A Self-Calibrating Tuning-Tracking Controller for Magnetic Field Loop Antennas

Magnetic loop antennas can be effective in locations where small and/or low-profile antennas are needed. But they have very narrow bandwidths and require frequent retuning. K6KM shares a method he devised to automate the process and provide remote control as well.

Electromagnetic waves travel in space from transmitter to receiver. They carry two components: an electric field and a magnetic field, which are perpendicular to each other. Traditional half-wave dipoles rely on resonating and coupling the electric field component while presenting a practical feedline impedance to couple to the transceiver for good power transfer efficiency. In the quest to reduce the physical size of the antennas, the dipole could be shortened and still radiate; however, the feedline will present a strong capacitive reactive component. The feedline reactive component can be compensated by adding an inductor. The inductor will add losses to the system which reduce the radiation resistance of the dipole. More importantly, given that the dipole acts as a capacitor, any environmental object at near-field will drastically change the characteristics of the antenna, making it impractical for use in more than one location, at which you can optimize it for its surrounding area.

A more robust solution was devised trying to work with the magnetic field component. Instead of a shortened dipole, a shortened magnetic loop was implemented. To be considered short, the physical length of the loop inductor has to be around one tenth of a wavelength.

Once excited by a sinusoidal RF current, the loop acts as a lump inductor, thus less sensitive to nearby objects. The feedline impedance becomes heavily inductive and a capacitor can be connected to compensate the reactive component. Notice that the LC combination becomes a very high Q circuit. It presents very high gain, which is beneficial to overcome the low radiation resistance, but it also provides a very narrow bandwidth. Very high currents are involved in the transmission of RF signals so stray resistances in the loop have to be minimized to avoid power losses. The radiation pattern of magnetic loop antennas is fairly directional at near field with nulls perpendicular to the loop.

In addition to amateur communication, these types of antennas are used for direction finding, AM