I need to do a little more reading, as, no surprise whatsoever, it seems the RF shielding behaviour of conductors - the mix of absorption loss vs reflection loss - the portion of the source/incident wave that is absorbed vs the portion that is reflected - is... complicated. It varies rather a lot with frequency and material thickness (skin-depth once again the culprit) as you can see from the graph below, which is copper. I need to find an equivalent for graphite.
Reflection loss btw is not the same thing as the similar sounding return loss in an RF transmission line context, indeed it's pretty much the opposite (return loss being the ratio of the incident wave to the reflected wave in dB, higher the better for lower internal reflections).
Copper sheet
Graphite foil vs Aluminium & Steel
Absorption-dominant radio-wave (0.2–2.0 GHz) attenuation loss is comparatively reported for materials of high electrical conductivity, namely metals (aluminum and steel) and graphite. These materials exhibit similarly high absorption loss (≤ 91.5%) and similarly low reflection loss (≥ 8.5%), both as fractions of the total loss in dB. The absorption loss is high (< 55 dB) and the reflection loss is low (< 10 dB) for both graphite and the metals. The absorption-dominant attenuation loss of these high-conductivity materials is in contrast to the notion that high conductivity (due to the high impedance mismatch with air) generally causes reflection-dominant attenuation loss. The metals and graphite are in foil form, with the graphite being thicker than the metals. The linear absorption coefficient (directly related to the absorption loss per unit thickness) is lower for graphite (≤ 93 mm⁻¹) than the metals (≤ 394 mm⁻¹), due to the greater thickness of the graphite. The absorption loss and fractional absorption loss contribution increase with increasing frequency, whereas the reflection loss decreases, as consistent with electromagnetic theory. On the other hand, from the viewpoint of the fractional loss in power, reflection dominates over absorption for all three materials in the entire frequency range. For shielding, the metals are more effective than graphite if the absorption loss per unit thickness (< 3400 dB/mm) is considered. For stealth, graphite is advantageous to the metals in the low reflection loss, though it is disadvantageous in the low absorption loss per unit thickness.
What is interesting is although reflection loss (the portion of the signal that is reflected) is low for both graphite and the metals there's a 2-3db difference, the metals being more reflective. Expect graphite felt is even less reflective due to multiple internal ' bounces' and associated absorption opportunities whilst copper would be more reflective than aluminium as it seems to be proportionate to electrical conductivity.
Reflection loss btw is not the same thing as the similar sounding return loss in an RF transmission line context, indeed it's pretty much the opposite (return loss being the ratio of the incident wave to the reflected wave in dB, higher the better for lower internal reflections).
Copper sheet
https://www.sciencedirect.com/topics/engineering/absorption-loss
Absorption loss - Tim Williams, in EMC for Product Designers (Fifth Edition), 2017
Absorption loss depends on the barrier thickness and its skin depth and is the same whether the field is electric, magnetic or plane wave: that is, it doesn’t depend on the wave impedance, in contrast to reflection loss. The skin depth in turn depends on the barrier material’s properties; steel, for instance, offers higher absorption than copper of the same thickness, at the lower frequencies where its relative permeability is high. At high frequencies, as Figure 15.2 shows, absorption becomes the dominant term, increasing exponentially with the square root of the frequency. Appendix D (section D.4) gives the formulae for the values of A, R and B for given material parameters.Graphite foil vs Aluminium & Steel
Absorption-dominant radio-wave (0.2–2.0 GHz) attenuation loss is comparatively reported for materials of high electrical conductivity, namely metals (aluminum and steel) and graphite. These materials exhibit similarly high absorption loss (≤ 91.5%) and similarly low reflection loss (≥ 8.5%), both as fractions of the total loss in dB. The absorption loss is high (< 55 dB) and the reflection loss is low (< 10 dB) for both graphite and the metals. The absorption-dominant attenuation loss of these high-conductivity materials is in contrast to the notion that high conductivity (due to the high impedance mismatch with air) generally causes reflection-dominant attenuation loss. The metals and graphite are in foil form, with the graphite being thicker than the metals. The linear absorption coefficient (directly related to the absorption loss per unit thickness) is lower for graphite (≤ 93 mm⁻¹) than the metals (≤ 394 mm⁻¹), due to the greater thickness of the graphite. The absorption loss and fractional absorption loss contribution increase with increasing frequency, whereas the reflection loss decreases, as consistent with electromagnetic theory. On the other hand, from the viewpoint of the fractional loss in power, reflection dominates over absorption for all three materials in the entire frequency range. For shielding, the metals are more effective than graphite if the absorption loss per unit thickness (< 3400 dB/mm) is considered. For stealth, graphite is advantageous to the metals in the low reflection loss, though it is disadvantageous in the low absorption loss per unit thickness.
What is interesting is although reflection loss (the portion of the signal that is reflected) is low for both graphite and the metals there's a 2-3db difference, the metals being more reflective. Expect graphite felt is even less reflective due to multiple internal ' bounces' and associated absorption opportunities whilst copper would be more reflective than aluminium as it seems to be proportionate to electrical conductivity.
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