Materials

Everything the app knows about what buildings are made of comes from one place:

Recommendation ITU-R P.2040, "Effects of building materials and structures on radiowave propagation above about 100 MHz", section 3 and Table 3.

It is a free download from the ITU and it is the reference the whole field uses. The table is reproduced here without adjustment.

How a material is described

Two numbers describe how a material responds to a radio wave, and both depend on frequency. P.2040 gives them as power laws, with frequency in GHz:

eps_r' = a * f^b            relative permittivity, dimensionless
sigma  = c * f^d            conductivity, siemens per metre

which combine into the complex relative permittivity that everything downstream uses:

eps_r = eps_r' - j * sigma / (2*pi*f*eps_0)

The real part says how much the wave slows down inside the material, which determines how it refracts and how strongly it reflects at each interface. The imaginary part says how much it is absorbed.

The table

Every entry has a frequency range it was characterised over. That range is part of the data and the app enforces it.

Material	a	b	c	d	Valid range
Vacuum / air	1	0	0	0	0.001 to 100 GHz
Concrete	5.24	0	0.0462	0.7822	1 to 100 GHz
Brick	3.91	0	0.0238	0.16	1 to 10 GHz
Plasterboard	2.73	0	0.0085	0.9395	1 to 100 GHz
Wood	1.99	0	0.0047	1.0718	0.001 to 100 GHz
Glass	6.31	0	0.0036	1.3394	0.1 to 100 GHz
Ceiling board	1.48	0	0.0011	1.075	1 to 100 GHz
Chipboard	2.58	0	0.0217	0.78	1 to 100 GHz
Plywood	2.71	0	0.33	0	1 to 40 GHz
Marble	7.074	0	0.0055	0.9262	1 to 60 GHz
Floorboard	3.66	0	0.0044	1.3515	50 to 100 GHz
Metal	1	0	1e7	0	1 to 100 GHz
Very dry ground	3	0	0.00015	2.52	1 to 10 GHz
Medium dry ground	15	−0.1	0.035	1.63	1 to 10 GHz
Wet ground	30	−0.4	0.15	1.3	1 to 10 GHz

Notice that b is zero for everything except the ground types. Over the ranges given, the real permittivity of ordinary building materials does not depend on frequency. All the frequency dependence you see in a wall comes from the conductivity and from the geometry of the layers.

Two entries worth watching

Brick stops at 10 GHz. The 6 GHz Wi-Fi band is inside that, so brick is fine for everything this app does. 60 GHz would not be.

Floorboard is only characterised from 50 GHz upward. Every Wi-Fi band is a large extrapolation from that, and the app flags it every time. Use Wood instead unless you have a measured value. This is the entry that catches people out, because the name sounds exactly like what you want for a timber floor.

Extrapolation is reported, not hidden

Ask for a material outside its stated range and you still get a number, because refusing would be less useful than warning. But the result is marked, the marking propagates up through the wall build-up to the coverage map, and the exported report names which materials were extrapolated.

The point is that you can tell the difference between a number backed by published measurements and a number produced by a power law running well past where anybody checked it.

What is deliberately missing

Several ordinary building products have no P.2040 entry, and this project does not invent one:

• PVC and uPVC window profiles
• Screed, floor tile, carpet
• Mineral wool and other cavity insulation
• EPS and other rigid insulation boards
• Aerated concrete blocks
• Low-emissivity coatings, which are handled a different way, see walls and glazing

If you need one of these, add it as a custom material. The app requires a provenance note, which is the point: the exported report can then say where the number came from, and a reader can judge it.

If you have a sourced value for any of these, that is the most useful contribution you can make to this project. See contributing.

Bulk absorption

For reference, the attenuation of a plane wave travelling inside a material, at normal incidence, following P.2040 equations 12 and 13:

alpha = (2*pi*f/c) * Im(sqrt(eps_r))    nepers per metre

converted to decibels per metre with a factor of 20 log10(e), about 8.686.

This is the bulk absorption only. The reflections at each interface, and the multiple reflections inside a layered construction, are handled separately by the transfer matrix. See walls and glazing.