Normally the resistivity of a metal is linear in degrees Kelvin. So, going from 300K to 1300K would cause a metal's resistance to increase by 1300/300 or a factor of 4.3. The RF skin depth will increase as the square root of this change or about 2. So, the leakage out of this Faraday cage will about double in amplitude (6 dB). In an ideal Faraday cage (ideal conductor), all of the RF is reflected back into the interior, none leaks out, and none is absorbed. In a real metal Faraday cage, most of the signal is reflected, the signal is attenuated substantially in going from inside to outside, and some of the power is absorbed in the metal. The amount that leaks out is proportional to the number of skin depths in thickness of the Faraday cage metal. The skin depth is 1/sqrt(pi F mu sigma), so it is inversely proportional to the square root of frequency. High frequencies have small skin depth and hence, the metal thickness being more skin depths in thickness, the signal transmitted is more attenuated going through the metal. Thus, low frequencies with big skin depths penetrate better and high RF frequencies penetrate less. That is why I said that it is unlikely that high RF frequencies would escape in a measurable way - given the thickness of the reactor metal (probably on the order of 1.5mm) above 100 kHz would probably be considered high frequency. And, as previously stated, about 6dB more will come out at 1000C than at room temp.
Bob Higgins On Fri, Oct 3, 2014 at 10:42 AM, Axil Axil <janap...@gmail.com> wrote: > Dear Bob > > Unlike myself, it is great that you are an expert in this subject that > greatly interests me. > > Please address this issue. Unlike the usual RF shielding applications > using stainless steel, the Hot-Cat reaches 1000C without active heat > removal. > > Bearing in mind that RF shielding protection is a function of the > conductivity of the metal, the electrical resistance of stainless steel is > a increasing function of temperature. How much RF protection is lost in a > 1000C stainless steel faraday cage as a function of the increasing > temperature of stainless steel. > > > http://www.aksteel.com/pdf/markets_products/stainless/austenitic/304_304L_Data_Sheet.pdf > > Electrical Resistivity of 304 stainless in > > (microhm-cm) > > 68°F (20°C) – 28.4 (72) > > 1200°F (659°C) – 45.8 (116) > > > On Fri, Oct 3, 2014 at 11:27 AM, Bob Higgins <rj.bob.higg...@gmail.com> > wrote: > >> Yes Axil, I am an RF engineer and I am aware of permalloy/supermalloy >> sheets used in EMI protection. These materials are added in sensitive >> instrument applications to shunt low frequency evanescent magnetic fields, >> not propagating RF EM fields. As I said, RF fields above about 1 kHz would >> be prevented from escaping from Rossi's hotCat by the hermetic stainless >> steel reactor enclosure acting as a Faraday cage. There will be no >> propagating RF escaping from the Rossi's reactor vessel. There is only the >> possibility of low frequency evanescent fields escaping. >> >> Bob Higgins >> >> >> On Fri, Oct 3, 2014 at 9:11 AM, Axil Axil <janap...@gmail.com> wrote: >> >>> >>> http://webcache.googleusercontent.com/search?q=cache:http://aip.org/tip/INPHFA/vol-7/iss-5/p24.pdf >>> >>> *How does magnetic shielding work?* >>> All EMI shielding materials are manufac- >>> tured from high-permeability alloys that con- >>> tain about 80% nickel; the alloys vary in the >>> composition of their remaining metals. They >>> are usually fabricated as foils or sheets and >>> are baked at 2,000 °F in a dry hydrogen-rich >>> atmosphere to anneal them. Annealing sig- >>> nificantly improves a material’s attenuation, >>> that is, its ability to absorb and redirect mag- >>> netic fields. >>> >>> A shielding alloy works by diverting a >>> magnetic flux into itself. The alloy redirects >>> the magnetic flux away from the sensitive >>> object and returns it to the north–south >>> field. Although the field from a magnet is >>> greatly reduced by a shield plate, the protec- >>> tive alloy itself is attracted to the magnet, >>> but with no ill effects. Closed shapes are the >>> most efficient for magnetic shielding—cylin- >>> ders with caps, boxes with covers, and simi- >>> lar enclosed shapes are the most effective >>> (see figure). >>> >>> Magnetic shielding materials offer a very- >>> high-permeability path for magnetic field >>> lines to travel through, directing them >>> through the thickness of the shielding alloy >>> and keeping them from going where they >>> are not wanted. It is important that the >>> shield should offer a complete path for the >>> field lines, so that they do not exit the mate- >>> rial in a place where they will cause unin- >>> tended interference. >>> >>> *What is the difference between RF shield -* >>> *ing and magnetic shielding?* >>> >>> Ra d i o-frequency (RF) shielding is >>> required when it is necessary to block high- >>> frequency (100 kHz and above) interference >>> fields. RF shields typically use copper, alu- >>> minum, galvanized steel, or conductive rub- >>> b e r, plastic, or paints. These materials work >>> at high frequencies by means of their high >>> c o n d u c t i v i t y. Unlike magnetic shields that >>> use their high permeability to attract mag- >>> netic fields, RF shielding has little or no >>> magnetic permeability. However, when they >>> are properly engineered and constructed, >>> magnetic-shield alloys become broadband >>> shields that protect against both EMI and >>> RF interference. >>> >> >
