24 Jan 2021. "A New Portable ELF SR Receiver"; Part 3: Magnetic Induction Coil.
16"A new portable ELF Schumann resonance receiver: design and detailed analysis of the antenna and the analog front-end", by Constantinos Votis; Part 3: Magnetic Induction Coil
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(Part 3 -- Magnetic Induction Antennae)
Experimental measurements on Schumann resonance magnetic field components involve antenna implementations that are usually induction coils with efficient magnetic properties and enhanced detection sensitivity.
Ferromagnetic core exhibits a relative magnetic permeability of the order of 105, providing high induced voltage amplitude.
Mumetal is a soft ferromagnetic material that exhibits a maximum magnetic permeability of the order of 2 × 105.
Our antenna implementation is based on induction coil architecture with mumetal core material.
That mumetal material (ASTM A753 Alloy 4) exhibits very small coercive force, very low core loss, and remanence.
Due to the fact that our core is a rod that is open-ended, the resultant equivalent magnetic permeability is much smaller than the mumetal relative magnetic permeability and is given by Eqs. (1) and (2).
Also, it strongly depends on the relative permeability of the core material as well as on the induction coil geometry through demagnetization factor N.
Equations (1) and (2) give demagnetization factor N and the resultant magnetic permeability (μ) of an induction coil.
where m is the length to diameter core ratio (m = 300/25 = 12 in this research) and μ r is the relative magnetic permeability of the core material.
In this work N = 0.0151.
In practice, choosing a quite large induction coil length is a benefit due to the resultant high magnetic permeability [23].
But taking into account that as the length of the induction coil increases, the ratio m also increases eliminating the demagnetization factor value N, as observed from Eq. (1). That yields to the fact that the resultant magnetic permeability (μ) exhibits quite strong dependence on relative magnetic permeability (μ r ) of the core material due to Eq. (2).
Moreover, the value of μ r presents crucial instability resulting from temperature or applied field frequency variations.
Therefore, the induction coil sensor performance may be crucially affected and degraded through variations of magnetic permeability μ.
As that material presents high relative magnetic permeability, Eq. 2 is modified to Eq. 3 and the induction coil magnetic permeability depends on the value of demagnetization factor N.
Figure 1 depicts a block diagram of the single induction coil, and Table 1 summarizes the corresponding geometry values.
The windings should be laid on the core within 70 to 90% of its total surface in order to take advantage of the maximum possible flux.
[FIGURE 1]
[TABLE 1]
The self-inductance, resistance, and capacitance of the single induction coil were firstly computed through the calculated value of the demagnetization factor as well as the value of other geometry parameters and core material characteristics.
Using Nagaoka’s formula in the case of an air-coil inductor and a correction factor λ which was proposed by Lukoschus [24] for core material inductors, the magnetic antenna self-inductance was calculated [25, 26].
The induction coil resistance was then calculated through [27].
The magnetic antenna self-capacitance exhibited a strong dependence on the geometry of the coil as well as on the wire insulator electric permittivity and the shielding that may be used between the coil layers.
Also, for the purpose of computing such a parameter, several extended computations [25] were needed.
In practice, the induction coil self-capacitance was measured experimentally.
Both calculated and measured values of the magnetic antenna self-inductance, resistance, and capacitance are summarized in Table 2.