// reference · antennas & rf systems · cwna ch.4
802.11 Antenna Reference
Antennas don't amplify - they focus. Understanding passive gain, polarization, and MIMO chains is the difference between a WLAN that covers well and one that transmits into walls. CWNA-109 Chapter 4 content, cross-referenced to IEEE 802.11-2020 and 802.11n.
// gain references - isotropic, dBi, dBd
Isotropic Radiator
A theoretical point source that radiates RF equally in all directions - a perfect sphere of energy. Does not exist in reality. Used as the universal reference. Think of the sun: radiates equally in all directions from a point source.
EIRP is defined relative to isotropic. All regulatory limits use dBi.
dBi
Gain in decibels referenced to an isotropic radiator. Measures how much more signal an antenna delivers in its best direction compared to what an isotropic radiator would deliver. A 0 dBi antenna has the same gain as isotropic. A standard dipole (rubber duck) = 2.14 dBi.
Industry standard. All regulatory documents use dBi. FCC power limits are in dBi.
dBd
Gain in decibels referenced to a half-wave dipole antenna. Some vendors specify antennas in dBd. Since a dipole has 2.14 dBi gain, converting is simple: dBi = dBd + 2.14. A 3 dBd antenna = 5.14 dBi.
Always convert to dBi before doing EIRP calculations or comparing antennas.
// antenna types - three categories
Omni-Directional 2-9 dBi typical
Definition
Radiates in all horizontal directions equally (360° azimuth), but with a flat, pancake-shaped pattern - limited vertical coverage. The rubber duck antenna on most indoor APs. More gain = flatter pancake = more range horizontally but less vertically.
Use cases
Indoor APs in open-plan offices. Ceiling-mounted APs in standard enterprise environments. Any scenario where 360° horizontal coverage is needed.
Field: A high-gain omni (9 dBi) with a very flat pattern can miss clients directly below a ceiling-mounted AP. Choose moderate gain (2-5 dBi) for ceiling mounts.
Semi-Directional 6-20 dBi typical
Definition
Focuses energy in one general direction with a wider beam than highly directional antennas. Types: patch (flat panel), sector (120° spread for cellular-style coverage), yagi (long-range beam). Used for corridors, outdoor coverage of specific areas, and building-to-building short links.
Use cases
Hospital corridors (patch aimed down hallway). Outdoor stadiums (sector panels). Parking garage coverage. Short outdoor point-to-point runs up to ~500m.
Field: Patch antennas are invisible to regulatory bodies because they look like standard APs. Sector antennas are the building block of outdoor Wi-Fi mesh and hotspot networks.
Highly Directional 20-30+ dBi typical
Definition
Very narrow beam (3-10° beamwidth) focused in one precise direction. Dish, parabolic, and grid antennas. Maximum range. Must be precisely aimed - a 1° pointing error at long range = massive coverage miss. Used for building-to-building outdoor bridges up to several kilometers.
Use cases
Point-to-point outdoor bridges. Long-range outdoor backhaul. Any link over 500m in open space.
Field: FCC rules for point-to-point links allow higher EIRP if the antenna gain exceeds certain thresholds. A high-gain dish may require reducing transmit power to stay within EIRP limits.
// beamwidth
The angle between the two -3 dB points on an antenna's radiation pattern. Where signal drops to half power (-3 dB) from the center. Two measurements:
H-plane (azimuth)
Horizontal beamwidth - looking at the antenna from above. Omni = 360°. Patch = 60-90°. Dish = 3-10°.
E-plane (elevation)
Vertical beamwidth - looking at the antenna from the side. Determines how flat the "pancake" is. Lower elevation beamwidth = more horizontal range.
Higher gain = narrower beam
The antenna focuses energy by making the beam narrower. A 20 dBi dish has ~5° beamwidth. A 5 dBi omni has ~60° elevation beamwidth.
// polarization
The orientation of the electric field wave from the antenna. Transmitter and receiver antennas must be on the same polarization for best signal. Mismatched polarization causes 20+ dB of signal loss.
Vertical
Electric field perpendicular to ground. Standard for omni indoor APs. Dipole pointing up.
Horizontal
Electric field parallel to ground. Used for some outdoor links. Patch antenna oriented sideways.
Circular (dual-pol)
MIMO APs use ±45° dual-polarization. Client can be in any orientation and still match one polarization axis. Best for enterprise indoor.
// mimo radio chains - the a×b:c notation
4×4:4 = 4 transmit chains × 4 receive chains : 4 spatial streams max
First number (a)
Number of transmit radio chains - how many antennas can transmit simultaneously
Second number (b)
Number of receive radio chains - how many antennas can receive simultaneously
Third number (c)
Maximum spatial streams - the ceiling on simultaneous data streams (c ≤ min(a,b))
Real example: a 2×2:2 client connecting to a 4×4:4 AP - the AP uses 2 of its 4 radio chains for this client. The extra 2 chains provide MRC diversity benefit even for a 2-stream client. This is why a 4×4 AP serves 2SS clients better than a 2×2 AP.
// mimo techniques - how multiple chains are used
Spatial Multiplexing (SM) 802.11n, ac, ax, be
Multiple independent data streams transmitted simultaneously on separate spatial paths (exploiting multipath). Each stream carries different data. Increases throughput proportionally to the number of streams. A 4SS link is theoretically 4× faster than 1SS at the same MCS.
Transmit Beamforming (TxBF) 802.11n (optional), ac/ax/be (standard)
AP adjusts the phase and amplitude of signals from multiple antennas to focus the beam toward a specific client. Client must send CSI (Channel State Information) feedback to enable explicit TxBF. Improves SNR at the client - enables higher MCS at greater range.
Maximal Ratio Combining (MRC) All 802.11n/ac/ax/be APs - always active on receive
Receive technique. Multiple antenna chains receive the same signal. The received copies are combined - weighted by their quality - to produce a better composite signal than any single antenna would receive. Extra receive chains always help, even with 1SS clients.
Space-Time Block Coding (STBC) 802.11n (optional)
Transmit diversity technique. Same data is encoded across multiple antennas simultaneously. The receiver reconstructs it using the redundant copies. Improves reliability without requiring channel feedback. Only between 802.11n-capable devices.
Cyclic Shift Diversity (CSD) 802.11n (mandatory for multi-chain)
Legacy-compatible transmit diversity. Transmits the same signal from multiple antennas with a slight time shift on each. Legacy (pre-802.11n) clients can receive this - they just see more signal options. No channel feedback required.
// vswr - voltage standing wave ratio
VSWR measures the impedance mismatch between the transmitter, cable, and antenna. Perfect match = 1:1. Any mismatch causes some power to reflect back toward the transmitter (return loss). A VSWR of 2:1 means ~11% of power is reflected. High VSWR can damage transmitters over time. Exam tip: VSWR 1:1 = perfect, VSWR 2:1 = acceptable, VSWR >3:1 = problem. Return Loss (dB) = 20 × log(VSWR+1 / VSWR-1).
See antenna gain in your PCAP
Radiotap header includes antenna signal and noise per chain - WiFi Analyser surfaces avg RSSI per client across the full capture.