# Permeability

 Permeability is a property of a medium or a region of space. On this page, I'm going to give its meaning as it relates to antenna theory, with only a small explanation on the physics behind it. The permeability, as we saw relates the magnetic flux density to the magnetic field. The permeability is given in units of Henries/meter; since Henries relates to inductance, a material with a higher permeability can be thought of as storing more magnetic energy. The permeability of a vacuum (or free space) is given by: This is roughly the permeability of air on Earth. Suppose now we are looking at a magnetic material (for instance, iron). This material will affect the magnetic field because the magnetic moments of the molecules that make up the material align themselves in the direction of an external magnetic field. In addition, these magnetic moments tend to remain even after the external field is turned off, leading to a permanent magnetization of some materials. The permeability of a medium is typically specified by a relative permeability: Materials are characterized as diamagnetic (relative permeability slightly less than 1.0), paramagnetic (relative permeability slightly more than 1.0), and ferromagnetic (high value for relative permeability, that also exhibits hysteresis). A table of common materials and their relative permeabilities is presented below.
MaterialRelative Permeability
Gold
0.999996
Mercury
0.999997
Water
0.99999
Air
1.000004
Aluminum
1.00002
Cobalt
255
Nickel
600
Steel
2000
Iron
4500

 Finally, as mentioned on the page on permittivity, the permeability also affects the speed of propagation of an electromagnetic wave in a medium with a relative permeability given as , and also its corresponding wavelength: In the above, c is the speed of wave propagation in the medium and is the speed of light in free space (or a vacuum); also is the wavelength of a wave at frequency f in the medium, the wavelength at the same frequency in a vacuum would be .

Antenna Definitions List

Fourier Transforms [external website]