General on microwave absorption

A practical and useful expression is the penetration depth (or power depth) dp. It is defined as the depth below the surface of a thick, plane “irradiated” surface of the material where the power density has diminished to about 37 % of the value at the surface. Since the decay is exponential, also 37 % of the total power will be deposited below this depth. In technical and scientific work, the so-called (complex) permittivity ε is used and consists of the (real) permittivity ε' (the old term “dielectric constant” is deprecated), and the loss factor ε" . A high real permittivity, such as for materials with a high water content, causes the interior wavelength to become times shorter. dp equals about 20×/ε" , in mm at 2450 MHz.

Under most practical circumstances, a dp between about 5 and 100 mm results in a good absorption, but dp up to 500 mm may be used in special industrial systems. – The absorption capability in non-magnetic metals is proportional to dp.

Microwave transparency is, in some standards, defined by ε' <7 and dp > about 3 m. This value is in the practical border region of possible processing (see above). A plate with food is not heated relatively.

The dp of non-magnetic metals is inversely proportional to the square root of the frequency and conductivity.

The table gives dp values in mm, m, and µm, respectively – all at 2450 MHz.

 Examples of penetration depth: absorption    (mm)  Examples of penetration depth: transparency    (m)
Examples of penetration depth: metals    (µm)
Water +20 °C 16 Micanite  28 Silver  (best of all materials) 0,33
Water +100 °C 80 Quartz, pure
(best of all materials)		 200 Aluminium, 100% 0,86
Cooked meat –10 °C 25 Fedspar china 5,8 Brass, 70 % Cu 1,4
Cooked meat +40 °C 11 Aluminium oxide ceram, for MW use 30 Zink, pure 1,24
Mash. potato +40 °C 8 Polyethene, 25 °C 40 Nickel, pure 1,3
Spruce 10 % moist.  77 PTFE (Teflon®) 90 Titanium, pure 3,3
Nitrile rubber, natural 650 Plexiglass, 30 °C
(not useable!)		 2 Stainless steel (304) 4,3
Rubber, SBR, vulc. 190 Borosilicate glass, 50 °C ca 8 See this site for a comprehensive table of metal conductivities
Sand soil, dry 2000 Ice  –12 °C 12 – –

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