RF Basics: Radio Frequency Fundamentals, Explained

What is RF?

Radio Frequency (RF) refers to electromagnetic waves from about 3 kHz to 300 GHz, between audio and visible light. RF is used for wireless comms – e.g. broadcast radio/TV, cellular, Wi-Fi, and radar. The ITU divides this band into named regions: LF, MF, HF, VHF, UHF, SHF, and EHF. Each has typical uses and wavelength ranges, as summarized below:

Band	Freq. Range	Wavelength	Common Uses
LF	30 – 300 kHz	10 – 1 km	Radio navigation beacons, time signals, RFID (long-range comms)
MF	0.3 – 3 MHz	1 – 0.1 km	AM broadcast radio, maritime/mobile comms (ground/sky waves)
HF	3 – 30 MHz	100 – 10 m	Shortwave/short-range broadcasting, amateur radio, ionospheric “skywave” links
VHF	30 – 300 MHz	10 – 1 m	FM radio, VHF TV, aviation/airband, marine, two-way radio (line-of-sight)
UHF	0.3 – 3 GHz	1 – 0.1 m	Cell phones, Bluetooth/Wi-Fi (2.4, 5 GHz), UHF TV, radar, GPS (1.5 GHz)
SHF	3 – 30 GHz	0.1 – 0.01 m	5G, satellite links, microwave radio, point-to-point WLAN (5–6 GHz)
EHF	30 – 300 GHz	10 – 1 mm	Millimeter-wave (mmWave) 5G, imaging/radar; mostly experimental (high atmospheric loss)

(MHz = megahertz, GHz = gigahertz; ITU band names and ranges from [74]).

Wavelength, Frequency, and the Speed of Light

The wavelength (λ) and frequency (f) of an RF wave are inversely related by λ = c/f, where c is the speed of light (c ≈ 2.998×10^8 m/s). Thus higher frequencies have shorter wavelengths. For example:

• At 900 MHz: λ = (3.00×10^8 m/s) / (9.00×10^8 Hz) ≈ 0.33 m (33 cm).
• At 2.4 GHz: λ = 3.00e8 / 2.4e9 ≈ 0.125 m (12.5 cm).
• At 5.0 GHz: λ = 3.00e8 / 5.0e9 ≈ 0.060 m (6.0 cm).
• At 28 GHz: λ = 3.00e8 / 28e9 ≈ 0.0107 m (10.7 mm).

These calculations use c ≈ 3×10^8 m/s. Wavelength helps determine antenna size (e.g. a 2.4 GHz Wi-Fi dipole is about 6 cm long).

dB, dBm, dBi, dBd — Decibel Fundamentals

Decibels (dB) express power ratios on a logarithmic scale:
$$\text{Gain (dB)} = 10\log_{10}\frac{P_{\text{out}}}{P_{\text{in}}}.$$
This compresses large ratios into convenient numbers. For example, a power ratio of 10:1 is 10 dB, 100:1 is 20 dB, and 2:1 is 3.01 dB. Conversely, a loss of 3 dB halves the power. Using dB lets us add gains/losses along a link easily (multiplication of ratios → addition of dB).

For absolute power, dBm and dBW units fix a reference. 0 dBm = 1 milliwatt (mW). Every 10 dB is a factor of 10 in power: e.g. 30 dBm = 1 W, 20 dBm = 0.1 W, –10 dBm = 0.1 mW.

Antennas use dB for gain: dBi = gain above an isotropic (ideal) radiator, while dBd = gain above a half-wave dipole (a real reference). A dipole has 2.15 dBi gain, so: dBd ≈ dBi – 2.15. Thus 0 dBd = 2.15 dBi (dipole gain) and 0 dBi = –2.15 dBd.

Ratio (power)	dB	dBm to mW
0.1	–10	–30 dBm = 0.001 mW
0.5	–3.01	–20 dBm = 0.01 mW
1	0	–10 dBm = 0.1 mW
2	3.01	0 dBm = 1 mW
10	10	10 dBm = 10 mW
100	20	20 dBm = 100 mW
1000	30	30 dBm = 1 W

Common dB conversions: a power ratio of 2 ≈ 3 dB, 10 ≈ 10 dB, 100 ≈ 20 dB, etc. (dBm table reference: 0 dBm =1 mW.)

Key RF Quantities

• Transmit Power (P_t) – the RF output of the transmitter, measured in watts or dBm. Typical handheld radios might output 1–30 dBm (1–1000 mW), while base stations use higher power (W).
• Antenna Gain (G) – how much an antenna focuses power in a direction, relative to isotropic (dBi) or dipole (dBd). A high-gain antenna (e.g. 15 dBi) concentrates energy and extends range.
• EIRP (Equivalent Isotropically Radiated Power) – transmitter power plus antenna gain (in dBm + dBi). It is the power an ideal isotropic antenna would need to match the field strength. For example, a 20 dBm transmitter with a 6 dBi antenna yields EIRP=26 dBm.
• Received Power (P_r) – the power at the receiver antenna terminals. In free space, the Friis formula applies: [P_r = P_t + G_t + G_r – \text{FSPL (dB)},] where FSPL is free-space path loss. (Practically, also account for cable/connector losses and polarization mismatch.)
• Noise Floor – the thermal noise power in the receiver bandwidth, given by k·T·B. At room temp (290 K), noise density is –174 dBm/Hz. For bandwidth B (Hz), noise floor ≈ –174 dBm + 10·log₁₀(B). E.g. 20 MHz BW: –174 + 10·log₁₀(20e6) ≈ –101 dBm.
• SNR (Signal-to-Noise Ratio) – the difference (in dB) between received signal and noise floor: SNR = P_r – P_noise. A higher SNR means clearer reception.

Worked example: Suppose P_t=30 dBm, G_t=9 dBi, G_r=3 dBi, at 2.4 GHz, distance 1 km. FSPL ≈ 100 dB, so P_r ≈30+9+3–100=–58 dBm. For 20 MHz BW, noise floor ≈–101 dBm, yielding SNR ≈ 43 dB.

How RF Signals Propagate

RF waves generally travel by line-of-sight or by interacting with the environment. In free space (open air, no obstacles) they obey the inverse-square law (power ∝1/d²). Obstacles and media cause additional effects:

• Reflection: Waves bounce off surfaces (buildings, ground, water). Reflected paths can interfere (constructively or destructively).
• Diffraction: Waves bend around edges (rooflines, hills). This “bend” enables non-line-of-sight links at lower frequencies.
• Scattering: Waves hitting rough/irregular objects scatter in multiple directions. Urban clutter often causes multipath fading.
• Absorption: Materials (and atmosphere) attenuate RF. Moisture, rain, and gases absorb energy, especially at higher frequencies (e.g. 60 GHz oxygen absorption).

Radio waves experience reflection, refraction, diffraction, scattering, and attenuation much like light. For example, ground waves (LF/MF) can follow Earth’s curvature via diffraction, while HF signals refract off the ionosphere for long skip links. Higher-frequency (UHF/SHF) signals mostly go line-of-sight and suffer greater attenuation in rain or foliage. Understanding these mechanisms is key to link planning and choosing frequencies for range.

Polarization (Linear, Circular) and Mismatch Loss

The polarization of an RF wave is the orientation of its electric field vector. For linear polarization, the field oscillates in one plane (e.g. vertical or horizontal); circular polarization has the field rotating (right- or left-hand rotation). Antennas are usually designed for a given polarization. Two antennas with the same polarization (both vertical, for instance) transfer energy efficiently; if one is rotated, less energy is coupled.

Figure: An RF wave’s electric (E) and magnetic (H) fields are perpendicular; linear polarization (here horizontal) is the orientation of the E-field.

A mismatch in polarization causes polarization loss. In theory two orthogonal linear polarizations (90° apart) give infinite loss, but real antennas leak some cross-polar signal. A rule of thumb: linear vs. circular mismatch costs about 3 dB, while slight misalignments or non-ideal conditions often introduce 5–20 dB loss. For example, a vertical antenna paired with one tilted 45° yields about 3 dB extra loss. Proper alignment (or using matched circular polarizations) maximizes received power.

Common Pitfalls for Beginners

• Impedance Mismatch: RF systems typically use 50 Ω. Mismatch (e.g. 50 Ω cable to 75 Ω antenna) causes reflections and standing waves, wasting power. Always match impedances (with a tuner or matching network) to minimize VSWR and loss.
• Cable/Connector Loss: Coaxial cable attenuates RF – higher frequency and thinner cable = more loss. For instance, RG‑58 loses ~0.5 dB/ft at 1 GHz. Every connector adds ~0.1–0.3 dB. Long runs or many adapters can dramatically cut link margin. Use low-loss cable (LMR‑400 etc) for high-frequency or long-distance links.
• Near-Field vs Far-Field: RF formulas (like Friis) assume far-field (plane-wave). The far-field starts roughly at the Fraunhofer distance d = 2D²/λ (D = largest antenna dimension). In the near-field (<d), the fields are reactive and simple path-loss formulas fail. Ensure you place antennas at least a few λ apart (and >>2D²/λ) to be safely in the far-field.
• Other Practical Issues: Neglecting connectors, weather effects, or using uncalibrated instruments can skew results. Also, regulations limit EIRP in many bands – don’t assume unlimited power or gain.

Where to Go Next

For more on these topics, see the AntennaMath learning pages on Antenna Basics, Path Loss, and Impedance Matching. To practice calculations, try our Free-Space Path Loss (FSPL), EIRP, and Link Budget calculators.

FAQ

What frequency range is considered “RF”?

RF covers roughly 3 kHz to 300 GHz. This spans traditional radio (kHz–MHz) up through microwave (GHz) and beyond. In other words, RF includes everything below infrared light and above the audio range.

How do I calculate wavelength from frequency?

Use λ = c/f, where c ≈ 3×10^8 m/s. For example, at f=100 MHz, λ = 3×10^8/1×10^8 = 3 m. At f=2.4 GHz, λ ≈ 0.125 m (12.5 cm). Always keep consistent units (Hz for f, m/s for c) when computing.

What does 0 dBm mean in watts?

0 dBm is defined as 1 milliwatt of power. Since every 10 dB is a factor of 10, +10 dBm = 10 mW, +20 dBm = 100 mW, +30 dBm = 1 W. Conversely, –10 dBm = 0.1 mW, –20 dBm = 0.01 mW, etc.

Why does antenna polarization matter?

Polarization is the orientation of the wave’s electric field. If transmit and receive antennas have mismatched polarization (e.g. one vertical, one horizontal), much of the signal is lost (often 3–20 dB). Matching polarization maximizes coupling. Circular vs linear mismatch always costs ~3 dB, and cross-polarized antennas leak only a small fraction of power.

What is free-space path loss (FSPL)?

FSPL is the power drop in a line-of-sight RF link due to spreading of the wave. In dB, FSPL ≈ 20·log₁₀(4πd/λ) (or 20·log₁₀(d) + 20·log₁₀(f) + constant). For instance, at 2.4 GHz over 1 km, FSPL ≈ 100 dB. This must be overcome by antenna gain or transmit power to achieve a usable received signal.