Magnetic Antenna

a loop antenna (usually multiturn) with a core of magnetic material, usually a magnetodielectric or a ferrite (ferrite antenna). Magnetic antennas are used mainly for the reception of radio waves in radio direction finding and navigation and very extensively for small radio broadcast receivers. Their directivity patterns are the same as for an ordinary loop antenna. The loop of a magnetic antenna is usually connected to a variable capacitor, thus forming a parallel resonant circuit, which is tuned to the operating wavelength, at the receiver’s input. When the electrical oscillations are very powerful (for example, in the transmitting mode), a strong electromagnetic field is produced in the core of a magnetic antenna, causing an undesirable change in its characteristics.

The core of a magnetic antenna is made in the form of a solid rod or, in the case of large antennas, is composed of separate sections. The insertion of a core in a loop (conductive winding) increases the electromotive force (emf) induced in the loop by a factor of N, the radiation impedance by N², and the loop inductance by approximately N. The value ofWis given by the formula N= μ_eff .b²/p², where μ_eff is the effective value of the magnetic permeability of the core, which depends on the original magnetic permeability μ₀ of the core material and the ratio of its length to its radius; b is the radius of the core; and ρ is the radius of the loop.

In addition to the favorable effect of increasing the emf, the insertion of a core in the loop is attended by increased thermal losses caused by the conduction currents induced in the core and by hysteresis loss. As a rule, the losses are higher when using materials having high values of magnetic permeability, and they increase as the operating wavelength is shortened. This restricts the useful range of magnetic antennas to the hectometer and kilometer wavelengths, and also the feasible values of N (for decameter wavelengths, for example, a factor of not more than several tens).