Wireless RF Fundamentals

Mental model

Wired Ethernet is a private hallway — your bits travel down a cable that no one else can touch. Wi-Fi is a shared public square — everyone’s bits ride invisible radio waves on the same channel, and only one device can broadcast at a time without collision.

That single sentence explains 80% of Wi-Fi behavior:

• Why Wi-Fi is half-duplex (CSMA/CA — listen before talking).
• Why throughput drops as more clients join (sharing airtime).
• Why a microwave kills your Wi-Fi (2.4 GHz oven, 2.4 GHz Wi-Fi, same band).
• Why moving from a phone-jam coffee shop to an empty office triples your speed.

Wi-Fi performance is mostly radio physics, then a thin layer of 802.11 protocol logic on top.

Bands — three Wi-Fi worlds

Band	Channels	Range	Congestion	Use
2.4 GHz	1, 6, 11 (only non-overlapping in N. America)	Long (penetrates walls)	High — Bluetooth, microwaves, IoT	IoT, legacy clients, when range matters
5 GHz	~25 non-overlapping (UNII-1/2/2e/3)	Shorter, more loss through walls	Lower	Default for modern enterprise Wi-Fi
6 GHz (Wi-Fi 6E / 7)	14 × 80 MHz non-overlapping	Similar to 5 GHz	Empty (only Wi-Fi 6E+ clients allowed)	New high-density deployments

2.4 GHz has only 3 non-overlapping channels (1, 6, 11) in North America. APs nearby on channels 2-5 interfere with channels 1 and 6 even though they have different numbers — channels in 2.4 GHz overlap.

5 GHz has many non-overlapping 20 MHz channels, making it the default for enterprise. Some channels (DFS — Dynamic Frequency Selection) must yield to radar; expect occasional channel changes.

6 GHz (Wi-Fi 6E and Wi-Fi 7) opened in 2020-2022. Massive clean spectrum — but only the newest clients can use it.

Channel width — speed vs density trade-off

802.11 lets you bond channels together for more throughput:

• 20 MHz — basic. ~150 Mbps per stream on 802.11ac.
• 40 MHz — 2 adjacent channels. ~300 Mbps.
• 80 MHz — 4 channels. ~600 Mbps. Default for ac/ax in residential.
• 160 MHz — 8 channels. ~1.2 Gbps. Great for one AP, terrible for dense deployments (uses half the 5 GHz spectrum).

Rule of thumb: in a high-density office or apartment, smaller channels = more APs that don’t step on each other = higher aggregate capacity. In a single-AP home, wider is fine.

RSSI vs SNR — the two numbers that matter

Wi-Fi signal strength is measured in dBm (decibel-milliwatts) — a logarithmic scale where bigger negative number = weaker.

RSSI (Received Signal Strength Indicator) — how loud the AP sounds at the client:

RSSI (dBm)	Quality	Realistic use
−30 to −50	Excellent	Right next to the AP
−50 to −65	Good	Same room as AP
−65 to −75	Fair	Voice/video struggles
−75 to −85	Poor	Connected but unusable
−85 and below	Disconnect	Below sensitivity floor

Noise floor — how loud all the random RF noise is at the same location. Usually around −90 dBm in clean environments, −85 in noisy ones.

SNR (Signal-to-Noise Ratio) = RSSI − noise floor. In dB.

SNR (dB)	Quality
> 40	Excellent
25 - 40	Good
15 - 25	Fair (data only)
< 15	Unreliable

High RSSI but low SNR = strong signal in a noisy room → still poor. SNR is the better single number to chase.

Free-space path loss

Radio signal weakens with distance. The formula (free-space, ignoring obstacles):

Loss (dB) = 20 × log10(distance) + 20 × log10(frequency) + 32.44

The practical takeaway: doubling distance loses 6 dB. Going from 5m to 10m, signal drops by 6 dB. Going from 10m to 20m, another 6 dB.

5 GHz suffers more path loss than 2.4 GHz at the same distance — about 7 dB more. That’s why 2.4 GHz “reaches further” even though it’s worse spectrum.

Walls add their own loss:

Obstacle	Loss (approx)
Drywall	3 dB
Wood door	4 dB
Brick wall	8 dB
Concrete wall	12 dB
Glass with metal coating	8-15 dB
Elevator shaft / metal cabinet	20-30 dB

Two concrete walls = 24 dB loss = signal cut to 1/250 of original.

Antenna patterns

APs don’t radiate evenly in all directions. Three common patterns:

• Omnidirectional — donut-shape, radiates outward equally in all horizontal directions. Default for ceiling-mount APs.
• Directional / patch — focuses energy in one direction. Used for long corridors, warehouse aisles, point-to-point outdoor links.
• Sector / yagi — narrow beam, used for outdoor bridge links or for covering one specific area without leaking.

Mounting matters. A ceiling-mount AP designed for ceilings, placed on a wall, broadcasts most of its energy into the wall behind it. Match the AP’s intended mount.

CSMA/CA — why Wi-Fi is “slow”

Wired Ethernet uses CSMA/CD (Collision Detection). Wi-Fi uses CSMA/CA (Collision Avoidance) — because wireless radios can’t transmit and receive simultaneously, so they can’t detect collisions while transmitting.

Steps:

1. Listen — is the channel idle?
2. If busy → back off random time, then listen again.
3. If idle → transmit, expect ACK.
4. No ACK = collision assumed → retry.

Effects:

• Maximum throughput is ~50-60% of raw rate (overhead from ACKs, contention, inter-frame spacing).
• More clients = more contention = lower per-client speed.
• One slow client (legacy 802.11g) drags everyone down — it holds airtime longer.

Real-world troubleshooting

User says “Wi-Fi is slow in the conference room.” The questions in order:

1. What’s the RSSI / SNR? (wlan show interfaces on Windows, airport -I on macOS). Anything worse than -70 dBm → coverage gap.
2. What’s the band? Stuck on 2.4 GHz when 5 GHz is available → push the client to dual-band.
3. How many clients on this AP? High-density = shared airtime.
4. Any DFS channel-switch events? Look at controller logs — sudden 30s blackouts.
5. Interference? Walk in with a Wi-Fi scanner (NetSpot, WiFi Explorer, inSSIDer). Look for non-Wi-Fi noise.

Common mistakes

1. Treating dBm as percentage. −60 dBm isn’t “60% strength.” It’s logarithmic — −60 is 1000× stronger than −90.

2. Over-powering APs. Cranking AP transmit power doesn’t help; clients can hear the AP but the AP can’t hear the client back. Power must match the weakest direction (uplink from client).

3. 2.4 GHz channel 3. Non-overlapping in 2.4 are 1, 6, 11. Any other channel overlaps and creates interference.

4. Wider channels = always better. 80 MHz on every AP in an office = constant overlap. Smaller channels with channel reuse beats wider channels in dense areas.

5. Ignoring sticky clients. Devices that connect to one AP and stay there even when a better one is closer. Solved with band steering, 802.11k/v assistance, sometimes a forced disassociation policy.

6. Mixing 802.11b clients with modern. A single 11b client downshifts the AP to 11b protection mode and tanks throughput for everyone. Turn off 802.11b data rates in 2026.

Lab to try tonight

1. Install a free Wi-Fi scanner (NetSpot home, inSSIDer, or iw dev wlan0 scan on Linux).
2. Walk through your home/office. Note RSSI in each room.
3. Identify your noise sources — count APs on each 2.4 GHz channel.
4. Try switching to a less-congested 5 GHz channel via your AP/router admin GUI.
5. Compare before/after with a speed test from the conference-room corner.
6. Bonus: if you have a spectrum analyzer (Wi-Spy or even Ubiquiti’s built-in tools), look for non-Wi-Fi noise — microwaves, Bluetooth, baby monitors. They don’t show in normal Wi-Fi scans but they wreck your channel.

Cheat strip

Concept	Plain English
2.4 / 5 / 6 GHz	Bands. 2.4 = range/congested. 5 = default. 6 = new and clean.
2.4 GHz non-overlapping	Only channels 1, 6, 11 (N. America)
Channel width	20/40/80/160 MHz. Wider = faster per AP, worse for density
RSSI	Signal strength at receiver in dBm. −50 great, −75 poor
SNR	RSSI minus noise. >25 dB = good. The number that actually predicts throughput
dBm scale	Logarithmic. Each −3 dB = half the signal
Path loss	Doubling distance = 6 dB drop. 5 GHz loses more than 2.4
CSMA/CA	Listen before talk. Why Wi-Fi maxes ~50-60% of raw rate
DFS channels	Must yield to radar — occasional 30s outages
Omni vs directional	Donut radiation vs focused beam — pick to match the area