2.6. Shielding¶
A practical version of a single-winding loop for test purposes is shown in Fig. 4.12. As a protection against electric field effects, the actual winding is additionally surrounded by a tubular metal shield that must have a slot at one point in order not to short-circuit the magnetic field [49]
Buie investigated the design and construction of a shielded receiving loop antenna. The designed antenna had approximately 60% increase in effective height and a 3 dB increase in terminal voltage over a conventional loop antenna. Capacitive loading of the shield gap was optimized at 14 - 34 kHz band and results were presented. In addition, optimum number of turns were optimized [10].
Patents
| Year | Name | Patent Number |
|---|---|---|
| 1942 | Loop antenna | US2292182 |
| 1961 | Electrostatically-shielded loop antenna | US2981950 |
2.6.1. Magnetic Shielding¶
Magnetic shielding can be made three different ways: high-permeability magnetic material, thick-walled conducting material or active compensation. Heinonen et. al., investigated a thick-walled conducting enclosure for biomagnetic measurements [25].
Commonly, high-permeability \(\mu\)-metal is used on the magnetically shielded enclosures and it is convenient from DC to radio-frequencies. However, \(\mu\)-metal is very sensitive to mechanical vibrations and degradation of permeability in mechanical stress.
The advantages of thick-walled conducting material are low price, easy and rigid construction. Its main disadvantage is that the attenuation is zero for static field and increasing function of frequency [25].