Effect of Antenna Height
Raising the antenna above the ground and above piles of metal opens up the Fresnel zone and reduces shadowing loss, so on most sites it gains more than increasing transmitter power. Considering that a 150 metre 433 MHz link needs a clear volume of roughly 5 metres radius at mid-path, a height difference of a few metres translates directly into dB.
Height has three separate contributions and they add up.
- The Fresnel zone opens up: the ground and low obstacles leave the volume around the direct path.
- Shadowing decreases: the load, a truck or a machine body no longer gets in between.
- Ground reflection weakens: the strongest component of multipath fading is usually the one coming off the ground.
On the other hand height is not unlimitedly good: taking the antenna very high can drop the vertical pattern null onto a nearby operator, and as the cable gets longer coaxial loss comes into play. For RG58 at 433 MHz this is about 0.44 dB/m, so 10 metres of cable costs 4.4 dB.
The practical balance is this: raise the antenna enough to clear the obstacles, but do not extend the cable length unnecessarily. In AXI's IP67 sets the antenna and cable gland being included in the set contents is what makes this adjustment possible.
Context in which this term is used: 433 MHz or 2.4 GHz?