RF & Antennas
Under the bits, it's all waves. Frequency, wavelength, decibels and gain are the words the physical layer speaks, and the antenna is the spot where current turns into radio.
Layers on this page
This page covers OSI layer 1, physical.
Six ideas that make RF make sense.
You don't need a physics degree. These six concepts, and the formulas behind them, cover most of what you'll meet when working with real wireless links.
Frequency
f = c / λHow many times per second the wave repeats, measured in hertz. Wi-Fi works at 2.4, 5 and 6 GHz, which is billions of cycles a second.
Wavelength
λ = c / f (c ≈ 3×108 m/s)The physical length of one cycle. It sets antenna size: the higher the frequency, the shorter the wavelength and the smaller the antenna.
dBm
dBm = 10·log10(P / 1 mW)Power relative to one milliwatt, on a log scale. 0 dBm is 1 mW, 20 dBm is 100 mW, and −70 dBm is a ten-millionth of a milliwatt.
dBi & gain
half-wave dipole ≈ 2.15 dBiGain relative to an ideal antenna that radiates equally in all directions. Antennas don't create power. They focus it, like a lens.
Polarization
loss factor = cos2θThe orientation of the electric field. A vertical antenna talking to a horizontal one can lose 20 dB or more. Point them the same way.
Radiation pattern
beamwidth = angle between −3 dB pointsA 3D map of where an antenna sends its energy. Gain in one direction always comes at the expense of another.
How long should the antenna be?
Antennas are sized in fractions of a wavelength. Enter a frequency, or pick a preset, to get the wavelength and the classic dipole and quarter-wave lengths.
Pick the shape for the job.
More gain means a narrower beam. An omni lights up the whole room, while a dish lights up one building five kilometres away. Values are typical ranges and vary by model.
| Antenna | Pattern | Typical gain | Beamwidth (H) | Where you'll see it |
|---|---|---|---|---|
| Dipole / “rubber duck” | Omnidirectional | 2–5 dBi | 360° | Home routers, USB adapters, handheld radios |
| Collinear omni | Omni, flattened | 6–12 dBi | 360° | Outdoor hotspots, masts, campus coverage |
| Patch / panel | Directional | 6–9 dBi | 60–80° | Wall-mounted APs, long hallways, warehouses |
| Sector | Directional | 14–18 dBi | 60–120° | Wireless ISP and cellular base stations |
| Yagi-Uda | Highly directional | 10–17 dBi | 25–50° | Point-to-point links, TV reception, ham radio |
| Parabolic dish / grid | Highly directional | 24–34 dBi | 3–10° | Long-range backhaul between buildings or towers |
Where the energy actually goes.
Add dB, don't multiply watts.
Radio power spans more than twenty orders of magnitude, from watts at the transmitter to picowatts at the receiver. Decibels turn all of that into simple addition and subtraction.
| dBm | Power | Where you'll meet it |
|---|---|---|
| +36 | 4 W | US EIRP limit, 2.4 GHz point-to-multipoint |
| +30 | 1 W | One watt, a common outdoor radio ceiling |
| +20 | 100 mW | EU EIRP limit at 2.4 GHz; a typical AP |
| +15 | ~32 mW | A typical phone's Wi-Fi transmit power |
| 0 | 1 mW | The reference point |
| −30 | 1 µW | Received a meter or two from an AP |
| −67 | ~0.2 nW | Design target for voice and video Wi-Fi |
| −90 | 1 pW | Around the noise floor; the link is gone |
- +3 dB ≈ double the power
- +10 dB = ten times the power
- −3 dB ≈ half the power
- +6 dB ≈ double the free-space range
- Gains and losses in dB simply add
EIRP: what actually leaves the antenna
EIRP = Ptx + Gant − Lcable20 dBm radio + 6 dBi antenna − 2 dB cable = 24 dBm EIRP (about 251 mW). Regulators limit EIRP, not just transmitter power.