ANALYZING PROPAGATION AND LINK BUDGET...
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RADIO WAVES :: PROPAGATION AND RANGE
> Range depends as much on the path and antennas as on the radio settings.
FREQUENCY AND WAVELENGTH
Frequency tells us how many cycles occur each second. Wavelength is the physical distance covered by one cycle in free space. The two are linked by the speed of light.
wavelength = speed of light / frequency
At 868 MHz:
λ ≈ 0.345 m
λ/2 ≈ 17.3 cm
λ/4 ≈ 8.6 cmHOW WAVES REACH THE RECEIVER
The strongest path is usually line of sight between the two antennas. Raising either antenna can transform a poor link.
Radio waves can bend around an obstacle or ridge, but the signal arriving behind it is weaker.
Buildings, ground, water, and metal can create several paths. These paths may reinforce or cancel one another.
Walls, soil, the human body, and especially wet vegetation absorb part of the energy.
Rough surfaces, foliage, vehicles, and dense urban structures spread energy in multiple directions.
Other short-range devices share the band. A strong local transmitter can mask a distant LoRa signal.
THE FRESNEL ZONE
A radio link needs more than a thin visual line between antennas. The first Fresnel zone is an ellipsoidal volume around the direct path. Obstacles entering this zone cause diffraction and additional loss, even when the two antennas appear visually aligned.
LINK BUDGET
Received power =
Transmit power
+ transmit antenna gain
- transmit cable and connector losses
- propagation loss
+ receive antenna gain
- receive cable and connector lossesA link works when the received power remains above the receiver sensitivity with enough margin for fading, movement, rain-wet vegetation, interference, and installation variations.
RSSI AND SNR
Received Signal Strength Indicator. A value closer to 0 dBm is stronger. Around -40 dBm is very strong; values below -120 dBm are weak but may still be usable with LoRa.
Signal-to-noise ratio. A positive value is comfortable. LoRa can demodulate some signals below the noise floor, so negative SNR does not automatically mean failure.
WHAT MOST OFTEN REDUCES RANGE
- Antennas placed on the floor, inside a metal cabinet, or behind reinforced concrete.
- One antenna vertical and the other horizontal.
- An antenna designed for 915 MHz used on 868 MHz, or the wrong connector/adapter.
- Long, thin coaxial cable between the radio and antenna.
- Buildings, hills, dense forest, wet leaves, or a person standing next to the antenna.
- High local noise from electronics, USB power supplies, computers, or other transmitters.
- Too much mesh traffic, causing collisions and congestion.
EUROPEAN 863–870 MHz BAND
European LoRa systems commonly use the 863–870 MHz short-range-device spectrum. This is not one uniform channel: sub-bands have different power, duty-cycle, and channel-access rules. LoRaWAN regional parameters and the device firmware must be configured for the deployment region.
Examples of common European SRD conditions:
863–868 MHz usually low power with duty-cycle or polite-access limits
868.7–869.2 MHz 25 mW e.r.p., often 0.1% duty cycle or LBT+AFA
869.4–869.65 MHz up to 500 mW e.r.p., up to 10% duty cycle or LBT+AFA
Always verify the currently applicable national and sub-band rules.PRACTICAL RANGE TEST
1. Fix one node at a known position and height.
2. Use the same radio profile on both nodes.
3. Record antenna type and orientation.
4. Walk or drive away while logging RSSI, SNR, and packet success.
5. Mark obstacles and terrain changes.
6. Repeat with the fixed antenna higher.
7. Change only one parameter at a time.