What are the noise floor and SNR, and how are they related to range?
The noise floor is the energy level present at the receiver input even when there is no signal; SNR is the received signal level minus the noise floor. What determines range is not the absolute signal but this difference. If the noise floor rises by 10 dB, the effective range shortens by about 3.16 times even if you do not change the transmitter power at all; the fact that 6 dB doubles the distance is calculated from the same scale.
Receiver sensitivity figures (typically -125 to -130 dBm in FSK) are measured under clean laboratory conditions. When the noise floor in the field rises above that value, the receiver's catalogue sensitivity becomes meaningless; the limit is no longer your hardware but the environment.
The calculation is simple. Free space path loss is found with the formula FSPL = 32.44 + 20*log10(d_km) + 20*log10(f_MHz); at 433.92 MHz it comes out as 65.2 dB for 100 m and 68.7 dB for 150 m. The link budget is then received power = transmitter power + transmit antenna gain - cable loss - FSPL - obstacle loss + receive antenna gain. As long as this value stays sufficiently above the noise floor the link holds; the difference between them is called the fade margin, and leaving 10-20 dB of margin is common engineering practice in field installations.
In practice this means:
- With a range complaint, look for noise sources first, do not try to increase the power.
- Raising the signal level and lowering the noise are mathematically equivalent; the second is usually cheaper.
- A system running at the limit behaves erratically depending on the time of day, the equipment on site and the weather. Leave margin.
Full topic: 433 MHz or 2.4 GHz?