Fresnel Zone
The Fresnel zone is the ellipsoidal volume surrounding the direct path between transmitter and receiver; most of the RF energy is carried inside it. Its radius at mid-path is found with r = ½·√(λ·D). On a 150 metre 433 MHz link this is about 5 metres, and about 2 metres at 2.45 GHz, and the aim is to keep at least 60% of it clear.
An obstacle entering the zone cuts part of the wave and disturbs the phase relationship; the result is direct attenuation. That is why the situation of "we can see each other but the remote control does not hold" is very common.
- Mid-path radius for a 100 metre link: about 4.2 m at 433 MHz, 2.9 m at 868 MHz, 1.7 m at 2.45 GHz.
- For a 150 metre link: about 5.1 m, 3.6 m and 2.1 m respectively.
- The one advantage of higher frequency shows up here: the short wavelength requires a thinner tube.
The practical application is simple: keep antennas away from the ground and from large metal surfaces by roughly this radius. Bringing the receiver antenna on top of the enclosure lid, to the most open point of the machine body, alone gives a noticeable range gain on most sites. Leaving the antenna close to the ground, by contrast, blocks the entire lower half of the zone.
Context where this term is used: 433 MHz or 2.4 GHz?