Wavelength (λ)
Wavelength is the physical distance a wave travels in one full cycle and in vacuum is found by the relation λ = 300 / f(MHz) metres. That is about 69 cm at 433 MHz, 35 cm at 868 MHz and 12.2 cm at 2.45 GHz. Antenna length, ground plane size and the effect of obstacles all scale with this length.
Wavelength is a reference ruler in RF. Whether an obstacle, a piece of metal or an antenna counts as "large" or "small" is decided not in centimetres but relative to λ.
- Antenna: for an efficient radiator the typical target is a length on the order of λ/4 or λ/2. Antennas going below λ/10 rapidly become inefficient.
- Ground plane: for the conductive surface under a monopole the practical target is a radius of at least λ/4.
- Obstacle: obstacles smaller than the wavelength are worked around by diffraction; obstacles much larger than λ leave a deep shadow.
That is why the 69 cm wavelength of 433 MHz can bend through a doorway or around the edge of a column, while the 12 cm of 2.45 GHz loses noticeably more in the same geometry. The rule is the same when choosing the antenna location in the field: keep the antenna at least a few centimetres, and if possible on the order of λ/4, away from metal surfaces.
Context in which this term is used: 433 MHz or 2.4 GHz?