Wavelength

Wavelength is the physical distance a radio wave travels during one complete cycle of its oscillation. It is found by dividing the speed of light (~300,000,000 m/s) by the frequency in hertz, so wavelength and frequency are inversely related: the higher the frequency, the shorter the wave. The 915 MHz band common to North American LoRa has a wavelength of about 33 cm; the 868 MHz European band, about 35 cm; 2.4 GHz Wi-Fi, about 12.5 cm. Every practical decision in radio — antenna length, mounting position, what the signal will and won't pass through — traces back to this one number.

Why it sets antenna size

Antennas are dimensioned in fractions of a wavelength, which is why wavelength is the first number an RF builder calculates. A classic half-wave dipole is one half-wavelength long end to end, and a quarter-wave whip (the stubby antenna on most mesh nodes) is a quarter of it — roughly 8 cm at 915 MHz. An antenna cut to the wrong length for its frequency radiates poorly and reflects power back into the radio, so matching antenna length to operating wavelength matters more for real-world range than transmit power does. This is also why "universal" antennas should be treated with suspicion: an antenna sold for 868 MHz is measurably mistuned at 915 MHz, and the few percent of length difference costs real decibels. When in doubt, check the antenna's tuned band against your region's, and remember that a well-matched antenna at both ends beats a power increase at one.

Wavelength and propagation

Wavelength also shapes how a signal behaves in the world. Longer waves (lower frequencies) diffract around obstacles and penetrate foliage and buildings better, while shorter waves travel more line-of-sight and are more easily blocked — one reason sub-GHz LoRa reaches farther through terrain than 2.4 GHz Wi-Fi at similar power. Objects comparable in size to the wavelength interact with it strongly: at 33 cm, human bodies, metal racks, and vehicle panels are all significant reflectors and absorbers, which is why moving a mesh node half a metre can transform a marginal link. Wavelength further determines the size of the Fresnel zone that must stay clear along a path for the link to achieve anything near line-of-sight performance.

Practical rules of thumb

Three habits follow directly from the physics. First, ground-plane and spacing effects operate in wavelength terms: keeping an antenna at least a quarter-wavelength clear of metal surfaces avoids detuning it. Second, height beats power — raising an antenna clears obstacles that no legal power increase can punch through, because diffraction losses at these wavelengths are brutal. Third, when comparing bands, think in trade-offs: lower frequency means longer wavelength, better penetration, and physically larger antennas; higher frequency means more bandwidth but a more fragile path. For off-grid Bitcoiners building Meshtastic or other mesh links as communication backups, these rules are most of what separates a network that works in a demo from one that works in a storm.

One number worth memorising: wavelength in metres is roughly 300 divided by the frequency in megahertz. That mental shortcut — 915 MHz is about a third of a metre, 146 MHz about two metres, 7 MHz about forty — instantly tells you how big antennas must be and how a band will behave. Inside coaxial cable, signals travel slower than in free space (the cable's velocity factor, typically 0.66–0.85), so elements cut from coax must be shortened accordingly — a classic gotcha when building antennas from online dimensions that assumed bare wire.

Wavelength underpins antenna design and path planning alike. See the dipole antenna it sizes, the Fresnel zone it governs, and SNR for how cleanly the arriving signal is heard.