Signal Propagation
Between two antennas, everything is in the way: distance, brick, people, water, the ground itself. Propagation asks what the journey does to your signal, and how much of it still lands.
Layers on this page
This page covers OSI layer 1, physical, and the air between antennas.
// unofficially, layer 0: the air itself
Even empty air charges you.
A signal leaving an antenna smears itself across a sphere that never stops growing, so the power landing on any one patch drops with the square of the distance. Free-space path loss is the bill that comes due with a perfectly clear path.
FSPL(dB) = 20·log10(dkm) + 20·log10(fMHz) + 32.44Double the range, lose 6 dB. Climb in frequency and you lose more again, because an antenna cut for a shorter wavelength presents a smaller aperture to the wave going by.
Six things that happen on the way.
Indoors, the direct path is one of dozens. What lands at the receiver is the sum of everything that bounced, bent, scattered and survived.
Reflection
Metal, glass and water throw signals back at you. That's how coverage gets around a corner, and it's where multipath is born.
Refraction
Waves bend crossing layers of different density. On long outdoor shots the atmosphere curls the beam gently earthward and buys you extra horizon.
Diffraction
Waves wrap around edges: rooftops, door frames, filing cabinets. Lower frequencies wrap better, which is why 2.4 GHz still gets into rooms 5 GHz has given up on.
Scattering
Rough surfaces and small clutter - foliage, rain, warehouse shelving - spray energy in every direction and rob the main path.
Absorption
Some materials turn RF straight into warmth. Water is greedy about it and people are mostly water, so a packed hall quietly costs you several dB.
Multipath
The same signal shows up by several routes, nanoseconds apart indoors. It used to mean fading. OFDM and MIMO flipped it into capacity.
Will this link work? Do the budget.
A link budget adds up every gain and loss between two radios. If the received signal beats what the receiver needs, with margin to spare, the link will hold. Change any value and the results update instantly.
What each wall costs you.
These are ballpark losses per obstacle, taken from typical site surveys. Real materials vary widely, so treat this table as a starting point and measure your own building.
| Obstacle | 2.4 GHz | 5 GHz | Notes |
|---|---|---|---|
| Interior drywall | ~3 dB | ~4 dB | Studs, pipes and wiring inside add more |
| Wooden door | ~3 dB | ~4 dB | Solid-core doors are worse than hollow ones |
| Clear glass window | 2–3 dB | 3–4 dB | Plain glass is nearly transparent to RF |
| Low-E coated glass | 20+ dB | 25+ dB | The metal-oxide coating reflects RF like a mirror |
| Brick wall | 6–8 dB | 10–12 dB | Wet brick absorbs noticeably more |
| Concrete wall | 10–15 dB | 15–20 dB | Rebar and thickness make a big difference |
| Human body | 3–5 dB | 4–6 dB | Mostly water. Crowds matter. |
| Elevator shaft / metal | 30+ dB | 30+ dB | Plan around it, not through it |
Line of sight isn't enough.
Radio energy fills a stretched football of space around the straight line between antennas. Let a roof edge, a tree crown or a swell in the ground poke into that volume and part of your signal cancels itself out. Keep 60% of the first zone clear and you can stop worrying.
Over a 1 km link at 2.4 GHz, the first Fresnel zone is about 5.6 m in radius at its widest. At 5.8 GHz it shrinks to about 3.6 m. Keep at least 60% of that radius clear of obstacles.
Why the "I can see it, so why is it flaky" link happens
Binoculars say the far antenna is right there, yet the link goes soft every afternoon. Ground clearance is the usual suspect. Warm air changes how hard the atmosphere bends your beam, the zone sinks, and a hill that behaved itself at dawn starts shaving your signal by three o'clock.
Before you mount anything.
Six habits that prevent most of the wireless trouble tickets I've ever seen.
- Survey first: measure RSSI and SNR where people actually sit, not next to the AP.
- A few well-placed APs at moderate power beat many APs at full power.
- Mount antennas above head height and away from metal surfaces.
- Match polarization at both ends of every point-to-point link.
- Budget at least 10 dB of fade margin indoors and 20 dB outdoors.
- Survey again once the furniture and the people have moved in.