1The Pool Moment
Before we define a single equation, look at this photograph.
The refraction myth — corrected. Refraction is the bending of a wave as it crosses between two media of different densities — the bent-pencil effect when a straw sits in a glass of water. The pool ring pattern is not that. It is caused by sunlight reflecting off multiple points on the wind-rippled water surface, with each reflection point acting as a secondary wave source. The rings form where those reflected wavefronts add (constructive) or cancel (destructive). In WiFi terms: refraction is mostly an outdoor / atmospheric problem. Reflection and interference are the dominant indoor story — which is what this module is about.
Why this matters for WiFi. The bright rings in that pool are the visual equivalent of RSSI peaks. The dark zones are the equivalent of RSSI nulls — positions where a client's received signal collapses. A client moving just a few centimeters can cross from a peak to a null. To understand why, you first need to understand the wave itself.
2The Four Properties of an EM Wave
Every electromagnetic wave — whether it carries a WiFi frame, an FM radio broadcast, or visible light — is fully described by four properties. Master these four and the physics of the rest of this module falls into place.
Frequency (f)
Cycles per second, measured in Hz. WiFi operates at 2.4 GHz, 5 GHz, and 6 GHz. Higher frequency = more energy per photon, shorter wavelength, more path loss per meter.
Wavelength (λ)
Physical distance of one complete cycle. Inversely related to frequency: λ = c / f. At 2.4 GHz, λ = 12.5 cm. At 5 GHz, λ = 6.0 cm. At 6 GHz, λ = 5.0 cm.
Amplitude (A)
Peak displacement from centre — corresponds to signal power. High amplitude = strong signal. When we talk about RSSI in dBm, we are measuring the received wave amplitude.
Phase (φ)
The starting position in a cycle, expressed in degrees (0°–360°). Two waves of the same frequency but different phase can add or cancel depending on their phase relationship.
| Property | Symbol | Unit | WiFi example |
|---|---|---|---|
| Frequency | f | Hz · MHz · GHz | 2.437 GHz = 802.11g/n/ax channel 6 |
| Wavelength | λ | m · cm | λ = 300 / 2,437 MHz = 12.3 cm at CH6 |
| Amplitude | A | V/m (field) · dBm (received) | −65 dBm = good RSSI for voice |
| Phase | φ | degrees (°) · radians | 180° phase shift = destructive interference |
The wavelength–frequency relationship: λ = c / f, where c = 3 × 10⁸ m/s (speed of light). Frequency and wavelength are inversely proportional — double the frequency, halve the wavelength. This is why 6 GHz antennas are physically smaller than 2.4 GHz antennas, and why 6 GHz signals experience more path loss per meter.
3EM Wave Explorer — See It, Change It
Reading about wave properties is useful. Watching them change in real time is better. The EM Wave Explorer lets you manipulate all four properties simultaneously and observe the results — including what happens when two waves with different phase relationships occupy the same space.
EM Wave Explorer v1.1
29 presets covering polarization, frequency, interference, MIMO concepts and more. Use Freehand mode to set Wave 1 and Wave 2 independently — adjust frequency, amplitude, and phase offset to observe constructive and destructive superposition live. Open in a new tab and return here for the guided activity below.
Open the EM Wave Explorer. In the Freehand panel, set Wave 1 and Wave 2 to the same frequency and amplitude. Work through the questions below.
0° phase offset: The two waves add perfectly — the combined amplitude doubles. This is constructive interference. In WiFi: a multipath echo arriving in-phase boosts the received signal above the direct path alone.
180° phase offset: The waves cancel completely — the combined amplitude is zero. This is destructive interference. In WiFi: a reflected echo arriving half a wavelength late creates a null where the client receives nothing regardless of transmit power.
90° phase offset: Partial cancellation — the combined amplitude is √2 × A (≈ 3 dB reduction). In WiFi: most multipath situations produce partial rather than complete cancellation, which is why RSSI variation across positions tends to be gradual rather than binary.
Different frequencies: No stable constructive or destructive interference — the phase relationship changes continuously. OFDM exploits this: each subcarrier is orthogonal to its neighbours, so they do not interfere with each other across the symbol boundary.
4Polarization & The WiFi Connection
Polarization describes the orientation of the electric field component of the wave — the plane in which it oscillates as it travels. It is the fifth wave characteristic that the EM Wave Explorer adds to the four above, and it has direct practical consequences for WiFi antenna selection and AP mounting orientation.
Polarization types
Vertical
Electric field oscillates up-down. Most ceiling-mount AP internal antennas use vertical polarization. Clients held upright receive best signal.
Horizontal
Electric field oscillates side-to-side. Wall-mount APs or flat patch antennas. A vertically polarized AP and horizontal client antenna suffer polarization mismatch loss (~20 dB).
Dual / Cross
Two orthogonal antennas ±45°. Used in enterprise APs to handle any client orientation. Also forms the basis of 2×2 MIMO spatial stream separation.
Circular
Rotating field — right-hand or left-hand. Useful for satellite and some outdoor links. Rare in enterprise WiFi but visible in the EM Wave Explorer presets.
Practical rule: Polarization mismatch between AP and client antenna can cause up to 20 dB of additional signal loss — equivalent to the difference between excellent RSSI (−50 dBm) and near-unusable (−70 dBm). This is why enterprise APs use dual ±45° cross-polarized antennas: they ensure reasonable polarization alignment regardless of how the client device is oriented or held.
The pool connection revisited. You now have the vocabulary to describe exactly what is happening in the pool photograph. The sunlight is the transmitter. The wind-rippled water surface creates multiple reflection points — each one a secondary wave source. Those reflected waves have the same frequency as the incident light but arrive at the pool floor with different path lengths, producing different phase relationships. Where the phase difference is 0° or a multiple of 360°, crests meet crests — constructive interference, bright rings. Where the phase difference is 180°, a crest meets a trough — destructive interference, dark zones. Replace sunlight with a 5 GHz 802.11ax signal and the pool floor with your client device, and you have just described multipath fading.
Answer from memory before revealing. These are the foundational facts that Module 3 builds on throughout.
Channel 36 wavelength: λ = c / f = 300,000,000 / 5,180,000,000 = 0.05792 m = 57.9 mm ≈ 5.8 cm.
3 cm path difference at 5 GHz: λ = 6.0 cm. λ/2 = 3.0 cm. A path length difference of exactly λ/2 produces a 180° phase shift — this is destructive interference. The reflected echo arrives perfectly out of phase with the direct signal and cancels it. This is a complete null.
Polarization mismatch loss: Approximately 20 dB. A vertically polarized transmit antenna and a horizontally polarized receive antenna are orthogonal — the received electric field component on the receive axis approaches zero.
±45° cross-polarized antennas: Client devices are held in many orientations — vertical, horizontal, tilted. A single vertical antenna suffers mismatch loss with any non-vertical client. ±45° cross-polarized pairs ensure the receive antenna is never more than 45° from the client's polarization axis, limiting worst-case mismatch loss to about 3 dB rather than 20 dB. The two orthogonal feeds also provide the independent spatial channels required for 2×2 MIMO.
1 · At 2.4 GHz, what is λ/2 — the path length difference that produces a 180° phase shift and a destructive null?
2 · Which wave property directly corresponds to received signal strength (RSSI)?
3 · The pool ring pattern (white rings on a sunlit pool floor) is primarily caused by: