// reference · rf propagation · cwna ch.2
RF Propagation Behaviors
RF behaves like light - it reflects, bends, and scatters. Understanding why is the difference between placing an AP that works and one that creates a dead zone. Every CWNA exam has 3-5 propagation questions. These are the behaviors, in order of importance for enterprise Wi-Fi deployment.
// the six rf propagation behaviors
Absorption
Definition
RF energy is converted to heat as it passes through a material. The energy is lost, not redirected. Absorption is frequency-dependent: higher frequencies absorb more through the same material.
Field examples
Human bodies (1-3 dB per person). Concrete walls (12-15 dB). Brick (3-8 dB). Drywall (1-2 dB). Water (very high - aquariums, water-filled walls, wet lumber). The famous "body fade": a client gets -5 dB the moment someone stands between it and the AP.
Reflection
Definition
Signal bounces off a flat surface and changes direction. The angle of incidence equals the angle of reflection (like light off a mirror). The reflected signal is weaker - the reflecting material absorbs some energy.
Field examples
Metal surfaces (filing cabinets, elevator doors, ductwork, metallic window film). Reflections cause multipath. 802.11n/ac/ax use MIMO to exploit reflections rather than fight them.
Refraction
Definition
Signal bends when passing through a medium of different density. Unlike reflection, the signal continues through - just at a different angle. Common in outdoor deployments where signals travel through different atmospheric layers.
Field examples
Atmospheric refraction bends signals toward the earth (helps outdoor range). Glass windows cause minor refraction. Rarely an issue indoors. CWNA exam: know the definition, not deployment impact.
Diffraction
Definition
Signal bends around the edges of obstacles. Allows Wi-Fi to "wrap around" corners and building edges to some degree. Lower frequencies diffract more than higher frequencies - one reason 2.4 GHz penetrates better than 5 GHz.
Field examples
Signal bending around the corner of a wall. Signals reaching clients around cubicle partitions. Helps 2.4 GHz coverage in cluttered environments. Diffraction causes additional path loss vs line-of-sight.
Scattering
Definition
Signal spreads in multiple directions when it hits an irregular or rough surface. Results in energy going in many directions, weakening the primary signal. Common cause of multipath in indoor environments.
Field examples
Rough concrete walls. Vegetation (leaves). Chain-link fences. Ceiling tiles with irregular surfaces. Unlike reflection (flat surface, predictable angle), scattering sends energy in unpredictable directions.
Multipath
Definition
Multiple copies of the same signal arrive at the receiver via different paths - direct line-of-sight plus reflected/diffracted/scattered copies. Each copy travels a different distance and arrives at a different time (delay spread). The receiver combines them - sometimes constructively (upfade), sometimes destructively (downfade, nulling).
Field examples
Ubiquitous indoors. Every reflected path creates a multipath copy. Pre-802.11n systems tried to avoid multipath. 802.11n/ac/ax MIMO deliberately exploits multipath to transmit multiple spatial streams simultaneously.
// multipath -- 4 possible outcomes at the receiver
Upfade
Multiple signals arrive in phase (0-120° phase difference) and combine constructively. Signal is STRONGER than free space. Rare but possible.
Downfade
Signals arrive partially out of phase (120-180°). Signal is weaker than free space. Most common multipath result.
Nulling (worst case)
Signals arrive exactly 180° out of phase and cancel completely. Signal drops to zero. Client disconnects. Moving the device 1-2 cm can resolve it.
Data Corruption
Signal is above sensitivity threshold but multipath delay spread causes ISI (inter-symbol interference). Frame errors, retransmissions. Guard Interval addresses this.
// how 802.11n/ac/ax turned multipath from enemy to friend
Pre-802.11n (SISO)
Single antenna. Multiple paths = phase cancellation = problem. Engineers tried to reduce reflections with antennas, AP placement, and absorbers.
802.11n/ac/ax (MIMO)
Multiple antennas. Each path becomes a separate spatial stream. Multipath is required - more reflections = more spatial streams = more throughput. This is why MIMO actually works better indoors than outdoors.
Key point for exam: 802.11n MIMO requires delay spread (multipath with time-separated copies) to differentiate spatial streams. In an anechoic chamber with no reflections, MIMO performs like SISO. This is why Wi-Fi degrades in outdoor line-of-sight environments.
// fresnel zone -- clearance requirement for outdoor links
The Fresnel zone is an ellipsoid around the direct line of sight between transmitter and receiver. RF energy doesn't travel in a straight line - it spreads and fills this zone. Any obstruction that penetrates more than 40% into the first Fresnel zone causes significant signal loss due to diffraction.
First Fresnel Zone radius: r = 17.32 × √(d / (4 × f))
where d = distance in km, f = frequency in GHz, r in metres
Simplified: at 2.4 GHz, 100m link → r ≈ 2.25m clearance needed at midpoint
where d = distance in km, f = frequency in GHz, r in metres
Simplified: at 2.4 GHz, 100m link → r ≈ 2.25m clearance needed at midpoint
60% rule
For acceptable performance, the first Fresnel zone must be 60% clear of obstructions. This is the standard design target for outdoor point-to-point links.
Higher freq = smaller Fresnel
5 GHz has a smaller first Fresnel zone than 2.4 GHz at the same distance. Less clearance needed - but more signal lost through materials.
Midpoint = widest
The Fresnel zone is widest at the midpoint between transmitter and receiver. A tree at the midpoint is more problematic than one at the endpoints.
Indoor: mostly irrelevant
Fresnel zones matter for outdoor point-to-point links. Indoors, the environment is so cluttered that Fresnel zone clearance is impossible to achieve - MIMO compensates via multipath.
// typical material attenuation values
| Material | 2.4 GHz loss | 5 GHz loss | Notes |
|---|---|---|---|
| Drywall (plasterboard) | 1-2 dB | 2-3 dB | Standard office partition - minimal impact |
| Wood door | 2-4 dB | 4-6 dB | Solid core worse than hollow |
| Glass (plain) | 2-3 dB | 3-5 dB | Low-E metallic coating = 10-25 dB loss |
| Brick wall | 3-8 dB | 8-15 dB | Highly variable by thickness and density |
| Concrete wall | 10-15 dB | 15-25 dB | Reinforced concrete worse - rebar reflects |
| Metal / Steel | 20-30 dB | 25-40 dB | Elevator shafts, server racks, metal studs |
| Human body | 3-4 dB | 5-6 dB | Body fade effect. Multiple people: up to -15 dB |
| Water / aquarium | 10-20 dB | 20-30 dB | Very high absorption at microwave frequencies |
| Foliage / trees | 2-10 dB | 5-20 dB | Varies with moisture, leaf density, season |
See signal propagation in your PCAP
WiFi Analyser tracks per-client RSSI trends - shows body fade, material loss, and multipath effects as RSSI variance patterns.