What the Fresnel zone is
Radio waves don't travel in a thin line. The first Fresnel zone is an ellipse-shaped region around the direct path, widest at the middle, where reflections from obstructions would arrive out of phase and weaken the signal. If at least 60% of the first Fresnel zone is clear, the link behaves almost as if it were in free space. The radius at any point is r = 17.32 × √(d₁ × d₂ ÷ (f × d)) metres, with distances in km and frequency in GHz.
Earth bulge
Over longer links, the Earth's curve rises between the antennas. Radio waves bend slightly with the atmosphere, which is allowed for with the k factor, normally 4/3. The clearance needed at the midpoint is the earth bulge plus 60% of the Fresnel radius, plus any trees or buildings.
Lower frequencies need more room
The Fresnel zone is wider at lower frequencies: for a 20 km link, about 16 m at 5.8 GHz but over 100 m at 145 MHz. That's one reason VHF links still work past obstructions by diffraction, while microwave links need a clear view. The link budget calculator works out the signal strength over the path.
Questions
What is the Fresnel zone?
An ellipse-shaped region around the direct path between two antennas, which must be mostly clear for full signal strength.
How much of the Fresnel zone needs to be clear?
At least 60% of the first Fresnel zone, plus the earth bulge on longer links.
Why does frequency change the Fresnel zone?
The radius is proportional to the square root of the wavelength, so lower frequencies have larger zones.
What is the radio horizon?
How far apart two antennas can be before the Earth's curve blocks the path, about 4.12 × (√h₁ + √h₂) km with heights in metres.