, or browse through the table of contents and find out how we’ve built it and how it performs.
Table Of Contents
The Beginning
Bill Of Materials (BOM)
Soldering Components
SWR Bridge
SWR Bridge in A RF Shielded Box
Metalwork
Assembly
First QSO
Region 1 SSB Field day
Extra Info
Rotary Encoder
A4975STB, A4988 or DRV8825?
Printed Circuit Board (PCB)
Another Soldering Session
Stepper Motor
Automatic Magnetic Loop Tuner – Kit
Smaller Enclosure, Green Display
IC-7300 Setup
Smaller Enclosure Finished
New Firmware Available (V4.09)
Capacitor Box
Arduino 1.8.11 & Teensyduino 1.50